Gas-phase chloroiodomethane short-time photodissociation dynamics in the A-band absorption and a comparison with the solution phase short-time photodissociation dynamics

Abstract

A gas-phase resonance Raman spectrum of chloroiodomethane has been obtained with excitation in the A-band absorption with similar resolution and the same experimental apparatus used to obtain solution-phase A-band resonance Raman spectra of chloroiodomethane reported previously. This allows a careful and detailed comparison of the gas-and solution-phase chloroiodomethane A-band resonance Raman spectra and their associated short-time photodissociation dynamics. The gas-phase A-band resonance Raman spectrum has four Franck-Condon active vibrational modes and a noticeable degree of multidimensionality in its photodissociation reaction. The A-band gas-phase resonance Raman intensities and absorption spectrum were simulated using a simple model and time-dependent wave-packet calculations. The motion of the wave packet on the excited state gas-phase potential surface was converted from dimensionless normal coordinates into easy-to-visualize internal coordinates using the results of normal coordinate calculations. The A-band early-time photodissociation dynamics are very similar to those of the solution phase in that the C-I bond lengthens, the I-C-Cl and H-C-I angles become smaller, and the H-C-Cl angles become larger. These short-time photodissociation dynamics are consistent with a simple impulsive 'semirigid' radical model description of the photodissociation. A comparison of the gas-phase and solution-phase short-time photodissociation dynamics suggests that solvation effects are not important for internal coordinate motions such as the C-I stretch and I-C-Cl angle where there are strong intramolecular forces but that solvation effects are noticeable for internal coordinate motions such as H-C-H angle and H-C-Cl angles which are weakly involved in the dissociation coordinate and have relatively weak intramolecular forces that are more easily perturbed by the solvent-solute forces. This implies that 'static' or mean-field solvation effects may be more significant for internal coordinate motions that have small intramolecular forces compared with internal coordinate motions that have much larger intramolecular forces from the photodissociation reaction.