EXCITED-STATE DYNAMICS OF CHLORINE DIOXIDE IN THE CONDENSED-PHASE FROM RESONANCE RAMAN INTENSITIES

Resonance Raman spectra of chlorine dioxide (OClO) dissolved in cycloh exane obtained with excitation throughout the B-2(1)-(2)A(2) electroni c 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 stru ctural evolution along all three coordinates occurs upon photoexcitati on. The electronic absorption and absolute resonance Raman cross secti ons are reproduced employing the time-dependent formalism for Raman sc attering using an anharmonic description of the (2)A(2), excited-state potential-energy surface. Analysis of the resonance Raman cross-secti ons demonstrates that both homogeneous and inhomogeneous broadening me chanisms are operative in cyclohexane. Comparison of the experimentall y determined, gas-phase (2)A(2) surface to that in solution defined by the analysis presented here shows that although displacements along t he symmetric stretch and bend are similar in both phases, evolution al ong the asymmetric stretch is dramatically altered in solution. Specif ically, employing the gas-phase potential along this coordinate, the p redicted intensity of the overtone transition is an order of magnitude larger than that observed. The analysis presented here demonstrates t hat the asymmetric stretch overtone intensity is consistent with a red uction in excited-state frequency along this coordinate from 1100 to 7 50 +/- 100 cm(-1). This comparison suggests that differences in evolut ion along the asymmetric stretch may be responsible for the phase-depe ndent reactivity of OClO. In particular, the absence of substantial ev olution along the asymmetric stretch in solution results in the ground -state symmetry of OClO being maintained in the (2)A(2) excited state. The role of symmetry in defining the reaction coordinate and the natu re of the solvent interaction responsible for modulation of the excite d-state potential energy surface are discussed.