Femtosecond TRIR Studies of ClNO Photochemistry in Solution: Evidence for Photoisomerization and Geminate Recombination

Abstract

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.

Effect of Solvent Polarity on the Vibrational Dephasing Dynamics of the Nitrosyl Stretch in an FeII Complex Revealed by 2D IR Spectroscopy

“…Recent work by Huber et al, have measured a positive correlation of the ν CO frequency and the AN in a dioxygen adduct of Vaska’s complex and have attributed it to interactions of the trans O 2 ligand and the solvents that modulate the metal–CO π-back-bonding interactions . The parameter E T N (30) has not been previously correlated with the frequency of the ν NO in SNP, but it has been correlated with the ν NO in ClNO by Bixby et al, where they see a similar positive correlation . Although there is a strong correlation with the frequency of the NO stretch and empirical measures of the solvent polarity, there is no correlation with the line width of the NO stretch or its vibrational lifetime.…”

“…The role of electric field fluctuations in the dephasing dynamics of charged species dissolved in polar solvents has been widely studied to understand the role of solvent fluctuations in chemistry and biology. There is growing experimental evidence that the collective dynamics of the solvent around small ionic solutes play the biggest role in determining the amplitudes and time scales of the vibrational frequency fluctuations of the solute. ,,− Tominaga and co-workers have performed systematic studies of measuring the FFCFs of the cyanide, nitrosyl, and azide stretching vibrations in ionic solutes dissolved in aqueous solvents. ,,− Their studies find a common time scale ranging from 1 to 1.5 ps for the frequency fluctuations for various ions regardless of the vibration being studied. This similar time scale has been obtained in experimental studies of the OH(OD) stretch of HOD in D 2 O(H 2 O). ,,, Molecular dynamics simulations have been used to obtain the FFCF of the azide stretch in D 2 O and the OH(OD) stretch of HOD in D 2 O(H 2 O). ,, The simulation studies have shown that the experimentally measured picosecond vibrational dephasing dynamics are driven by electric field fluctuations originating from collective reorganization of the hydrogen bonding network.…”

Comparing molecular photofragmentation dynamics in the gas and liquid phases

“…23 The recent literature contains many studies of the UV photofragmentation of prototypical small molecules, e.g. I 2 , [24][25][26][27][28][29] HgI 2 , 30-37 I 3 À , 19,[38][39][40][41][42] ICN, 11,[43][44][45][46][47] ClNO, [48][49][50] OClO, [51][52][53][54][55][56][57][58][59][60] HOCl 61,62 and CH 3 OCl, 63 and haloalkanes like CF 2 I 2 , 64-66 CH 2 I 2 , [67][68][69][70][71][72] and CH 2 Br 2 , 73,74 that seek more quantitative insights into VET and/or recombination mechanisms and, more generally, to explore the synergies and the obvious differences between molecular photofragmentations occurring in the gas and condensed phases. 13,75 The present perspective article seeks to explore such issues by comparing and contrasting the UV photofragmentation behaviour of a limited family of molecules -phenols, thiophenols and selected ether/thi...…”

Time-Resolved Infrared Absorption Studies of the Solvent-Dependent Photochemistry of ClNO

“…Resonance Raman (RR) studies established that the frequencies of the N–Cl stretch (ν 2 ) and bend (ν 3 ) transitions are solvent dependent, shifting to lower frequency in more polar solvents . Fourier transform infrared (FTIR) absorption studies of the NO stretch (ν 1 ) fundamental transition observed that this transition shifts to higher frequency and the line width increases with an increase in solvent polarity . The influence of solvent in ClNO photochemistry was investigated using femtosecond pump–probe spectroscopy .…”

“…Recent time-resolved infrared (TRIR) studies have also explored the photochemisty of ClNO in acetonitrile. Here, the evolution in optical density in the region of the ν 1 transition of ClNO was measured following 266 nm photoexcitation . Photoinduced depletion of ground-state ClNO and subsequent geminate recombination of the primary photofragments resulting in ClNO reformation was measured with a quantum yield of 0.54 ± 0.06.…”