Femtosecond TRIR studies of CINO photochemistry in solution: Evidence for photoisomerization and geminate recombination

Article abstract of DOI:10.1021/jp8100283

The photochemistry of nitrosyl chloride (ClNO) in the solution phase is investigated using Fourier transform infrared (FTIR) and ultrafast time-resolved infrared (TRIR) spectroscopies. The NO-stretch fundamental transition for ClNO dissolved in cyclohexan

Full text of DOI:10.1021/jp8100283

3886
J. Phys. Chem. A 2009, 113, 3886–3894
Femtosecond TRIR Studies of ClNO Photochemistry in Solution: Evidence for
Photoisomerization and Geminate Recombination†
Teresa J. Bixby, Joshua D. Patterson, and Philip J. Reid*
Department of Chemistry, UniVersity of Washington, Box 351700, Seattle, Washington 98195
ReceiVed: NoVember 13, 2008; ReVised Manuscript ReceiVed: December 16, 2008
The photochemistry of nitrosyl chloride (ClNO) in the solution phase is investigated using Fourier transform
infrared (FTIR) and ultrafast time-resolved infrared (TRIR) spectroscopies. The NO-stretch fundamental
transition for ClNO dissolved in cyclohexane, carbon tetrachloride, chloroform, dichloromethane, and
acetonitrile is measured, with the frequency and line width of this transition demonstrating a strong dependence
on solvent polarity. Following the photolysis of ClNO dissolved in acetonitrile at 266 nm, the subsequent
optical-density evolution across the entire width of the NO-stretch fundamental is measured. Analysis of the
optical-density evolution demonstrates that geminate recombination of the primary photofragments resulting
in the reformation of ground state ClNO occurs with a quantum yield of 0.54 ( 0.06. In addition, an increase
in optical density is observed at 1860 cm-1 that is assigned to the NO-stretch fundamental transition of the
photoisomer, ClON, having a formation quantum yield of 0.07 ( 0.02. This work represents the first definitive
observation of ClNO photoisomerization in solution. Finally, essentially no evidence is observed for significant
vibrational excitation of the NO fragment following photodissociation, in marked contrast to the behavior
observed in the gas phase. An environment-dependent dissociation scheme is proposed in which the interplay
between solvent polarity and the location of the ground state potential-energy-surface minimum along the
Cl-N coordinate provides for the optical preparation of different excited states thereby affecting the extent
of NO vibrational excitation following photolysis.
Introduction
of ClNO is characterized by a series of bands, labeled the K
through A as one proceeds from low to high energy, beginning
∼650 nm and extending into the UV.40,41 The A-band dominates
the absorption spectrum, with a maximum in the absorption
cross section occurring at 196 nm in the gas phase. Photoex-
citation of gaseous ClNO resonant with the A-band results
predominately in dissociation to form Cl and vibrationally
excited NO.42-54 The observation of vibrational energy in the
NO photofragment up to V ) 7 even at modest photolysis
energies implies significant excited state evolution along the
N-O stretch coordinate in the gas phase.25,44,51-53,55 Only a few
studies exist regarding the condensed phase photochemistry of
ClNO. Low-temperature matrix isolation experiments on ClNO
have observed a significant shift and broadening of the A-band
absorption in comparison to the gas phase. Additionally, nearly
exclusive formation of the photoisomer, ClON, following
photoexcitation is observed.12,56 In solution, broadening and
bathochromic shifting of the A-band is observed, but the
integrated intensity of the band remains constant indicating that
no new transitions are being accessed. Resonance Raman
intensity analysis studies demonstrated that excited state struc-
tural evolution in the A-band is dominated by the N-Cl stretch
and bend coordinates. The increased broadening of the A-band
with increased solvent polarity was attributed to an increase in
the N-Cl bond length and corresponding shift of the ground
state potential energy surface minimum along the N-Cl
coordinate.57,58 This assertion is supported by reduction in N-Cl
stretch frequency with increasing solvent polarity. In contrast,
the frequency of the N-O stretch does not undergo a significant
frequency shift between the gas phase and argon matrixes. The
resonance Raman studies found little scattering intensity along
A significant challenge in atmospheric chemistry is to
understand the fundamental aspects of phase-dependent pho-
tochemical reactivity. The environment-dependent reactivity of
halooxides, such as chlorine dioxide (OClO), have attracted a
significant amount of attention due to their role as a reservoir
species for stratospheric atomic chlorine.1-4 The recent discovery
of a global stratospheric OClO layer has illustrated the
importance of understanding the aspects that govern the potential
of OClO and other halogen-containing species to release atomic
chlorine.5 Photoexcitation of gaseous OClO results primarily
in the formation of OCl and O photofragments with a small
amount of Cl and O2 also formed.6-8 However, production of
the photoisomer, ClOO, has been observed in low-temperature
matrixes,9-12 and intermediate behavior with substantial forma-
tion of OCl/O, Cl/O2, and photoisomer observed in solution.13-23
Another halooxide, dichlorine monoxide (ClOCl), demonstrates
photochemical behavior similar to OClO. While gaseous ClOCl
forms ClO and Cl upon photoexcitation,24-32 the isomer, ClClO,
is produced in low-temperature matrixes. In solution, intermedi-
ate behavior is observed with geminate recombination of the
ClO and Cl photofragments occurring in addition to ClClO
formation.10,33-37 To further investigate phase-dependent pho-
tochemistry in relatively simple molecular systems, we have
initiated a series of studies involving nitrosyl halides, and in
particular nitrosyl chloride (ClNO).38
Nitrosyl chloride is formed in the troposphere by the reaction
of sea salt and NO2 gas, and in the stratosphere by the reaction
of HONO on frozen HCl surfaces.39 The UV-vis absorption
† Part of the “George C. Schatz Festschrift”.
