Excited-State Reaction Dynamics of Chlorine Dioxide in Water from Absolute Resonance Raman Intensities

Abstract

Resonance Raman spectra of aqueous chlorine dioxide (OClO) are measured for several excitation wavelengths spanning the photochemically relevant 2B1−2A2 optical transition. A mode-specific description of the optically prepared, 2A2 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 v symmetry on the 2A2 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 2B1−2A2 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 O3 - 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.

Molecular dynamics study of the weakly solvent dependent relaxation dynamics following chlorine dioxide photoexcitation

“…The role of solvent in the excited-state reaction dynamics of OClO has been explored using absolute resonance Raman intensity analysis ͑RRIA͒. [15][16][17][18] These studies have demonstrated that structural evolution along the asymmetric-stretch coordinate is restricted in solution result-ing in the preservation of ground-state C 2v symmetry in the excited state. The preservation of C 2v symmetry is a primary factor for the enhanced Cl production in condensed environments.…”

Calculation of absorption and resonance Raman cross sections of ClO2 in the gas‐phase and in solution: including Duschinsky effects

“…Esposito et al [8] have obtained the absorption and resonance Raman spectra of OClO dissolved in cyclohexane at resonance with the X 2 B 1 -Ã 2 A 2 electronic transition. The absorption and resonance Raman spectra of OClO at resonance with the X 2 B 1 -Ã 2 A 2 electronic transition in the gas-phase [11] and in the solvents such as water, [10] chloroform, [12] and cyclohexane [8] were obtained. For analysis of resonance Raman intensities, the excited-state potential energy surfaces were modeled as harmonic for both symmetric bending and asymmetric stretch coordinates, and cubic anharmonic for the symmetric stretch coordinate.…”

“…Experimental absorption spectrum of OClO in water (Reference[10]) solid curve and the calculated spectrum for displaced-distortedrotated (dashed curve) harmonic oscillator model using the parameters ofTables 3 and 4.…”

“…Experimental (Reference[10]) and calculated resonance Raman excitation profile of the symmetric stretch fundamental (a) and overtone (b) for OClO in water. Calculations were performed using parameters ofTables 3 and 4.…”

“…The first two terms refer to fundamental transitions and the latter three terms correspond to overtone transitions. b Calculated resonance Raman cross sections using rotated-distorteddisplaced oscillator model and parameters ofTables 3 and 4. c Taken from Reference[10]. d Taken from Reference[12].…”