The disrete excitation of nonequivalent CH oscillators—a local mode analysis of the high energy overtone spectra of the alkanes

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The overtone spectra of the normal alkanes, propane to n-heptane, and several branched alkanes are measured in the liquid or gas phase from 6000 to 15 000 Å. Each overtone band is principally composed of peaks corresponding to the different CH oscillator types in the molecule CH3, CH2, or CH. The localized character of the overtone bands increases with increasing energy. The relative intensity of the CH2 peak to the CH3 peak correlates with the number of CH2 hydrogens in the molecule. Combination bands involving CH local modes and a lower frequency normal mode are identified. The relative intensities of the components of these bands parallel that of the pure CH-stretching overtone bands. Combination bands between different CH-stretching local modes are also observed and tentatively assigned. Deconvolution of the main overtone bands for ΔvCH=3, 4, and 5 of the normal alkane spectra gives further information about individual local mode peaks, particularly with regard to band shape and bandwidth. The local mode frequencies ω1 and diagonal local mode anharmonicity constants X11 for the molecules are obtained from a local mode analysis of both the observed spectral data and deconvolution results. These two parameters form consistent sets for a given CH oscillator type.