CH stretching local mode behavior in alkanes: A global analysis of mono- and fully hydrogenated ethanes, propanes, and butanes up to 6 quanta of CH stretching

Abstract

Existing vibrational data for monohydrogenated C2 to C4 alkanes up to 4 quanta in CH stretching, and data at 6 quanta of excitation in the fully hydrogenated species, are combined in this study, along with CH deformation overtones and CD2 (or CD) stretching fundamentals, to enable a global local mode refinement of CH stretching to be made, to include the effects of both Fermi resonance and interbond coupling effects. Refinement to data over 14 isotopic ethanes, propanes and isobutanes is achieved in terms of consistent CH stretching vibration and anharmonicity parameters and stretch–bend resonance parameters. Data available on n-butanes fully support the findings. CH stretching vibration wave numbers correlate very closely with the structural characteristics in CH3, CH2, and CH groups. The associated CH stretching anharmonicities, however, are found to correlate with the type of CH bond present, primary (−59.6 cm−1), secondary (−62.1 cm−1), or tertiary (−64.5 cm−1), rather than with the individual CH stretching vibration wave numbers, as previously thought. A vibrational dependence of the Fermi resonance parameter is required to reproduce the isolated CH stretching manifolds of CH(CD3)3 and CH(CH3)3 up to 6 quanta in CH stretching.