CH stretching overtone spectra of trimethyl amine and dimethyl sulfide

Trimethyl amine (TMA) exhibits the largest known difference in CH bond lengths within a methyl group, due to what is known as the lone pair trans effect. Dimethyl sulfide also exhibits this effect, but to a far lesser extent, making it ideal for comparison to TMA. In this paper, the first through fourth overtone spectra of N(CH3)(3), N(CD3)(3), N(CD2HKD3)(2), N(CH3)(CD3)(2), N(CD3)(CH3)(2) and S(CH3)(2) are reported and all major bands are assigned. The intensities of the observed bands are compared to intensities predicted by the harmonically coupled anharmonic oscillator local mode model. Good correlation is found between the experimental intensities and those predicted with the local mode model and HF/6-311++G(2d,2p) calculated dipole moment functions. An increase in the ability to resolve peaks as methyl groups are deuterated suggests that the lone pair mediates increased coupling between methyl groups. (C) 2003 Elsevier B.V. All rights reserved.