* To whom correspondence should be addressed. E-mail: preid@
chem.washington.edu.
10.1021/jp8100283 CCC: $40.75
2009 American Chemical Society
Published on Web 02/11/2009

Photochemistry in Solution
J. Phys. Chem. A, Vol. 113, No. 16, 2009 3887
the N-O stretch consistent with modest excited state structural
evolution along this coordinate upon photoexcitation.
To further investigate the condensed phase reactivity of ClNO
following A-band photoexcitation, Cooksey et al. performed
femtosecond pump-probe experiments on ClNO in solution.38
These studies observed a photoinduced depletion of ClNO and
subsequent photoproduct formation in all solvents studied:
acetonitrile, chloroform, and dichloromethane. Photoproduct
assignment and quantification of the geminate recombination
quantum yield was hampered by overlapping absorption bands
and assumptions of possible photoproduct locations based on
data collected from matrix phases and other solvents. Modeling
of the optical-density evolution in acetonitrile demonstrated that
one photoproduct formed was the Cl:acetonitrile charge-transfer
complex with quantum yield ∼0.55. However, the band shape
and absorptivity of the photoproduct in chloroform was incon-
sistent with production of the Cl:chloroform charge-transfer
complex. The photoproduct in chloroform was therefore as-
signed to the photoisomer.
Building on this previous pump-probe study, we report here
static Fourier transform infrared (FTIR) and ultrafast time-
resolved infrared (TRIR) studies designed to elucidate the
environment-dependent photochemistry of ClNO. The IR ab-
sorption cross section of the N-O stretch fundamental of ClNO
in various solvents was measured, and is observed to shift to
higher energies and broaden significantly with an increase in
solvent polarity. The evolution in optical density throughout
the spectral region of the N-O stretch of ClNO in acetonitrile
was monitored following A-band photoexcitation at 266 nm.
Photoinduced depletion and subsequent geminate recombination
of the primary Cl and NO photoproducts is observed, as well
as the formation of a stable photoproduct. The geminate
recombination quantum yield is determined to be 0.54 ( 0.06,
consistent with earlier pump-probe work. The photoproduct
is assigned to the structural isomer, ClON, with an associated
quantum yield of ∼0.07. Little evidence is observed for
vibrational excitation along the NO-stretch coordinate reinforc-
ing the assertion that the energetics and displacement of the
ground and excited states are significantly modified in solution
relative to gas phase. In summary, the results presented here
provide the first detailed insight into photoproduct formation
and geminate recombination of ClNO in solution.
Figure 1. Schematic of the UV-pump, IR-probe spectrometer. Ab-
breviations are as follows: Ti:S, Ti:sapphire pumped regenerative
amplifier; CP, mechanical chopper; DFG, difference frequency genera-
tor; DS, delay stage; MCT, mercury/cadmium/telluride detectors; OPA,
optical parametric amplifier; S, sample cell; SP, spectrograph; TP, time
plate; XT, ꢀ-BBO crystal.
temporal delay of the pump relative to the probe. Polarization
of the pump beam was set to 54.7° relative to the probe by
using a zero-order half-wave plate minimizing contributions
from rotational dynamics to the observed optical-density evolu-
tion. The pump field (6 µJ/pulse) was weakly focused to a 1-mm
diameter at the sample. The change in sample optical density
was determined to be linearly dependent over a 5-fold change
in pump power. Far field collimation of the 5.5-µm probe field
was achieved using two off-axis parabolic mirrors (Janos). The
collimated probe field was direct to a 50/50 CaF2 beamsplitter
with the reflected and transmitted beams corresponding to the
signal and reference, respectively. The signal was focused on
the sample using an off-axis parabolic mirror. Spatial and
temporal overlap of pump and signal fields was verified by
monitoring the change in optical density in a Si wafer following
excitation at 266 nm. The observed optical response was
modeled to establish an instrument time resolution of 200 fs.
After passing through the sample, the signal was delivered
to a 0.25-m single-stage spectrograph (Jarrell Ash) equipped
with a 100-groove/mm grating (λblaze ) 5.6 µm). The reference
arm also passed through the spectrograph, but vertically
displaced from the signal. Identical components of the signal
and reference fields were isolated with a spectral resolution of
4 cm-1. Multiple orders of refracted light from a HeNe laser
were used to calibrate the spectrograph, and further calibration
was confirmed by taking the IR absorption spectrum of acetone
and ClNO with our instrument and comparing this information
directly to those obtained by a Bruker Vector 33 FTIR
spectrometer with a demonstrated agreement of (1 cm-1. Upon
exiting the spectrograph, the signal and reference were focused
onto a pair of LN2 cooled MCT detectors (Infrared Associates)
with ZnO lenses. The output of each detector was sent to a
gated integrator, and the integrator outputs were subtracted on
a shot-to-shot basis. A mechanical chopper, placed in the pump
field, was operated at half the repetition rate of the amplifier to
block every other pump pulse. Successive probe signals were
subtracted to determine the pump-induced change in optical
density. Each recorded time point was averaged over 4000 laser
shots, with each measurement consisting of the average of 3-7
time traces.
