Cavity Ringdown Spectroscopy of cis-cis HOONO and the HOONO/HONO 2 Branching Ratio in the Reaction OH + NO2 + M

Article abstract of DOI:10.1021/jp034407c

The termolecular association reaction OH + NO₂ + M was studied in a low-pressure discharge flow reactor, and both HONO₂ and HOONO products were detected by infrared cavity ringdown spectroscopy (IR-CRDS). The absorption spectrum of the fundamental ν₁ band of the cis-cis isomer of HOONO (pernitrous or peroxynitrous acid) was observed at 3306 cm ^-1, in good agreement with matrix isolation studies and ab initio predictions. The rotational contour of this band was partially resolved at 1 cm^-1 resolution and matched the profile predicted by ab initio calculations. The integrated absorbances of the ν₁ bands of the cis-cis HOONO and HONO₂ products were measured as a function of temperature and pressure. These were converted to product branching ratios by scaling the experimentally observed absorbances with ab initio integrated cross sections for HOONO and HONO₂ computed at the CCSD(T)/cc-p VTZ level. The product branching ratio for cis-cis HOONO to HONO₂ was 0.075 ± 0.020(2σ) at room temperature in a 20 Torr mixture of He/Ar/N₂ buffer gas. The largest contribution to the uncertainty is from the ab initio ratio of the absorption cross sections, computed in the double harmonic approximation, which is estimated to be accurate to within 20%. The branching ratio decreased slightly with temperature over the range 270 to 360 K at 20 Torr. Although trans-perp HOONO was not observed, its energy was computed at the CCSD(T)/cc-pVTZ level to be E₀ = +3.4 kcal/mol relative to the cis-cis isomer. Statistical rate calculations showed that the conformers of HOONO should reach equilibrium on the time scale of this exeriment. These results suggested that essentially all isomers had converted to cis-cis HOONO; thus, the reported branching ratio is a lower bound for and may represent the entire HOONO yield.

Full text of DOI:10.1021/jp034407c

6974
J. Phys. Chem. A 2003, 107, 6974-6985
Cavity Ringdown Spectroscopy of cis-cis HOONO and the HOONO/HONO2 Branching
Ratio in the Reaction OH + NO2 + M
Brian D. Bean,† Andrew K. Mollner, Sergey A. Nizkorodov,‡ Gautham Nair, and
Mitchio Okumura*
Arthur Amos Noyes Laboratory of Chemical Physics, MC 127-72, California Institute of Technology,
Pasadena, California 91125
Stanley P. Sander*
NASA Jet Propulsion Laboratory, MC 183-901, California Institute of Technology, Pasadena, California 91109
Kirk A. Peterson
Department of Chemistry, Washington State UniVersity, Pullman, Washington 99164-4630
Joseph S. Francisco
Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907A
ReceiVed: February 17, 2003; In Final Form: June 27, 2003
The termolecular association reaction OH + NO2 + M was studied in a low-pressure discharge flow reactor,
and both HONO2 and HOONO products were detected by infrared cavity ringdown spectroscopy (IR-CRDS).
The absorption spectrum of the fundamental ν1 band of the cis-cis isomer of HOONO (pernitrous or
peroxynitrous acid) was observed at 3306 cm-1, in good agreement with matrix isolation studies and ab initio
predictions. The rotational contour of this band was partially resolved at 1 cm-1 resolution and matched the
profile predicted by ab initio calculations. The integrated absorbances of the ν1 bands of the cis-cis HOONO
and HONO2 products were measured as a function of temperature and pressure. These were converted to
product branching ratios by scaling the experimentally observed absorbances with ab initio integrated cross
sections for HOONO and HONO2 computed at the CCSD(T)/cc-pVTZ level. The product branching ratio for
cis-cis HOONO to HONO2 was 0.075 ( 0.020(2σ) at room temperature in a 20 Torr mixture of He/Ar/N2
buffer gas. The largest contribution to the uncertainty is from the ab initio ratio of the absorption cross sections,
computed in the double harmonic approximation, which is estimated to be accurate to within 20%. The
branching ratio decreased slightly with temperature over the range 270 to 360 K at 20 Torr. Although trans-
perp HOONO was not observed, its energy was computed at the CCSD(T)/cc-pVTZ level to be E0 ) +3.4
kcal/mol relative to the cis-cis isomer. Statistical rate calculations showed that the conformers of HOONO
should reach equilibrium on the time scale of this exeriment. These results suggested that essentially all
isomers had converted to cis-cis HOONO; thus, the reported branching ratio is a lower bound for and may
represent the entire HOONO yield.
Introduction
rate of reaction 1 has important consequences for tropospheric
ozone production and photochemical smog. In polluted urban
airsheds with high ratios of NOx to volatile organic compounds
(VOC), the rate of reaction 1 is one of the single most influential
parameters determining peak ozone mixing ratios.3,4 At high
enough NOx levels, urban ozone levels decrease with increasing
NOx, because elevated NOx levels reduce OH concentrations
through reaction 1 and lead to reduced VOC oxidation rates.
Tonnesen5 has shown that model predictions of ozone produc-
tion can be almost inversely proportional to changes in the rate
coefficient for reaction 1 under these conditions. This negative
feedback complicates ozone attainment strategies, which must
often balance regulation of VOC vs NOx emissions. Further-
more, the uncertainty in the rate of reaction 1 is the major
contribution to model uncertainties in predicted ozone at high
NOx levels. This uncertainty propagates to scales such as the
Maximum Incremental Reactivity, a measure of the ozone-
The free radical chain termination reaction
OH + NO2 + M f HONO2 + M
(1)
is important in oxidation chemistry throughout the lower
atmosphere. Radicals in both the HOx and NOx families mediate
the destruction of ozone in the stratosphere and are central to
the formation of photochemical smog in the troposphere.1,2
Reaction 1 reduces the concentrations of these species by
converting them into the stable reservoir species nitric acid. The
* Corresponding Authors. E-mail (Okumura): mo@its.caltech.edu. E-
mail (Sander): Stanley.P.Sander@jpl.nasa.gov.
† Current address: Department of Chemistry, University of Wisconsin-
Madison, Madison, WI 53706.
‡ Current address: Department of Chemistry, University of California
at Irvine, Irvine, CA 92697.
10.1021/jp034407c CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/19/2003

Cavity Ringdown Spectroscopy of cis-cis HOONO
J. Phys. Chem. A, Vol. 107, No. 36, 2003 6975
Golden and Smith,31 Matheu and Green,33 and Troe34 have
modeled the pressure and temperature dependence of k1 and
k2. Since the submission of this paper, there has been a new set
of master equation calculations by Golden, Barker, and Lohr.35
While all sets of calculations suggest that HOONO formation
should compete with the HONO2 channel in the reaction OH
+ NO2, the resulting branching ratios at 1 atm and 298 K vary
from >1% (Matheu and Green), to 2.5% (Troe), to 30% (Golden
and Smith). However, rate constants, falloff curves, and product
yields cannot be calculated quantitatively from first principles,
because our knowledge of the transition states, relaxation
processes, and potential surfaces (especially for HOONO
formation) is too limited. These models rely to some extent on
fitting experimental data, and thus may not accurately predict
branching ratios without direct measurements of HOONO.
Figure 1. Energy level diagram of the reaction OH + NO2. The cis-
cis and trans-perp isomers of HOONO are shown. Energies for
HOONO are from CCSD(T) calculations.
There have been extensive calculations36-45 of the energetics
and conformations of peroxynitrous acid. From MP2 calcula-
tions, McGrath and Rowland found three stable forms of
HOONO, the cis-cis, cis-trans, and trans-perp isomers (distin-
guished by the HOON and OONO torsional angles). However,
recent results at the highest levels of theory (QCISD(T)/cc-
pVTZ) indicate that only the cis-cis and trans-perp conforma-
tions are stable stationary points.46 The best purely theoretical
estimate of the O-O bond energy of cis-cis HOONO is ∆2H0
) -19.8 kcal/mol at the CCSD(T)/CBS limit with corrections.44
This estimate is based on the computed ∆ H0(NO2), which is
forming potential of individual VOCs.6,7 In the stratosphere, both
HOx and NOx radicals are involved in catalytic cycles that
deplete ozone. Reaction 1 couples these catalytic cycles, and is
one of several key reactions that lead to a more complex
dependence of stratospheric ozone on NOx levels. Sensitivity
analyses have shown that differences in recommended rates for
reaction 1 have a significant effect on mid-latitude ozone
concentrations.8
Despite extensive studies over a wide range of temperatures
and pressures,9-26 significant uncertainties remain at atmo-
spherically relevant pressures and temperatures. Much attention
has recently focused on the yield for a second minor product
channel
f
0
in error by 1.5 kcal/mol. A perhaps more accurate assessment,
based on ∆fH00 (cis-cis HOONO) - ∆ H0(HONO2) ) 29.0
f
0
kcal/mol, gives ∆2H0 ) -18.3 kcal/mol. In either case, this
binding energy is large enough to expect that cis-cis HOONO
will be a relatively stable molecule at room temperature (see
Figure 1). The energy of the trans-perp isomer is computed to
be 1-3.2 kcal/mol above the cis-cis form. The transition state
for isomerization between these isomers, the perp-perp isomer,
has an MP2 energy (including ZPE correction) of 11.4 kcal/
mol relative to the cis-cis isomer.37
OH + NO2 + M f HOONO + M
(2)
producing HOONO (peroxynitrous or pernitrous acid), a weakly
bound isomer of nitric acid. The NASA Panel for Data
Evaluation27 and the IUPAC Gas Kinetic Data Evaluation28,29
summarize the current evidence for this channel, but neither
source currently makes a recommendation for its inclusion in
atmospheric models. A significant branching fraction into
channel 2 is of importance in the atmosphere. Formation of
HOONO would reduce the efficiency of the OH + NO2 + M
reaction as a sink for OH and NO2 radicals, because HOONO
would rapidly dissociate or photolyze back to reactants.
While Leighton30 had suggested the existence of the HOONO
channel in the gas phase, Robertshaw and Smith9 first proposed
that a significant HOONO yield could resolve a discrepancy in
the pressure dependence of reaction 1. The effective bimolecular
rate constant k1 has an anomalous falloff curve; the observed
HOONO has been studied extensively in the condensed phase,
though the most direct spectroscopic studies come from matrix-
isolation work. Lee and colleagues47-50 reported vibrational and
UV absorption spectra of HOONO trapped in rare gas matrixes.
The observed infrared bands matched frequencies and intensities
predicted by ab initio calculations for the cis-cis and trans-perp
isomers. The cis-perp isomer is predicted to have vibrational
frequencies similar to those of the trans-perp isomer, but it was
ruled out on the basis of intensity patterns.
In the gas phase, two earlier attempts at infrared absorption
spectroscopy failed to detect HOONO. Burkholder et al.51
attempted to prepare HOONO in a fast-flow multipass cell using
a variety of methods ranging from microwave discharge to direct
synthesis with H2O2 and NOBF4. They estimated an upper limit
for HOONO yield of 5% based on sensitivity arguments, and
an HONO2 yield of 75% + 25%/-10% from mass balance.
More recently, Dransfield et al.52 attempted to detect HOONO
formed by reaction 1 with reaction modulation FTIR spectros-
copy, but found no unidentified bands that could be assigned
to HOONO in the range 700-3200 cm-1. They estimated an
upper limit for HOONO of 3-4% based on mass balance
arguments.
∞
high-pressure rate constant k1 is significantly larger than the
high-pressure limit extrapolated from low-pressure data. The
discrepancy can be removed if the HOONO channel is
insignificant at low pressures, but becomes comparable to the
HONO2 channel at high pressures. Golden and Smith31 modeled
the OH + NO2 reaction and showed that the high- and low-
pressure data could be described quantitatively by statistical
models that included HOONO. Hippler et al.32 have recently
revised the high-pressure rate constants in He and N2 downward
by ≈30%, but the discrepancy remains.
An energy level diagram illustrating these two channels is
shown in Figure 1. At low pressures, the initial HOONO
association adduct will redissociate more quickly than HONO2
due to the former’s lower density of states, and bound HOONO
will be formed in low yields. At high pressures, the weakly
bound complex will be stabilized rapidly and the HOONO yield
could be comparable to, or even exceed, the HONO2 yield.
Direct evidence for formation of HOONO in the gas phase
by reaction 2 has been reported only recently. In 2001, Donahue
et al.53 described a series of 18OH isotopic scrambling experi-
ments that supported the existence of reaction 2 and led to an
estimate of the low-pressure branching ratio of k2°/k1°) 0.17
at room temperature. Nizkorodov and Wennberg54 then reported

