Intramolecular easily polarizable N+H ⋯ N hydrogen bonds with NN′-dipiperidylalkane perchlorates. Proton magnetic resonance and infrared investigations

Abstract

Five NN′-dipiperidylalkanes were prepared and their perchlorates were studied in [²H₃]acetonitrile solutions by p.m.r. and i.r. spectroscopy. Structurally symmetrical N⁺H ⋯ N ⇌ N ⋯ H⁺N hydrogen bonds are formed if such formation is not sterically hindered. The symmetry of the potential in these hydrogen bonds makes them easily polarizable. In contrast to results with intermolecular bonds no continuum is, however, observed since, first, the distribution of the hydrogen bond lengths is narrower for intramolecular than for intermolecular hydrogen bonds and secondly, these N⁺H ⋯ N ⇌ N ⋯ H⁺N hydrogen bonds are screened against the local electrical fields and against coupling with low wavenumber vibrations in their environment by the piperidine rings.