The relative intensity contributions of axial and equatorial CH bonds in the local mode overtone spectra of cyclohexane

Vapor phase room temperature overtone spectra of cyclohexane are measured with conventional near infrared spectroscopy for the lower overtones, ΔvCH=2–4, and with intracavity dye laser photoacoustic spectroscopy for the higher overtones, ΔvCH=5–7. The relative intensity of the axial to equatorial peak is explained in terms of the local mode model of harmonically coupled anharmonic oscillators, where all modes but the two CH-stretching modes are neglected. The dipole moment function is expanded in the two CH-stretching coordinates, where the expansion coefficients are determined from ab initio molecular orbital calculations. This simple calculation, which contains no adjustable parameters, can account very well for the observed relative intensity of axial and equatorial pure local mode peaks in the overtone spectra from ΔvCH=3–7. We also investigate different ways of obtaining the dipole moment derivatives, and find that a fourth order series expansion around the equilibrium geometry is a good approximation when calculating the relevant matrix elements.