6976 J. Phys. Chem. A, Vol. 107, No. 36, 2003
Bean et al.
Figure 2. Overview of the experimental apparatus. A microwave discharge is used to produce H atoms that are introduced into an Ar flow through
the injector tip (exploded view), which then mixes with an NO2/N2 flow. The H atoms react by H + NO2 f OH + NO to form OH radicals. The
OH radicals react with NO2 to form HOONO and HONO2, within 1 cm. Infrared absorption spectra of products are detected downstream in the
ringdown cavity, formed by two high-reflectivity mirrors (dashed lines, R ) 99.98% at 2.8 µm).
the first spectroscopic observation of gas-phase HOONO from
the reaction of OH and NO2. They observed the 2ν1 overtone
spectrum of HOONO formed in a low-pressure discharge flow
cell using action spectroscopy, detecting OH by LIF after
predissociation of the overtone-excited molecules. They ob-
served several bands which they tentatively assigned to the
isomers of HOONO and combination bands. They estimated
that the HOONO yield was 5 ( 3% at 253 K and 20 Torr. In
2002, Hippler et al.32 reported kinetic evidence for channel 2.
They observed a biexponential decay in the OH LIF signal in
the reaction OH + NO2 at 5-100 bar and 430-475 K, which
they modeled as rapid loss of OH by reactions 1 and 2, followed
by slower decomposition of HOONO by the reverse rate
k-2. They found the enthalpy of reaction 2 at this temperature
range to be ∆H2(0) ) -19.8 kcal/mol, in good agreement with
the theoretical predictions of the cis-cis HOONO dissociation
energy D0.
to predict the rate that the trans-perp HOONO isomer converts
to the cis-cis form and compute the trans-perp HOONO energy.
From these results, we infer that the observed cis-cis yield most
likely represents the total HOONO yield.
Experiment
An overview of the experimental apparatus is shown in Figure
2. The flow cell comprised two regions, a discharge flow reactor
in which the OH + NO2 reaction took place, and a long path
spectroscopic detection region. In the reaction region, hydrogen
atoms were formed by microwave discharge and thermalized
by injection into a flow of pre-cooled argon. This gas was then
mixed with NO2 seeded in N2 to produce OH radicals by the
reaction
H + NO2 f OH + NO
(3)
In principle, the branching ratio between reactions 1 and 2
can be measured by infrared absorption spectroscopy of both
HOONO and HONO2, but the difficulty of detecting HOONO
at such low concentrations (<1012 cm-3) has been compounded
by possible spectral interference from the high concentrations
of HONO2 and NO2. However, the lowest energy isomer, cis-
cis HOONO, can be distinguished from HONO2 in the 3 µm
region, because the OH stretch is significantly red-shifted
relative to that of HONO2. Ab initio calculations indicate that
this shift arises from a hydrogen-bonding interaction between
the H atom and the terminal O atom which occurs only in the
cis-cis conformation (see Figure 1).36,37 Lo and Lee observed
this band at 3285 cm-1 in Ar matrix.55 In contrast, they observed
the ν1 band of the trans-perp isomer at 3563.4 and 3545.7 cm-1
(different substitution locations), nearly coincident with the OH
with NO2 in excess. The OH products of reaction 3 then reacted
with the NO2. The resulting mixture of products flowed into
the detection region, where they were detected by infrared
absorption from 2.6 to 3.4 µm using cavity ringdown spectros-
copy. Experiments were performed over the pressure range 5
to 40 Torr and the temperature range 240 to 360 K.
The cell was constructed of Pyrex and quartz, and the interior
walls were coated with halocarbon wax to minimize wall
reactions. The entire system was evacuated by a 52-cfm
mechanical pump through a stopcock and an LN2-trapped
foreline. Two capacitance pressure transducers were located at
either end of the detection region to verify constant pressure
throughout the cell during measurements. All of the gases used
in these experiments were UHP grade (99.999%) or higher
purity. Six mass flow transducers coupled with needle valves
enabled us to vary the flows of the reactant gases as well as the
purge flows over the ringdown mirrors. The flows and estimated
concentrations of reactants are listed in Table 1. The linear flow
velocity was typically 100-200 cm/s, and the total residence
time in the cell was typically 200 ms.
stretch of nitric acid at 3550.0 cm-1
.
In this work, we report a joint experimental/theoretical study
to measure the cis-cis HOONO-to-HONO2 branching ratio from
the reaction OH + NO2 + M, which we denote k2(c-c)/k1, in
a low-pressure discharge flow reactor. We have exploited the
high sensitivity of infrared cavity ringdown spectroscopy (IR-
CRDS) to observe directly the absorption spectra of cis-cis
HOONO as well as HONO2 in the 3 µm region. The branching
ratio can be determined from the integrated intensities of the
OH stretch modes of both species with the aid of ab initio
vibrational band strengths calculated at the QCISD and CCSD-
(T)/cc-pVTZ level. Finally, we perform statistical calculations
A. OH + NO2 Reaction Region. H-atoms were produced
in a microwave discharge source. H2 was mixed in a 1:200 ratio
with He that had been passed through a LN2-cooled zeolite trap
to remove condensable impurities. The mixture then flowed
through a quartz capillary (4-mm i.d.) in which a discharge was
maintained by an Evenson microwave cavity. The capillary
extended collinearly into the main reactor cell (2.5-cm i.d.) in

