CHD2 spectra and Fermi resonance effects in the CH3 and CD3 stretching regions. Symmetrical CH3 groups

Abstract

Abstract The use of CH stretching frequencies in CHD2-substituted methyl compounds is proposed as an approximate method for estimating the unperturbed positions of CH3 stretching bands affected by Fermi resonances involving CH3 deformation overtones and combinations. A single unperturbed CH3 stretching frequency is also required. With additional assumptions, positions of CD3 stretching nodes can also be predicted. Predictions of the unperturbed positions of νCH(sym) are made for the following compounds containing symmetrical methyl groups for which CHD2 data are available: CH3-F, Cl, Br, I, CN3, CCH: CH4 and C2H6: M(CH3)4 where M = C, Si, Ge, Sn, Pb. Alternative means of assessing the degree of Fermi resonance are reviewed, including use of infrared intensities, change of environment, heavy isotope shifts, high resolution studies and anharmonicity. The scanty evidence provided is in fair agreement with the results from the CHD2 method. The latter is recommended as an indispensable tool in understanding the vibrational spectra of all methyl compounds.