Resonance Raman intensity analysis of chlorine dioxide, nitrosyl chloride, and isopropyl nitrate in solution

The solution-phase photochemistry of chlorine dioxide (OClO), nitrosyl chloride (ClNO), and isopropyl nitrate (IPN) is studied using resonance Raman spectroscopy. Specifically, absolute resonance Raman intensity analysis (RRIA) is used to describe the excited-state structural evolution that occurs upon photoexcitation for these molecules in solution. For OClO, RRIA studies demonstrate that excitation resonant with the 2B₁ to 2A₂ transition results in structural evolution along the symmetric-stretch and bend coordinates. In addition, the homogeneous linewidth for this transition is found to be the same in water, cyclohexane, and chloroform within experimental error. Fluorescence quantum yield studies demonstrate that the solvent independence of the homogeneous linewidth reflects a common timescale for pure dephasing in these solvents. The solvent response to OClO photoexcitation is modeled using the viscoelastic continuum model for non-polar solvation, and the dephasing times predicted by this model are shown to be consistent with solvent-independence of the homogeneous linewidth. In studies of ClNO photochemistry, the resonance Raman depolarization ratios demonstrate that more than one state contributes to the observed scattering when employing excitation resonant with the absorption band centered at ∼200 nm, commonly referred to as the “A-band”. Given this observation, the resonance Raman and absorption cross sections are modeled using two excited states. The results presented here demonstrate that the excited-state reaction dynamics are dominated by evolution along the N-Cl stretch and bend coordinates consistent with cleavage of the N-Cl bond. Comparison of the modeling results for ClNO dissolved in cyclohexane and acetonitrile demonstrates that both the homogeneous linewidth and the excited-state structural evolution along the N-Cl coordinate are solvent dependent. Finally, studies of IPN photochemistry in cyclohexane and acetonitrile demonstrate that the excited-state structural evolution is dominated by motion along the NO-stretch and NO₂-asymmetric-stretch coordinates consistent with N-O bond cleavage. Furthermore, more extensive evolution along the NO stretch is observed in acetonitrile relative to cyclohexane suggesting that the excited-state reaction dynamics of IPN are solvent dependent.