Experimental Section
A schematic of the femtosecond UV pump, IR probe
spectrometer is presented in Figure 1. A Ti:sapphire oscillator
(K&M Laboratories) pumped by the frequency-doubled output
of a Nd:VO4 laser (Coherent Verdi V) was used to seed a
regenerative Ti:sapphire amplifier (Spectra Physics Spitfire). The
output of the amplifier consisted of 50-fs pulses (full-width at
half-maximum) centered at 800 nm with an energy of 1 mJ/
pulse and repetition rate of 1 kHz. The amplifier output was
split into two beams with a 60/40 beamsplitter. The higher
energy beam was directed to an optical parametric amplifier
(OPA, Quantronix TOPAS) to generate signal and idler fields
at 1397 and 1895 nm, respectively. The signal and idler fields
were overlapped in a AgGaS2 crystal (type I) for difference
frequency generation (DFG) producing the probe field centered
at 5.35 µm with an energy of 1 µJ/pulse. The lower energy
beam from the amplifier output was frequency-tripled using a
series of ꢀ-BBO crystals (types I and II) to generate the 266-
nm pump field.
ClNO was prepared as described in the literature.58 Briefly,
8.75 g of sodium nitrite dissolved in 12.5 mL of H2O was added
dropwise to 50 mL of concentrated hydrochloric acid. The
The pump beam was delivered to a retroreflector mounted
on a motorized delay stage (Newport ES300) to allow for

3888 J. Phys. Chem. A, Vol. 113, No. 16, 2009
Bixby et al.
resulting ClNO gas was passed through a series of drying tubes
packed with sodium nitrite, moist potassium chloride, and
calcium chloride, then bubbled directly into a chilled flask
containing the solvent of interest. The sample was delivered to
a Teflon flow cell (Harrick Scientific) equipped with BaF2
windows. BaF2 windows were chosen instead of the more
commonly used CaF2 windows for their high damage threshold
to aid in reducing coherent artifacts. Further, the focus of the
pump beam was adjusted to minimize the coherence signal and
solvent spectra indicate no evidence for this feature. A 100-µm
cell path length was employed for TRIR studies involving
transitions corresponding to ClNO, while studies involving the
evolution in solvent vibrational modes were performed with use
of a 56-µm path length. Sample concentrations, adjusted between
30 and 60 mM were verified using UV-vis absorption.
Normalize of the resulting evolution in optical density was
accomplished by dividing the probe intensity measured at the
beginning and end of every scan. The absolute evolution in
optical density was then analyzed by fitting the data scans to a
sum of exponentials using the Levenberg-Marquardt algorithm.
Goodness of fit was assessed by visual inspection of the residuals
and reduced ꢁ2 values. The reported errors in extracted kinetic
parameters represent one standard deviation of the mean
determined from a minimum of 15 measurements. The variation
in the absolute change in optical density among scans at a given
wavelength is less than 10% and the data presented in Figure 3
represent an average of a minimum of 15 scans for each
frequency.
The molar extinction coefficient of the NO-stretch funda-
mental transition of ClNO in the solvents of interest was
determined using a Fourier transform infrared spectrometer
(Bruker Vector 33). The ClNO samples were placed in an
adjustable path length IR cell fitted with CaF2 windows. Path
lengths ranging from 300 to 400 µm were employed to ensure
that measurements were below an absorbance of 1.0. Absor-
bance contributions from the solvent were removed by direct
subtraction of the neat solvent spectra. Reported spectra were
averaged over a minimum of 7 measurements.
Results
Absorption Spectra. The solvent-subtracted infrared absorp-
tion spectra of the NO-stretch fundamental (ν1) transition of
ClNO in cyclohexane, carbon tetrachloride, chloroform, dichlo-
romethane, and acetonitrile are presented in Figure 2A. The
absorption maxima in cyclohexane, carbon tetrachloride, chlo-
roform, dichloromethane, and acetonitrile are located at 1805,
1815, 1836, 1845, and 1869 cm-1, respectively. These data
demonstrate that the transition shifts to higher frequency with
an increase in solvent polarity. For reference, the ET(30) polarity-
scale values are 30.9 (cyclohexane), 32.4 (carbon tetrachloride),
39.1 (chloroform), 40.7 (dichloromethane), and 45.6 kcal/mol
(acetonitrile). A linear relationship between transition frequency
and solvent polarity was observed (Figure 2B). Inspection of
Figure 2A reveals that the shape and width of the transition are
also solvent dependent. For example, in cyclohexane the line
shape is Lorentzian with a width of 10.8 cm-1 at the full width
at half-maximum (fwhm). However, as solvent polarity is
increased the line shape becomes more Gaussian, and the
breadth of the transition increases from 10.8 cm-1 in cyclohex-
ane to 24 cm-1 in carbon tetrachloride, 44 cm-1 in chloroform,
43 cm-1 in dichloromethane, and 55 cm-1 in acetonitrile.
