Resonance Raman study of the A-band short-time photodissociation dynamics of axial and equatorial conformers of iodocyclopentane

We have obtained resonance Raman spectra of iodocyclopentane in cyclohexane solution at three excitation wavelengths resonant with the A-band absorption. The A-band resonance Raman spectral bands can be assigned to fundamentals, overtones, and combination bands of seven axial conformer and eight equatorial conformer Franck-Condon active modes. The resonance Raman and absorption cross sections were simultaneously simulated using wave packet calculations and a simple model. The best fit parameters of the simulations and the normal mode descriptions were used to determine the A-band short-time photodissociation dynamics of the axial and equatorial conformers of iodocyclopentane. The axial and equatorial conformers exhibit noticeably different short-time photodissociation dynamics that suggest that the C-I bond cleavage process is conformation dependent. The axial conformer short-time photodissociation dynamics have larger changes in the carbon-carbon stretch and three carbon atom bending motions as well as the torsional motion about the alpha and beta carbon atom bond. The CCI bending motions for the axial and equatorial conformers of iodocyclopentane as well as previously reported results for the equatorial conformer of iodocyclohexane are significantly smaller than CCI bending motions found for most noncyclic iodoalkanes examined so far. This suggests that the cyclic backbone restricts the initial motion of the C-I bond cleavage along the CCI bend in iodocycloalkanes compared to the noncyclic iodoalkanes. (C) 1999 American Institute of Physics. [S0021-9606(99)00948-4].