Photodissociation of alkyl iodides in solution: Substituent effects on the early-time dynamics

Abstract

Resonance Raman spectra, including absolute scattering cross sections, have been measured for ethyl, isopropyl, and tert-butyl iodides in cyclohexane solution at seven to ten wavelengths between 303 and 200 nm. Spectra of fully deuterated ethyl iodide have also been obtained at five wavelengths. Spectra excited in the 300-250 nm region, on resonance with the directly dissociative A state, are dominated by long overtone progressions in the nominal C-I stretching mode near 500 cm-1. In all three molecules the fundamental of the C-I stretch is unexpectedly weak relative to the overtones when excited near the peak of the A band. This is shown to arise from interference between the A-state resonant part of the fundamental Raman amplitude and preresonant contributions from higher electronic states. In addition to the C-I stretching activity, A-state excitation generates significant intensity in fundamentals, overtones, and combination bands of modes nominally assigned as bending and CC stretching vibrations, suggesting a multidimensional character to the reaction coordinate. The absorption spectra and A-state resonant Raman intensities are modeled successfully through wave-packet propagation on a multidimensional locally harmonic potential with a preresonant contribution to the fundamental intensities included. The short-time photodissociation dynamics are then examined by using the normal-mode coefficients to convert the wave-packet motion from dimensionless normal coordinates into internal coordinates. It is found that while the dominant motion during the first 10 fs involves stretching of the C-I bond, other stretching and bending motions are also involved, although the precision of these conclusions for isopropyl and tert-butyl iodides is limited by the indeterminacy in the signs of the normal-mode displacements obtained from the intensity analysis. Comparison of the results for normal and perdeuterated ethyl iodide is used to resolve most of the sign indeterminacies for this molecule. The present results are compared and contrasted to conclusions of previous studies of energy partitioning in the vapor-phase photodissociation. © 1991 American Institute of Physics.