Femtosecond pump-probe studies of nitrosyl chloride photochemistry in solution

Article abstract of DOI:10.1021/jp062069k

We present a femtosecond pump-probe study of the primary events of nitrosyl chloride (ClNO) photochemistry in solution. Following 266 nm photolysis, the resulting evolution in optical density is measured for ClNO dissolved in acetonitrile, chloroform, and dichloromethane. The results demonstrate that photolysis results in the production of a photoproduct that has an absorption band maximum at 295 nm in acetonitrile and 330 nm in chloroform and dichloromethane. To determine the extent of Cl production, comparative photochemical studies of methyl hypochlorite (MeOCl) and C1NO are performed. Photolysis of MeOCl in solution results in the production of the Cl:solvent charge-transfer complex; therefore, a comparison of the spectral evolution observed following MeOCl and ClNO photolysis under identical photolysis conditions is performed to determine the extent of Cl production following ClNO photolysis. We find that similar to the gas-phase photochemistry, Cl and NO formation is the dominant photochemical channel in acetonitrile. However, the photochemistry in chloroform and dichloromethane is more complex, with a second product formed in addition to Cl and NO. It is proposed that in these solvents photoisomerization also occurs, resulting in the production of ClON. The results presented here represent the first detailed examination of the solution phase photochemistry of ClNO.

Full text of DOI:10.1021/jp062069k

J. Phys. Chem. A 2006, 110, 8613-8622
8613
Femtosecond Pump-Probe Studies of Nitrosyl Chloride Photochemistry in Solution
Catherine C. Cooksey, Kevin J. Johnson, and Philip J. Reid*
Box 351700, Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195
ReceiVed: April 3, 2006; In Final Form: May 9, 2006
We present a femtosecond pump-probe study of the primary events of nitrosyl chloride (ClNO) photochemistry
in solution. Following 266 nm photolysis, the resulting evolution in optical density is measured for ClNO
dissolved in acetonitrile, chloroform, and dichloromethane. The results demonstrate that photolysis results in
the production of a photoproduct that has an absorption band maximum at 295 nm in acetonitrile and 330 nm
in chloroform and dichloromethane. To determine the extent of Cl production, comparative photochemical
studies of methyl hypochlorite (MeOCl) and ClNO are performed. Photolysis of MeOCl in solution results
in the production of the Cl:solvent charge-transfer complex; therefore, a comparison of the spectral evolution
observed following MeOCl and ClNO photolysis under identical photolysis conditions is performed to determine
the extent of Cl production following ClNO photolysis. We find that similar to the gas-phase photochemistry,
Cl and NO formation is the dominant photochemical channel in acetonitrile. However, the photochemistry in
chloroform and dichloromethane is more complex, with a second product formed in addition to Cl and NO.
It is proposed that in these solvents photoisomerization also occurs, resulting in the production of ClON. The
results presented here represent the first detailed examination of the solution phase photochemistry of ClNO.
Introduction
the K through A bands as one proceeds from low to high
energy.57,58 The A-band dominates the absorption spectrum and
A significant challenge in atmospheric chemistry is to
understand the fundamental aspects of phase-dependent pho-
tochemical reactivity. For example, the environment-dependent
reactivity of halooxides, such as chlorine dioxide (OClO), has
attracted a significant amount of attention due to the contribution
of these compounds to the stratospheric chlorine budget.1-4
Photoexcitation of gaseous OClO results predominately in the
formation of ClO and O, with a small amount of Cl and O2
also produced.5-7 However, the photochemical production of
the ClOO isomer is observed in low-temperature matrixes,8-10
and photolysis of solution-phase OClO results in both ClO/O
and ClOO production.11-21 Another halooxide, dichlorine
monoxide (ClOCl), demonstrates phase-dependent photochemi-
cal reactivity similar to OClO. In the gas phase, direct
dissociation to form Cl and ClO is the dominant photochemical
pathway,22-30 whereas, photoexcitation in low-temperature
matrixes and in solution results in the formation of the isomer,
ClClO.9,31-35
Given the substantial information regarding the phase-
dependent photochemical reactivity of halooxides, it is reason-
able to ask if similar environment-dependent reactivity is
observed for related compounds. To address this question, we
have investigated the condensed-phase photochemistry of ni-
trosyl chloride (ClNO). ClNO participates in both stratospheric
and tropospheric chemistry.36,37 Surface reactions of HONO on
frozen HCl result in the formation of ClNO in the upper
troposphere and stratosphere, whereas the reaction of sea salt
with NO2 provides for ClNO production in the lower tropo-
sphere.38 The photochemistry of gaseous ClNO is dominated
by formation of the photoproducts, NO (2Π ( 1/2, (3/2) and Cl
(2P1/2,3/2), with near-unity quantum yield.25,36,39-56 The absorption
spectrum of ClNO consists of a series of bands referred to as
has an absorption maximum of 196 nm in the gas phase.
Following A-band photoexcitation vibrationally excited NO is
produced.23,40,41,50,51,53 A bimodal distribution of NO vibrational
energy was observed, suggesting that dissociation is mediated
by two or more electronic excited states.50,53 The production of
the Cl (2P3/2) and Cl* (2P1/2) following ClNO photolysis has
also been interpreted in terms of multiple excited states residing
within the A-band.52,54,55 This hypothesis is supported by
theoretical studies of ClNO.45,59-61 However, UV resonance
Raman intensity analysis studies of gaseous ClNO could be
explained in terms of a single excited state.56
There are only a handful of studies regarding the photochem-
istry of ClNO in condensed environments. A study of ClNO
surface films found that 193 nm photodissociation results
predominantly in dissociation to form Cl and NO, similar to
the behavior observed in the gas phase.62 However, matrix
isolation studies of ClNO observed the formation of the
structural isomer, ClON.63,64 Only recently has the solution-
phase photochemistry of ClNO been investigated. Resonance
Raman studies of ClNO dissolved in cyclohexane and aceto-
nitrile have observed scattered intensity consistent with excited-
state structural evolution along all three normal coordinates.65,66
These studies also found that the resonance Raman depolariza-
tion ratios measured at excitation wavelengths spanning the
A-band are less than 1/3, consistent with the presence of at least
two excited states.
In this paper, we describe femtosecond pump-probe studies
of solution-phase ClNO. Following 266 nm photoexcitation, the
optical-density evolution from 256 to 375 nm is measured. The
absorption spectra of ClNO and its potential photoproducts are
presented in Figure 1. To determine the dependence of the
photochemistry on environment, experiments were performed
on ClNO dissolved in acetonitrile, chloroform, and dichlo-
romethane. In all solvents, the photoinduced depletion of
* To whom correspondence should be addressed. E-mail: preid@
chem.washington.edu.
10.1021/jp062069k CCC: $33.50 © 2006 American Chemical Society
Published on Web 06/16/2006

8614 J. Phys. Chem. A, Vol. 110, No. 28, 2006
Cooksey et al.
Figure 2. Schematic of the pump-probe spectrometer used in this
study. The following abbreviations have been used: BS, beam splitter;
OPA, optical parametric amplifier; BBO, frequency doubling â-barium
borate crystal; DP, Brewster dispersion prism; PL, calcite polarizer; S,
sample; 3x, frequency tripler; DM, dichroic mirror; DL, optical delay
line; C, chopper; λ/2, zero-order half-wave plate.
of an optical parametric amplifier (OPA, Quantronix TOPAS).
The fourth harmonic of the signal field derived from the OPA
was used to generate all other probe wavelengths (292, 304,
311, 322, 331, 354, and 375 nm). All frequency conversions
were performed using appropriate â-BBO crystals. Two CaF2
prisms were used to spectrally isolate the probe, and to
compensate for group-velocity dispersion. A beam-splitter
separated the probe into sample and reference beams with the
probe polarization defined using a calcite polarizer. An optical-
delay line allowed for temporal delay between the pump and
probe pulses. Rotational contributions to the measured optical-
density evolution were minimized by orienting the pump
polarization to 54.7° relative to the probe with a zero-order half-
wave plate. The instrument response as measured by the optical
Kerr effect in water was 350 ( 50 fs at all probe wavelengths.