Cavity Ringdown Spectroscopy of cis-cis HOONO
J. Phys. Chem. A, Vol. 107, No. 36, 2003 6977
laser in a pair of KTP crystals.60 The OPA produced an IR beam
with energy E ≈ 1 mJ/pulse and diameter d ≈ 2 mm after
transmission through three filters to remove the residual 532
and 630 nm light. The IR line width was Γ ≈ 1 cm-1. The IR
wavelength was typically calibrated using the frequencies of
known HONO and HNO3 peaks in the spectrum. The IR beam
was aligned into the cavity, and the residual beam exiting the
cell was focused tightly onto a 1-mm diameter InSb detector
(Infrared Associates). Each ringdown trace was amplified and
then accumulated with a 300 MHz bandwidth digital oscil-
loscope with 8-bit digitizer/12-bit averager (Tektronix TDS 320).
For each point in a spectrum, typically 32 ringdown traces were
averaged by the oscilloscope before transfer to the PC, which
then fit the data to a single-exponential function and constant
baseline by the Levenberg-Marquardt algorithm.61
The ringdown lifetime of radiation trapped in an evacuated
optical cavity of length L with mirrors of reflectivity R is τ0 )
L/c(1 - R), where c is the speed of light.62-64 Typical empty-
cell ring-down times were τ0 ≈ 10 µs, in good agreement with
the stated mirror reflectivity (99.98% at the peak wavelength).
In the presence of an absorber, the lifetime is given by τ )
L/c(1 - R + RLs), where R ) [X]σX, σX is the extinction
coefficient of the absorber X, and LS is the absorption path
length through the sample. The measured extinction coefficient
is then
TABLE 1: Typical Flows and Concentrations for Branching
Ratio Measurements
concentration/
species
flow/sccm
cm-3 (300 K, 20 Torr)
He
Ar
N2
NO2
H2
615
515
242
13
2.85 × 1017
2.38 × 1017
1.12 × 1017
6.0 × 1015
1.38 × 1015
3
which pre-cooled Ar gas flowed. The discharge gas was injected
in jets perpendicular to the Ar flow through small holes near
the capillary tip. Turbulent mixing of the discharge gas with
the Ar flow ensured an even distribution of H atoms in the
reactor and helped to dissipate heat from the discharge.
A mixture of 4% NO2 in N2 by volume (Matheson, certified
gravimetrically) was introduced through a 4-mm i.d. inlet
approximately 1 cm downstream from the end of the quartz
discharge tube. We adjusted the location of the H-atom injection
with respect to the N2/NO2 entrance to allow for the mixing
and cooling of the discharge flow, while keeping wall loss of
H-atoms to a minimum. At the concentrations used in this
experiment, the reaction of H and NO2 was essentially complete
within microseconds, while the subsequent association reaction
of OH + NO2 + M was complete in <500 µs. Thus, products
from reactions 1 and 2 were formed within a few millimeters
along the flow, well before the gas entered the spectroscopy
region. Although H + NO2 is known to produce high yields of
vibrationally excited OH,56 NO2 is an effective quencher.
Furthermore, OH(V) + NO2 will produce highly excited transient
adducts which will not stabilize to form products.
The temperature of the reaction region was measured with a
calibrated type “T” thermocouple placed in the center of the
reaction zone. The temperature at the point of reaction was
slightly higher than the reactor walls, because the He/H2/Ar
mixture had not fully equilibrated. Thermal gradients led to
temperature variations of up to (3° C across the reaction region;
these uncertainties are reflected in abscissa error bars in the
temperature dependence.
B. Detection of HOONO and HONO2. Infrared absorption
spectra of the reaction products were obtained in a long-path
cell situated downstream of the reaction region, 8 cm from the
NO2 injection point. Most spectra were recorded in a room-
temperature cell (1.9-cm i.d.) which had a cavity length L )
81.5 cm and a sample absorption path length Ls ) 51 cm through
the reactant gas mixture. The absorption path length Ls was
determined by the distance from the gas inlet port to the pump-
out port. We occasionally used a second jacketed cell (L ) 67
cm, LS ) 32 cm, 2.5-cm i.d., 20 cm distance from the NO2
inlet port), which allowed us to vary the detection cell
temperature in order to investigate temperature-dependent wall
loss and residence time effects.
L
cLs
1
1
τ0
R )
-
(
)
τ
The possible dependence of the measured extinction coefficient
on the concentration of absorber will be discussed in detail
below.
Spectra of reaction products were typically recorded by
scanning the spectral region of interest with the discharge on
and off, and subtracting the two scans. This subtraction
eliminated background due to species present in the absence of
reaction, as well as to the wavelength-dependent ringdown loss
of the mirrors.
We estimated the sensitivity of our CRDS setup from ∆τ,
the minimum detectable change in τ.63 In a typical run, we found
∆τ/τ0 ≈ 2% (averaging 32 laser shots), giving a minimum
detectable absorption of Rmin ≈ 1 × 10-7 cm-1 (1.8 × 10-7
cm-1 xHz) for mirrors with reflectivity R ) 99.97%. For the
detection of HONO2 with the ν1 band, which has an integrated
intensity of 9.5 × 10-18 cm/molecule,65 the minimum detectable
concentration of nitric acid was [HONO2]min ≈ 5 × 1011
molecules/cm3 at a single point in the R-branch. Integration over
the OH stretching band further improved the sensitivity by
roughly N1/2, where N ≈ 20 is the number of data points making
up fwhm of the band.
We investigated the effect of the purge gas flow rate on the
detection of products. The linear flow velocity of Ar through
each of the purged volumes was ≈3 cm/s, or 0.6% of the total
flow. We measured both product absorption signals while
independently varying the purge flow at the inlet and outlet ends
of the detection cell from 0.5% to 9%. There was a gradual
decrease (approximately 20%) in the absorption by both HONO2
and HOONO as we increased the purge flow rates to 9%, but
the fractional yield of HOONO remained constant. Thus, while
some reaction gas mixture entered the purged volume at the
flow rates used in this experiment, these gases were flushed
out quickly and did not affect branching ratio measurements.
Absorption spectra of the reaction products were recorded
by pulsed cavity ringdown spectroscopy.57-59 A mid-infrared
laser pulse was injected into an optical cavity made by two high-
reflectivity infrared mirrors (R ) 99.98%, centered at λ ) 2.8
µm, 6 m radius of curvature) sealed to the ends of the detection
cell. Each mirror was separated from the main flow of gas by
a purge volume approximately 15 cm long. Dry argon flowed
over the mirrors (8 sccm over each mirror) and through the purge
volumes to protect the mirrors and to minimize losses in
reflectivity due to deposition.
Computational Methodology
We produced tunable (2.6-3.4 µm) mid-IR in a two-stage,
optical parametric amplifier (OPA)60 by mixing the second
harmonic of an Nd:YAG laser and the output of a tunable dye
A. Ab Initio Calculations of Structure, Energetics, and
Intensities. Ab initio electronic structure calculations were

6978 J. Phys. Chem. A, Vol. 107, No. 36, 2003
Bean et al.
TABLE 2: Computed Molecular Constants for the cis-cis and trans-perp Conformational Isomers of HOONO
cis-cis
trans-perp
QCISD
QCISD(T)
cc-pVTZa
CCSD(T)
cc-pVTZ
MP2
QCISD
cc-pVTZ
CCSD(T)
cc-pVTZ
MP2
cc-pVTZa
6-31G(d)b
6-31G(d)b
∆E0 (kcal/mol)
3.4
2.9c
A (MHz)
21726
8133
5917
21262
8041
5835
21310
8081
5859
3521
1630
1458
969
838
723
419
523
20900
7949
5759
3547d
1595
1476
991
858
776
418
532
55716
5060
4709
54127
4977
4627
3780
1736
1412
992
816
497
366
302
52576
4902
4550
3841e
1720
1408
999
811
443
373
311
B
C
ν1 (cm-1
)
3496(a′)
1649(a′)
1461(a′)
939(a′)
796(a′)
618(a′)
397(a′)
515(a′′)
363(a′′)
ν2
ν3
ν4
ν5
ν6
ν7
ν8
ν9
383
360
211
219
a Refs 43,44. b McGrath and Rowland, ref 37. c MP2/6-311+G(3df,2p), ref 37. d Frequencies calculated with the 6-311G(2df,2p) basis set.
e Frequencies calculated with the 6-311G(d,p) basis set.
performed to predict (a) structures, vibrational frequencies, and
energies of the cis-cis and trans-perp isomers of HOONO, and
(b) band intensities of HOONO, HO2NO2, and HONO2.
Calculations were performed with the Gaussian 9866 and
MOLPRO67 programs. Coupled cluster theory68 with single and
double excitations, with perturbative estimates of connected
triples CCSD(T), was used along with the quadratic configu-
ration interaction method (QCISD(T) and QCISD).69 The
QCISD(T) method is correct through fourth-order terms, and it
includes the same fifth-order terms as the CCSD(T) method.
In the absence of large multireference effects or large single
excitation amplitudes, the QCISD(T) method should be very
reliable,69 and its performance is very similar to that of CCSD-
(T).70 The calculations were performed with the Dunning
correlation-consistent polarized valence basis sets. The two basis
sets used throughout this study are the cc-pVDZ basis set
(3s2p1d for C, N, and O; 2s1p for H) and then the cc-pVTZ
basis set (4s3p2d1f for C, N, and O; 3s2p1d for H).71,72 From
many benchmark calculations with extended basis sets, the
CCSD(T) and QCISD(T) methods have shown that high
accuracy can be achieved for the molecular structure and
vibrational frequencies.73,74 Vibrational frequencies were cal-
culated with both the QCISD and CCSD(T) methods. Intensities
were calculated with the QCISD method and scaled to estimate
the CCSD(T) intensities by the factor 0.968. The scale factor
was determined from a calibration set of molecules of well-
known intensities and also from calculation of intensities for
the calibration set of molecules using the CCSD(T) method.
The CCSD(T) intensities were calculated with MOLPRO. The
calibration set was HCN, H2O, CH4, and CH3O. A total of 14
intensities were used in the scale factor determination.
energy cis-cis isomer. For the current calculation, we used
parameters for the perp-perp transition state for rotation of the
OONO dihedral angle, which lies between the trans-perp and
cis-perp isomers. The energy of the perp-perp isomer was
computed at the MP2/6-311G(3df,2p) level with MP2/6-31G-
(d) frequencies by McGrath and Rowland (note that the MP2
energies listed in their Table 2 are not corrected for zero-point
energy).37 We employed the trans-perp and cis-cis vibrational
frequencies from the same calculation, but we used our CCSD-
(T) calculation of the trans-perp energy. We also performed
some calculations of the initial branching ratios for reactions 1
and 2, using transition state parameters based on those of Golden
and Smith.31 Energy transfer collisions were described by the
single-exponential model75 with R ) 45 cm-1 + 0.005
E
upper(cm-1), and Lennard-Jones parameters ꢀ ) 180 K and σ
) 4.2 Å2.
Results
A. Spectroscopy of OH + NO2 Products. Figure 3 shows
a room-temperature scan from 3200 to 3700 cm-1 taken in 0.2
cm-1 steps with the flow cell conditions shown in Table 1. The
full-scale spectrum, shown in the inset, is dominated by the ν1
HONO2 transition. With the discharge off, the HONO2 spectrum
was approximately 8-10 times weaker. This small background
HONO2 signal, which was always present in the cell during
the experiments, arose largely from slow desorption from
passivated walls. Several other absorption bands were evident
in this region. All of the bands had partially resolved rotational
structure. The smaller bands could be assigned to known
transitions: the 2ν2 band of HONO2, the 2ν2 and ν1 bands of
both cis-HONO and trans-HONO, and the 2ν3 transition of NO2.
B. Transition Dipole Moment Vector. Additional calcula-
tions were carried out to determine the orientation of the
vibrational transition dipole moment for the ν1 band of HOONO.
The electrostatic dipole moment was computed at the B3LYP/
cc-pVTZ level using Gaussian 98 for several displacements from
equilibrium along the Q1 normal coordinate using the L matrix
given at the equilibrium geometry. The resulting dipole vector
functions were fit to a polynomial and then numerically
integrated over harmonic oscillator wave functions to obtain
the three components of the transition dipole matrix element in
the principal axis frame.
There remained one unidentified band centered at 3306 cm-1
.
B. Assignment of the cis-cis HOONO Band. Figure 4 shows
the spectrum from 3250 to 3375 cm-1 after subtraction of the
empty-cell background. The sharp features at 3260 and 3372
cm-1 were assigned to the Q-branches of the 2ν2 transitions of
cis- and trans-HONO, respectively. A clearly defined set of P,
Q, and R subbands centered at 3306 cm-1 was identifiable
between these features. We assigned this band to the ν1 ) 0 f
1(O-H stretch) transition of cis-cis HOONO for the following
reasons.
C. Calculations of Isomerization Rates. We computed the
formation and isomerization rates of the isomers of HOONO
using the program MultiWell.75,76 We focused in particular on
the rate of isomerization from the trans-perp isomer to the lower
(1) Chemistry. This band was not observed if the discharge
was off, even with all reagent gases (Table 1) present. We did
not observe any absorption in this region when we recorded
experimental reference spectra of either HONO or HONO2

