Infrared intensities of CH stretching bands in partially deuterated compounds, and effects of conformation and substitution

Abstract

Integrated infrared intensities were measured in the gas phase for the CH stretching bands in the following compounds: C 2H 6, C 2D 5H, CH 3CH 2CH 3, CD 3CD 2CHD 2, (CD 3) 2CHD, (CH 3) 3CH, (CD 3) 3CH, (CH 3) 3CD, (CD 3) 2CHD 2CD, c-C 6D 11H*, c-C 6H 12, CHD 2Cl, CHD 2I, CHD 2CD 2Cl*, CHD 2CHDCl, CH 3CH 2Cl, CHD 2CD 2Br*, CH 3CH 2Br, CHD 2OH*, (CHD 2) 2O* and CHD 2NH 2*. For the asterisked species, separate intensities were determined for each of the two bands seen. The additivity of νCH intensity, in inverse relation to the degree of deuteration of the bonds of a given type, appears to be good. In nearly every case, the undeuterated species has an intensity slightly greater than that predicted from the partially deuterated species. This is useful for checking the internal consistency of the intensity data, and for identifying likely error in earlier data for c-CH 6H 12. Intensities per CH bond in the methyl and methylene groups of acyclic alkanes agree well with the gas phase ones derived indirectly from a series of undeuterated alkanes. Marked effects of conformation on νCH intensities are found in cyclohexane, ethyl halides and in the CH 3O and CH 3N compounds. In the latter the high intensities associated with CH bonds trans to lone pairs of electrons can be understood in terms of the Gussoni—Zerbi electrooptical parameter theory. The lowering effect of halogen on αCH bond intensity in ethyl chloride and bromide is similar to that in the corresponding methyl compounds. Marked, but differing lowering of intensity is also found in the βCH bonds in the methyl groups which are gauche and trans to halogen, respectively. The abundances of conformers of CHD 2O- and CHD 2N- species are significantly affected by zero-point energy differences.