The sample was delivered to a fused-silica flow cell equipped
with CaF2 windows having a path of 0.095 or 0.2 cm, and the
flow rate was sufficient to replace the sample volume between
photolysis events.
The pump-induced change in sample optical density was
measured as follows. UV-enhanced photodiodes (Advanced
Photonix, Inc. SD 200-13-23-242) detected the sample and
reference beams. The photodiode outputs were processed using
separate gated integrators (Stanford Research Systems SR250),
and the integrator outputs were subtracted on a shot to shot
basis. A mechanical chopper frequency modulated the pump
beam at half the amplifier repetition rate. The difference in
gated-integrator outputs was delivered to a computer where the
voltage difference between adjacent laser shots was measured
to determine the pump-induced change in optical density. An
individual scan consisted of 1000-2000 laser shots per time
point, with two or three scans averaged to produce the data
shown here. All solvents demonstrated a minor contribution to
the optical-density evolution at zero delay. This solvent-only
contribution was removed by subtracting the measured solvent-
only response from that obtained with ClNO present. Subtraction
of the solvent contribution to the ClNO spectra was considered
complete when the solvent contributions occurring at zero time
were removed. Kinetic parameters describing the optical-density
evolution were determined by fitting the experimental evolution
to a sum of exponentials convolved with the instrument response
using the Levenberg-Marquardt algorithm. Visual inspection
of the residuals and ø2 values were used to judge the goodness
of the fit. The reported errors correspond to one standard
deviation from the mean of multiple measurements taken (g3
measurements).
Figure 1. Absorption spectrum of ClNO and potential photoproducts.
The molar absorptivity values for ClNO correspond to this species
dissolved in acetonitrile.66 The absorption spectrum of the structural
isomer, ClON, is for this species isolated in a low-temperature argon
matrix,64 and the absorption spectrum of the Cl:H2O charge-transfer
complex is taken from the literature.75 The dashed line indicates the
excitation wavelength used in this study.
ground-state ClNO is followed by the formation of photoproduct
with an absorption band centered at 295 nm in acetonitrile, and
330 nm in chloroform and dichloromethane. The band reaches
a maximum in optical density ∼4 ps after photolysis and decays
on the ∼1 ns time scale. To differentiate between possible
assignments for the photoproduct, a photochemical study of
methyl hypochlorite (MeOCl) dissolved in dichloromethane was
performed. Recent work by Crim and co-workers has demon-
strated that photoexcitation of MeOCl results in the production
of Cl; therefore, these studies allow for characterization of the
optical-density evolution accompanying production of the Cl:
solvent charge-transfer complex.67 A comparison of the transient
absorption spectra for MeOCl and ClNO measured under
identical photolysis conditions, and calculations of the predicted
optical-density change accompanying Cl production are per-
formed to determine the quantum yield for formation of the
Cl:solvent charge-transfer complex. We find that ClNO pho-
tolysis in acetonitrile results predominantly in dissociation to
form Cl and NO, with a portion of these fragments undergoing
geminate recombination to re-form the parent molecule. The
photochemistry in chloroform and dichloromethane is more
complex. Specifically, in addition to Cl and NO production
another photoproduct channel is observed. It is proposed that
in these solvents, photoisomerization also occurs, resulting in
the production of the structural isomer, ClON. The results
presented here represent the first detailed study of ClNO
photochemistry in solution.
Experimental Methods
Figure 2 presents a schematic of the femtosecond pump-
probe spectrometer employed in these studies. An amplified Ti:
sapphire laser (Spectra Physics Hurricane) produced 120 fs
pulses (full width at half-maximum) centered at 800 nm with
an energy of 750 µJ and a repetition rate of 1 kHz. The amplifier
output was separated into two beams, with 40% of the output
used to generate the 266-nm pump field through third-harmonic
generation (Spectra Physics TP-IA). The various probe wave-
lengths were generated with the remainder of the amplifier
output as follows. For the 266 nm probe, the amplifier output
was frequency tripled. Probe wavelengths of 256 and 284 nm
were generated from the second harmonic of the sum-frequency
field derived from the residual pump and signal or idler fields
Pump and probe pulse energies of 10.0 and 0.3 µJ, respec-
tively, were employed with all optical-density changes deter-
mined to scale linearly with a 4-fold increase in pump power.
ClNO samples were prepared as described in the literature.65,68

Nitrosyl Chloride Photochemistry in Solution
J. Phys. Chem. A, Vol. 110, No. 28, 2006 8615
Figure 3. Normalized absorption spectra of ClNO dissolved in
acetonitrile (ACN),66 dichloromethane (CH2Cl2), and chloroform (CHCl3).
The estimated absorption band for ClNO in chloroform (CHCl3 est.) is
also shown.
UV-vis absorption was used to determine the purity and optical
density of the ClNO solutions. For this study, sample optical
densities of 0.7, as measured using a 0.2 cm path length, were
employed. To prevent degassing of ClNO from solution, the
stock solution was kept in the refrigerator and the sample
solution was chilled in an ice bath during the experiment.
Decomposition of the stock solution was not evident as
determined by vis-UV spectra obtained before and after each
experiment; however, fresh stock solutions were routinely
employed to ensure sample integrity. For experiments in which
the absolute change in optical density was determined, fresh
samples of ClNO were used for each individual measurement
at a given probe wavelength. To reduce error associated with
subtraction of the solvent contribution from the optical-density
evolution, absolute optical-density changes were measured at
the lowest pump energies possible (1.6-7.2 µJ). This resulted
in essentially no solvent contribution for all probe wavelengths
except 256 and 266 nm. At these wavelengths, solvent absorp-
tion is the greatest at the shortest delay times (1 ps) and
decreases with increased delay. The maximum optical-density
change due to the solvent is 29 ( 10% and 84 ( 9% of the
maximum change in optical density change of ClNO (with
solvent subtracted) at 1.2 ps for probe wavelengths of 256 and
266 nm, respectively. It should be noted that photoproduct
absorption is minimal in this wavelength region.
The optical-density evolution corresponding to the Cl:solvent
charge-transfer complex was determined by the photodissocia-
tion of methyl hypochlorite (MeOCl) in dichloromethane at 266
nm with pulse energies of 0.54 µJ. The transient absorption
spectrum of the Cl:solvent charge-transfer complex was mea-
sured using probe wavelengths from 292 to 375 nm. The
preparation of MeOCl in dichloromethane was performed
according to the literature.67,69 Stock solutions were routinely
made to ensure sample integrity. Fresh samples, having a MeOCl
concentration of 1.1 M, were derived daily using the stock
solution.
Figure 4. Time-resolved pump-probe dynamics of ClNO dissolved
in acetonitrile. Probe wavelengths and optical density scale are indicated.
The solid line corresponds to the best fit to the data at a given probe
wavelength by a sum of exponentials convolved with the instrument
response. Corresponding fit parameters are presented in Table 1.
dichloromethane is complicated by solvent absorption at wave-
lengths less than ∼210 nm. To minimize solvent absorption, a
25 µm path length cell was employed in the measurement, and
the A-band maximum in dichloromethane was determined to
be 213 nm (46 948 cm-1). However, the maximum in chloro-
form was still difficult to resolve; therefore, an estimate for the
absorption maximum was obtained as follows. Assuming that
the shape of the A-band in acetonitrile is similar to that for
ClNO in chloroform, a 2395 cm-1 shift of the absorption in
acetonitrile to higher energy in chloroform is required to match
the red-edge of the A-band, resulting in a predicted absorption
maximum of 206 nm (48 544 cm-1) in chloroform. Solvent
dependence of the A-band absorption maximum has been
previously observed.66 The maxima of the A-band for ClNO in
the gas phase, cyclohexane, and acetonitrile are located at 196,
206, and 217 nm, respectively, demonstrating a bathochromic
shift of the A-band with increasing solvent polarity. For the
solvents in the present study, the ET(30) polarity-scale values
are 39.1 kcal/mol (chloroform), 40.7 kcal/mol (dichloromethane),
and 45.6 kcal/mol (acetonitrile),70 also consistent with a
bathchromic shift of the A-band with increasing solvent polarity.
