UV resonance Raman studies of C1NO in solution

Article abstract of DOI:10.1016/S0009-2614(02)00920-X

The first resonance Raman spectra of nitrosyl chloride (ClNO) in solution obtained with excitation resonant with the A-band are reported. For ClNO dissolved in cyclohexane, intensity is observed for transitions corresponding to all three normal coordinates: the bend, NCl stretch and NO stretch. This observation demonstrates that excited-state structural evolution occurs along these coordinates following photoexcitation resonant with the A-band. Resonance Raman depolarization ratios measured at excitation wavelengths spanning the A-band are reported. The depolarization ratios are found to be less that 1/3 consistent with at least two electronic transitions contributing to the A-band absorption intensity.

Full text of DOI:10.1016/S0009-2614(02)00920-X

24 July 2002
Chemical Physics Letters 361 (2002) 49–56
www.elsevier.com/locate/cplett
UV resonance Raman studies of ClNO in solution
*
Bethany P. Barham, Philip J. Reid
Department of Chemistry, P.O. Box 351700, University of Washington, Seattle, WA 98195, USA
Received 2 April 2002; in final form 15 May 2002
Abstract
The first resonance Raman spectra of nitrosyl chloride (ClNO) in solution obtained with excitation resonant with the
A-band are reported. For ClNO dissolved in cyclohexane, intensity is observed for transitions corresponding to all three
normal coordinates: the bend, NCl stretch and NO stretch. This observation demonstrates that excited-state structural
evolution occurs along these coordinates following photoexcitation resonant with the A-band. Resonance Raman
depolarization ratios measured at excitation wavelengths spanning the A-band are reported. The depolarization ratios
are found to be less that 1/3 consistent with at least two electronic transitions contributing to the A-band absorption
intensity. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction
bands extending from ꢀ650 nm into the UV
[15,16]. Consistent with earlier studies, these bands
are referred to as the K–Abands as one proceeds
from low to high energy. Studies of gaseous ClNO
have provided a detailed understanding of the
dissociation dynamics that occur following pho-
toexcitation resonant with all transitions except
for the A-band ðkmax ¼ 196 nm) [17,18]. The sub-
stantial cross-section and breadth of the A-band
suggests that more than a single electronic transi-
tion contributes to the observed band intensity.
Consistent with this suggestion, photodissociation
studies have observed an evolution in photofrag-
ment anisotropy with increased actinic energy
consistent with multiple states participating in the
dissociation dynamics [11,13]. It should be noted
that an evolution in photoproduct anisotropy can
occur when a substantial amount of energy is de-
posited into rotational energy of diatomic frag-
ment, even when a single-state is involved in the
photochemistry [19–21]. However, studies of the
The photochemistry of nitrosyl halides such as
nitrosyl chloride (ClNO) is of interest due to the
participation of these compounds in stratospheric
and tropospheric chemistry [1,2]. ClNO is formed
in the lower atmosphere by the reaction of sea salt
(NaCl) with NO2, and in the upper troposphere/
stratosphere via surface reactions of HONO on
frozen HCl. In the gas-phase, the photochemistry
of ClNO is dominated by dissociation to NO and
Cl which occurs with near-unity quantum yield
[1,3–14]:
hm
2
2
ClNO!NOð PÆ1=2;Æ3=2Þ þ Clð P1=2;3=2
The visible-ultraviolet (UV) absorption spectrum
Þ
of ClNO is characterized by a series of absorption
* Corresponding author. Fax: +206-685-8665.
E-mailaddress: preid@chem.washington.edu (P.J. Reid).
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S0009-2614(02)00920-X