Cavity Ringdown Spectroscopy of cis-cis HOONO
J. Phys. Chem. A, Vol. 107, No. 36, 2003 6979
Figure 3. Absorption spectrum of the products of OH + NO2 at 20 Torr and 300 K recorded by infrared cavity ringdown spectroscopy. No
background has been subtracted; the varying baseline is due to the underlying variations in ringdown mirror reflectivity. The laser line width is 1
cm-1 and the step size of the scan is 0.2 cm-1. The full scale spectrum, shown in the inset, shows that nitric acid is the dominant product in the
reaction OH + NO2. Weaker bands in the magnified scan assigned to known transitions are marked: HONO2 (b), HONO ((), or NO2 (9). The
only unassigned band, at 3306 cm-1, is attributed to the ν1 transition of cis-cis HOONO (*).
based on their preliminary assignment of a band at 6365 cm-1
to the V1 ) 0 f 2 overtone transition of cis-cis HOONO. Their
overtone assignment, taken in conjunction with our observation
of the fundamental band, would give ωe ) 3430 cm-1 and ωexe
) 62 cm-1 for the V1 band of HOONO. This harmonic frequency
lies close to ωe ) 3521 cm-1, predicted by our CCSD(T)/cc-
pVTZ calculations for the cis-cis isomer of HOONO. The
observed frequency did not match the OH stretch frequencies
of the other stable isomer(s) (or of the predicted saddlepoints)
of HOONO, all of which have frequencies close to that of nitric
acid (ωexp ) 3550 cm-1).
Finally, the integrated band intensity was approximately 3%
of the integrated intensity of the ν1 band of HNO3. This was
consistent with expectations of a relatively low HOONO yield,
assuming the two bands had similar absorption cross sections.
Figure 4. Spectrum of the cis-cis HOONO ν1 band and adjacent 2ν2
A quantitative assessment of the HOONO yield is given below.
C. Ab Initio Calculations. There have been a number of ab
initio calculations on the isomers of HOONO. Here we
performed QCISD, QCISD(T), and CCSD(T)/cc-pVTZ calcula-
tions on cis-cis and trans-perp conformers of HOONO, to have
a consistent set of high-level spectroscopic constants for the
two bound isomers. A third isomer, the cis-perp conformation,
was not found to be stable. Computational results on the
torsional potentials will be reported elsewhere.46
Table 2 lists results for the rotational constants and vibrational
frequencies of the two isomers, along with a comparison with
the MP2 calculations of McGrath and Rowland.37 The number-
ing of the normal modes of the trans-perp isomer are ordered
in descending frequency, while the modes of the cis-cis isomer,
which has Cs symmetry, are based first on the symmetry a′ and
then a′′. In general, the frequencies and rotational constants are
close to the MP2 results.
In addition, we report the CCSD(T)/cc-pVTZ energy of the
trans-perp isomer relative to the lowest energy cis-cis geometry
to be ∆E0 ) +3.4 kcal/mol, including zero-point correction.
The dissociation energy of the cis-cis isomer has been reported
at a higher level of theory; the best estimate is 18.3 kcal/mol.44
To determine the relative yields of HOONO and HONO2,
we needed the relative absorption cross sections of the respective
fundamental ν1 transitions. The band strength of nitric acid has
been measured, but not that of HOONO. We therefore relied
on ab initio intensities computed at equivalent levels of theory
bands of cis and trans-HONO, with the ringdown baseline subtracted.
The thick line is the experimental data, and the gray line is the sum of
three simulated spectra: cis-cis HOONO (thin line, centered at 3306
cm-1, cis-HONO (short dash, 3260 cm-1), and trans-HONO (long dash,
3372 cm-1).
generated from bubbled samples. Furthermore, the band was
not present if either NO2 or H2 was removed with the discharge
on. We found no detectable spectra of H2O2 or HO2, which
have band origins closer to 3400 cm-1, or of HO2NO2, which
has an absorption at 3540 cm-1. Thus, this band arose from a
reaction product but could not be attributed to any of the primary
reactant or product species (HONO, NO, NO2, HONO2) or other
conceivable products (H2O2, HO2NO2).
(2) Deuterium Substitution. When D2 was substituted for H2
in the discharge flow, the band at 3306 cm-1 disappeared but
the two adjacent DONO 2ν2 (NO2 stretch overtone bands) were
still readily observable. These results indicate that this mode
most likely involved a hydride X-H stretch.
(3) Band Position and Intensity. The frequency of the band
origin, 3306 cm-1, red-shifted relative to the OH stretch of
HONO2 by 245 cm-1, was in good agreement with the position
expected for the cis-cis isomer of HOONO from both experi-
ment and theory. The observed band center was close to the
3285 cm-1 origin of the ν1 fundamental of cis-cis HOONO
observed in matrix isolation experiments; the difference can be
accounted for by matrix-induced shifts. It also matched the
fundamental position estimated by Nizkorodov and Wennberg,54

6980 J. Phys. Chem. A, Vol. 107, No. 36, 2003
Bean et al.
TABLE 3: Calculated Integrated Intensities for ν1 (O-H
Stretch) Vibrational Transitions in km mol-1, and
Comparison with Previous Experimental Values
cis-cis trans-perp
HONO2
QCISD/cc-pVTZa 98.6
CCSD(T)/cc-pVTZ 95.4
experiment
57.0 ( 2.8b
HOONO HOONO HOONO2
34.3
33.2
50.1
48.6
52.8
51.2
36.2 ( 4.4c
a Scaled by 0.968 to estimate CCSD(T) intensities (see text). b Ref
65. c Ref 87.
to determine the ratio of the absorption strengths of cis-cis
HOONO and HONO2. To estimate the error that may be
associated with such a calculation, we also computed the
intensity for pernitric acid, HO2NO2, a molecule of similar
composition which, like HOONO, possesses an OH stretch
coupled to a peroxide bond.
Table 3 shows the integrated intensities for the OH stretches
in HOONO, HONO2, and HO2NO2 computed with the QCISD-
(T)/cc-pVTZ and CCSD(T)/cc-pVTZ methods, as well as a
comparison to published values. The QCISD(T) calculations
were scaled to estimate the CCSD(T) intensities. While the
calculated intensities of HONO2 and HO2NO2 are greater than
the experimental values by 67% and 41%, respectively, the
computed HONO2-to-HO2NO2 intensity ratios are 1.86 at the
CCSD(T) level of theory, while the experimental ratio is 1.57.
Thus, the computed ratiossthough not the absolute magnitudess
agree with experiment to within 18%.
The CCSD(T) calculations gave an intensity ratio of 2.87 for
HONO2 to cis-cis HOONO, and 1.96 for HONO2 to trans-perp
HOONO; the scaled QCISD(T) ratios were nearly identical. The
integrated absorbance of HOONO must be corrected by this
ratio to determine the relative HOONO-to-HONO2 product
concentrations. The comparison with pernitric acid suggests that
this value should be accurate to within roughly 20%. This
estimation of the error is based on a single calibration point;
hence, we cannot estimate the statistical uncertainty, but IR
absorbances calculated at this level of theory, even within the
double harmonic oscillator approximation, are typically accurate
to within 20%.77,78 By estimating the ratio of intensities of
similar stretch modes (OH stretches), systematic errors such as
anharmonicity are at least partially corrected, though the cis-
cis HOONO has an apparent hydrogen-bonding interaction not
present in either HONO2 or HO2NO2.
Figure 5. Comparison of the experimental cis-cis HOONO ν1 spectrum
(with background and HONO 2ν2 overtone bands subtracted) and a
simulation (grey line) computed as a hybrid band with ratio Ib/Ia ) 2.3
using ab initio rotational constants.
The modeled spectra matched reference spectra of HONO taken
with this apparatus reasonably well. Figure 4 shows the fit of
the modeled cis- and trans-HONO spectra.
In Figure 5, a scan of HOONO from 3245 to 3380 cm-1 taken
at 0.2 cm-1 steps with the background and HONO contributions
subtracted, is compared with the simulation based on the ab
initio predictions and a Ib/Ia ratio of 2.3. The simulation is in
good agreement with the observed spectrum. We varied the
orientation of the transition dipole moment, but found that the
spectrum could best be fit as a hybrid band of a-type and b-type
transitions. A range of Ib/Ia values from 1 to 4 was acceptable,
because the Q branch blended with the P and R branches,
making it difficult to estimate the relative area of the Q branch.
This range is consistent with the predicted ratio Ib/Ia ) 2.3:1.
The observed HOONO spectrum has a broader Q branch
which partially blends with the P and R branches and a broader
R branch shaded to the blue. We attribute these discrepancies
to (a) the use of identical upper and lower rotational constants,
and (b) contributions from sequence bands, progressions, or hot
bands of the low-frequency modes of the loosely bound ring
structure. The Q branch, in particular, is not expected to be as
sharp as predicted by the simple rigid-rotor approximation, but
the relative intensity roughly does match the observed spectrum.
Blue-shading of the spectrum may arise from intramolecular
hydrogen bond tightening in the vibrationally excited state,
leading to a more compact geometry and slightly larger upper
state rotational constants. Such effects are well documented in
hydrogen-bonded complexes. More important, however, will be
the significant population in excited low-frequency vibrations.
Our CCSD(T)/cc-pVTZ calculations predict three low-frequency
modessthe a′′ HOON torsion, the a′′ ONOO torsion, and the
Lower-level B3LYP calculations were performed to predict
the orientation of the vibrational transition dipole moment for
the V1 ) 0 f 1 transition of cis-cis HOONO. The transition
dipole moment vector computed as described above was (µa,
µb, µc) ) (0.041 D, 0.064 D, 0.0 D). This vector differs slightly
from the direction of the OH bond. This calculation then predicts
a hybrid band with a 2.3:1 ratio in b-type to a-type character.
D. Modeling the HOONO ν1 Band. With these ab initio
predictions, we modeled the expected rotational contours of the
ν1 band of cis-cis HOONO using the asymmetric rotor program
ASYROT.79 We made the rigid rotor approximation and used
the same set of rotational constants for the cis-cis isomer (from
the ab initio calculations) for both the upper and lower states,
because we had no information on the change in geometry upon
excitation of the V1 ) 0 f 1 transition.
a′ NOO bendsat 383 cm-1, 523 cm-1, and 419 cm-1
,
respectively. The OH torsion, in particular, will break the
hydrogen bond and lead to substantially blue-shifted sequence
bands.
Figure 4 shows the total fit of the summed HOONO and
HONO contributions. Together, the three spectra account for
most of the extinction in this region. If we assume that the
remaining area can be attributed to HOONO sequence bands,
we estimate the total HOONO integrated signal to be 14%
greater than the area of the simulated spectrum alone. This
additional area is primarily on the blue side, and is comparable
to the expected contribution from torsionally excited modes.
The additional absorption is thus most likely to be due to
sequence bands.
To compare the simulation with the 3306 cm-1 spectrum,
we first had to subtract the contributions to the observed
spectrum from the adjacent 2ν2 bands of cis-HONO and trans-
HONO, whose wings overlapped with the HOONO band. We
modeled the rotational contours of the HONO bands,79 using
asymmetric rotor constants from high-resolution FTIR data.80,81
E. Lifetime of HOONO. The detection of cis-cis HOONO
was performed at room temperature downstream of the reaction
region, well after reactions 1 and 2 were complete. Typical total