TRIR Studies of ClNO in Acetonitrile. The absolute
evolution in optical density following ClNO photoexcitation
dissolved in acetonitrile is presented in Figure 3. The range of
Figure 2. (A) IR absorption spectra of ClNO in the region of the NO-
stretch fundamental transition in various solvents. (B) Plot of NO-stretch
peak versus solvent polarity. Solvents are represented as follows:
cyclohexane, CXN, filled diamond; carbon tetrachloride, CTC, filled
square; chloroform, CLF, filled circle; dichloromethane, DCM, open
square; acetonitrile, MeCN, open diamond.
probe frequencies (1820 to 1920 cm-1) span the entire breadth
of the NO-stretch fundamental transition (see Figure 2A). At
all probe wavelengths a photoinduced reduction in optical
density is observed consistent with photolysis of ground state
ClNO. However, the temporal onset of this reduction varies
across the absorption band. For frequencies between 1820 and
1840 cm-1, an increase in optical density is observed prior to
depletion suggesting that an absorptive species may be present
on the low-energy side of the NO-stretch fundamental. The
initial depletion of ground state ClNO is followed by partial
optical-density recovery at all probe frequencies. Residual
depletion in optical density remains constant out to the longest
delay times investigated (90 ps), with amplitudes that vary from
30 to 56% across the band. The partial recovery in optical
density following photoinduced depletion is indicative of
geminate recombination of the primary photoproducts resulting

Photochemistry in Solution
J. Phys. Chem. A, Vol. 113, No. 16, 2009 3889
TABLE 1: Kinetic Parameters Determined from Analysis of
TRIR Data for ClNO Dissolved in Acetonitrile
probea
(cm-1
)
A1
τ1 (ps)
A2
τ2 (ps)
b
1920
1910
1900
1890
1880
1869
1860
1850
1840
1836
1830
1820
-0.47 ( 0.03
-0.38 ( 0.02
-0.44 ( 0.02
-0.42 ( 0.03
-0.38 ( 0.01
-0.36 ( 0.01
-0.31 ( 0.01
-0.32 ( 0.03
-0.36 ( 0.01
-0.38 ( 0.02
-0.39 ( 0.02
-0.57 ( 0.02
1.57 ( 0.19
1.85 ( 0.11
1.94 ( 0.11
2.05 ( 0.11
2.18 ( 0.14
2.54 ( 0.15
2.84 ( 0.19
2.84 ( 0.19
3.52 ( 0.23
3.86 ( 0.22
2.97 ( 0.24
3.27 ( 0.26
-0.53 ( 0.03
-0.62 ( 0.02
-0.56 ( 0.02
-0.58 ( 0.03
-0.62 ( 0.01
-0.64 ( 0.01
-0.69 ( 0.01
-0.68 ( 0.03
-0.58 ( 0.06
-0.62 ( 0.02
-0.61 ( 0.02
-0.43 ( 0.02
10 000 ( 0
10 000 ( 0
10 000 ( 0
10 000 ( 0
10 000 ( 0
10 000 ( 0
10 000 ( 0
10 000 ( 0
10 000 ( 0
10 000 ( 0
10 000 ( 0
10 000 ( 0
a Wavelengths at which optical density evolution was measured.
b Amplitudes are normalized such that ∑|Ai| ) 1. Errors represent
one standard deviation of the mean of all measurements (g10) at a
given probe wavelength.
The initial increase in optical density at lower probe energies
combined with the frequency dependence of the time constant
for ground state-depletion recovery warranted a more detailed
analysis of the optical-density evolution. A second view of the
optical-density evolution is provided by the two-dimensional
contour plot shown in the top panel of Figure 4. The lower
panel of this figure presents the infrared absorption spectra of
ClNO dissolved in acetonitrile, as well as the deconvolved
absorbance contributions from ClNO and acetonitrile over the
spectral range investigated. The figure demonstrates that the
absorbance over this region is dominated by the NO stretch
fundamental transition (ν1), with solvent contributing from 1820
to 1840 cm-1. The solvent feature, centered at 1830 cm-1
,
corresponds to the overtone of the acetonitrile C-C stretch. The
top panel illustrates that the maximum reduction in optical
density occurs at 1880 cm-1, with most of the evolution
completed within ∼5 ps. Note that there is little to no evolution
in the observed band shape despite the fact that it is asymmetric
and the maximum of depletion does not converge to the
maximum of the absorption band. As will be discussed later,
the absence of band shape evolution is indicative of little to no
excess vibrational energy being deposited into the NO stretch
of geminately recombined ClNO.
Transient Absorption Spectrum. To further investigate the
early time absorptive feature as well as the source of the asym-
metric optical density depletion following excitation, the
transient absorption spectrum of ClNO dissolved in acetonitrile
was constructed from the contour plot in Figure 4. Taking
“slices” along the time axis of the contour plot yields the
transient absorption spectrum shown in Figure 5A. The asym-
metry of the band and the shift in maximum reduction in optical
density relative to the absorption peak are consistent with the
presence of an optical density increase on the low-frequency
side of the NO-stretch fundamental transition. To investigate
this feature, the contribution of ground state ClNO depletion to
the optical density evolution was determined using the high-
frequency side of the NO stretch. The highest energy probe
frequencies (1890 to 1920 cm-1) exhibit behavior consistent
with depletion and recovery of ground state ClNO exclusively.