50
B.P. Barham, P.J. Reid / ChemicalPhysics Letters 361 (2002) 49–56
Cl-product velocity distribution have demon-
strated that the anisotropy is independent of the
kinetic energy released into the products consistent
with the multiple-state model [14]. From a theo-
retical perspective, ab initio studies have predicted
the presence of multiple transitions in this wave-
length region [7,22,23]. However, recent UV reso-
nance Raman studies of gaseous OClO found that
the evolution in scattering intensity with excitation
wavelength could be interpreted in terms of a
single-state dominating the A-band intensity [24].
Therefore, the number of electronic states partici-
pating in the photodissociation dynamics of gas-
eous ClNO following A-band excitation remains
unclear.
In contrast to the gas-phase, information re-
garding ClNO photochemistry in condensed envi-
ronments is limited. Studies of ClNO surface
photochemistry have observed NO formation
suggesting that similar to the gas-phase, photoex-
citation leads predominately to N–Cl bond cleav-
age [25]. However, matrix isolation studies have
observed the formation of ClON following ClNO
photoexcitation suggesting that photoisomeriza-
tion may also occur in condensed environments
[26,27]. Matrix isolation studies of ClNO:water
complexes have also demonstrated the formation
of HONO/HCl [28]. Finally, the photochemical
reaction dynamics of ClNO in solution are essen-
tially unknown.
We have been using resonance Raman intensity
analysis to study the solution-phase excited-state
reaction dynamics of ClNO that occur following
A-band photoexcitation. Earlier Raman spectra of
gaseous and matrix isolated ClNO were obtained
using excitation wavelengths that were non-reso-
nant or pre-resonant with the A-band [29–31]. In
these studies, differences in transition frequencies
were observed for gas-phase versus matrix-isolated
ClNO suggesting that the ground-state potential
energy surface is sensitive to environment. Re-
cently, resonance Raman spectra of gaseous ClNO
employing A-band excitation were reported [24].
Scattering corresponding to the bend and NCl-
stretch coordinates was observed demonstrating
that excited-state structural evolution occurs along
these coordinates following photoexcitation.
Scattering corresponding the NO-stretch was not
observed suggesting that structural evolution does
not occur along this coordinate following A-band
excitation. Acorresponding study of ClNO in so-
lution would show how the presence of solvent
impacts these dynamics.
We report here the first resonance Raman
spectra of ClNO in solution. Specifically, reso-
nance Raman spectra of ClNO dissolved in cy-
clohexane employing excitation resonant with the
A-band are presented. Cyclohexane was chosen
for study since the absorption spectrum of ClNO is
minimally shifted in this solvent relative to the gas-
phase. Therefore, this solvent provides an envi-
ronment somewhat comparable to that of gaseous
ClNO. The results presented here show that sig-
nificant intensity is observed for transitions in-
volving the NCl stretch consistent with dissociative
evolution along this coordinate upon photoexci-
tation. We also provide the first evidence for res-
onance Raman intensity corresponding to the NO
stretch consistent with modest excited-state struc-
tural evolution along this coordinate. As men-
tioned above, it has been proposed that the
A-band is comprised of more than one electronic
transition. The presence of multiple transitions can
be investigated using resonance Raman depolar-
ization ratios. Adepolarization ratio of a 1/3
suggests that a single-state makes the dominant
contribution to the observed scattering, and devi-
ations from this value provide evidence that mul-
tiple states contribute to the observed scattering.
We have found that the ClNO depolarization ra-
tios are less than 1/3 consistent with the A-band
being composed of more than a single electronic
transition.
2. Experimental
2.1. Materials
Nitrosyl chloride (ClNO) was synthesized using
literature methods [32]. Briefly, 8.75 g of sodium
nitrite (99.9% reagent grade, Aldrich) was dissolved
in 12.5 ml of water, and added drop-wise to 50 ml
of concentrated hydrochloric acid (Fisher). The
solution was stirred continuously, and the pro-
duction of ClNO was evidenced by the evolution of

