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 in vestigated using time-resolved resonance Raman spectroscopy. Stokes spectra are measured as a function of time following photoexcitation using degener ate pump and probe wavelengths of 398 nm. For OClO dissolved in ethanol, su bpicosecond geminate recombination occurs resulting in the reformation of g roundstate OClO with a quantum yield of 0.5 +/- 0.1. Following recombinatio n, intermolecular-vibrational relaxation of OClO occurs with a time constan t of 31 +/- 10 ps. For OClO dissolved in TFE, recombination occurs with a t ime 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 recombi nation, and slower vibrational relaxation for OClO in TFE is interpreted in terms of greater self-association of the solvent. Degenerate pump-probe ex periments are also presented that demonstrate decay of the Cl-solvent charg e-transfer complex on the similar to1-ns time scale in ethanol and TFE. Thi s time is significantly longer than the abstraction times observed for othe r systems demonstrating that Cl hydrogen abstraction from alcohols occurs i n the presence of a significant energy barrier. (C) 2001 Elsevier Science B .V. All rights reserved.