Energy (cm-1)

Mo

lar E

xtin

ctio

n C

oef

ficie

nt

(M-1

cm-1

)

1750 1770 1790 1810 1830 1850 1870 1890 1910 1930 1950 1970 19900

100

200

300

400

500

600

700

800

900

1000 Cyclohexane

Carbon Tetrachloride

Chloroform

Dichloromethane

Acetonitrile

−3 −2 −1 0 1 2 3 4 5−1

Time (ps)

∆OD

(mO

D)

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

= O= Cl

= N -6

-5

-4

-3

-2

-1

0

1

2

1820 1840 1860 1880 1900 1920

0.75 ps

1 ps

2 ps

5 ps

15 ps

Frequency (cm-1)

∆OD

(mO

D)

A

1820 1840 1860 1880 1900 1920

0.75 ps

1 ps

1.5 ps

2 ps

5 ps

15 ps

Frequency (cm-1)

∆OD

(mO

D)

-1

0

1

2

3

4

5

6

7

-2

B

Static FTIR Absorption

Figure 5: A) The absolute change in optical den-sity measured at specific delay times following photoexcitation of ClNO at 266 nm. B) Optical-density increase corresponding to photoproduct formation following ClNO photolysis. The increase in optical density is centered at 1860 cm-1, roughly 10 cm-1 lower in frequency than the NO-stretch fundamental in ClNO. This feature decays on the 5-ps timescale to a value of 2 mOD that remains constant out to the longest delay times investigated (50 ps).

Figure 3: Optical density evolution of ClNO dissolved in acetonitrile following photoexcitation at 266 nm. Measured at probe frequencies ranging from1820 cm-1 to 1920 cm-1.

Figure 4: Contour plot of TRIR optical-density evolution observed in the region of the NO-stretch fundamental transition of ClNO dis-solved in acetonitrile following 266-nm pho-tolysis. The color scale representing the change in optical den-sity is provided at the top of the figure. The bottom panel presents FTIR spectra of the sample (solid), acetoni-trile only (short dashed), and the NO-stretch (long dash).

Figure 1: IR absorption spectra of ClNO in the region of the NO stretch fundamental transition in various solvents. As the solvent polarity is increased, the fre-quency of the transition increases consistent with an increase in the NO bond strength and corre-sponding decrease in strength of the NCl bond.

−2 0 2 4 6 8 10 12 14

1920 cm-1

1910 cm-1

1900 cm-1

1890 cm-1

1880 cm-1

1869 cm-1

1860 cm-1

1850 cm-1

1840 cm-1

1836 cm-1

1830 cm-1

1820 cm-1

0

0

0

0

0

0

0

0

0

0

0

0

Time (ps)

∆OD

(mO

D)

= 2 mO.D.

Following 266-nm excitation, a reduction and subse-quent recovery in optical density is observed at all probe wavelengths. Additionally, at probe wavelengths be-tween 1820 cm-1 and 1840 cm-1 an initial increase in opti-cal density is evident, peaking at 0.75 ps. The depletion is consistent with loss of ground-state ClNO due to photoly-sis and the recovery indicates partial geminate recombi-nation of the photoproducts. The maximum reduction in optical density is at 1880 cm-1, and most of the evolution is completed within ~5 ps. There is no evolution in the band shape as observed despite the fact that it is asymmetric and shifted relative to the absorption band. This may indi-cate little to no excess vibrational energy is being depos-ited into the NO stretch of geminately recombined ClNO.

DS

CPXT

XT

TP

OPA DFG

SP

MCT

S

Amplified

Ti:SapphireFigure 2: A schematic of the UV-pump, IR-probe spectrom-eter used in this study. The following abbreviations have been used: CP: mechanical chopper, DS: delay stage, XT: BBO crystal, TP: time plate, OPA: optical parametric ampli-fier, DFG: difference frequency generator, S: sample, SP: spectrograph, MCT: a pair of LN

2 mercury/ cadmium/ tellu-

ride detectors.

Understanding the fundamental aspects of phase-dependent photochemical reactivity is a central challenge in environmental chemistry. For example, the environment-dependent reactivity of haloox-ides, such as chlorine dioxide (OClO), has attracted a significant amount of attention due to the contri-bution of these compounds to the stratospheric chlorine budget.1 Photoexcitation of gaseous OClO results primarily in the formation of OCl and O.2 In contrast, ClOO production is observed in low tem-perature matrixes.3 Finally, in solution OCl/O, Cl/O

2 and photoisomer production are all observed.4

Similar complexity is observed in nitrosyl-halide chemistry, and our lab has recently investigated the photochemistry of nitrosyl chloride (ClNO)(Scheme 1).

