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, ν_σ, 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 ν_σ 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.