Investigation of the Franck-Condon region photodissociation dynamics of linear and cyclic nitroalkanes using resonance Raman spectroscopy

Abstract

of thesis entitled INVESTIGATION OF THE FRANCK-CONDON REGION PHOTODISSOCIATION DYNAMICS OF LINEAR AND CYCLIC NITROALKANES USING RESONANCE RAMAN SPECTROSCOPY submitted by Mei Sze HUNG for the degree of Master of Philosophy at the University of Hong Kong in June 1998 We have obtained B-band resonance Raman spectra of nitroethane, 1nitropropane,nitrocyclopentane and nitrocyclohexane in the solution phase (cyclohexane solution) at excitation wavelengths of 204.2 nm and 217.8 nm and in the vapour phase at an excitation wavelength of 217.8 nm. For nitrocyclopentane and nitrocyclohexane, absolute resonance Raman cross section measurements have also been obtained. The B-band resonance Raman spectra have most of their intensity in the nominal NO2 symmetric stretch progression (nvN02). The initial short-time (femtosecond) photodissociation dynamics of the excited electronic state associated with the B-band absorption change mainly in the two N ~ 0 bond lengths in the Franck-Condon region. The absorption spectra and the resonance Raman intensities were simulated using timedependent wavepacket calculations and a simple model. These calculations suggest that very fast electronic dephasing (most likely due to predissociation and/or solvent collisions) competes with wavepacket motion out of the Franck-Condon region and/or there is a significant change in the transition dipole moment along the nominal NO2 symmetric stretch coordinate, that must be taken into account to correctly model the higher overtone resonance Raman intensities. Since the resonance Raman intensity patterns and calculation parameters are very similar for gas and solution phase nitroethane, 1nitropropane,nitrocyclopentane and nitrocyclohexane, electronic dephasing due to solvent collisions does not appear to be a significant factor in determining the higher resonance Raman overtones. Moreover, this probably suggests that there are very small substituent effects on the B-band resonance Raman spectra of nitroalkanes Comparisons between Bband resonance Raman spectra and short-time photodissociation dynamics of nitroalkanes with A-band resonance Raman spectra and short-time phtodissociation dynamics of iodoalkanes are also presented.