The photochemistry of solution phase ClNO was recently investigated using femtosecond pump-probe spectroscopy.5 In these studies photoinduced depletion of ClNO and recovery via geminate re-combination was observed. In addition, photoproduct formation in all solvents (acetonitrile, chloro-form, and dichloromethane) was found, but definitive assignment of the photoproducts was not achieved. To address this issue, we have initiated a series of FTIR and ultrafast TRIR studies of ClNO pho-tochemistry in solution.

Results and Discussion

The solvent-dependence of the N-O stretch fundamental transition frequency was studied. The transition demonstrates a positive solvatochromic shift that is linear with solvent polarity (E

T(30) scale).

A corresponding decrease in the N-Cl stretch fundamental frequency is also observed in the resonance Raman spectrum.6

Energy (cm-1)

Tim

e (p

s)

1900

15

10

1820 1830 1850 1870 1890 1910 19201860 18801840

Ab

sorb

ance

0.4

0.6

0.810

5

0.2

Optical Density Color Scale (mOD)

-3.5-3-2.5-2-1.5-1-0.50 -4.5-4

The maximum reduction in opti-cal density does not correspond to the peak of the N-O stretch funda-mental. Therefore, an optical density increase must exist on the low fre-quency side of the N-O stretch. To in-vestigate this possibility, ClNO deple-tion was removed by adding back the ground-state absorption spec-trum until the reduction in OD for probe frequencies > 1900 cm-1 was removed (Figure 5 B). The peak of the absorptive feature occurs at 1860 cm-1 and may correspond to the pho-toisomer (ClON), free NO, or vibra-tionally excited ClNO produced by geminate recombination.

Cl:ACN Charge-Transfer Complex

The weak absorption feature appearing below 1836 cm-1 in Figure 5 B suggests possible modi-fication of the solvent modes upon formation of the chlorine-acetonitrile (Cl:ACN) charge-transfer complex. To further inves-tigate this possibility we have probed the C-N stretch of acetoni-trile (Figure 6).

Figure 6: TRIR optical density evolution follow-ing ClNO photolysis in acetonitrile. The probe frequencies employed are on the high-energy (blue, 2260 cm-1) and low-energy (red, 2240 cm-1) side of the CN-stretch fundamental transi-tion (2250 cm-1).

We have observed the unexpected environment-dependence of the NO stretch of ClNO. The line-width is significantly increased and the frequency dramatically blue-shifted wih increasing solvent po-larity. We have observed the photo-induced depletion and subsequent geminate recombination fol-lowing photoexcitation of ClNO. Formation of a photoproduct is seen, which we have assigned as the first observation of the photoisomer, ClON, in solution phases.

Conclusions

Introduction

Department of Chemistry, University of Washington, Box 351700, Seattle, WA 98195

Teresa J. Bixby, Joshua D. Patterson, Philip J. Reid

Time-Resolved Infrared Pump-Probe Spectroscopy of ClNO in Acetonitrile

hν+

∆+

Scheme 1: The reaction pathways available to ClNO upon photoexcitation. In the gas phase dissociation to form Cl and NO dominates, whereas in matrixes photoisomerization to form ClON is observed.

Experimental Setup

Acknowledgements:

This work was funded by the NSF.

3. C. J. Pursell, J. Conyers, P. Alapat, and R. Parveen. J. Phys. Chem. 99, 10433 (1995).4. R. C. Dunn, E. C. Richard, V. Vaida, and J. D. SImon. J. Phys. Chem. 95, 6060 (1991).5. C. C. Cooksey, K. Johnson, and P. J. Reid. J. Phys. Chem. A. 110, 8613 (2006).6. B. P. Nyholm and P. J. Reid. J. Phys. Chem. B. 108, 8716 (2004).

References:1. C. C. Cooksey and P. J. Reid. Photochem. Photobio. 80, 386 (2004).2. H. F. Davis and Y. T. Lee. J. Chem. Phys. 105, 8142 (1996).

Time-Resolved IR Absorption Study