Spectroscopy of the C–H Stretching Vibrational Band in Selected Organic Molecules

Abstract

The vibrational spectroscopy of C-H stretches in organic molecules is of considerable importance for the characterization of these systems and for exploration of their properties. These stretches are strongly anharmonic, and thus methods including anharmonicity have to be used. The vibrational self-consistent field (VSCF) is applied to the following organic compounds: acetone, dimethylacetylene, neopentane, toluene, ethylene, and cyclopropane. The computed spectra are compared to new experimental data, including Raman measurements of all molecules except cyclopropane and IR of acetone, neopentane, and ethylene. A high level of agreement is found for all of the molecules. The characteristic features of CH3 and CH2 groups are studied and analyzed in detail. A reliable, unambiguous assignment of vibrational modes to spectral peaks is provided. Several characteristic features of CH3 and CH2 vibrations in polyatomic molecules are clarified, providing easier assignments for different types of organic molecules.

MIR and NIR group spectra of n-alkanes and 1-chloroalkanes

“…1a) appear four peaks assigned to the symmetric stretching vibrations of CH 2 at 2860 cm -1 and CH 3 at 2873 cm -1 , and the corresponding asymmetric vibrations of CH 2 at 2922 cm -1 and CH 3 at 2958 cm -1 . The bands due to the methyl groups appear at the same positions, while the methylene bands are more or less shifted, confirming previous observations [6,7]. This shift suggests that the CH 2 band consists of at least two components located at slightly different positions, however in the raw spectra these components are not resolved.…”

“…Snyder et al evidenced that the methylene band of the crystalline polyethylene (at 2922 cm -1 ) was affected by the chain conformation, whereas the methyl band (at 2938 cm -1 ) was influenced by the solvent [6]. Another conclusion was obtained by Šebek et al from the studies of the CH stretching band in selected organic molecules in the liquid and gas phase [7]. It was shown that the CH 3 symmetric and asymmetric frequencies are almost independent on the surroundings and the corresponding bands always appear at the same position.…”

New spectral assignment of n‐propanol in the C―H stretching region

“…However, it is still a challenge to predict an accurate spectrum in the C―H stretching region because of large anharmonic effect and Fermi resonance interaction, which make the comparison between theoretical and experimental spectra difficult. To establish a direct contrast with the observed spectrum, several research groups recently developed theoretical models to simulate spectral details in the C―H stretching region . For example, Gerber group developed anharmonic vibrational self‐consistent field method and the C―H stretching spectra were well reproduced without any scaling for a serial of organic molecules, such as long chain hydrocarbons, dodecane .…”

“…To establish a direct contrast with the observed spectrum, several research groups recently developed theoretical models to simulate spectral details in the C―H stretching region . For example, Gerber group developed anharmonic vibrational self‐consistent field method and the C―H stretching spectra were well reproduced without any scaling for a serial of organic molecules, such as long chain hydrocarbons, dodecane . Based on a local mode Hamiltonian that incorporates cubic stretch–bend coupling, Sibert group recently developed a first‐principle model for accurately describing the Fermi resonance coupling in the alkyl C―H stretching region …”

“…In this work, we examine the spectra of another important alcohol molecule, n‐propanol, in the C―H stretching region, as part of a program designated to provide reliable spectral groundwork for alcohols and better develop the C―H vibration as a probe of molecular structures. This is an area of research continuing to receive considerable interest …”