Highly excited vibrational states of molecules by thermal lensing spectroscopy and the local mode model. II. Normal, branched, and cycloalkanes

Abstract

The C–H stretching overtones of n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane, n-undecane, n-dodecane, 2-methylbutane, 2-methylpentane, 3-methylpentane, 2,2,4-trimethylpentane, cyclopentane, cyclohexane, methylcyclohexane, cycloheptane, and cyclooctane in the liquid phase are reported as seen by conventional absorption (5600–15 800 cm−1) and thermal lensing (14 700–18 500 cm−1) spectroscopy. A local mode (LM) model based on uncoupled, anharmonic C–H oscillators is used to analyze the observed spectra. Relatively strong, well-resolved absorption peaks are observed and are assigned within the LM model as the ’’pure’’ vibrational overtones of nonequivalent C–H oscillators in the molecule. The relative intensities of the absorption bands assigned to primary and secondary C–H oscillators at a given level of excitation reflect the relative numbers of primary and secondary hydrogens in the molecule. Both LM anharmonicity constants and LM mechanical frequencies are obtained from the overtone analyses. A gradual increase in the bandwidths of the pure LM states with increasing vibrational quantum number is seen for the n-alkanes. The effect which the environment exerts on the C–H bond is demonstrated in the spectra of cyclopentane and cyclohexane, where the absorption of the axial and equatorial C–H bonds is resolved. Relatively less intense bands which appear in the spectra are interpreted within the LM model as combination bands. The partially resolved bands which appear as shoulders on the high-energy side of the ’’pure’’ LM overtones are interpreted as combinations in which the vibrational quanta are distributed over two LM modes. The more prominent combination bands are assigned as combinations of the C–H bending mode with the ’’pure’’ LM C–H stretching mode. Relatively large anharmonic coupling coefficients are obtained from a fit to the energies of this latter type of combination band. The magnitude of these coupling coefficients is thought to indicate the importance of the combination motion in the deactivation of the highly vibrationally excited molecule. Finally, the intensities of the combination bands relative to the pure LM overtones are seen to decrease with increasing vibrational quanta, in accord with the prediction of the LM model.