Influence of stereochemistry, screening, and deuteration of long and short intramolecular easily polarizable hydrogen bonds on their infrared continua

Abstract

HClO4 salts of N,N′-tetramethylalkanes (compounds 1–6) and of N,N′-tetraalkyl-o-xylildiamines (compounds 7–11) and HAuCl4 salts of N,N′-tetramethyl-o-xylildiamine-di-N-oxide (compound 12) and of o-xylildipiperidyl-di-N-oxides (compounds 13–14) were studied by nmr and ir spectroscopy. It is shown that in the compounds 3, 4, 5, and 7–11, long intramolecular [Formula: see text] hydrogen bonds with large proton polarizability and in compounds 12–14, short intramolecular [Formula: see text] hydrogen bonds with large proton polarizability are formed. The study of the compounds 1–6 shows that the most favorable conditions for the formation of the intramolecular easily polarizable hydrogen bonds occur with compound 4, i.e., if these hydrogen bonds are present in the 7-membered rings. In the case of compounds 7–14 it is shown that the continua usually observed with easily polarizable hydrogen bonds vanish when these bonds become screened from their solvent environments. Then, instead of continua only bands of the fundamental transitions are observed, with the long [Formula: see text] bonds in the region 2800–1800 cm−1 and with the short [Formula: see text] bonds in the region 1200–850 cm−1. This result demonstrates that for the occurrence of the ir continua, the interaction of the easily polarizable hydrogen bonds with their solvent environments is absolutely necessary. Furthermore, it is shown with the long as well as the short hydrogen bonds that the intensity of the continua decreases strongly with deuteration, demonstrating that in both cases the deuteron polarizabilities are much smaller than the proton polarizabilities of the hydrogen or deuterium bonds, respectively. From this observation it is concluded that the proton potentials of these structurally symmetrical hydrogen bonds are considerably deformed by their molecular surroundings.