No abstract

Tetramethylsilane (TMS) Spectra were obtained with a Cary 14-spectropholometer. The intensities in the primary C-H LM overtones and the neighboring sidebands of TMS are given and discussed in terms of the Universal cconcept of intensities. (AIP)

A thermo-optical spectrometer is used to resolve absorption peaks due to nonequivalent C–H local mode vibrations and combination bands in several molecules containing aromatic, primary, and secondary hydrogens. Although the bandwidths vary, a constant absorption cross section per C–H is observed for the transitions to the v=3,4,5, and 6 overtone levels. This result stands in contrast to the behavior of the fundamental transitions, where C–H stretching intensities are seen to vary by as much as a factor of four between aromatic and aliphatic C–H bonds.

Absorption spectra of CH3OH, CD3OH, CH3OD, and CD3OD in the region 15 800–17 400 cm−1 are reported as seen by thermal lensing spectroscopy. Dilution studies of the protonated methanols reveal a relatively narrow (FWHM=200 cm−1) absorption peak at 16 500±10 cm−1, which is assigned to the fourth overtone (v=5) of the nonhydrogen-bonded O–H stretching vibration. The energy of this overtone is in good agreement with the predictions made by a simple one dimensional anharmonic oscillator model.

Side bands appear in the local-mode (LM) overtone vibrational spectra of many substances, including the aromatic and aliphatic hydrocarbons. Many of these side bands are interpretable as being LM–LM combination bands of various C–H oscillators in these molecules. The simultaneous excitation of two LM’s represents the most commonly observed transition of the many LM–LM combinations possible. However, such transitions are strictly forbidden as long as the dipole moment and Hamiltonian operators are separable within the LM subspace. The consequences of the breakdown of each of these separabilities is investigated. Application is made to detailed new spectra for benzene (containing the prototypical aromatic C–H oscillator) and tetramethylsilane (containing a prototype of the aliphatic primary C–H oscillator). Multisite terms in the dipole operator are treated empirically and are able to rationalize the intensities of binary LM combination bands. Inter-LM coupling terms in the Hamiltonian are taken from previous ab initio calculations and normal mode analysis. The Hamiltonian is diagonalized within a truncated basis of LM (anharmonic) eigenfunctions. The new eigenfunctions explain not only the appearance of the side bands correctly, but also account for energy splittings in the spectra, particularly conspicuous at low quantum numbers. Separability for both operators is found to be a good zeroth-order approximation for both molecules, but somewhat less so for tetramethylsilane whose methyl substituents contain three LM oscillators sharing a common atom.