Cavity Ringdown Spectroscopy of cis-cis HOONO
J. Phys. Chem. A, Vol. 107, No. 36, 2003 6981
residence times were 200 ms; thus, any measurements of
absorption by HOONO could only be correlated with HOONO
concentration if the HOONO lifetime relative to decomposition
at room temperature was significantly longer than 200 ms. We
varied the residence time from 75 to 250 ms, but we saw no
change in the absolute HOONO signal. If the lifetime of
HOONO were τ ) 200 ms, we would have expected to see the
HOONO signal change by a factor of 1.7 over this time range.
Given an uncertainty in HOONO detection of 20%, we estimate
that the lifetime must be τ > 500 ms. Thus, we conclude that
the HOONO concentration was not changing appreciably over
the time scale of this experiment. These conclusions apply as
well to mechanisms which might increase cis-cis HOONO
population, e.g., isomerization of trans-perp HOONO to cis-
cis HOONO during transit through the detection cell.
F. Characterization of Wall Losses. In addition to decom-
position of cis-cis-HOONO, loss of either HOONO or HONO2
on the walls of the cell would result in erroneous product yields.
While we saw no significant HOONO loss processes at room
temperature, we found small decreases in the HOONO and
HONO2 signal at temperatures between 260 and 280 K.
These losses, on the order of 10-20%, increased when a
cooled extension was inserted, indicating that some wall
deposition or reaction was occurring. At T < 250 K, substantial
reversible deposition of HONO2 occurred. Our results are in
qualitative agreement with unpublished wall loss measurements
for HOONO by Fry et al.82 We therefore limited our HOONO/
HONO2 branching measurements to temperatures above 270
K.
G. Integrated Absorbances of HOONO and HONO2.
Integrated absorbances of the ν1 bands of both HOONO and
HONO2 were obtained by scanning the respective ν1 bands
several times, recording discharge-on and discharge-off spectra,
taking the difference, and then summing the runs. The bands
were then numerically integrated.
We typically scanned each HOONO spectrum over the range
3250 to 3375 cm-1 and fit the observed bands to a composite
spectrum from the simulated cis-HONO trans-HONO, and cis-
cis HOONO spectra, allowing contributions from each of the
three molecules to vary independently. We then used the area
under the HOONO curve, scaled to account for the total area
of the observed HOONO spectrum in Figure 5, to obtain the
integrated HOONO absorption
were typically 50 times stronger than the HOONO spectra, and
the resulting ringdown lifetime was an order of magnitude
greater than the baseline ringdown time.
We therefore performed a series of experiments to examine
the ringdown spectra of varying concentrations of HONO2 from
nitric acid mixed with Ar at 20 Torr. We found that the overall
absorption band shape varied slightly with HONO2 concentra-
tion. The band shape agreed well with modeled contours at low
HONO2 concentrations, but the P and Q branches appeared to
decrease relative to the R branch at higher concentrations.
Hence, some apparent nonlinearity in the absorption measure-
ment appeared to be occurring.
The use of a low-resolution laser for cavity ringdown
spectroscopy may introduce a potential error. As Zalicki and
Zare63 and Yalin and Zare83 have shown, when the ratio of the
absorption bandwidth to laser bandwidth is V < 100, the
ringdown decay becomes nonexponential. At a given frequency,
the laser samples some ensemble of narrow, partially resolved,
and unresolved absorption features within the line width of 1
cm-1. The total signal is the sum of frequency components νi ,
each contributing a signal with a decay time τi determined by
the absorption R(νi). The resulting ringdown signal in general
will not be a single exponential. In a typical experiment, the
total signal is then a fit to an effective overall decay, which
underestimates the absorption. As the spectrum becomes
increasingly congested, however, narrow features blend and the
lines can become unresolvable. If the spectral features become
much wider than the laser line width, the cavity ringdown
spectrum should approach the convoluted low-resolution spec-
trum. Alternatively, Zalicki and Zare also showed that if the
absorbance was weak compared to cavity loss, then the
integrated intensitysthough not the individual line shapeswas
correct to within 10%. Furthermore, as Hodges et al.84 have
shown, the spectral profile of YAG-pumped dye lasers is not
Gaussian, but composed of fluctuating lines at the cavity mode
spacing, which tends to ameliorate the underestimation.
The ν1 band of nitric acid is known to have resolvable
Doppler-broadened features, but it is highly congested and has
not yet been assigned.85 In the current experiment, the underlying
HONO2 spectrum typically consisted of a set of narrow lines
with a Voigt profile from the convoluted Doppler and pressure-
broadened line shapes. The ringdown spectra that we recorded
possessed partially resolved features.
We therefore determined the deviation of the integrated
absorbance measured by CRDS in our branching ratio experi-
ments in a separate experiment, in which we measured the
absolute integrated intensity of the ν1 band as a function of
HONO2 concentration. N2 was bubbled through nitric acid and
its concentration measured by a 185 nm Hg lamp before dilution
with N2 and passage into the CRDS cell. A detailed description
of this experiment, as well as simulations of ringdown spectra
based on high-resolution spectra of HONO2, will be reported
elsewhere.86 The true integrated intensity was measured at low
HONO2 concentrations, and compared to HONO2 absorbance
measured at the HONO2 concentrations obtained in the dis-
charge. From these measurements, we concluded that the current
branching ratio experiments underestimated the absolute infrared
intensity of the ν1 band of HONO2 by 17%.
IHOONO ) Ls
RHOONO (V) dV
∫
We obtained integrated absorption intensities of nitric acid
in a similar manner, scanning from 3490 to 3610 cm-1. It was
essential to subtract the discharge-off spectrum, as there was a
significant amount of background HONO2 accounting for up
to 10-15% of the total nitric acid. The spectrum in this region
consisted of the main fundamental ν1 band of HONO2, as well
as a sequence band, and a small contribution from the ν1 band
of trans-HONO. To obtain the integrated absorption of HONO2,
a spectrum of pure bubbled HONO2 of comparable intensity
was fit to our spectra and then integrated. We verified that a
combination of the bubbled HONO2 and trans-HONO spectra
adequately represented the observed spectral intensity in the
region from 3490 to 3610 cm-1
.
In contrast, it is likely that the integrated absorbance of the
ν1 band of cis-cis HOONO is correct. First, the HOONO
absorption is approximately 10 times less than the mirror loss,
thus meeting the criterion of Zalicki and Zare for weak
absorption. Second, we expect that this band is highly congested
at room temperature. The observed spectrum is relatively
The magnitude of the HONO2 absorption raised concerns
about possible systematic errors in the observed absorbance of
the nitric acid spectrum. The peak HONO2 absorption was R l
e 0.002 (at the Q branch), so that the spectrum did not appear
to be saturated in a single pass. However, the HONO2 spectra

