Solvation effects on the A-band photodissociation of dibromomethane: Turning a photodissociation into a photoisomerization

Abstract

We have obtained A-band resonance Raman spectra of dibromomethane in the gas and solution phase and nanosecond time-resolved resonance Raman spectra of dibromomethane photoproducts. The A-band resonance Raman spectra suggest the short-time dynamics are similar to other dihalomethanes that are known to have direct photodissociation reactions in the gas phase. Several power dependent A-band resonance Raman bands were tentatively assigned to the C - Br stretch overtone progression of the CH 2Br radical which has a strong absorption band that is coincident with the A-band resonance Raman excitation wavelength. Two-color nanosecond time-resolved resonance Raman spectra (266.0 nm pump/341.5 nm probe) were obtained and comparison of the vibrational frequencies to the results for density functional theory calculations indicate that the iso-CH 2Br - Br species is mainly responsible for the transient photoproduct absorption band ∼360 nm. Our preliminary results for A-band photoexcitation of dibromomethane in conjunction with previously reported diiodomethane results suggest that solvation effects (probably via recombination of the CH 2Br and Br fragments within a solvent cage) lead to noticeable production of the isodibromomethane photoisomerization photoproduct observed in the nanosecond time-resolved resonance Raman spectra. © 2000 American Chemical Society.