Results
Absorption Spectra. The absorption spectra of ClNO dis-
solved in acetonitrile, dichloromethane, and chloroform are
presented in Figure 3.The large absorption evident in the figure
is referred to as the A-band. The maximum of the ClNO A-band
in acetonitrile is 217 nm (46 083 cm-1).66 Determination of the
absorption maximum of ClNO dissolved in chloroform and
ClNO in Acetonitrile. The photoinduced evolution in optical
density for ClNO dissolved in acetonitrile is presented in Figure
4. To simplify presentation, the results are grouped in the

8616 J. Phys. Chem. A, Vol. 110, No. 28, 2006
Cooksey et al.
TABLE 1: Kinetic Parameters Determined from Analysis of the Pump-Probe Data for ClNO Dissolved in Acetonitrile
b
λpra (nm)
A1
τ1 (ps)
A2
τ2 (ps)
A3
τ3 (ps)
A4
τ4 (ps)
offsetd
offset
256
266
284
292
304
311
322
331
354
375
-0.69 ( 0.12c
-0.79 ( 0.14
3.7 ( 1.0
0.41 ( 0.17
0.31 ( 0.12
0.02 ( 0.01
0.27 ( 0.08
0.49 ( 0.16
0.24 ( 0.07
0.31 ( 0.07
0.39 ( 0.06
0.27 ( 0.06
0.07 ( 0.03
0.08 ( 0.07
-0.11 ( 0.08
-0.46 ( 0.07
-0.28 ( 0.08
-0.51 ( 0.08
-0.34 ( 0.12
-0.22 ( 0.11
-0.26 ( 0.07
-0.50 ( 0.13
-0.48 ( 0.22
7.4 ( 1.9
5.2 ( 2.1
3.4 ( 1.1
2.9 ( 0.9
3.4 ( 1.8
1.7 ( 0.4
2.5 ( 1.2
2.4 ( 0.3
2.1 ( 0.9
0.08 ( 0.05
0.27 ( 0.09
0.23 ( 0.09
0.26 ( 0.04
0.35 ( 0.06
0.40 ( 0.06
0.47 ( 0.02
0.43 ( 0.10
0.44 ( 0.15
23.5 ( 2.6
16.0 ( 2.1
21.3 ( 5.4
11.1 ( 5.6
16.7 ( 2.3
14.2 ( 1.3
12.0 ( 1.7
10.9 ( 3.1
10.7 ( 4.8
>1000
780 ( 150
800 ( 90
770 ( 30
910 ( 210
620 ( 60
590 ( 350
>1000
a Wavelength at which the transient change in optical density was measured. b Amplitudes are normalized such that Σ|Ai| ) 1. c Errors represent
one standard deviation from the mean of all measurements (g3) employed at a given wavelength. d A time constant of 10 000 ps was included to
represent any persistent offset in optical density at long delay times.
following wavelength regions: 256-266 nm, 266-284 nm, and
284-375 nm.
of probe wavelength measured at specific delay times is
presented in Figure 6A. This figure illustrates the two main
wavelength regions of interest: the decrease and partial recovery
in optical density observed at 256 nm and the large increase in
optical density centered at ∼295 nm. The decrease in optical
density at 256 nm occurs immediately after photolysis, consistent
with the photoinduced depletion of ground state ClNO. The
optical density partially recovers to a value of -3 mOD by 10
ps and remains constant out to the longest delays investigated.
This evolution in optical density may be indicative of ClNO
re-formation by geminate recombination. Time scans employing
finer time spaces demonstrate that the maximum optical-density
decrease following photolysis is -42 ( 5 mOD. Comparison
of this value to the -3 mOD of depletion observed at longer
times provides an upper limit of 93% for the amount of the
initially depleted ClNO that may be re-formed. Optical-density
evolution accompanying the vibrational relaxation of re-formed
ClNO is obscured by the photoproduct absorption band at 295
nm that is observed within 1 ps of ClNO photolysis, and reaches
a maximum of 31 ( 5 mOD by 5 ps.
256 nm e λpr < 266 nm. In this region, a photoinduced
reduction in optical density (“depletion”) and subsequent
recovery is observed. The depletion is instrument-response
limited consistent with a loss of ground-state ClNO accompany-
ing photolysis. The optical-density depletion partially recovers
to reveal a -3 ( 1 m optical density (OD) depletion that persists
out to the longest delay times investigated (500 ps). The recovery
in optical density was best fit by a single-exponential having a
time constant of 3.7 ( 1.0 ps, with the persistent decrease in
optical density reproduced using a long time offset. Time
constants determined at this and the other probe wavelengths
for ClNO dissolved in acetonitrile are provided in Table 1.
266 nm e λpr < 284 nm. At 266 nm, a pump-induced
reduction in optical density is observed at early times consistent
with the photoinduced depletion of ground-state ClNO. An
increase in optical density is observed following the initial
depletion, reaching a maximum value of 6 ( 2 mOD by ∼15
ps. Decay follows until a persistent optical-density change of
∼2.5 mOD is reached. The optical-density evolution at 266 nm
was best fit by a sum of three exponentials. The biphasic
appearance has an instrument-response limited time constant
and a slower constant of 7.4 ( 1.9 ps. The third exponential
term corresponds to optical-density decay with a time constant
of 23.5 ( 2.6 ps. Finally, a long-time offset was included to
reproduce the persistent increase in optical density.
284 nm e λpr e 375 nm. At all probe wavelengths in this
region, an initial increase in optical density is observed followed
by a decay. The optical-density increase was well fit by a single
exponential with a time constant that decreases from 7.4 ( 1.9
to 2.1 ( 0.9 ps as the probe wavelength increases from 284 to
375 nm. The decay is biphasic, consisting of a shorter time
constant that varies from 21.3 ( 5.4 ps at 292 nm to 10.7 (
4.8 ps at 375 nm, and a longer component of ∼1 ns at all
wavelengths. This longer time constant should be considered
an estimate due to the limited range of delay times investigated.
It is important to note that the maximum change in optical
density is observed at shorter delay times as the probe
wavelength increases. Specifically, at 284 nm the optical density
reaches a maximum value at ∼12 ps, whereas, at 375 nm a
maximum is reached at ∼4 ps. This evolution in optical density
is more readily apparent in two-dimensional contour plot of the
transient absorption spectrum presented in Figure 5A. Here,
modest blue-shifting and narrowing of the photoproduct band
is evident, consistent with photoproduct production followed
by vibrational relaxation of this species on the ∼20 ps time
scale.
ClNO in Chloroform. The photoinduced evolution in optical
density for ClNO dissolved in chloroform is presented in Figure
7. Consistent with the previous discussion, the results are
discussed in the following wavelength regions: 256-266 nm,
266-284 nm, and 284-375 nm.
256 nm e λpr < 266 nm. The evolution in optical density in
this region is characterized by an initial decrease in optical-
density consistent with photoinduced depletion of ground-state
ClNO. This is followed by a near complete recovery of optical
density, which occurs with a time constant of 8 ( 1 ps. A long-
time offset in optical density was included to account for the
small residual offset in optical density observed at long delays.
Table 2 provides the time constants determined at this and other
probe wavelengths for ClNO dissolved in chloroform.
266 nm e λpr < 284 nm. Similar to the behavior observed
for ClNO in acetonitrile, an initial reduction in optical density
is observed, consistent with the photoinduced depletion of
ground-state ClNO. The pump-induced depletion is followed
by an increase in optical density that reaches a maximum of 10
mOD by ∼25 ps. A decay in optical density is then observed
until a value of ∼1 mOD is reached. The optical-density
evolution was best fit with a sum of two exponentials, with the
increase in optical density occurring with a time constant of
10.1 ( 1.7 ps and subsequent decay with a time constant of
36.6 ( 21.1 ps. Finally, a long-time offset was included in the
fitting procedure to reproduce the persistent increase in optical
density.
