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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

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posted on 14.07.2011, 00:00 authored by S. Kong, A. O. Borissova, S. B. Lesnichin, M. Hartl, L. L. Daemen, J. Eckert, M. Yu. Antipin, I. G. Shenderovich
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 CDF3/CDClF2 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.