6982 J. Phys. Chem. A, Vol. 107, No. 36, 2003
Bean et al.
region. Typical uncertainties in T due to radial variations of up
to (3 K are shown, but the weak temperature dependence leads
to little resulting uncertainty in the yield of HOONO.
I. Uncertainties. The scatter in the data shown in Figures 6
is due to random errors that include the effects of (a) noise in
the collected spectra; (b) uncertainty in the estimation of the
integrated band intensities from fitting the band of HOONO,
HONO, and HONO2; and (c) variations in conditions of the
reaction region. As discussed above, there is a temperature
uncertainty of up to (3 °C. Our uncertainty in the pressure is
1%. However, the branching ratio depends only weakly on
temperature, and thus the error propagated into the branching
ratios is negligible.
In addition to the random experimental errors, there are
several uncertainties. The largest uncertainty arises from the
calculated ratio of absorption cross sections, which we estimate
is on the order of 20% on the basis of the difference between
the computed and observed ratios of ν1 band strengths of
HONO2 and HO2NO2. A second uncertainty lies in our
correction of the integrated HONO2 absorbance due to the use
of the cavity ringdown method. We estimate the error in this
correction to be on the order of 10%. Finally, if we conserva-
tively assume that there is resolvable structure in the HOONO
spectrum, then the weak absorbance condition of Zalicki and
Zare63 leads to 10% uncertainty in the HOONO intensity.
Adding these uncertainties in quadrature to the 2σ random error
of 0.009(12%), we find that the branching ratio for cis-cis
HOONO vs HONO2 products is
Figure 6. Observed ratio of cis-cis HOONO to HONO2 products in
the reaction OH + NO2 as a function of temperature, at 20 Torr (See
Table 1 for gas composition). The experimentally measured ratios of
the integrated ν1 absorption bands were scaled by the computed ratio
of absorption cross sections. The abscissa uncertainties are the 2σ
random errors associated with temperature variations. One point (square)
was the mean of measurements on several days.
smooth, with no detectable sub-structure. cis-cis HOONO is
predicted to be an asymmetric rotor with Ray’s parameter κ )
-0.71, and the ν1 band is predicted to be an a-b hybrid. Since
the OH bond participates in an intramolecular hydrogen-bond,
the rotation-vibration interactions cannot be neglected. Finally,
several low-frequency modes, including two torsional modes,
will have significant populations that will increase the spectral
congestion. HOONO has three modes with harmonic frequencies
under 500 cm-1, whereas the lowest frequency harmonic mode
in nitric acid is slightly below 500 cm-1. Under these conditions,
the “low-resolution” cavity ringdown spectrum of HOONO is
more likely to both reproduce the overall shape of the band
and yield the correct integrated absorbance.
H. Determination of the cis-cis HOONO/HONO2 Branch-
ing Ratio. We recorded the integrated absorptions of cis-cis
HOONO and HONO2 at a variety of reaction temperatures at
20 Torr, and pressures from 4 to 40 Torr at 300 K. While the
temperature of the reaction region was varied, all spectroscopic
measurements were performed with the detection cell held at
room temperature. Thus, the band contours were independent
of reaction conditions, and wall losses in the detection region
were negligible. Furthermore, as we demonstrated above, the
HOONO lifetimes were sufficiently long that HOONO extinc-
tion coefficients were unaffected by the residence time. Thus,
the column density in the spectroscopy should accurately reflect
the product yield. The HONO2 integrated absorption was scaled
by 1.17 to correct for the error due to cavity ringdown
measurement. The HONO2 concentrations did not vary consid-
erably in the data sets, and we found that use of the same
correction factor did not introduce a significant error.
k2(c-c)/k1 ) 0.075 ( 0.020(2σ)
at room temperature and 20 Torr of the He/Ar/N2 mixture.
J. Search for ν1 Band of trans-perp HOONO. Our
spectroscopic measurements in the 3500-3650 cm-1 range can
also provide a coarse upper bound on the trans-perp HOONO
concentration. From the matrix isolation studies, the ν1 band of
trans-perp HOONO is near 3550 cm-1, under the OH stretch
of nitric acid. Our calculations predict that this band is 50%
weaker than the ν1 band of HONO2. This band is likely to be
a c-type transition, because the OH bond lies nearly perpen-
dicular to the plane of the heavy atoms. Thus, the trans-perp
OH stretch band should exhibit a sharp Q branch that would be
readily observed within the HONO2 absorption (unless it lies
directly under the HONO2 Q branch). From the absence of any
anomalous features or other large discrepancies in the vicinity
of the HONO2 spectrum, we infer that the branching fraction
of the trans-perp isomer is < 20%.
K. Statistical Calculation of the trans-perp f cis-cis
Isomerization Rate. We simulated the isomerization process,
assuming that these isomers are separated by the transition state
with geometry corresponding to the perp-perp saddle point
computed at the MP2/6-311G(3df,2p) level by McGrath and
Rowland.37 This transition state has an energy of 11.4 kcal/mol
relative to the cis-cis isomer at the MP2 level, including zero-
point energy. For consistency in the density of states calcula-
tions, we used their MP2 vibrational frequencies and geometries
for all three conformers, cis-cis, trans-perp, and perp-perp, but
used our best CCSD(T) estimate for the trans-perp energy
relative to the energy of cis-cis HOONO.
HOONO-to-HONO2 branching ratios were then obtained by
weighting the observed absorption intensity ratios by 2.87, the
ratio of the ab initio absorption band strengths computed at the
CCSD(T)/cc-pVTZ level.
Measurements of the branching ratio were performed at room
temperature and 20 Torr on several different days. The varia-
tions, most likely arising from source conditions, were (0.025.
Averaging over these results, we determined the 300 K
branching ratio to be k2(c-c)/k1 ) 0.075 ( 0.009, where the
quoted uncertainty is the 2σ standard error of the mean.
Figure 6 shows our measurements of the branching ratio of
cis-cis HOONO to HONO2 over the temperature range 270 K
to 360 K at 20 Torr total pressure. The cis-cis HOONO yield
appeared to be almost independent of temperature; the slope of
the temperature profile was -1.7((2.4) × 10-4/K (2σ). The
temperatures plotted were from the thermocouple in the reaction
We found that the nascent HOONO molecules establish a
rapid (k > 1010 s-1) microcanonical equilibrium between the
two isomers as they are slowly (kcollision ≈ 108 s-1 at 13 Torr)
driven down the internal energy ladder by collisions. Because
of its smaller moments of inertia (centrifugal barrier effect) and
larger degree of “floppiness”, the trans-perp isomer has a density

Cavity Ringdown Spectroscopy of cis-cis HOONO
J. Phys. Chem. A, Vol. 107, No. 36, 2003 6983
of states comparable to that of the cis-cis isomer at energies
above the isomerization barrier. Thus, a large fraction of excited
HOONO molecules (≈30%) initially fall into the trans-perp
well. Once the internal energy of the HOONO molecules is
below the isomerization barrier, vibrational energy is equili-
brated with translations on a microsecond time scale under our
experimental conditions. This initial yield of vibrationally
thermalized HOONO isomers depends critically on the relative
density of states of the two isomers just above the top of the
barrier at low pressures. At higher pressures, collisional
relaxation may become competitive with isomerization rates.
After the HOONO molecules relax to an initial c-c/t-p
population, the trans-perp HOONO isomerize on a slower time
scale of 0.1-10 ms (over the temperature range of interest),
and the c-c/t-p populations approach thermal equilibrium.
Using the 11.4 kcal/mol barrier for the cis-cis to trans-perp
isomerization and the 3.4 kcal/mol energy separation between
the isomers (both zero-point corrected) and assuming a doubly
degenerate reaction path, we find isomerization lifetimes of 0.35
ms and 1.5 ms at 300 and 260 K, respectively. Even with a
zero-point corrected perp-perp energy of 13.5 kcal/mol and
assuming a single reaction path, the lifetime for trans-perp
relative to isomerization was 18 ms at 300 K. Thus, trans-perp
HOONO is expected to convert to the lowest energy conformer
rapidly relative to the characteristic residence time of products
in the flow cell (200 ms). Therefore, the initial yields of the
HOONO isomers from OH + NO2 + M, which are difficult to
compute, will not be detected in our experiment.
If isomerization of the trans-perp isomer to the cis-cis form
is indeed rapid compared to the residence time, we can calculate
the total yield of HOONO in reactions 1 and 2. From the
energetics and frequencies predicted by our CCSD(T) calcula-
tions in Table 2, we find Keq (300 K) ) [trans-perp HOONO]/
[cis-cis HOONO] ) 0.005. Thus, at equilibrium only a
negligible fraction the HOONO will be in the trans-perp
conformation. Even at the highest temperature reported here,
the trans-perp isomer contributes less than 1.5% to the total
population at equilibrium.
While the cis-perp isomer is not bound in CCSD(T) calcula-
tions,46 its geometry may be accessed by low (V9 g 1) torsional
levels of the NOOH torsion. This torsional motion breaks the
intramolecular hydrogen bond, and the large predicted change
in the OH vibrational frequency with torsional angle suggests
that the sequence bands involving V9 will be significantly blue-
shifted. The excited torsional modesswhich may account for
10-30% of the cis-cis HOONO populationsmay be the cause
of the intensity on the blue side of the spectrum in Figure 5.
However, we cannot rule out that some of the sequence bands
cannot be detected within the 3306 cm-1 band. Thus, the
observed yield obtained from the integrated absorbance of the
3306 cm-1 band may underestimate the HOONO yield, though
by at most 10-30%. Only detailed modeling of the torsional
levels with accurate ab initio potentials, or direct observation
of the sequence bands, will allow us to settle this question.
We conclude that the observed cis-cis yield accounts for
essentially all HOONO formed, i.e., that k2/k1 ) 0.075 at 20
Torr and 300 K for the reaction OH + NO2 + M.
Discussion
A. Total HOONO Yield. The current experiment measures
the branching ratio for the formation of one HOONO isomer,
the lowest energy cis-cis conformation, relative to formation
of nitric acid. These results thus put a rigorous lower bound on
the total HOONO yield, as other isomers of HOONO may have
been formed but were undetected.
Two other HOONO isomers have been predicted, both with
OH stretch bands that would overlap with that of nitric acid.
Recent calculations conclude that the second lowest energy form,
the cis-perp isomer, is an inflection point, not a minimum, along
the torsional potential about the peroxy bond. This result is
consistent with the absence of spectroscopic evidence for the
cis-perp isomer in matrixes. We consider first the possible
formation of the trans-perp isomer, whose energy is predicted
to be ∆E0 ) +3.4 kcal/mol relative to the cis-cis isomer, and
then briefly discuss possible problems arising from torsionally
excited cis-cis HOONO.
B. Comparison with Previous Measurements. Both
Burkholder et al. and Dransfield et al. were unable to detect
HOONO by FT-IR spectroscopy. Burkholder et al. placed an
upper limit of 5% on the HOONO yield,51 but assumed equal
ν1 cross sections with nitric acid; if our CCSD(T) intensities
are used, this upper limit becomes 14%. Our yield also agrees
with their HONO2 yield of 75% + 25%/-10% determined from
mass balance. Dransfield et al. scanned a wider spectral range
under a variety of conditions;52 they reported an estimate from
the mass balance of the HOONO yield to be 0.0 ( 4.1% at 298
K and 60 Torr. Curiously, their published spectra end at 3200
cm-1. Our experiments suggest that the concentrations of
HOONO in their experiments were near the limits of the
reported sensitivities. In addition, their measurements were
performed at higher pressures, where HOONO yields may be
higher but collisional dissociation will be faster.
Nizkorodov and Wennberg54 reported a HOONO yield of 5
( 3% at 253 K and 20 Torr. These measurements were
performed under conditions similar to our own, but were based
on detection of HOONO alone. Their uncertainty arose because
the authors assumed that the HOONO overtone cross-sections
were the same as that of HO2NO2, and that HOONO dissociated
with unit quantum efficiency. There was also some uncertainty
in the assignment of the peaks, as no rotational structure was
resolved. They assigned peaks in a cluster of bands to the ν1
overtones of the cis-cis, cis-perp, and trans-perp isomers; more
recent results82 suggest that these are all combination bands,
hot bands, and/or Fermi resonances of the cis-cis isomer. Given
these assumptions, their yield is in quite good agreement with
the current findings.
Any trans-perp HOONO products formed by reaction 2
would most likely isomerize to the cis-cis form, because the
computed isomerization barrier (11.4 kcal/mol) is predicted to
be much lower than the dissociation threshold (18.3 kcal/mol).
The absence of any change in the cis-cis HOONO abundance
with residence time is consistent with one of three hypotheses:
(a) the trans-perp form equilibrates rapidly with cis-cis, prior
to entry into the detection volume, (b) no trans-perp isomer is
formed, or (c) the trans-perp isomer isomerizes very slowly
compared to the residence time.
Our RRKM calculations with MultiWell support the first
scenario, that trans-perp isomers rearrange rapidly and are in
equilibrium with cis-cis HOONO in the detection volume. We
calculate that the rate is >103 s-1 at room temperature, and is
sufficiently fast, even if the zero-point corrected barrier is as
high as 13.5 kcal/mol. Furthermore, the calculations assume that
the perp-perp conformation is the only transition state, but other
pathways may exist.
Donahue et al. estimated a low-pressure branching ratio of
0.17 from their isotope exchange measurements, but this
estimate is rather weakly constrained by data.53 Furthermore,
since the high-pressure data had not been corrected, they