B.P. Barham, P.J. Reid / ChemicalPhysics Letters 361 (2002) 49–56
51
an orange-brown gas. The ClNO gas passed
through a condenser and drying tube packed with
sodium nitrite (99.9% reagent grade, Aldrich),
moist potassium chloride (Baker), and calcium
chloride (Fisher). The product gas was collected in
a flask submerged in a dry ice/acetone bath, re-
sulting in a deep red/bright yellow liquid slurry of
ClNO (boiling point: )6 °C). ClNO was allowed to
warm, and the gaseous ClNO produced upon
warming was bubbled through cyclohexane to
produce the ClNO/cyclohexane stock solution.
Purity was determined by static vis-UV absorption.
To construct the sample, the stock solution was
diluted until a ClNO concentration of 0.1–4 mM
was reached, the specific concentration depending
on excitation wavelength. Stock solutions were
made approximately once a week to ensure sample
integrity.
The absorption cross-section for ClNO in cy-
clohexane was determined as follows. The prepa-
ration of ClNO was performed as described above,
and neat ClNO was isolated as a liquid using a dry
ice/acetone bath. Calibrated capillary pipettes were
precooled in liquid nitrogen and used to transfer
the liquid ClNO into volumetric flasks for dilution
and determination of the cross-section.
each experiment. The concentration changed less
than 10% for a single experiment. All ClNO scat-
tering intensities were found to increase linearly
with incident power.
2.3. Depolarization ratios
Resonance Raman depolarization ratios were
determined at 282.4, 252.7, 228.7 and 217.8 nm.
The depolarization ratio is defined as the intensity
of scattered light with polarization perpendicular
to that of the excitation light divided by the in-
tensity of scattered light with polarization parallel
to that of the excitation light [33]. An a-BBO-po-
larizer was put in the path of the excitation beam
to define the incident polarization. Also, the sap-
phire windows on the sample cell were replaced
with non-birefringent CaF2 windows to maintain
the polarization of the incident and scattered fields.
The polarization of the scattered light was ana-
lyzed using a large aperture (1.5 cm) Glan–Taylor
calcite polarizer placed before the polarization
scrambler at the entrance to the spectrograph.
Measurement of the depolarization ratio of the
802 cmÀ1 cyclohexane transition was performed at
each wavelength to correct for the non-ideal ex-
tinction of the polarizer as described in detail
elsewhere [34]. Transition intensities were deter-
mined by both simple peak integration and by
fitting the bands to a Gaussian function convolved
with a Lorenztian instrument response. Either
approach was found to produce identical results
within the experimental error.
2.2. Resonance Raman spectra
Excitation wavelengths of 282.4, 252.7, 228.7,
217.8 and 192.2 nm were obtained from the hy-
drogen shifted, second and third harmonic output
from a Nd:YAG laser (Spectra Physics GCR 170).
ClNO in cyclohexane was flowed through a quartz
cell equipped with sapphire windows. The sample
flow rate was sufficient to replace the illuminated
sample volume between excitation pulses. The
scattered light was collected by refractive UV-
quality optics and delivered to a 0.75-m spectro-
graph (Acton) where it was dispersed using either a
1200-groove/mm classically ruled, 2400-groove/
mm holographic, or 3600-g/mm holographic
grating. Detection of the scattered light was ac-
complished using a 1100 Â 300 pixel, back-thin-
ned, liquid nitrogen cooled CCD detector
(Princeton Instruments). AHP diode array spec-
trometer was used to monitor the sample concen-
trations (0.1–4 mM for ClNO) before and after
3. Results and discussion
3.1. Absorption spectra
Fig. 1 presents the absorption spectrum of
gaseous ClNO, and of ClNO dissolved in cyclo-
hexane. The large transition observed in this
wavelength region is commonly referred to as the
A-band [15,16]. The breadth and absence of
structure for this transition suggests that rapid
dissociation occurs following photoexcitation res-
onant with this transition. The gas-phase spectrum
is in agreement with that reported in the literature