Therefore, the concentration of depleted ClNO was determined
by these four frequencies, and its contribution was removed to
construct the photoproduct spectra absorption presented in
Figure 5B. To confirm the ability to decouple the photoproduct
spectrum from the transient absorption spectra, subtraction of
the predicted ClNO contribution was performed by using only
the three highest probe energies, only the two highest probe
Figure 3. Evolution in optical density following photoexcitation at
266 nm of ClNO dissolved in acetonitrile. Probe frequencies at which
the data was obtained are indicated. The circles represent the experi-
mental data, while the solid lines correspond to the best fit to the data
by a sum of exponentials with fit parameters provided in Table 1.
in ground state ClNO reformation. The best fit to the data was
accomplished employing a sum of exponentials, with one
exponential fixed at 10 000 ps to match the long-time offset in
optical density. Time constants for recovery also vary across
the band, ranging from 1.57 to 3.86 ps, with the fastest dynamics
observed at higher probe energies and the slowest at lower probe
energies. The kinetic parameters derived from fitting are
presented in Table 1.

3890 J. Phys. Chem. A, Vol. 113, No. 16, 2009
Bixby et al.
Figure 4. (Top) Contour plot of TRIR optical-density evolution observed in the NO-stretch fundamental of ClNO dissolved in acetonitrile. The
absolute change in optical density is indicated by the contour lines, with the color scale shown above the plot. (Bottom) The IR absorption spectra
of ClNO in acetonitrile (solid), acetonitrile only (short dashed), and the NO-stretch of ClNO (long dash).
energies, and only the highest probe energy. No difference in
the position of any feature in the photoproduct spectrum appears
and the absolute optical density changes less than 10%. The
figure demonstrates an instrument-response-limited increase in
optical density with subsequent decay to a constant positive
offset within ∼5 ps. The peak of the absorptive feature is at
1860 cm-1 and persists out to the longest delays investigated
suggesting formation of a stable photoproduct. Additionally, a
energy of the absorption spectrum, and decrease in the N-Cl
stretch and bending fundamental frequencies in polar solvents.
Resonance Raman intensity analysis studies in both gas and
solution phases observed modest to no evolution along the N-O
stretch coordinate.57-60 In addition, the N-O stretch frequency
of ClNO in matrix phases is reported to lie within a few
wavenumbers of gas phase measurements.12,56,61,62 Those studies
suggest that the environmental dependence of the N-O stretch
is modest. However, we have demonstrated that the N-O stretch
fundamental frequency increases from 1805 to 1869 cm-1,
broadens from 11 to 55 cm-1 at full-width at half-maximum
(fwhm), and changes from a purely Lorentzian line shape to a
more Gaussian line shape in going from cyclohexane to
acetonitrile. It is apparent that ClNO undergoes dramatic
structure modifications with environment, and these modifica-
tions should result in a corresponding change in the reaction
dynamics in solution relative to the gas phase.
weaker absorption is seen to appear from 1820 to 1836 cm-1
,
possibly signifying a change in absorptivity of the solvent
transition. We will argue below that this behavior reflects
formation of the chlorine-acetonitrile charge-transfer complex.
Discussion
ClNO Bonding in Solution. The central goal of this study
was to investigate the condensed phase reaction dynamics of
ClNO. Previous studies suggest that the reaction dynamics of
ClNO may be significantly perturbed in solution relative to the
gas phase as a consequence of solvent-induced structural
changes, and thus changes in the electronic states, of ClNO.58
For example, in the UV absorption spectrum of ClNO the
strongest or “A-band” transition broadens and shifts from 198
nm in the gas phase to 206 nm in cyclohexane and 218 nm in
acetonitrile. Comparison of the integrated intensity of the A-band
in these solvents demonstrated that the overall intensity is
conserved; therefore, the same electronic transitions are being
accessed with excitation into this band. The increase in transition
breadth with increasing solvent polarity was attributed to
weakening of the N-Cl bond and a corresponding shift of the
ground state potential-energy-surface minimum along the N-Cl
stretch coordinate. This is consistent with the shift to lower
Insight into the nature of the NO-stretch solvent dependence
is provided by considering theoretical descriptions of ClNO
bonding. Ab initio SCF and CISD calculations predict that the
N-Cl bond is extraordinarily long (1.973 Å) and the dipole
moment of ClNO is large (2.49 D), consistent with substantial
ionic character.63,64 Furthermore, work by Meredith et al.
describes the electronic structure of ClNO as a weak interaction
between the singly occupied p orbital of Cl with the singly
occupied 2π antibonding orbital of NO. The long N-Cl bond
distance leads to poor overlap of these two out-of-plane orbitals
and the high electronegativity of Cl results in a partial charge
transfer from the NO to the Cl. This behavior results in an
enhanced NO bond strength and bonding character similar to
that of NO+. These calculations also show that any perturbation

Photochemistry in Solution
J. Phys. Chem. A, Vol. 113, No. 16, 2009 3891
stretch. It is important to note that this involves very subtle
changes in the character of NO. The frequency of the NO
fundamental transition of ClNO ranges from 1805 to 1869 cm-1
in the solvents studied here, but the fundamental frequency of
NO+ in acetonitrile is 2355 cm-1. Even in the most polar solvent
studied, the N-Cl bond distance is still close enough that it
does not allow for complete electron transfer.