6984 J. Phys. Chem. A, Vol. 107, No. 36, 2003
Bean et al.
consequently overestimated k2∞ for HOONO, and this discrep-
ancy will have had an impact on their low-pressure extrapola-
tion.
Hippler et al. measured a high-pressure branching ratio in
He of 0.33 at 430 K, which fell to 0.15 at 5 bar He.32 Since
HOONO yields are predicted to be further reduced at lower
pressures, these results are qualitatively consistent with the
present data. They did not estimate rate constants in the low-
pressure limit, so a direct comparison would require the pressure
and temperature dependences.
Comparisons with the earlier modeling results are more
tenuous, since the calculations are sensitive to input parameters.
Troe’s calculation,34 for example, may have underestimated the
HOONO yield by using too low a binding energy. Matheu and
Green33 attempted to model both forms of HOONO; they find
the low-pressure ratio k2/k1 g 0.02. Our results are in good
agreement with Golden and Smith,31 who predict k2/k1 ) 0.1
in the low-pressure limit; this rate was arrived at by multiplying
the computed cis-cis rate by three to account for the three
isomers. Our results are in satisfactory agreement with recent
work by Golden, Barker, and Lohr,35 the most detailed master
equation modeling calculations to date. From their fitted
parameters, we compute a branching ratio of 0.057 at 13 Torr
of N2.
computed cross sections leads to the largest contribution to the
uncertainty in the branching ratio. We find a branching ratio of
k2/k1 ) 0.075 ( 0.020(2σ) at room temperature and 20 Torr
(He/Ar/N2 buffer gas). This branching ratio decreases weakly
with increasing temperature over the range 270-360 K.
Although a second isomer of HOONO may be formed, statistical
rate calculations show that the higher energy conformer, trans-
perp HOONO, is expected to isomerize rapidly on the time scale
of the current experiment, and will be in its equilibrium
abundance, computed to be < 1.5% of the total population. A
small fraction of ν9 (HOON-torsion) excited molecules that
access the cis-perp geometry may have sequence bands that
are significantly blue-shifted and lie under the HONO2 band,
and thus be undetected. Thus, the current measurements provide
a firm lower bound for and may be a good estimate of the total
yield of HOONO.
Acknowledgment. This work was supported by NASA
Grants NAG5-11657 and NAGW-3911 and the Upper Atmo-
spheric Research and Tropospheric Chemistry Programs. A.K.M.
acknowledges support of an NSF Graduate Research Fellowship,
G.N. support by a Caltech SURF Fellowship, and G.N. and
S.A.N. by the Camille and Henry Dreyfus Postdoctoral Program
in Environmental Chemistry. Calculations were supported by
the NASA JPL Supercomputing Project. We thank D. M.
Golden and J. R. Barker for sharing their results prior to
publication and A. Goldman for providing spectral data.
C. Atmospheric Implications. The low-pressure branching
ratios reported here provide an important constraint on the low-
0
pressure k2 /k10 branching ratio for models, and are a lower limit
on HOONO yields in the atmosphere. The gas mixture, He:Ar:
N2, at 20 Torr is equivalent to approximately 13 Torr of dry air
after collision efficiencies â of the gases are taken into account.
The HOONO yield at pressures relevant to the atmosphere will
be higher than the yields reported here. Doubling or tripling of
this yield could significantly affect atmospheric models. How-
ever, while calculations predict that HOONO yields will increase
to high levels at atmospheric pressures, one cannot yet predict
with full confidence the true falloff curves for HOONO.
Laboratory measurements of reaction 1 generally detect only
loss of reactants (typically OH under pseudo-first-order condi-
tions); however, the reported rate coefficient may be either k1
or (k1 + k2), depending on whether HOONO has collisionally
dissociated on the time scale of the measurements. The
implications of the current branching ratio will then depend on
the specific experiment.
References and Notes
(1) Perkins, K. K.; Hanisco, T. F.; Cohen, R. C.; Koch, L. C.; Stimpfle,
R. M.; Voss, P. B.; Bonne, G. P.; Lanzendorf, E. J.; Anderson, J. G.;
Wennberg, P. O.; Gao, R. S.; Del Negro, L. A.; Salawitch, R. J.; McElroy,
C. T.; Hintsa, E. J.; Loewenstein, M.; Bui, T. P. J. Phys. Chem. A 2001,
105, 1521.
(2) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics:
From Air Pollution to Climate Change; 1998.
(3) Gao, D. F.; Stockwell, W. R.; Milford, J. B. J. Geophys. Res.-D
1996, 101, 9107.
(4) Gao, D. F.; Stockwell, W. R.; Milford, J. B. J. Geophys. Res.-D
1995, 100, 23153.
(5) Tonnesen, G. S. Atmos. EnV. 1999, 33, 1587.
(6) Yang, Y. J.; Stockwell, W. R.; Milford, J. B. EnViron. Sci. Technol.
1995, 29, 1336.
(7) Bergin, M. S.; Russell, A. G.; Milford, J. B. EnViron. Sci. Technol.
1998, 32, 694.
(8) Smith, G. P.; Dubey, M. K.; Kinnison, D. E.; Connell, P. S. J.
Phys. Chem. A 2001, 105, 1449.
(9) Robertshaw, J. S.; Smith, I. W. M. J. Phys. Chem. 1982, 86, 785.
(10) Westenberg, A. A.; DeHaas, N. J. Chem. Phys. 1972, 57, 5375.
(11) Anderson, J. G.; Kaufman, F. Chem. Phys. Lett. 1972, 16, 375.
(12) Anderson, J. G.; Margitan, J. J.; Kaufman, F. J. Chem. Phys. 1974,
60, 3310.
(13) Harris, G. W.; Wayne, R. P. J. Chem. Soc., Faraday Trans. 1 1975,
71, 610.
(14) Howard, C. J.; Evenson, K. M. J. Chem. Phys. 1974, 61, 1943.
(15) Morley, C.; Smith, I. W. M. J. Chem. Soc., Faraday Trans. 2 1972,
68, 1016.
(16) Anastasi, C.; Bemand, P. P.; Smith, I. W. M. Chem. Phys. Lett.
1976, 37, 370.
(17) Anastasi, C.; Smith, I. W. M. J. Chem. Soc., Faraday Trans. 2
1976, 72, 1459.
(18) Atkinson, R.; Perry, R. A.; Pitts, J. N., Jr. J. Chem. Phys. 1976,
Conclusions
This work establishes the formation of HOONO as a minor
product channel in the reaction OH + NO2, consistent with
recent experimental observations by Donahue,53 Nizkorodov,54
and Hippler.32 Our experiments unambiguously identify HOONO
by mid-infrared spectroscopy and provide a direct measure of
the branching ratio in the temperature range 270-360 K. We
have exploited the sensitive technique of cavity ringdown
spectroscopy to detect the absorption spectrum of the ν1 band
of cis-cis HOONO, centered at 3306 cm-1. The rotational
envelope, obtained after subtracting the contributions from the
neighboring HONO ν2 overtones, agrees well with the simulated
spectrum derived from high-level ab initio calculations. We
determined the HOONO:HONO2 product branching ratio k2/k1
for the OH + NO2 + M reaction as a function of reaction
pressure and temperature from measurements of the integrated
absorption of the ν1 bands of cis-cis HOONO and HONO2.
These measurements were converted to branching ratios using
the ratio of calculated integrated intensities of HOONO and
HONO2 bands at the CCSD(T)/cc-pVTZ level. The use of
65, 306.
(19) Wine, P. H.; Kreutter, N. M.; Ravishankara, A. R. J. Phys. Chem.
1979, 83, 3191.
(20) Troe, J. J. Phys. Chem. 1979, 83, 114.
(21) Forster, R.; Frost, M.; Fulle, D.; Hamann, H. F.; Hippler, H.;
Schlepegrell, A.; Troe, J. J. Chem. Phys. 1995, 103, 2949.
(22) Donahue, N. M.; Dubey, M. K.; Mohrschladt, R.; Demerjian, K.
L.; Anderson, J. G. J. Geophys. Res. D 1997, 102, 6159.
(23) Donahue, N. M.; Demerjian, K. L.; Anderson, J. G. J. Phys. Chem.
1996, 100, 17855.
(24) Fulle, D.; Hamann, H. F.; Hippler, H.; Troe, J. J. Chem. Phys. 1998,
108, 5391.

