Spectroscopic evidence for an intramolecular C–H⋯N hydrogen bond: infrared and Raman spectroscopy and ab initio molecular orbital calculations of N, N-dimethylpropylamine and propyltrimethylammonium bromide

Abstract

The infrared and Raman spectra were measured for N, N-dimethylpropylamine (DPA) in an argon matrix, the liquid and glassy states and aqueous solution, and for propyltrimethylammonium bromide (PTAB) in the solid state and aqueous solution. The ab initio molecular orbital calculations were performed on possible conformers of DPA and propyltrimethylammonium ion. In an argon matrix, the conformational stability of DPA is in the order G′G>GT>TT, which agrees with the prediction by ab initio calculations. The experimentally confirmed high stability of the G′G conformer indicates that an appreciably strong attractive interaction occurs between the methyl hydrogen atom and the nitrogen atom. This interaction may be called the intramolecular 1,4-C–H⋯N hydrogen bonding. In the liquid and glassy states, the GT conformer is the most stable. In aqueous solution of DPA, the GT and TT conformers are more stabilized than in the neat liquid, possibly because of the formation of more significant intermolecular hydrogen bonds between the nitrogen atom of DPA and water molecules. For PTAB, only the T conformer exists in the solid state, and this conformer is more stable than the G conformer in aqueous solution. The GG and TG conformers of DPA and the G conformer of PTAB are significantly less stabilized by steric repulsions between the CH 3(N) and (C)CH 3 groups.