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 (15)N NMR correlation. The frequency of the protonic out-of-plane bending mode is 822 cm(-1) in agreement with Novak's correlation.

The properties of the intramolecular hydrogen bonds of doubly 15N‐labeled protonated sponges of the 1,8‐bis(dimethylamino)naphthalene (DMANH+) type have been studied as a function of the solvent, counteranion, and temperature using low‐temperature NMR spectroscopy. Information about the hydrogen‐bond symmetries was obtained by the analysis of the chemical shifts δH and δN and the scalar coupling constants J(N,N), J(N,H), J(H,N) of the 15NH15N hydrogen bonds. Whereas the individual couplings J(N,H) and J(H,N) were averaged by a fast intramolecular proton tautomerism between two forms, it is shown that the sum |J(N,H)+J(H,N)| generally represents a measure of the hydrogen‐bond strength in a similar way to δH and J(N,N). The NMR spectroscopic parameters of DMANH+ and of 4‐nitro‐DMANH+ are independent of the anion in the case of CD3CN, which indicates ion‐pair dissociation in this solvent. By contrast, studies using CD2Cl2, [D8]toluene as well as the freon mixture CDF3/CDF2Cl, which is liquid down to 100 K, revealed an influence of temperature and of the counteranions. Whereas a small counteranion such as trifluoroacetate perturbed the hydrogen bond, the large noncoordinating anion tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate B[{C6H3(CF3)2}4]− (BARF−), which exhibits a delocalized charge, made the hydrogen bond more symmetric. Lowering the temperature led to a similar symmetrization, an effect that is discussed in terms of solvent ordering at low temperature and differential solvent order/disorder at high temperatures. By contrast, toluene molecules that are ordered around the cation led to typical high‐field shifts of the hydrogen‐bonded proton as well as of those bound to carbon, an effect that is absent in the case of neutral NHN chelates.

The structure and spectroscopic properties of the 1:1 complexes of substituted pyridines with benzoic acid and phenol derivatives in aprotic solvents are studied using B3LYP functional combined with the polarizable continuum model approximation. Two extreme structures are investigated: the state without (HB) and with proton transfer (PT). In the presence of an external electric field the O...N distance is contracted and the PT state does appear. The PT state of both the pyridine-benzoic and the pyridine-phenol complexes displays the only IR-active band in the 2800-1800 frequency region, which is located around 2000 cm(-1). However, the nature of the band is different for these two complexes. In the pyridine-benzoic acid complex it is practically a pure stretching vibration of the HN(+) group, while in the pyridine-phenol complex it is the mixed vibration of the bridging proton. A specific feature of the PT state in the pyridine-phenol complex is an IR-intensive band near 600 cm(-1), associated with the asymmetric stretching vibrations of the O(-)...HN(+) fragment. Its intensity is reciprocally proportional to the O...N distance. The appearance of this band provides an efficient criterion to differentiate between the HB and PT states of the 1:1 complexes of phenols with pyridines in aprotic solvents.

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