284 nm e λpr e 375 nm. The evolution observed in this
wavelength region is characterized by an initial increase in
optical density followed by a biphasic decay. The initial optical-
Transient Absorption Spectrum. The absolute change in
optical density for ClNO dissolved in acetonitrile as a function

Nitrosyl Chloride Photochemistry in Solution
J. Phys. Chem. A, Vol. 110, No. 28, 2006 8617
Figure 5. Contour plots of the transient absorption spectrum of (A) ClNO dissolved in acetonitrile, (B) ClNO dissolved in chloroform, (C) MeOCl
dissolved in dichloromethane and (D) ClNO dissolved in dichloromethane. The absolute change in optical density is indicated by the contour lines,
with the color scale shown above the each plot. Photolysis energies were 10.0 µJ in acetonitrile and chloroform, and 0.54 µJ for ClNO and MeOCl
in dichloromethane.
density increase was fit by a single exponential with time
constants ranging from 3.2 ( 1.1 ps at 331 nm to 9.9 ( 1.8 ps
at 284 nm. The shortest decay time constant varied from 10.0
( 4.2 ps at 375 nm to 34.4 ( 11.4 ps at 304 nm. The longer
decay time constant was ∼1 ns at all probe wavelengths, which
should be viewed as an estimate given the limited delay-times
investigated. In contrast to the behavior observed in acetonitrile,
the photoproduct absorption band demonstrates limited spectral
evolution as illustrated in the contour plot of the optical-density
evolution presented in Figure 5B. Blue-edge enhancement of
the photoproduct absorption band is apparent at early times,
however, significant early-time optical-density enhancement and
decay on red-shifting of the absorption maximum consistent with
vibrational relaxation is not observed.
Transient Absorption Spectrum. The transient absorption
spectrum of ClNO dissolved in chloroform is presented in Figure
6B. Two main wavelength regions of interest are evident: the
initial decrease and recovery in optical density observed at 256
nm and the large increase in optical density observed at 331
nm. Photolysis results in an initial decrease in optical density
consistent with depletion of ground-state ClNO. Experiments
with finer time spacings demonstrate that the magnitude
depletion amplitude is -21 ( 6 mOD. The depletion recovers
to zero within the error of the measurement (3 mOD) and
remains constant out to the longest delays. The photoproduct
absorption band centered at 331 nm appears within 1 ps of ClNO
photolysis reaching a maximum of 81 ( 12 mOD by 3 ps. The
absence of residual depletion associated with ground-state ClNO
prohibits defining a lower limit for the geminate recombination
quantum yield; however, the magnitude of the photoproduct
band optical density is 4 times larger than the reduction in
optical-density associated with ClNO depletion such that the
photoproduct must have a molar absorptivity that is significantly
greater than that of ClNO at 256 nm.
Cl:Solvent Charge-Transfer Complex Study. To explore
the possibility that the photoproduct band observed following
ClNO photolysis corresponds to the Cl:solvent charge-transfer
complex, photochemical studies of MeOCl were performed.
Recent work by Crim and co-workers has established that
photolysis of this species at 266 nm results in the production
of Cl such that MeOCl is a convenient photochemical Cl
precursor.67 Unfortunately, the instability of MeOCl in chloro-
form prevented its use in this solvent. Instead, the Cl:solvent
charge-transfer study was performed in dichloromethane. The
similarity of dichloromethane and chloroform in terms of
molecular structure and solvent polarity, as well as the preexist-
ing photochemical studies in this solvent make dichloromethane
an attractive alternative solvent for study. A contour plot of the

8618 J. Phys. Chem. A, Vol. 110, No. 28, 2006
Cooksey et al.
Figure 6. Transient absorption spectrum of (A) ClNO dissolved in
acetonitrile and (B) ClNO dissolved in chloroform. The time delay for
each spectrum is given in the figure.
Figure 7. Time-resolved pump-probe dynamics of ClNO dissolved
in chloroform. Probe wavelengths and optical density scale are
indicated. The solid line corresponds to the best fit to the data at a
given probe wavelength by a sum of exponentials convolved with the
instrument response. Corresponding fit parameters are presented in
Table 2.
pump-probe dynamics observed following MeOCl photoexci-
tation in dichloromethane is presented in Figure 5C. At all probe
wavelengths an instrument-response-limited increase in optical
density is observed followed by a longer-time optical-density
increase that occurs with a time constant that varies from 0.99
( 0.07 ps at 311 nm to 2.9 ( 0.8 at 375 nm. After reaching a
maximum, a decay in optical density is observed with a time
constant that varies from 20.0 ( 7.1 ps at 340 nm to 41.1 (
11.2 ps at 304 nm. A long-time offset in optical density follows
and persists out to the longest delays investigated (50 ps). The
time constants measured at the individual probe wavelengths
are provided in Table 3. The absorption band of the charge-
transfer complex is broad and centered at ∼340 nm, reaching a
maximum optical density change of 7 mOD by ∼5 ps.
is slower, and the decay in optical density assignable to the
photoproduct is more rapid than that observed for the Cl:solvent
charge-transfer complex produced following MeOCl photolysis.
In summary, the results presented here indicate that in all
solvents ClNO photolysis results in the production of at least
one photoproduct that persists out to 500 ps. The photoproduct
band has a maximum of 295 nm in acetonitrile, but in
chloroform and dichloromethane the maximum is shifted to 330
nm. Photolysis of MeOCl in dichloromethane demonstrates that
the Cl:solvent charge-transfer complex in this solvent has an
absorption maximum close to 330 nm supporting assignment
of the ClNO photoproduct in chloroform and dichloromethane
to the Cl:solvent charge-transfer complex. However, the pho-
toproduct band shapes, the kinetics for optical-density evolution,
and the magnitude of the optical-density increase corresponding
to photoproduct formation differ significantly between ClNO
and MeOCl under identical photolysis conditions complicating
the assignment of the ClNO photoproduct to the Cl:solvent
complex. The maximum change in optical density of the ClNO
photoproduct is solvent dependent, with the optical density being
∼2.7 times larger in chloroform compared to acetonitrile (Figure
6). Finally, the maximum optical density change of the ClNO-
photoproduct peak in chloroform is 4 times larger than the
decrease in optical density due to photoexcitation of ClNO,
The pump-probe dynamics for ClNO dissolved in dichlo-
romethane were measured under photolysis conditions identical
to those employed for MeOCl, and a contour plot of the pump-
probe dynamics in this solvent is presented in Figure 5D. The
time constants for optical-density evolution determined at the
individual probe wavelengths are provided in Table 4. Following
photoexcitation, an instrument-response limited increase in
optical density is observed followed by an additional longer-
time increase that occurs with a time constant that varies from
1.6 ( 0.4 ps at 340 nm to 4.3 ( 3.0 ps at 304 nm. A subsequent
decay in optical density is observed, occurring with a time
constant that varies from 12.8 ( 3.8 ps at 322 nm to 25.6 (
13.5 ps at 304 nm to reveal an optical-density offset that is
constant out to the longest delays investigated. Comparison of
the pump-probe kinetics for MeOCl and ClNO in dichlo-
romethane demonstrates that photoproduct formation for ClNO

Nitrosyl Chloride Photochemistry in Solution
J. Phys. Chem. A, Vol. 110, No. 28, 2006 8619
TABLE 2: Kinetic Parameters Determined from Analysis of the Pump-Probe Data for ClNO Dissolved in Chloroform
b
λpra (nm)
A1
τ1 (ps)
A2
τ2 (ps)
A3
τ3 (ps)
A4
τ4 (ps)
offsetd
offset
7600 ( 4200
1430 ( 580
980 ( 470
730 ( 190
880 ( 440
780 ( 490
1100 ( 430
1250 ( 1240
256
266
284
292
304
311
322
331
354
375
-0.92 ( 0.06c
7.9 ( 1.0
-0.04 ( 0.08
0.09 ( 0.03
0.18 ( 0.09
0.36 ( 0.16
0.45 ( 0.11
0.27 ( 0.07
0.56 ( 0.09
0.53 ( 0.07
0.41 ( 0.08
0.10 ( 0.06
-0.62 ( 0.03
-0.44 ( 0.06
-0.30 ( 0.08
-0.23 ( 0.10
-0.43 ( 0.05
-0.19 ( 0.05
-0.21 ( 0.05
-0.30 ( 0.04
-0.47 ( 0.04
10.1 ( 1.7
9.9 ( 1.8
7.2 ( 4.7
4.9 ( 3.4
6.7 ( 3.9
3.2 ( 1.6
3.2 ( 1.0
5.4 ( 1.5
6.7 ( 3.1
0.29 ( 0.02
0.37 ( 0.04
0.34 ( 0.10
0.32 ( 0.07
0.29 ( 0.03
0.24 ( 0.05
0.25 ( 0.03
0.29 ( 0.04
0.48 ( 0.03
36.6 ( 21.1
16.8 ( 5.0
27.2 ( 13.4
34.4 ( 11.4
16.5 ( 6.3
12.6 ( 6.9
19.2 ( 6.8
12.2 ( 3.7
10.0 ( 4.2
a Wavelength at which the transient change in optical density was measured. b Amplitudes are normalized such that Σ|Ai| ) 1. c Errors represent
one standard deviation from the mean of all measurements (g3) employed at a given wavelength. d A time constant of 10 000 ps was included to
represent any persistent offset in optical density at long delay times.
