Excited-State Dynamics of Chlorine Dioxide in the Condensed Phase from Resonance Raman Intensities

Abstract

Resonance Raman spectra of chlorine dioxide (OClO) dissolved in cyclohexane obtained with excitation throughout the 2B1−2A2 electronic transition are presented. Resonance Raman intensity corresponding to all vibrational degrees of freedom (the symmetric stretch, bend, and asymmetric stretch) is observed, demonstrating that excited-state structural evolution along all three coordinates occurs upon photoexcitation. The electronic absorption and absolute resonance Raman cross sections are reproduced employing the time-dependent formalism for Raman scattering using an anharmonic description of the 2A2, excited-state potential-energy surface. Analysis of the resonance Raman cross-sections demonstrates that both homogeneous and inhomogeneous broadening mechanisms are operative in cyclohexane. Comparison of the experimentally determined, gas-phase 2A2 surface to that in solution defined by the analysis presented here shows that although displacements along the symmetric stretch and bend are similar in both phases, evolution along the asymmetric stretch is dramatically altered in solution. Specifically, employing the gas-phase potential along this coordinate, the predicted intensity of the overtone transition is an order of magnitude larger than that observed. The analysis presented here demonstrates that the asymmetric stretch overtone intensity is consistent with a reduction in excited-state frequency along this coordinate from 1100 to 750 ± 100 cm-1. This comparison suggests that differences in evolution along the asymmetric stretch may be responsible for the phase-dependent reactivity of OClO. In particular, the absence of substantial evolution along the asymmetric stretch in solution results in the ground-state symmetry of OClO being maintained in the 2A2 excited state. The role of symmetry in defining the reaction coordinate and the nature of the solvent interaction responsible for modulation of the excited-state potential energy surface are discussed.