Short‐time photodissociation dynamics of A‐band and B‐band bromoiodomethane in solution: An examination of bond selective electronic excitation

We have obtained resonance Raman spectra and absolute Raman cross section measurements at eight excitation wavelengths in the A‐band and B‐band absorptions of bromoiodomethane in cyclohexane solution. The resonance Raman intensities and absorption spectra were simulated using a simple model and time‐dependent wave packet calculations. Normal mode vibrational descriptions were used with the results of the calculations to find the short‐time photodissociation dynamics in terms of internal coordinates. The A‐band short‐time photodissociation dynamics indicate that the C–I bond becomes much longer, the C–Br bond becomes smaller, the I–C–Br angle becomes smaller, the H–C–Br angles become larger, the H–C–I angles become smaller, and the H–C–H angle becomes a bit smaller. The B‐band short‐time photodissociation dynamics indicate the C–Br bond becomes much longer, the C–I bond becomes slightly longer, the I–C–Br angle becomes smaller, the H–C–I angles become larger, the H–C–Br angles become smaller, and the H–C–H angle becomes slightly smaller. Both the A‐band and B‐band short‐time photodissociation dynamics appear to be most consistent with an impulsive ‘‘semi‐rigid’’ radical model qualitative description of the photodissociation with the CH2Br radical changing to a more planar structure in the A‐band and the CH2I radical changing to a more planar structure in the B band. We have carried out a Gaussian deconvolution of the A‐band and B‐band absorption spectra of bromoiodomethane, as well as iodomethane and bromomethane. The absorption spectra, resonance Raman intensities, and short‐time photodissociation dynamics suggest a moderate amount of coupling of the C–I and C–Br chromophores.