TABLE 3: Kinetic Parameters Determined from Analysis of the Pump-Probe Data for CH3OCl Dissolved in Dichloromethane
b
λpr a (nm)
A1
τ1 (ps)
A2
τ2 (ps)
A3
τ3 (ps)
304
311
322
331
340
354
375
-0.41 ( 0.28c
-0.19 ( 0.04
-0.22 ( 0.06
-0.36 ( 0.01
-0.24 ( 0.07
-0.32 ( 0.04
-0.30 ( 0.20
1.48 ( 0.48
0.99 ( 0.07
1.14 ( 0.20
1.04 ( 0.15
2.02 ( 0.77
1.34 ( 0.41
2.94 ( 0.78
0.32 ( 0.17
0.55 ( 0.16
0.44 ( 0.14
0.36 ( 0.10
0.31 ( 0.04
0.36 ( 0.02
0.47 ( 0.26
41.1 ( 11.2
40.0 ( 23.4
35.7 ( 21.6
33.5 ( 10.4
20.0 ( 7.1
34.8 ( 7.7
21.5 ( 3.2
0.28 ( 0.15
0.26 ( 0.17
0.34 ( 0.12
0.29 ( 0.09
0.47 ( 0.06
0.26 ( 0.15
0.23 ( 0.16
offsetd
offset
offset
offset
offset
offset
offset
a Wavelength at which the transient change in optical density was measured. b Amplitudes are normalized such that Σ|Ai| ) 1. c Errors represent
one standard deviation from the mean of all measurements (g3) employed at a given wavelength. d A time constant of 10 000 ps was included to
represent any persistent offset in optical density at long delay times.
TABLE 4: Kinetic Parameters Determined from Analysis of the Pump-Probe Data for ClNO Dissolved in Dichloromethane
b
λpra (nm)
A1
τ1 (ps)
A2
τ2 (ps)
A3
τ3 (ps)
304
311
322
331
340
354
375
400
-0.27 ( 0.05c
-0.26 ( 0.03
-0.19 ( 0.08
-0.19 ( 0.11
-0.31 ( 0.07
-0.31 ( 0.02
-0.33 ( 0.03
-0.35 ( 0.04
4.3 ( 3.0
3.4 ( 0.9
4.6 ( 1.6
3.7 ( 2.4
1.6 ( 0.4
2.0 ( 0.5
2.7 ( 0.7
11.3 ( 2.1
0.37 ( 0.13
0.31 ( 0.04
0.32 ( 0.08
0.29 ( 0.06
0.27 ( 0.06
0.22 ( 0.02
0.27 ( 0.06
0.57 ( 0.07
25.6 ( 13.5
14.0 ( 5.0
12.8 ( 3.8
22.6 ( 10.5
18.2 ( 6.6
14.0 ( 3.5
16.9 ( 7.4
22.9 ( 2.5
0.36 ( 0.15
0.36 ( 0.18
0.48 ( 0.14
0.52 ( 0.14
0.42 ( 0.07
0.47 ( 0.04
0.41 ( 0.06
0.08 ( 0.04
offsetd
offset
offset
offset
offset
offset
offset
offset
a Wavelength at which the transient change in optical density was measured. b Amplitudes are normalized such that Σ|Ai| ) 1. c Errors represent
one standard deviation from the mean of all measurements (g3) employed at a given wavelength. d A time constant of 10 000 ps was included to
represent any persistent offset in optical density at long delay times.
suggesting that the molar absorptivity of the photoproduct at
330 nm is at least 4 times larger than that of ClNO at 256 nm
(Figure 6).
of interest, this species most likely does not contribute to the
optical density evolution observed in this experiment and is
subsequently removed from consideration as the photoproduct.
Dissociation is the dominant photochemical pathway for
gaseous ClNO; therefore, it is reasonable to expect that Cl is
produced following ClNO photolysis in solution. In solution,
Cl will interact with the solvent to form the Cl:solvent charge-
transfer complex, with the absorption band maximum of this
species correlating with the ionization potential of the solvent.72
The absorption maximum for the Cl:chloroform charge-transfer
complex is at 330 nm.72 This wavelength is essentially identical
to the ClNO photoproduct absorption-band maximum in chlo-
roform supporting assignment of the photoproduct to the Cl:
solvent charge-transfer complex. To test this assignment, we
performed a photochemical study of MeOCl dissolved in a
similar solvent, dichloromethane. MeOCl is a convenient
photochemical precursor for Cl.67,73,74 Furthermore, chloroform
and dichloromethane have essentially the same ionization
potential (11.37 and 11.32 eV) such that the location of the
Cl:solvent charge-transfer band in these solvents should be
almost identical.72 Finally, the pump-probe dynamics of MeOCl
in dichloromethane have previously been investigated by Crim
and co-workers, and they found that photolysis of MeOCl at
Discussion
One of the main goals for this study is to examine the
photochemistry of solution-phase ClNO. As mentioned in the
Introduction, photochemical studies of gaseous ClNO employing
wavelengths resonant with the A-band have shown that Cl and
NO formation is the dominant photochemical pathway.25,36,39-56
In contrast, the structural isomer (ClON) is the only photoprod-
uct observed in low-temperature matrix studies.63,64 The results
presented here represent the first detailed measurements of ClNO
photochemistry in solution. The experimentally observed optical-
density evolution that occurs following ClNO photolysis
demonstrates that a photoproduct is formed that has an absorp-
tion maximum of 295 nm in acetonitrile and 330 nm in
chloroform. There are three possible assignments for this
photoproduct: NO, the Cl:solvent charge-transfer complex, and
the photoisomer ClON. The absorption band of gas-phase NO
consists of a series of discrete rotation bands, which begin at
about 230 nm and continue into the far UV.71 Because the
absorption of NO is well removed from the wavelength region

8620 J. Phys. Chem. A, Vol. 110, No. 28, 2006
Cooksey et al.
