Inequivalent C–H oscillators of gaseous alkanes and alkenes in laser photoacoustic overtone spectroscopy

The overtone spectra of the C–H stretching vibrations of several gaseous alkanes and alkenes were observed using laser photoacoustic spectroscopy. Resolvable peaks are seen for each chemically or sterically equivalent C–H bond and are assigned using the local mode model. The fifth overtone transition energies decrease linearly with increasing C–H bond lengths. Spectral shifts corresponding to 0.001 Å bond length differences are observed. Linear correlations of bond length and anharmonicity with fundamental C–H stretching frequency allow a Morse potential for CH bonds to be defined in terms of a single parameter. The integrated cross section per C–H oscillator for the fifth overtone spectra varied mostly within a factor of 2 about the average value of (1.08±0.28)×10−23 cm2 cm−1.