The notable increase in the vibrational frequency of the NO-
stretch fundamental is accompanied by a significant increase
in line width and change in line shape. In going from
cyclohexane to acetonitrile, the 11 cm-1 fwhm Lorentzian
transition picks up considerable Gaussian character and increases
to 55 cm-1 fwhm. An increase in the width of the vibronic
A-band of ClNO has previously been explained as an increase
in the homogeneous linewidth of ClNO when comparing
cyclohexane to acetonitrile. For this transition the homogeneous
linewidth (Γ) is composed of two components: the excited state
vibrational lifetime and solvent-induced pure dephasing. It is
likely that the increase in width of the NO stretch in more polar
environments arises from enhancement of the vibrational-
relaxation rate, and also an increase in the distribution of NO
bond strengths corresponding to increases in homogeneous and
inhomogeneous broadening, respectively. It is important to note
that the ground state energetics of ClNO are environment
dependent and our TRIR studies provide insight as to the extent
these modifications perturb the reaction dynamics.
ClNO Photochemistry in Solution. Photochemical studies
of gaseous ClNO have found that direct dissociation to form
Cl and NO is the dominant photoproduct channel following
A-band photoexcitation.42,43,45-54 Furthermore, the NO photof-
ragment is produced with significant vibrational energy, with
population of states as high as n ) 7 observed following 337-
nm photoexcitation.25,44,51-53,55 A strikingly different picture
emerges in low-temperature matrixes, where studies have shown
the structural isomer (ClON) to be the only photoproduct
formed.12,56 In solution, the case is less clear. Resonance Raman
intensity analysis studies of ClNO in cyclohexane and aceto-
nitrile found evidence of modest excited state evolution along
the NO stretch coordinate.57,58 This observation suggests that
the excited state of ClNO, and correspondingly the photochem-
istry derived from this state, is considerably modified in solution
relative to the gas phase. In an effort to further elucidate the
environment-dependent photochemistry of ClNO, UV pump-
probe experiments of ClNO in acetonitrile and chloroform were
performed.38 Evidence for geminate recombination and photo-
product formation in both solvents was observed; however,
quantification and assignment of the photochemical species
produced was difficult to ascertain from the transient optical
density evolution. However, these earlier studies provide an
important point of contact for our own effort to investigate the
geminate recombination quantum yield, vibrational excitation,
and photoproduct assignment following photoexcitation of the
A-band of ClNO in acetonitrile.
Geminate Recombination. In the UV pump-probe study of
ClNO, 93% recovery of initially depleted ClNO based on
observed optical-density evolution at 256 nm was reported.
However, with the uncertainty in photoproduct locations and
band widths this number was taken as an upper limit for
geminate recombination. Further investigating this issue, the
measured optical density change as a function of probe
wavelength at 50 ps was reproduced assuming direct dissociation
to Cl and NO as the predominant pathway and a quantum yield
for Cl production of 0.55. The results presented here are
consistent with this analysis. Specifically, although the observed
Figure 5. (A) Transient IR absorption spectrum of ClNO dissolved
in acetonitrile. The time delay for each spectrum is given in the figure.
(B) Transient IR absorption difference spectrum. Each spectrum is the
difference between the measured absorption and the predicted ClNO
absorption spectrum at each time delay.
such that there is increased separation between the N and Cl in
the ground state enhances charge transfer making the NO bond
more NO+-like. The ionic nature of this interaction suggests
that the N-Cl bond length and fundamental frequency will
depend greatly on solvent polarity. This prediction is supported
by frequency shifts observed in the resonance Raman spectrum
where the frequency of the N-Cl fundamental decreases from
332 cm-1 in the gas phase to 325 cm-1 in cyclohexane and 310
cm-1 in acetonitrile.58 In addition, Meredith’s work predicts that
the NO bond in ClNO should be stronger relative to molecular
NO due to the charge transfer to the Cl p orbital reducing the
population in the 2π antibonding orbital of NO. Other studies
support this idea, placing the NO bond length in ClNO (1.13
Å) between that of molecular NO (1.162 Å) and NO+ (1.078
Å).63,64 We propose that an increase in solvent polarity weakens
and lengthens the N-Cl bond and consequently enhances the
extent of charge transfer from the 2π antibonding orbital of NO
to the p orbital of Cl. The reduction of electron density in the
antibonding orbital of NO subsequently shifts the bonding
characteristics of the NO moiety closer to that of NO+ with a
corresponding increase in the vibrational frequency of the NO

3892 J. Phys. Chem. A, Vol. 113, No. 16, 2009
Bixby et al.
evolution in optical density across the band demonstrates a range
of recoveries, from 30 to 56%, analysis of the photoproduct-
free probe wavelengths yields a geminate recombination quan-
tum yield of 0.54 ( 0.06, consistent with the pump-probe
results.
relative to the Cl-N bond length of the parent, stretching from
1.973 to 2.228 Å, and the N-O bond length consequently contracts
from 1.13 to 1.113 Å. The isomer is therefore even more likely to
be responsive to changes in environment and exhibit bathochromic
shifts similar to that observed in ClNO.