266 nm resulted in formation of the Cl:solvent charge-transfer
complex with an absorption maximum of 330 nm.67 The pump-
probe dynamics for MeOCl dissolved in dichloromethane were
compared to those observed for ClNO dissolved in this same
solvent and under identical photolysis conditions. The photo-
product absorption maxima for MeOCl and ClNO are essentially
identical (Figure 5C,D), supporting assignment of the ClNO
photoproduct to the Cl:solvent charge-transfer complex. How-
ever, the kinetics, magnitude of the optical-density change, and
the band shapes associated with the photoproduct differ
significantly between ClNO and MeOCl. Specifically, for ClNO
photoproduct formation is slower, and decay of this species is
more rapid in comparison to MeOCl (Tables 3 and 4). Also,
for ClNO the long-time increase in optical density is twice as
large as that observed for MeOCl. Both of these observations
call into question assignment of the ClNO photoproduct in
chloroform and dichloromethane to the Cl:solvent charge-
transfer complex.
To further investigate the assignment of the Cl:solvent charge-
transfer complex as the ClNO photoproduct, we attempted to
model the transient absorption spectrum of ClNO in chloroform
by assuming that Cl production is the dominant photochemical
pathway. In the model we also assume that the surrounding
solvent shell provides a barrier for some of the departing
photofragments, resulting in geminate recombination to re-form
the parent molecule, with Cl photofragments that escape the
solvent cage resulting in formation of the charge-transfer
complex. The photolysis-induced change in optical density
(∆OD(λ)) can be expressed as
Figure 8. Calculation of the transient absorption spectrum of (A) ClNO
dissolved in chloroform and (B) ClNO dissolved in acetonitrile. The
calculation is based on a photochemical mechanism involving only
geminate recombination and Cl production, as described in the text.
The calculated spectra, quantified in terms of the chlorine quantum
yield (ΦCl), are compared to the measured transient absorption spectra
taken with a 50 ps time delay.
was 0.095 cm, and ꢀClNO at 256 nm was 1197 M-1 cm-1. Using
∆OD(λ) )
these quantities and Beer’s law, [ClNO]ex ) 1.8 × 10-4
M
corresponding to 4% depletion. Beginning with this concentra-
tion as an upper limit for Cl production (i.e., complete cage
escape), we explored various values of ΦCl in an attempt to
reproduce the transient absorption spectrum. Figure 8A presents
a comparison of the calculated and experimental optical-density
change observed at 50 ps delay. The figure demonstrates that
the magnitude of the optical-density increase can be reproduced
by assuming ΦCl ) 0.86; however, the breadth of the Cl:solvent
charge-transfer band is significantly greater than that of the
ClNO photoproduct. There are a few issues that should be
considered at this point. First, we have assumed a maximum
molar absorptivity value for the Cl:chloroform charge-transfer
complex of 4000 M-1 cm-1, consistent with the corresponding
value in water.75 However, the Cl:benzene charge-transfer
complex has a maximum molar absorptivity value of only 2000
b{ꢀCl(λ)[Cl] - ꢀClNO(λ)([ClNO]ex - [ClNO]ref)} (1)
where b is the path length and ꢀCl(λ) and ꢀClNO(λ) are the
wavelength-dependent molar absorptivities of the Cl:solvent
charge-transfer complex and ClNO, respectively. The concentra-
tion of the Cl:solvent charge-transfer complex is given by [Cl].
The concentration of ClNO re-formed following photolysis is
given by [ClNO]ref, and the concentration of initially photo-
excited ClNO is given by [ClNO]ex. The amount of Cl produced
and the amount of ClNO that geminately recombines is equal
to the total amount of the initially photoexcited ClNO:
[ClNO]ex ) [ClNO]ref + [Cl]
(2)
Applying this constraint to eq 1 and using the definition of the
quantum yield of chlorine production (ΦCl ) [Cl]/[ClNO]ex),
the following expression for the change in optical density is
obtained:
M-1 cm-1 76
; thus, the molar absorptivity of water may represent
an upper limit for charge-transfer complexes. Also, we have
assumed that the Cl:solvent complex absorption bandwidths are
identical for water and chloroform. However, the bandwidths
vary from ∼4000 cm-1 to ∼10 000 cm-1 (full width half-
maximum) depending on solvent.72,77 In comparison, the
∆OD(λ) ) b{ꢀCl(λ) - ꢀClNO(λ)}ΦCl[ClNO]ex
(3)
Application of eq 3 requires knowledge of the molar absorptivity
of the Cl:solvent charge-transfer complex, however these values
are not known for Cl:chloroform. Therefore, approximate values
were derived as follows. The absorption band of the Cl:water
charge-transfer complex has an absorption maximum at 310 nm
(32 258 cm-1); therefore, this band was shifted by 1955 cm-1
to lower energy to match the absorption maximum of the Cl:
chloroform complex of 330 nm (30 303 cm-1).72 The Cl:water
and Cl:dichloromethane charge-transfer complexes have similar
bandwidths supporting this approximation.67,75 Next, the con-
centration of initially photolyzed ClNO ([ClNO]ex) was deter-
mined as follows. The maximum depletion observed at 256 nm
in chloroform was -21 mOD, the cell path length in this solvent
bandwidth of the ClNO photoproduct band is only ∼2600 cm-1
.
Therefore, assignment of the ClNO photoproduct in chloroform
to the Cl:solvent charge-transfer complex is far from clear.
To further explore the extent of Cl production following
ClNO photolysis, an identical comparison of the predicted and
experimental optical-density change for ClNO dissolved in
acetonitrile was performed. As with chloroform, the molar
absorptivity values are not known for the Cl:acetonitrile charge-
transfer complex; therefore, the values for the Cl:water complex
were again employed. Using a linear correlation between the
Cl:solvent charge-transfer band maxima and the solvent ioniza-
tion potential72 and the literature value for the ionization
potential for acetonitrile (12.20 eV), we calculated the absorption

Nitrosyl Chloride Photochemistry in Solution
J. Phys. Chem. A, Vol. 110, No. 28, 2006 8621
maximum to be 275 nm (36 364 cm-1). However, a better fit
to the data was obtained using an absorption maximum of 280
nm (35 714 cm-1). The maximum depletion observed at 256
nm was -42 mOD, the path length used for the studies of ClNO
in acetonitrile was 0.2 cm, and the ClNO molar absorptivity at
256 nm is 3508 M-1 cm-1. Therefore, [ClNO]ex ) 6.0 × 10-5
M corresponding to 4.6% depletion. Figure 8B presents the
comparison between the predicted and measured optical-density
change for ClNO in acetonitrile at 50 ps delay. In contrast to
chloroform, reasonably good agreement was obtained by as-
suming ΦCl ) 0.55. The ability of the calculation to reproduce
the measured optical-density change provides support for the
dominance of the Cl and NO photochemical channel in
acetonitrile. According to the calculation, the maximum change
in optical density due to the ClNO photoproduct corresponds
to ΦCl ) 0.75; however, the apparent quantum yield decreases
to 0.55 by 50 ps following photolysis. The decay of chlorine
may be due to diffusive geminate recombination, consistent with
previous studies of the dynamics of Cl:solvent charge-transfer
complexes.67,78
(ν2) and 332 cm-1 (ν3) in the gas phase to 585 cm-1 (ν2) and
319 cm-1 (ν3) in an Ar-matrix, respectively.63,64,82 In solution,
the ν2 and ν3 fundamental transitions shift from 585 and 325
cm-1 in cyclohexane to 545 and 310 cm-1 in acetonitrile.65,66
With respect to the structural isomer, ClON is predicted to have
an even shorter NO bond length (1.127 Å) than diatomic NO
whereas the ClO bond length is predicted to be extremely long
(2.273 Å).83 Calculations also suggest that the ionic contribution
to the bonding in ClNO and ClON may be a key factor in
relative stability of the isomer.81,83 Given the ionic nature of
ClO bond, the environment surrounding ClON is expected to
significantly influence geometry and state energetics leading
to solvatochromic behavior. A similar hypothesis has also been
made for the structural isomer of dichlorine monoxide, ClClO.35
Here the absorption band of ClClO in perfluorohexane was
proposed to be 55 nm red shifted compared to the absorption
band of ClClO in matrix isolation due to the environmental
dependence on the Cl-Cl bond strength.
