Intermolecular hydrogen bonding in chlorine dioxide photochemistry: A time-resolved resonance Raman study

The geminate-recombination and vibrational-relaxation dynamics of chlorine dioxide (OClO) dissolved in ethanol and 2,2,2-trifluoroethanol (TFE) are investigated using time-resolved resonance Raman spectroscopy. Stokes spectra are measured as a function of time following photoexcitation using degenerate pump and probe wavelengths of 398 nm. For OClO dissolved in ethanol, subpicosecond geminate recombination occurs resulting in the reformation of groundstate OClO with a quantum yield of 0.5 +/- 0.1. Following recombination, intermolecular-vibrational relaxation of OClO occurs with a time constant of 31 +/- 10 ps. For OClO dissolved in TFE, recombination occurs with a time constant of 1.8 +/- 0.8 ps and a quantum yield of only 0.3 +/- 0.1. The intermolecular-vibrational-relaxation time constant of OClO in TFE is 79 +/- 27 ps, The reduced geminate-recombination quantum yield, delayed recombination, and slower vibrational relaxation for OClO in TFE is interpreted in terms of greater self-association of the solvent. Degenerate pump-probe experiments are also presented that demonstrate decay of the Cl-solvent charge-transfer complex on the similar to1-ns time scale in ethanol and TFE. This time is significantly longer than the abstraction times observed for other systems demonstrating that Cl hydrogen abstraction from alcohols occurs in the presence of a significant energy barrier. (C) 2001 Elsevier Science B.V. All rights reserved.