52
B.P. Barham, P.J. Reid / ChemicalPhysics Letters 361 (2002) 49–56
Fig. 1. Absorption spectrum of ClNO in cyclohexane (solid)
and in gas-phase (dotted). Excitation wavelengths employed in
this study are indicated.
[16]. The maximum absorption cross-section of
ClNO in cyclohexaÀn1e was determined to be
ꢀ2
0:48 A at 48500 cm ðkmax ¼ 206 nmÞ. Compar-
ison of the gas-phase and cyclohexane spectra
demonstrates that the absorption maximum is
shifted 2480 cmÀ1 (10 nm) to lower energy in cy-
clohexane. In addition, the transition becomes
broader in cyclohexane (ꢀ9000 cmÀ1 full-width at
half maximum); however, the integrated intensity
of the A-band in both the gas and condensed
phase is the same (within experimental error). This
suggests that the increased breadth is not due to
the appearance of a new electronic transition, but
is instead due to a change in the energetics and/or
displacements of excited-state or states involved
in the electronic transitions that comprise the
A-band.
Fig. 2. Resonance Raman spectrum of ClNO in cyclohexane
obtained at 282.4, 217.8 and 192.2 nm excitation. Solvent lines
are marked with asterisks and hydrogen lines are denoted with
diamonds. Note that directly on resonance (217.8 and 192.2
nm) transitions corresponding to all three vibrational modes
(m1, NO stretch, m2, NCl stretch and m3, bend) are observed.
assignments and descriptions employed here are in
agreement with those described by Bell et al. and
others; [26,30,31] however, other mode assign-
ments have been proposed [24]. The transition
frequencies observed in cyclohexane, and those
reported for gaseous ClNO are presented in Table
1. Comparison of the transition frequencies dem-
onstrates that the frequencies in cyclohexane are
reduced by approximately 10–20 cmÀ1 relative to
the gas-phase demonstrating that the ground-state
potential energy surface is significantly modified in
solution. In addition to transitions involving the
3.2. ClNO in cyclohexane Raman intensity analysis
Representative resonance Raman spectra of
ClNO dissolved in cyclohexane obtained with ex-
citation at 282.4, 217.8 and 192.2 nm are presented
in Fig. 2. The location of these excitation wave-
lengths with respect to the A-band is indicated in
Fig. 1. Inspection of Fig. 2 demonstrates that the
spectra are dominated by intensity corresponding
to the bend and NCl stretch, similar to the be-
havior observed for gaseous ClNO [24]. Mode

B.P. Barham, P.J. Reid / ChemicalPhysics Letters 361 (2002) 49–56
53
Table 1
Resonance Raman transition frequencies for ClNO dissolved in
cyclohexane and gaseous ClNO
tions following photoexcitation at 248 nm led
them to conclude that the A-band is comprised of
more than one excited-state. Photofragmentation
studies of Skorokhodov et al. [14] employing ex-
citation at 235 nm observed the production of
ground and spin–orbit excited Cl with a bimodal
distribution of translational energy. This result
taken in combination with earlier studies resulted
in a three-state model for the A-band. Finally, ab
initio studies predict that multiple states lie in the
energetic vicinity of A-band; however, two singlet–
singlet transitions are expected to carry the ma-
jority of the oscillator strength [7,23].
Understanding the dissociation dynamics that
occur following A-band photoexcitation is de-
pendent on identifying the number of states that
participate in the dissociation process. This issue
can be addressed through the measurement and
analysis of resonance Raman depolarization ra-
tios. The resonance Raman depolarization ratio is
defined as the intensity of scattered light with po-
larization perpendicular to that of the excitation
light divided by the intensity of light scattered with
polarization parallel to that of the excitation light.
The simplest two-state model for the A-band
consistent with the ab initio results is obtained in
the limit where the A-band is comprised of two
orthogonal transitions with no vibronic coupling
between these surfaces. In this limit, the depolar-
ization ratio (q) expressed as polarizability tensor
elements is defined as [33]:
Transition
Transition
frequency in
cyclohexane
Transition
frequency in the
gas-phasea
(cmÀ1
)
(cmÀ1
)
m3
m2
2m3
325
585
640
895
950
330
598
662
926
m2 þ m3
3m3
984
2m2
1184
1249
1506
1569
1640
1768
m2 þ 2m3
2m2 þ m3
m2 þ 3m3
5m3
3m2
m1
1215
1750
1800
a Taken from [24].
bend and NCl stretch, resonance Raman intensity
corresponding to the NO stretch is also observed
(1800 cmÀ1). These data represent the first obser-
vation of resonance Raman intensity involving this
coordinate. The pattern of resonance Raman in-
tensities observed here demonstrates that excited-
state structural evolution occurs along all three
vibrational coordinates upon photoexcitation.
Although the NO-stretch fundamental transition
is observed, attempts to observe the corresponding
overtone transitions were unsuccessful suggesting
that only modest evolution occurs along this co-
ordinate upon A-band photoexcitation. Therefore,
the resonance Raman results presented here dem-
onstrate that the initial excited-state structural
evolution of ClNO is dominated by evolution
along the bend and NCl stretch with only modest
evolution occurring along the NO stretch.
Iper
Ipar
3R2
10R0 þ 4R2
q ¼
¼
;
where
ꢀ
ꢀ2
1
R0 ¼ axx þ ayy
;
ꢀ
ꢀ
3
It has been proposed that the A-band is com-
prised of more than a single electronic transition.
In photodissociation studies employing 193 nm
excitation, Haas et al. [11] observed a bimodal
NO-fragment energy distribution with differing
photoproduct recoil anisotropies suggesting that
two electronic transitions are in close proximity at
this excitation wavelength. This hypothesis was
further explored in subsequent studies by Felder
and Morley [13] where the observation of distinct
photofragment anisotropy and energy distribu-
n
o
ꢀ
ꢀ2
ꢀ
ꢀ
ꢀ
ꢀ2
ꢀ
1
3
2
R2 ¼
axx À ayy þ jaxxj þ ayy
:
ꢀ
These expressions are representative of the fol-
lowing picture. The presence of two-electronic
transitions gives rise to two corresponding ele-
ments of the polarizability tensor (axx and ayy). If
the A-band were comprised of a single electronic
transition, then only a single tensor element would
be non-vanishing and q ¼ 1=3. To the extent a
second, orthogonal transition contributed to

