Overtone spectra of C–H oscillators in cold molecules

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Abstract
The C–H stretch overtone spectra of methane (5–0), ethylene (5–0 and 6–0), ethane (5–0 and 6–0), propyne (4–0 and 5–0 acetylenic and 5–0 methyl C–H stretches), allene (5–0), propane (5–0 and 6–0), cyclopropane (5–0 and 6–0), dimethyl ether (5–0), and isobutane (5–0) have been recorded at temperatures between 143 and 189 K, depending on the molecule. A comparison is made to the spectra obtained at room temperature, with the goal of improved understanding of the band shapes. The temperature dependence of most of the observed bands is found to be significantly less than that expected for ‘‘simple’’ bands. For these small to medium size hydrocarbons, the temperature independence of the overtone bands is found to correlate loosely with the density of states and with the degree of saturation. Other factors are important determinants of spectral widths and temperature independence as well, such as conformational inequivalence of the C–H oscillators, and the number and positions of the oscillators. It is concluded that the vast majority of hydrocarbon C–H stretch high overtone bands have upper states which are extensively mixed with other states. This is the case even for most of the relatively small hydrocarbons. This mixing produces a broadening effect and greatly increases the transition density, thereby diluting the oscillator strength of the rovibrational transitions from that of the zero-order approximation. The Fermi resonance type of interaction appears to be of greater importance than the Coriolis type in determining the appearance of the high overtone bands.