A second consideration with respect to assignment of the
photoproduct absorption in chloroform to ClON is the magnitude
of the observed optical density change. Inspection of the
transient absorption spectra for ClNO dissolved in chloroform
reveals that the reduction in optical density due to ground-state
depletion of ClNO is 4 times less than the increase in optical
density associated with photoproduct formation. This compari-
son implies that the maximum molar absorptivity of the
photoproduct is at least 4 times greater than the molar absorp-
tivity of ClNO at 256 nm. This is consistent with the relative
absorptivity of halooxide isomers. Specifically, the molar
absorptivities of ClOO and ClClO are more than twice as large
as their more stable forms, OClO and ClOCl.32,84 Thus, the
ClNO-photoproduct band in chloroform could correspond to
formation of the isomer, ClON.
The results and analysis presented above can be used to
develop a picture of the photochemical reaction dynamics of
ClNO in solution. The transient absorption spectrum of ClNO
in acetonitrile was successfully reproduced by assuming Cl and
NO production occurs with a quantum yield of 0.75. A subset
of the initially produced Cl and NO undergoes geminate
recombination in this solvent, resulting in the re-formation of
ClNO. In chloroform the photochemistry appears to be more
complex. A portion of the optical density evolution can also be
assigned to Cl and NO production; however, the width of the
dominant photoproduct absorption band is not consistent with
formation of the Cl:solvent charge-transfer complex. Instead,
we have proposed that another photochemical channel is
responsible for the photoproduct absorption band centered at
330 nm in this solvent whereas formation of the Cl:solvent
charge-transfer complex contributes to the low absorbing
background. Clarification and confirmation of the photochemical
reaction dynamics of ClNO require spectroscopic techniques
capable of unequivocal structural identification of the photo-
products, and time-resolved vibrational spectroscopy studies are
currently underway.
The model described above assumes that only Cl and NO
are produced; therefore, the possibility of other photochemical
channels being operative in this solvent was considered. It
remains reasonable to expect that the production of Cl and NO
occurs in this solvent given the dominance of this photoproduct
channel in the gas phase. Furthermore, Figure 8A demonstrates
that there is a broad low-lying optical density increase that could
be consistent with formation of the Cl:solvent charge-transfer
complex (ΦCl ) 0.42). However, the species responsible for
the main photoproduct band in chloroform remains in question.
The final potential photoproduct to consider is the structural
isomer, ClON. The production of ClON following ClNO
photolysis in low-temperature Ar and N2 matrixes has been
observed.63,64 In this previous study, the absorption spectrum
of ClNO before and after photolysis was obtained, and the
formation of a new broad absorption band centered at 417 nm
was observed and assigned to ClON.64 Given this observation,
assignment of the solution-phase photoproduct to ClON requires
that the absorption maximum of this species undergo an 85 nm
shift in chloroform relative to low-temperature matrixes. How
reasonable is it to expect such a large shift in the absorption
maximum of this species?
Insight into the environmental sensitivity of the ClON
absorption spectrum can be obtained by considering the behavior
of the parent species, ClNO. Experimental and theoretical studies
have shown that the absorption spectrum of ClNO is sensitive
to the local environment. For gaseous ClNO the A-band
maximum occurs at 196 nm, and in acetonitrile the maximum
shifts to 217 nm.65,66 This behavior was attributed to the
environmental dependence of the NCl bond length.66 Calcula-
tions indicate that in ClNO, the NCl bond length (1.98 Å) is
elongated, whereas the NO bond length (1.13 Å) is close to
that of molecular NO (1.151 Å), suggesting that the Cl is only
loosely bound to NO.79 Furthermore, it has been suggested that
nitrosyl halides are best described by an ionic structure
corresponding to X- (X) F, Cl, Br) interacting with NO+.80,81
Assuming substantial ionic character of the ClN bond in ClNO,
the absorption spectrum of this species should be sensitive to
the environment. Environmental sensitivity of the ClN bond
length is reflected by variation of the ClNO vibrational
frequencies with solvent. Specifically, the NO-stretch funda-
mental transition (ν1) does not change significantly between gas-
phase (1799.7 cm-1) and Ar matrixes (1805 cm-1); however,
the bend and ClN fundamental transitions shift from 596 cm-1
Acknowledgment. The National Science Foundation is
acknowledged for their support of this work (CHE-0350191).
Acknowledgment is also made to the donors of the Petroleum
Research Fund, administered by the American Chemical Society.
C.C.C. gratefully acknowledges support from the Ford Founda-
tion.
References and Notes
(1) Rowland, F. S. Annu. ReV. Phys. Chem. 1991, 42, 731.
(2) Renard, J. B.; Pirre, M.; Robert, C.; Huguenin, D. J. Geophys. Res.-
Atmos. 1998, 103, 25383.

8622 J. Phys. Chem. A, Vol. 110, No. 28, 2006
Cooksey et al.
(3) Solomon, S.; Borrmann, S.; Garcia, R. R.; Portmann, R.; Thomason,
L.; Poole, L. R.; Winker, D.; McCormick, M. P. J. Geophys. Res.-Atmos.
1997, 102, 21411.
(4) Donsig, H. A.; Herridge, D.; Vickerman, J. C. J. Phys. Chem. A
1998, 103, 9211.
(5) Vaida, V.; Simon, J. D. Science 1995, 268, 1443.
(6) Reid, P. J. J. Phys. Chem. A 2002, 106, 1473.
(7) Cooksey, C. C.; Reid, P. J. Photochem. Photobiol. 2004, 80, 386.
(8) Arkell, A.; Schwager, I. J. Am. Chem. Soc. 1967, 89, 5999.
(9) Rochkind, M. M.; Pimentel, G. C. J. Chem. Phys. 1967, 46, 4481.
(10) Mueller, H. S. P.; Willner, H. J. Phys. Chem. 1993, 97, 10589.
(11) Thogersen, J.; Jepsen, P. U.; Thomsen, C. L.; Poulsen, J. A.; Byberg,
J. R.; Keiding, S. R. J. Phys. Chem. A 1997, 101, 3317.
(12) Philpott, M. J.; Charalambous, S.; Reid, P. J. Chem. Phys. Lett.
1997, 281, 1.
(13) Thogersen, J.; Thomsen, C. L.; Poulsen, J. A.; Keiding, S. R. J.
Phys. Chem. A 1998, 102, 4186.
(14) Poulsen, J. A.; Thomsen, C. L.; Keiding, S. R.; Thogersen, J. J.
Chem. Phys. 1998, 108, 8461.
(15) Philpott, M. J.; Hayes, S. C.; Reid, P. J. Chem. Phys. 1998, 236,
207.
(16) Hayes, S. C.; Philpott, M. J.; Reid, P. J. J. Chem. Phys. 1998, 109,
2596.
(17) Hayes, S. C.; Philpott, M. P.; Mayer, S. G.; Reid, P. J. J. Phys.
Chem. A 1999, 103, 5534.
(18) Thomsen, C. L.; Philpott, M. P.; Hayes, S. C.; Reid, P. J. J. Chem.
Phys. 2000, 112, 505.
(19) Thomsen, C. L.; Reid, P. J.; Kieding, S. R. J. Am. Chem. Soc. 2000,
122, 12795.
(20) Philpott, M. J.; Hayes, S. C.; Thomsen, C. L.; Reid, P. J. Chem.
Phys. 2001, 263, 389.
(21) Hayes, S. C.; Thomsen, C. L.; Reid, P. J. J. Chem. Phys. 2001,
115, 11228.
(22) Edgecombe, F. H. C.; Norrish, R. G. W.; Thrush, F. R. S.; Thrush,
B. A. Proc. R. Soc. London A 1957, 243, 24.
(23) Basco, N.; Dogra, S. K. Proc. R. Soc. London A 1971, 323, 401.
(24) Sander, S. P.; Friedl, R. R. J. Phys. Chem. 1989, 93, 4764.
(25) Chichinin, A. I. Chem. Phys. Lett. 1993, 209, 459.
(26) Nelson, C. M.; Moore, T. A.; Okumura, M.; Minton, T. K. J. Chem.
