Excited-state reaction dynamics of chlorine dioxide in water from absolute resonance Raman intensities

Resonance Raman spectra of aqueous chlorine dioxide (OClO) are measured for several excitation wavelengths spanning the photochemically relevant (2)B(1)-(2)A(2) optical transition. A mode-specific description of the optically prepared, (2)A(2) potential energy surface is derived by simultaneous analysis of the absolute resonance Raman and absorption cross sections. The resonance Raman spectra are dominated by transitions involving the symmetric stretch and bend coordinates demonstrating that excited-state structural evolution occurs predominately along these degrees of freedom. Scattering intensity is not observed for transitions involving the asymmetric stretch, demonstrating that excited-state structural evolution along this coordinate is modest. The limited evolution along the asymmetric stretch coordinate results in the preservation of C(2 nu) symmetry on the (2)A(2) surface. It is proposed that this preservation of symmetry is responsible for the increase in Cl photoproduct quantum yield in solution relative to the gas phase. Analysis of the absolute scattering cross sections also demonstrates that the homogeneous line width for the (2)B(1)-(2)A(2) optical transition in water is essentially identical to that in cyclohexane; however, the extent of inhomogeneous broadening increases dramatically in aqueous solution. Comparison of the spectroscopic properties of OClO to the properties of isoelectronic O(3)(-) is made to elucidate the origin of the solvent response to OClO photoexcitation. It is suggested that solvent-solute dipole-dipole coupling and intermolecular hydrogen bonding represent the largest components of the solvent coordinate.