Intramolecular NH⋯N–⇌–N⋯HN hydrogen bonds with large proton polarizability in monosalts of diamines

Abstract

Five tetrabutylammonium monosalts of diamines were prepared and studied in [²H₃]-acetonitrile solutions by n.m.r. and i.r. spectroscopies. With monosalts of NN′-dimethyl-o-xylyldiamine 1,NN′-dihexyl-o-xylyldiamine 2 and NN′-dimethyl-1,4-diaminobutane 4, strong intramolecular NH⋯N^–⇌^–N⋯HN hydrogen bonds are formed. With compounds 1 and 4 intense continua in the i.r. spectra demonstrate that these bonds show large proton polarizability.

By comparison of the monosalt of NN′-dimethyl-1,3-diaminopropane 3 with compounds 4 and 5 it is shown that these hydrogen bonds are particularly stable if they are present in seven-membered rings. With compound 4, these intramolecular hydrogen bonds are still formed but they are weaker and bent because of steric factors, as shown by molecular models. They still, however, show proton polarizability. With compound 5, these intramolecular hydrogen bonds are no longer formed to a noticeable extent.

By comparison of the i.r. spectra of compounds 1 and 2 it is demonstrated that the interaction of hydrogen bonds having large proton polarizability with their solvent environments is a necessary prerequisite for the i.r. continua, as has been previously postulated theoretically.