Vibrational Energy Deposition. The studies presented here
provide insight into the extent of vibrational excitation ac-
companying the reformation of ClNO via geminate recombina-
tion. Specifically, the 4-cm-1 experimental resolution combined
with the significant anharmonicity of the NO stretch allow for
a clear determination of excess vibrational energy deposition
into the NO-stretch coordinate. The anharmonicity of the NO
stretch remains relatively unperturbed with changes in environ-
ment: 18 in the gas phase, 17.5 in argon matrixes, and 18.75 (
0.29 cm-1 in cyclohexane, carbon tetrachloride, chloroform, and
dichloromethane. The n ) 1 f 2 transition is therefore predicted
to be at 1836 cm-1. This transition, although overlapping the
red edge of the fundamental transition, is well within the spectral
region investigated here (Figure 3). However, there is little
evidence for vibrationally excited ClNO. Vibrational relaxation
in this case, in such a polar solvent, is expected to be very
efficient and is not expected to present itself on a 50- to 100-ps
timescale. The width of this transition (55 cm-1) in acetonitrile
supports this assertion, indicating that the vibrational lifetime
is enhanced relative to cyclohexane. The static nature of the
spectra on this timescale suggests that if excess vibrational
energy is being deposited along this coordinate following
geminate recombination the relaxation is fast enough to be
obscured by the absorption of the solvent mode and the
photoproduct within the first few picoseconds. Further, previous
UV pump-probe studies reveal no evidence for vibrational
relaxation on the 50- to 100-ps time scale and resonance Raman
data show little to no evolution along this coordinate. While
this observation is surprising given the extensive vibrational
excitation of the NO photofragment observed in the gas phase,
it is not entirely unexpected when one considers the evidence
suggesting that the energetics and displacement of the potential
energy surfaces are significantly modified in solution. Therefore,
the time-resolved infrared studies further suggest that the excited
state is significantly modified along the N-O stretch coordinate
in condensed environments such that structural evolution along
this coordinate is significantly restricted in solution. The
photochemical impacts of this behavior are discussed below.
Photoproduct Production. Construction of the photoproduct
absorption spectrum (Figure 5B) reveals a feature at 1860 cm-1
that persists out to the longest delays investigated demonstrating
that the corresponding photoproduct is stable on the ∼100-ps
timescale. Vibrationally excited ClNO does not absorb at 1860
cm-1. Further, the efficiency of vibrational relaxation is expected
to be high in such a polar solvent so the lack of evolution in the
absorption band on ∼100-ps timescale suggests that the cause of
the asymmetry in Figure 5A is not vibrational relaxation. Only
two possibilities remain for the photoproduct: NO and the structural
isomer ClON. In acetonitrile NO absorbs at 1870 cm-1, inconsistent
with the transition observed here. This leaves the isomer as the
remaining possibility for the photoproduct. In argon matrixes, the
NO stretch fundamental of ClON is observed at 1842 cm-1, lying
between the absorption of ClNO (1805 cm-1) and that of free NO
(1870 cm-1).12 Is assignment of the isomer to the observed
photoproduct reasonable if one has to invoke an 18-cm-1 shift in
the NO stretch transition of this species? In ClON, the Cl is
predicted to be even more weakly bound to the NO with nearly
complete transfer of the NO 2π electron to the singly occupied
valence p orbital of the Cl. The Cl-O bond length is elongated
With assignment of the photoproduct to ClON, partitioning
of photoproduct pathways can be performed. Hallou et al. have
measured the extinction coefficient of the NO-stretch funda-
mental transition of ClON and ClNO in argon matrixes and
found that the transition is 4.0((0.5)-fold stronger in ClON
relative to ClNO.12 A comparison of the maximum depletion
of ClNO to the maximum absorption of ClON can therefore
estimate the extent of isomer formation and elucidate the
partitioning of photoproduct pathways. This analysis reveals that
only ∼7% of initially excited ClNO results in ClON production.
With geminate recombination accounting for 54% of initially
excited ClNO, the remaining ∼39% must go on to form Cl and
NO. This partitioning is consistent with the results of earlier
UV pump-probe studies from our laboratory where following
267-nm photoexcitation of ClNO in acetonitrile a broad pho-
toproduct peak centered at 295 nm with a small shoulder at
∼330 nm was observed. Despite uncertainty in photoproduct
positions and band widths, arguments were made that the main
photoproduct peak corresponded to the Cl:acetonitrile charge-
transfer complex. The observed offset in optical density at 50
ps was reproduced assuming that Cl and NO were the only
photoproducts formed with a quantum yield Φ ) 0.55.
Moreover, we propose that the small shoulder at 330 nm is due
to the isomer, ClON. Even with ∼7% production, the isomer
should appear due to its large absorption cross section, about 4
times that of ClNO in the UV.
Careful examination of the photoproduct spectrum (Figure
5B) reveals the presence of a weak absorption from 1820 to
1836 cm-1 that persists out to the longest delays investigated.
This absorption lies in the region of the C-C stretch overtone
of acetonitrile, and we propose that this feature corresponds to
formation of the Cl:acetonitrile charge-transfer complex (CTC).
In support of this, the evolution in optical density of the
acetonitrile CN stretch following ClNO photoexcitation was
measured. Centered at 2250 cm-1, the CN-stretch is well
removed from any transition corresponding to ClNO. Prelimi-
nary observations of the optical density evolution of the CN-
stretch at 2240 and 2260 cm-1 following photolysis of ClNO
at 266 nm are presented in Figure 6. The optical-density
evolution that occurs between 0 to 1.5 ps arises from the optical
response of the cell windows. However, this evolution is
followed by a persistent optical-density offset that was not
observed in either the CN-stretch or the CC-stretch overtone in
neat acetonitrile samples. The offset is positive for both probe
frequencies which span the CN-stretch transition, suggesting
that the overall transition intensity has increased due to
formation of the Cl:solvent CTC. This measurable perturbation
provides not only a novel way to monitor formation of the Cl:
solvent CTC, but also a way to obtain information regarding
the structure of this transient species. By investigating multiple
solvent modes, a picture of the interaction between Cl and
solvent could emerge.