54
B.P. Barham, P.J. Reid / ChemicalPhysics Letters 361 (2002) 49–56
Table 2
Depolarization ratios for ClNO in cyclohexane
a
Excitation
wavelength (nm)
qðm3Þ
qðm2Þ
qð2m3Þ
282.4
252.7
228.7
217.8
0:19 Æ 0:02b 0:21 Æ 0:03 0:33 Æ 0:06
0:20 Æ 0:03 0:13 Æ 0:01 0:16 Æ 0:04
0:20 Æ 0:02 0:09 Æ 0:01 0:13 Æ 0:04
0:22 Æ 0:01 0:11 Æ 0:01 0:13 Æ 0:01
a Raman depolarization ratios are defined as the scattered
intensity with polarization perpendicular to that of the incident
radiation divided by the scattered intensity with polarization
parallel to that of the incident radiation.
b Errors represent one standard deviation from the mean 3–5
measurements depending on excitation wavelength.
A-band intensity, the second tensor element be-
comes appreciable and q ¼ 1=3. The depolariza-
tion ratios for the NCl stretch ðm2Þ, the bend ðm3),
and the bend overtone (2m3) transitions measured
at 282.4, 252.7, 228.7 and 217.8 nm are presented
in Table 2 and depicted in Fig. 3. The figures
demonstrate that the depolarization ratios are
significantly less than 1/3 consistent with more
than one excited-state contributing to the observed
scattering.
It should be noted that the deviation of the
depolarization ratios from the single-state limit of
1/3 could reflect pre-resonance contributions from
states located in the far-UV. The importance of
this effect was studied by calculating the expected
depolarization dispersion curve for the bend fun-
damental transition by modeling the A-band as
being due to a single electronic transition, and
employing a pre-resonant expression to model
polarizability contributions from states located in
the far-UV. The polarizability contribution of the
single-state involved in the A-band transition was
determined using the time-dependent formalism as
described in detail elsewhere [34]. Parameters for
this state were chosen such that the breadth of the
absorption spectrum and absolute scattering cross-
sections for the bend fundamental transition were
reproduced. The pre-resonant contribution from
the far-UV state was modeled using the following
expression:
Fig. 3. Depolarization ratios versus excitation wavelength for
NCl-stretch fundamental (m2), the bend (m3), and the bend
overtone (2m3).
where Meg is transition length for the transition of
interest, x is mode frequency, D is the displace-
ment of the excited-state potential energy surface
minimum relative to the ground state along the
coordinate of interest, E00 is the energy of the ex-
cited-state from which the pre-resonant intensity is
derived, and E1 is the incident field energy. The
far-UV absorption spectrum of ClNO demon-
strates transition intensity at ꢀ150 nm with a
cross-section that is roughly half that of the A-
band [35]. Therefore values of E00 ¼ 65000 cmÀ1
x
E020 þ E12
;
2
aðE1Þ ¼ Me2g
D
ꢀ
and Meg ¼ 0:2 Awere employed. Using the
pffiffi
ðE020 À El2Þ
ground-state frequency of the bend (325 cmÀ1),
2