Phys. 1994, 100, 8055.
(27) Nickolaisen, S. L.; Miller, C. E.; Sander, S. P.; Hand, M. R.;
Williams, I. H.; Francisco, J. S. J. Chem. Phys. 1996, 104, 2857.
(28) Moore, T. A.; Okumura, M.; Minton, T. K. J. Chem. Phys. 1997,
107, 3337.
(44) Ticktin, A.; Bruno, A. E.; Bruhlmann, U.; Huber, J. R. Chem. Phys.
1988, 125, 403.
(45) Bai, Y. Y.; Ogai, A.; Qian, C. X. W.; Iwata, L.; Segal, G. A.;
Reisler, H. J. Chem. Phys. 1989, 90, 3903
(46) Bell, A. J.; Pardon, P. R.; Frey, J. G. Mol. Phys. 1989, 67, 465.
(47) Bell, A. J.; Frey, J. G. Mol. Phys. 1990, 69, 943.
(48) Ogai, A.; Qian, C. X. W.; Reisler, H. J. Chem. Phys. 1990, 93,
1107.
(49) Cao, J. Y.; Wang, Y. F.; Qian, C. X. W. J. Chem. Phys. 1995,
103, 9653.
(50) Haas, B. M.; Felder, P.; Huber, J. R. Chem. Phys. Lett. 1991, 180,
293.
(51) Gillan, I. T. F.; Denvir, D. J.; Cormican, H. F. J.; Duncan, I.;
Murrow, T. Chem. Phys. 1992, 167, 193.
(52) Kim, H. L.; Mo, Y.; Matsumi, Y.; Kawasaki, M. Bull. Korean
Chem. Soc. 1992, 13, 162.
(53) Felder, P.; Morley, G. P. Chem. Phys. 1994, 185, 145.
(54) Kawasaki, M.; Suto, K.; Sato, Y.; Matsumi, Y.; Bersohn, R. J. Phys.
Chem. 1996, 100, 19853.
(55) Skorokhodov, V.; Sato, Y.; Suto, K.; Matsumi, Y.; Kawasaki, M.
J. Phys. Chem. 1996, 100, 12321.
(56) Mackey, J. L.; Johnson, B. R.; Kittrell, C.; Le, L. D.; Kinsey, J. L.
J. Chem. Phys. 2001, 114, 6631.
(57) Goodeve, C. F.; Katz, S. Proc. R. Soc. A 1939, 172, 432.
(58) Ballash, N. M.; Armstrong, D. A. Spectrochim. Acta 1974, 30A,
941.
(59) Solgadi, D.; Lahmani, F.; Lardeux, C. Chem. Phys. 1983, 79, 1983.
(60) Lacombe, S.; Loudet, M.; Dargelos, A.; Camou, J. M. Chem. Phys.
2000, 258, 1.
(61) Yamashita, T.; Kato, S. J. Chem. Phys. 2003, 119, 4251.
(62) Ning, C.; Pfab, J. Chem. Phys. Lett. 1993, 216, 87.
(63) Hallou, A.; Schriver-Mazzuoli, L.; Schriver, A.; Chaquin, P. Chem.
Phys. 1998, 237, 251.
(64) Maier, G.; Reisenauer, H. P.; De Marco, M. Chem. Eur. J. 2000,
6, 800.
(65) Barham, B. P.; Reid, P. J. Chem. Phys. Lett. 2002, 361, 49.
(66) Nyholm, B. P.; Reid, P. J. J. Phys. Chem. B 2004, 108, 8716.
(67) Elles, C. G.; Cox, M. J.; Barnes, G. L.; Crim, F. F. J. Phys. Chem.
A 2004, 108, 10973.
(68) Pass, G.; Sutcliffe, H., Practical Inorganic Chemistry: Preparation,
Reactions and Instrumental Methods; Chapman and Hall: London, 1968.
(69) Jenner, E. L. J. Org. Chem. 1962, 27, 1031.
(70) KnoVel SolVents - A Properties Database; Wypych, G., Ed.;
ChemTec Publishing: Toronto, 2000.
(29) Tanaka, Y.; Kawasaki, M.; Matsumi, Y.; Fujiwara, H.; Ishiwata,
T.; Rogers, L. J.; Dixon, R. N.; Ashfold, M. N. R. J. Chem. Phys. 1998,
109, 1315.
(30) Smith, G. D.; Tablas, F. M. G.; Molina, L. T.; Molina, M. J. J.
Phys. Chem. A 2001, 105, 8658.
(31) Chi, F. K.; Andrews, L. J. Phys. Chem. 1973, 77, 3062.
(32) Johnsson, K.; Engdahl, A.; Nelander, B. J. Phys. Chem. 1995, 99,
3965.
(33) Gane, M. P.; Williams, N. A.; Sodeau, J. R. J. Chem. Soc., Faraday
T. 1997, 93, 2747.
(71) Okabe, J. Photochemistry of Small Molecules; Wiley-Interscience:
New York, 1978.
(72) Chateauneuf, J. E. Chem. Phys. Lett. 1989, 164, 577.
(73) Schindler, R. N.; Liesner, M.; Schmidt, S.; Kirchner, U.; Benter,
T. J. Photochem. Photobiol. A 1997, 107, 9.
(74) Krisch, M. J.; McCunn, L. R.; Takematsu, K.; Butler, L. J.; Blase,
F. R.; Shu, J. J. Phys. Chem. A 2004, 108, 1650.
(75) Klaning, U. K.; Wolff, T. Ber. Bunsen-Ges. Phys. Chem. 1985,
89, 243.
(76) Treinin, A.; Hayon, E. J. Am. Chem. Soc. 1975, 97, 1716.
(77) Buhler, R. E. J. Phys. Chem. 1972, 76, 3220.
(78) Thomsen, C. L.; Madsen, D.; Poulsen, J. A.; Thogersen, J.; Knak
Jensen, S. J.; Keiding, S. R. J. Chem. Phys. 2001, 115, 9361.
(79) Cazzoli, G.; Esposti, C. D.; Palmieri, P.; Simeone, S. J. Mol.
Spectrosc. 1983, 97, 165.
(80) Pauling, L. The Nature of the Chemical Bond; Cornell University:
New York, 1960.
(81) Meredith, C.; Quelch, G. E.; Schaeffer, H. F. J. Chem. Phys. 1992,
96, 480.
(82) McDonald, J. K.; Merritt, J. A.; Kalasinsky, V. F.; Heusel, H. L.;
(34) Esposito, A. P.; Reid, P. J.; Rousslang, K. W. J. Photochem.
Photobiol. A Chem. 1999, 129, 9.
(35) Cooksey, C. C.; Reid, P. J. J. Phys. Chem. A 2003, 107, 5508.
(36) Colburn, C. B. DeVelopments in Inorganic Nitrogen Chemistry;
Elsevier Scientific Publishing Co.: New York, 1973.
(37) Finlayson-Pitts, B. J.; Pitts, J. N., Chemistry in the Upper and Lower
Atmosphere; Academic Press: New York, 2000.
(38) Luick, T.; Heckert, R.; Schulz, K.; Disselkamp, R. J. Atmos. Chem.
1999, 32, 315.
(39) Engelman, R.; Rouse, P. E. J. Mol. Spectrosc. 1971, 37, 240.
(40) Werner, L.; Wunderer, B.; Walther, H. Chem. Phys. 1981, 60, 109.
(41) Moser, M. D.; Weitz, E.; Schatz, G. C. J. Chem. Phys. 1983, 78,
757.
Durig, J. R. J. Mol. Spectrosc. 1986, 117, 69.
(83) Lee, T. J. Chem. Phys. Lett. 1994, 223, 431.
(84) 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; Jet Propulsion Laboratory: Pasadena, CA, 1994.
(42) Bechara, J.; Morrow, T.; McGrath, W. D. Chem. Phys. Lett. 1985,
122, 605.
(43) Bruno, A. E.; Bruhlmann, U.; Huber, J. R. Chem. Phys. 1988, 120,
155.