THE RELATIVE INTENSITY CONTRIBUTIONS OF AXIAL AND EQUATORIAL CH BONDSIN THE LOCAL MODE OVERTONE SPECTRA OF CYCLOHEXANE

Vapor phase room temperature overtone spectra of cyclohexane are measu red with conventional near infrared spectroscopy for the lower overton es, DELTA-upsilon(CH) = 2-4, and with intracavity dye laser photoacous tic spectroscopy for the higher overtones, DELTA-upsilon(CH) = 5-7. Th e relative intensity of the axial to equatorial peak is explained in t erms of the local mode model of harmonically coupled anharmonic oscill ators, where all modes but the two CH-stretching modes are neglected. The dipole moment function is expanded in the two CH-stretching coordi nates, where the expansion coefficients are determined from ab initio molecular orbital calculations. This simple calculation, which contain s no adjustable parameters, can account very well for the observed rel ative intensity of axial and equatorial pure local mode peaks in the o vertone spectra from DELTA-upsilon(CH) = 3-7. We also investigate diff erent ways of obtaining the dipole moment derivatives, and find that a fourth order series expansion around the equilibrium geometry is a go od approximation when calculating the relevant matrix elements.