Probing the photoisomerization of CHBr3 and CHI3 in solution with transient vibrational and electronic spectroscopy.

Transient infrared absorption spectroscopy monitors condensed-phase photodissociation dynamics of 30 mM CHBr3 and 50 mM CHI3 in liquid CCl4. The experiments have picosecond time resolution and monitor the C-H stretch region of both the parent polyhalomethanes and their photolytically generated isomers. The C-H stretching transitions of these isomers, in which the emergent halogen atom returns to form a C-X-X bonding motif, appear about 9 ps after photolysis for iso-CHBr2-Br and in about 46 ps for iso-CHI2-I. These time scales are consistent with, but differ from, the time evolution of the transient electronic absorption spectra of the same samples, highlighting the subtle differences between monitoring the vibrational and electronic chromophores. The specificity of using vibrational transitions to track condensed-phase reaction dynamics permits reassessment of the transient electronic spectrum of photolysis in neat CHBr3, which has an additional prompt feature near 400 nm. Calculations show that this feature, which arises from a precursor to the isomer, is a charge-transfer transition of a contact pair between the nascent Br fragment and a nearby CHBr3 molecule. Dilution and solvent studies show that transition is independent of the solvent. The iso-CHBr2-Br transition wavelength, however, shifts over the range of 400 to 510 nm depending on the solvent. Time-dependent density functional calculations faithfully reproduce these trends.