Cavity Ringdown Spectroscopy of cis-cis HOONO
J. Phys. Chem. A, Vol. 107, No. 36, 2003 6985
(25) Brown, S. S.; Talukdar, R. K.; Ravishankara, A. R. Chem. Phys.
Lett. 1999, 299, 277.
(26) D’Ottone, L.; Campuzano-Jost, P.; Bauer, D.; Hynes, A. J. J. Phys.
(60) Reid, S. A.; Tang, Y. Appl. Opt. 1996, 35, 1473.
(61) Press:, W. H.; Teukolsky, W. T.; Vetterling, W. T.; Flannery, B.
P. Numerical Recipes in Fortran, 2nd ed.; Cambridge University Press:
Cambridge, 1992.
Chem. A 2001, 105, 10538.
(62) O’Keefe, A.; Deacon, D. A. G. ReV. Sci. Instrum. 1988, 59, 2544.
(63) Zalicki, P.; Zare, R. N. J. Chem. Phys. 1995, 102, 2708.
(64) Romanini, D.; Lehmann, K. K. J. Chem. Phys. 1993, 99, 6287.
(65) Lange, K. R.; Wells, N. P.; Plegge, K. S.; Phillips, J. A. J. Phys.
Chem. A 2001, 105, 3481.
(27) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.;
Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina,
M. J. Chemical Kinetics and Photochemical Data for Use in Stratospheric
Modeling: Evaluation Number 12. Jet Propulsion Laboratory, 1997.
(28) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F.; Kerr, J.
A.; Rossi, M. J.; Troe, J. J. Chem. Phys. Ref. Data 1997, 26, 1329.
(29) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Crowley, J. N.; Hampson,
R. F.; Kerr, J. A.; Rossi, M. J.; Troe, J. Evaluated kinetic and photochemical
data for atmospheric chemistry. In IUPAC subcommittee on gas kinetic data
eValuation for atmospheric chemistry, 2002.
(66) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, A.10 ed.;; Gaussian, Inc.:
Pittsburgh, PA, 2001.
(67) Werner, H. J.; Knowles, P. J. “MOLPRO is a package of ab initio
programs written with contributions from J. Almlo¨f, R. D. Amos, A.
Bernhardsson, A. Berning, P. Celani, D. L. Cooper, M. J. O. Deegan, A. J.
Dobbyn, F. Eckert, S. T. Elbert, C. Hampel, G. Hetzer, T. Korona, R. Lindh,
A. W. Lloyd, S. J. McNicholas, F. R. Manby, W. Meyer, M. E. Mura, A.
Nicklass, P. Palmieri, K. A. Peterson, R. M. Pitzer, P. Pulay, G. Rauhut,
M. Schu¨tz, H. Stoll, A. J. Stone, R. Tarroni, P. R. Taylor, T. Thorsteinsson.,”
University of Sussex, 2000.
(68) Raghavachari, K.; Trucks, G. W.; Pople, J. A.; Head-Gordon, M.
Chem. Phys. Lett. 1989, 157, 479.
(69) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. J. Chem. Phys.
1987, 87, 5968.
(70) Lee, T. J.; Rendell; Taylor, P. R. J. Phys. Chem. 1990, 94, 5463.
(71) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007.
(72) Kendall, R. A.; Dunning, T. H., Jr.; Harrison, R. J. J. Chem. Phys.
1992, 96, 6796.
(73) Martin, J. M. L. J. Chem. Phys. 1999, 100, 8186.
(74) Bartlett, R. J.; Stanton, J. F. In ReViews of Computational Chemistry;
Lipkowitz, K. B., Boyd, D. B., Eds.; VCH Publishers: New York, 1994;
Vol. 5, p 65.
(75) Barker, J. R. Int. J. Chem. Kinet. 2001, 33, 232.
(76) Barker, J. R. “MultiWell-1.3.1 software,”, 2003.
(77) Galabov, B.; Yamaguchi, Y.; Remington, R. B.; Schaefer, H. F. I.
J. Phys. Chem. 2002, 106, 819.
(78) Thomas, J. R.; DeLeeuw, B. J.; Vacek, G.; Crawford, T. D.;
Yamaguchi, Y.; Schaefer, H. F. I. J. Chem. Phys. 1993, 99, 403.
(79) Judge, R. H.; Clouthier, D. J. Comput. Phys. Commun. 2001, 135,
293.
(80) Guilmot, J. M.; Melen, F.; Herman, M. J. Mol. Spectrosc. 1993,
160, 401.
(81) Guilmot, J. M.; Godefroid, M.; Herman, M. J. Mol. Spectrosc. 1993,
160, 387.
(82) Fry, J.; Nizkorodov, S. A.; Okumura, M.; Wennberg, P. O. In
preparation.
(83) Yalin, A. P.; Zare, R. N. Laser Phys. 2002, 12, 1065.
(84) Hodges, J. T.; Looney, J. P.; van Zee, R. D. Appl. Opt. 1996, 35,
4112.
(30) Leighton, P. A. Photochemistry of air pollution; Academic Press:
New York, 1961.
(31) Golden, D. M.; Smith, G. P. J. Phys. Chem. A 2000, 104, 3991.
(32) Hippler, H.; Nasterlack, S.; Striebel, F. Phys. Chem. Commun. Phys.
2002, 4, 2959.
(33) Matheu, D. M.; Green, W. H., Jr. Int. J. Chem. Kinet. 2000, 32,
245.
(34) Troe, J. Int. J. Chem. Kinet. 2001, 33, 878.
(35) Golden, D. M.; Barker, J. R.; Lohr, L. L. J. Phys. Chem. A,
submitted.
(36) McGrath, M. P.; Francl, M. M.; Rowland, F. S.; Hehre, W. J. J.
Phys. Chem. 1988, 92, 5352.
(37) McGrath, M. P.; Rowland, F. S. J. Phys. Chem. 1994, 98, 1061.
(38) Tsai, H. H.; Hamilton, T. P.; Tsai, J. H. M.; vanderWoerd, M.;
Harrison, J. G.; Jablonsky, M. J.; Beckman, J. S.; Koppenol, W. H. J. Phys.
Chem. 1996, 100, 15087.
(39) Sumathi, R.; Peyerimhoff, S. D. J. Chem. Phys. 1997, 107, 1872.
(40) Doclo, K.; Rothlisberger, U. Chem. Phys. Lett. 1998, 297, 205.
(41) Chakraborty, D.; Park, J.; Lin, M. C. Chem. Phys. 1998, 231, 39.
(42) Jitariu, L. C.; Hirst, D. M. Phys. Chem. Commun. Phys. 1999, 1,
983.
(43) Li, Y.; Francisco, J. S. J. Chem. Phys. 2000, 113, 7976.
(44) Dixon, D. A.; Feller, D.; Zhan, C.-G.; Francisco, J. S. J. Phys.
Chem. A 2002, 106, 3191.
(45) Houk, K. N.; Condroski, K. R.; Pryor, W. A. J. Am. Chem. Soc.
1996, 118, 13002.
(46) Francisco, J. S. Unpublished results.
(47) Cheng, B. M.; Lee, J. W.; Lee, Y. P. J. Phys. Chem. 1991, 95,
2814.
(48) Chen, W. J.; Lo, W. J.; Cheng, B. M.; Lee, Y. P. J. Chem. Phys.
1992, 97, 7167.
(49) Lo, W.-J.; Lee, Y. J. Chem. Phys. 1994, 101, 5494.
(50) Koch, T. G.; Sodeau, J. R. J. Phys. Chem. 1995, 99, 10824.
(51) Burkholder, J. B.; Hammer, P. D.; Howard, C. J. J. Phys. Chem.
1987, 91, 2136.
(52) Dransfield, T. J.; Donahue, N. M.; Anderson, J. G. J. Phys. Chem.
A 2001, 105, 1507.
(53) Donahue, N. M.; Mohrschladt, R.; Dransfield, T. J.; Anderson, J.
G.; Dubey, M. K. J. Phys. Chem. A 2001, 105, 1515.
(54) Nizkorodov, S. A.; Wennberg, P. O. J. Phys. Chem. A 2002, 106,
855.
(55) Lo, W.-J.; Lee, Y. P. J. Chem. Phys. 1994, 101, 5494.
(56) Irvine, A. M. L.; Smith, I. W. M.; Tuckett, R. P.; Yang, X. F. J.
Chem. Phys. 1990, 93, 3177.
(57) O’Keefe, A.; Deacon, D. A. G. ReV. Sci. Instrum. 1988, 59, 2544.
(58) Berden, G.; Peeters, R.; Meijer, G. Int. ReV. Phys. Chem. 2000,
19, 565.
(59) Brown, S. S.; Wilson, R. W.; Ravishankara, A. R. J. Phys. Chem.
2000, 104, 4976.
(85) Goldman, A.; Rinsland, C.; Perrin, A.; Flaud, J. M. J. Quant.
Spectrosc. Radiat. Transfer 1998, 60, 351.
(86) Mollner, A. K.; Nair, G.; Sander, S. P.; Okumura, M. In preparation.
(87) May, R. D.; Friedl, R. R. J. Quant. Spectrosc. Radiat. Transfer
1993, 50, 257.