Multistate Photochemical Reaction Dynamics of ClNO in Solution: An Absolute Resonance Raman Intensity Analysis Study

Abstract

The excited-state reaction dynamics of nitrosyl chloride (ClNO) are studied using absolute resonance Raman intensity analysis. The absolute resonance Raman cross sections for ClNO dissolved in cyclohexane and acetonitrile are measured at several excitation wavelengths spanning the absorption band commonly referred to as the “A band” (λmax ≈ 200 nm). The resonance Raman and absorption cross sections are modeled using the time-dependent formalism. Resonance Raman depolarization ratios are also measured and are found to be consistent with at least two electronic transitions participating in the scattering process. Therefore, the standard time-dependent formalism approach was modified by incorporating two excited states into the analysis, with state contributions deconvolved through modeling of the depolarization ratios in addition to the absolute resonance Raman and absorption cross sections. The spectroscopic observables are well reproduced using this two-state model. The analysis presented here demonstrates that the photoexcitation of solution-phase ClNO results in a substantial evolution of the N−Cl stretch coordinate consistent with the dissociation of the N−Cl bond. Significant structural evolution is also observed along the bend, with minimal excited-state structural evolution observed along the NO stretch. The structural evolution along the dissociative N−Cl stretch coordinate is found to be solvent-dependent, and the origin of this dependence is related to changes in the ground-state equilibrium geometry as a function of solvent environment. Finally, the homogeneous line width undergoes a significant increase in acetonitrile relative to cyclohexane, and this increase is proposed to reflect the modification of the excited-state interactions and nonadiabatic relaxation dynamics.