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Dimerization and solvent‐assisted proton dislocation in the low‐barrier hydrogen bond of a Mannich base: a low‐temperature NMR study

Using low‐temperature UV, 1H and 15N NMR spectroscopy, we studied the easily polarizable O—H··· N↔Oδ−···H··· Nδ+↔O−··· H—N+ hydrogen bond of the labeled Mannich base[15N] 2‐(N,N‐diethylaminomethyl)‐3,4,6‐trichlorophenol (Cl3MB) dissolved in dichloromethane and in a 2 : 1 mixture of CDF3 and CDClF2. Whereas at high temperature the molecule is present as a monomer exhibiting an intramolecular hydrogen bond, at low temperature two different cyclic hydrogen‐bonded dimers are formed which have been observed for the first time by NMR in the slow hydrogen bond exchange regime. The proton chemical shifts and the scalar 1H, 15N coupling constants indicate a quasi‐symmetric low‐barrier hydrogen bond in the monomer but polar O−··· H—N+ hydrogen bonds exhibiting a considerable charge separation in the case of the dimers. The thermodynamics of the association process were elucidated by UV and NMR spectroscopy. Interestingly, it was found that the addition of methanol does not change substantially the monomer–dimer equilibrium but leads to a considerable shift of the proton in the monomer towards the nitrogen atom. The effect is smaller but also observable for the dimers. In order to assign molecular structures to the observed species, DFT calculations were performed. They reveal a variety of both monomeric and dimeric conformations of Cl3MB exhibiting different hydrogen bond geometries. In particular, two monomeric conformations, bent and planar, were found which exhibit almost the same energy and two proton‐transfer dimeric species of similar energy and different dipole moments. Copyright © 2001 John Wiley & Sons, Ltd.