Proton polarizability of N+ H ? N ? N ? H+ N hydrogen bonds and far-infrared continua of nitrogen-base systems. A fourier-transform infrared spectroscopic study

Abstract

0.2 mol dm–3 HCl, HBr, Hl, HClO4 and HBF4 solutions in pyridine are studied in the far-infrared region. In the case of the first two systems, N+ H ⋯ Cl– and N+ H ⋯ Br–, hydrogen-bond stretching vibrations, νσ, are observed at 208 cm–1 and 148 cm–1, respectively. With the other acids structurally symmetrical N+ H ⋯ N ⇌ N ⋯ H+ N hydrogen bonds with large proton polarizability are formed. With all these systems the hydrogen-bond vibration νσ is found at 137 cm–1. Note that with all these systems the IR continuum caused by these structurally symmetrical hydrogen bonds with proton polarizability extends from higher wavenumbers to only ca. 200 cm–1. Thus, almost all hydrogen bonds in the solutions are already polarized by the local fields to such an extent that the proton transition 00–10 no longer couples with the hydrogen-bond vibration 00–01. In the case of the deuterated systems the 00–10 transition is at lower wavenumbers and thus, with a large number of less strongly polarized hydrogen bonds, the 00–10 and 00–01 transitions couple. Owing to this coupling effect the intensity of the hydrogen-bond vibration is much less. Since the deuterium transition 00–10 is at smaller wavenumbers, and as a result of the coupling effect, the continuum extends toward smaller wavenumbers. Finally, asymmetrical N+1 H ⋯ N2 hydrogen bonds are studied. It is shown how these asymmetrical hydrogen bonds change to symmetrical N+1 H ⋯ N1⇌ N1⋯ H+ N1 bonds with decreasing content of the N2 base.