B.P. Barham, P.J. Reid / ChemicalPhysics Letters 361 (2002) 49–56
55
E00 ¼ 34500 cmÀ1 and Meg ¼ 0:8 for state 1, and
E00 ¼ 39200 cmÀ1 and Meg ¼ 0:82 for state 2. In
addition, displacements along the all three coor-
dinates were taken to be equal in both states, and
adjusted to reproduce the breadth of the overall
absorption band as well as the absolute scattering
cross-sections for the bend and N–Cl stretch. The
agreement between the model and experiment is
good, and demonstrates that a two-state model is
capable of reproducing the depolarization date for
excitation wavelengths directly resonant with the
A-band. Ratios measured at lower excitation en-
ergies are underestimated in the model potentially
due to contributions from the weak B-band at
these wavelengths. The comparison between the
data and two-state model clearly demonstrates
that the resonance Raman depolarization ratios
are consistent with the A-band being comprised of
at least two electronic transitions. We are currently
developing a two-state model for the A-band that
is capable of reproducing the absorption spectrum,
absolute scattering cross-sections, and depolariza-
tion ratios.
values of D ranging from 0.25 to 2.5 were investi-
gated. Arepresentative fit to the depolarization
dispersion data for the bend fundamental transi-
tion employing this model for D ¼ 1:0 is presented
in Fig. 4a. The agreement between theory and
experiment is poor, and demonstrates that pre-
resonance contributions from states located in the
far-UV are incapable of reproducing the observed
depolarization ratios. Furthermore, pre-resonant
contributions are expected to cause little or no
deviation to the depolarization dispersion curve in
this wavelength region of the A-band.
We have also performed a preliminary calcula-
tion in which the A-band is modeled as being
comprised of two orthogonal transitions. Fig. 4b
presents the results for this two-state model with
4. Conclusion
We have presented the first resonance Raman
spectra of ClNO in solution. The spectra of
ClNO dissolved in cyclohexane demonstrate that
significant structural evolution occurs along the
bend and NCl stretch upon A-band photoexci-
tation. For the first time, resonance Raman
intensity corresponding to the NO-stretch co-
ordinate was observed; however, the limited in-
tensity for this transition suggests that the
excited-state structural evolution that occurs
along this coordinate following photoexcitation is
modest. Finally, resonance Raman depolarization
ratios were obtained and found to be consistent
with the A-band being comprised of at least two
separate electronic transitions. Currently, reso-
nance Raman intensity analysis studies are un-
derway to observe ClNO in additional solvents,
and to determine depolarization ratios at other
excitation wavelengths in an attempt to obtain a
mode-specific model of the excited-states that
comprise the A-band.
Fig. 4. Depolarization dispersion curves for the bend funda-
mental transition derived using the resonant/pre-resonant (a)
and a two-resonant-state models (b) for the A-band. Experi-
mental bend depolarization ratios (points) are indicated. Details
of the models are presented in the text.

56
B.P. Barham, P.J. Reid / ChemicalPhysics Letters 361 (2002) 49–56
[15] C.F. Goodeve, S. Katz, Proc. R. Soc. A172 (1939) 432.
Acknowledgements
[16] N.M. Ballash, D.A. Armstrong, Spectrochim. Acta 30A
(1974) 941.
The National Science Foundation is acknowl-
edged for their support of this work (CHE-
0091320). Acknowledgment is also made to the
donors of the Petroleum Research Fund, admin-
istered by the American Chemical Society. P.J.R. is
the recipient of an Alfred P. Sloan Fellowship, and
is a Cottrell Scholar of the Research Corporation.
[17] H. Reisler, C. Qian, in: J.M. Bowman (Ed.), Advances in
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