Geometry and spectral properties of the protonated homodimer of pyridine in the liquid and solid states. a combined nmr, x-ray diffraction and inelastic neutron scattering study

The structure and spectral signatures of the protonated homodimer of pyridine in its complex with a poorly coordinating anion have been studied in solution in CDF ₃/CDClF ₂ down to 120 K and in a single crystal. In both phases, the hydrogen bond is asymmetric. In the solution, the proton is involved in a fast reversible transfer that determines the multiplicity of NMR signals and the sign of the primary H/D isotope effect of -0.95 ppm. The proton resonates at 21.73 ppm that is above any value reported in the past and is indicative of a very short hydrogen bond. By combining X-ray diffraction analysis with model computations, the position of the proton in the crystal has been defined as d(N-H) = 1.123 Å and d(H•••N) = 1.532 Å. The same distances have been estimated using a ¹⁵N NMR correlation. The frequency of the protonic out-of-plane bending mode is 822 cm ^-1 in agreement with Novak's correlation.