Environmental influences on the formation and on the proton potentials of easily polarizable N+H ⋯ N ⇌ N ⋯ H+N hydrogen bonds

Abstract

The structurally symmetrical N⁺H ⋯ N ⇌ N ⋯ H⁺N bond in pyridinium + pyridine and in o-xylyldiamine hydroperchlorate is studied in different organic solvents, using i.r. and n.m.r. spectroscopy. With the pyridinium + pyridine solutions, the complex-formation constants are determined from the chemical shift of the NH⁺ proton signals as function of the base concentration, as well as from three different i.r. bands. The results of both methods show very good agreement. The complex-formation constant increases in the series of solvents: acetone, nitriles and nitroalkanes. This observation is explained by decreasing interaction of the free pyridinium with the different solvents.

The proton polarizability of these hydrogen bonds is very great. Thus, according to theory, the proton potentials should be strongly deformed in solution. The pyridine bands show, however, that the delocalization of the proton in the N⁺H ⋯ N ⇌ N ⋯ H⁺N bonds is still so large that it influences both pyridine molecules to the same extent. This is in good agreement with the fact that in all systems intense i.r. continua are observed, indicating large proton polarizability due to delocalisation of the proton in the hydrogen bonds.