EnWironment-Dependent Photochemistry. Our laboratory has
explored the solution phase photochemistry of halooxides, and
in particular OClO.7,8,14,17-23,36,37,58,65,66 It is interesting at this
point to compare the chemistry of this species to that of ClNO.
The geminate recombination quantum yield for OClO in
acetonitrile following 401-nm excitation is 0.53 ( 0.16, almost
identical with that of ClNO. Although it is tempting to propose

Photochemistry in Solution
J. Phys. Chem. A, Vol. 113, No. 16, 2009 3893
Figure 6. Evolution in optical density at 2240 cm–1 (solid line) and
2260 cm–1 (dashed line); 10 cm-1 to the red and the blue of the CN-
stretch fundamental of acetonitrile, following photoexcitation of ClNO
at 266 nm.
commonality in the photochemistry of these two species, it is
important to keep in mind that the geminate recombination
quantum yield of OClO is highly dependent on photolysis
energy. Gas phase studies have demonstrated that more trans-
lational energy was imparted to the photofragments with
increasing actinic energy, with increased translational energy
translated into enhanced cage escape (that is, reduced geminate
recombination) in solution. For ClNO, gas phase studies indicate
that with increasing photolysis energy there is an increase in
vibrational energy content of the NO fragment, and a slight
decrease in translational energy imparted to the photofragments
NO and Cl.54 Skorokhodov et al. proposed that following A-band
photoexcitation, two exit channels along the dissociative
coordinate are accessed, and that these channels diverge at
relatively short N-Cl bond distances. A schematic of the
proposed dissociation is presented in Figure 7. In this model
an avoided crossing of the S5 and S4 excited states is responsible
for defining the two dissociative pathways. One pathway results
in high translational energies imparted to the photoproducts with
little excitation along the NO stretch. The other pathway results
in less photoproduct translational energy and significantly more
NO-stretch excitation. With increasing photolysis energy, Sko-
rokhodov et al. observed an increase in the vibrational energy
of the NO fragment, and a corresponding decrease in the
translational energy of the Cl fragment. Recall, no evidence was
found here for NO vibrational excitation following ClNO
photolysis in acetonitrile in contrast to the behavior observed
in the gas phase. Why would this be? In solution, the N-Cl
bond length is elongated and the N-O bond length decreased
in comparison to that of the gas phase. Resonance Raman studies
of ClNO in cyclohexane and acetonitrile observed a weakened
N-Cl bond in more polar solvents, corresponding to a shift of
the ground state potential energy surface minimum along the
reaction coordinate to larger N-Cl bond distances.58 We propose
that this shift results in preparation of the excited state beyond
the avoided crossing such that the pathway corresponding to
enhanced translational energy imparted to the photofragments
is accessed and minimal NO vibrational excitation occurs. This
hypothesis can be tested by comparative studies in nonpolar
solvents where the ground state potential-energy-surface mini-
mum should shift to shorter N-Cl bond lengths resulting in
Figure 7. Dissociation scheme for ClNO. Dashed lines represent gas
phase and solid lines represent a polar solvent environment. The plot
of ε(E) on the left side presents the ClNO absorption spectra and
expected energy of the electronic states. The avoided crossing between
the S5 and S4 states is marked by a circle.
optical preparation of the excited state such that the vibrational-
energy deposition into NO is increased.
Conclusions
We have performed FTIR studies of ClNO dissolved in
cyclohexane, carbon tetrachloride, chloroform, dichloromethane,
and acetonitrile. The spectra demonstrate that the energetics of
the NO-stretch fundamental transition are highly solvent de-
pendent. Specifically, the transition displays a linear shift to
higher peak frequencies and significantly broadens with increas-
ing solvent polarity. This solvent dependence is due to more
efficient transfer of the 2π antibonding electron of NO to the
valence p orbital of Cl in higher polarity solvents. Subsequently
the NO fundamental exhibits enhanced vibrational relaxation
rates and an increase in the distribution of NO bond energies.
We also performed the first TRIR investigation of ClNO in
solution. Following 266-nm photoexcitation of ClNO in aceto-
nitrile, the evolution in optical density in the photoproduct-free
region of the band demonstrates a consistent geminate recom-
bination quantum yield of 0.54 ( 0.06. The transient absorption
spectra also reveal the instrument-response limited appearance
of a photoproduct centered at 1860 cm-1, and we have assigned
this feature to formation of the structural isomer, ClON. By
using the relative extinction coefficients for the NO stretch in
ClON to ClNO, the quantum yield for isomer production is
∼0.07. Little evidence is observed for significant vibrational
excitation along the NO-stretch coordinate, this observation
being consistent with previous resonance Raman studies that
demonstrated the energetics and displacement of the ground and

3894 J. Phys. Chem. A, Vol. 113, No. 16, 2009
Bixby et al.
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We have proposed a scheme for the photodissociation process
in solution derived from the proposal of Skorokhodov et al.
wherein there are two photoproduct channels that arise from
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Enhanced charge transfer from the NO antibonding 2π orbital
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Acknowledgment. The National Science Foundation
(CHE-0350191) is acknowledged for their support of this work.
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