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

Abstract

− · · ·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 Cl 3 MB 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.

Polarized IR spectra of resonance assisted hydrogen bond (RAHB) in 2-hydroxyazobenzenes

“…For Mannich bases, as follows from detailed studies [42], dimerization leads to a drastic change of the potential shape, i.e. to the formation of the polar proton transfer state.…”

A review with comprehensive data on experimental indirect scalar NMR spin–spin coupling constants across hydrogen bonds

“…[58] Biological molecules and related compounds (Table 21). [59] Note that the situation depicted for 26 is intramolecular (Section 2) but we have described it here because those of 27 and 28 are intermolecular. low temperature were used for these experiments; it appears that all coupling constants decrease upon complex formation.…”

Regulation of Transcription by Synthetic DNA‐Bending Agents

“…More in detail, we are analysing the variation of the dimerization of BTZ exerted by the presence of the amines. Indeed, the second binding constant as shown in Table 2 for all the complexes can be seen as the formation of a BTZ dimer in which one of the two BTZ molecules is involved in an ‘extremely polarized’ version of an hydrogen bond, which is sometimes referred to as zwitterionic hydrogen bond (Scheme ) 65–67. The second series of binding constants, corresponding to the binding of BTZ to the benzotriazolate–ammonium complex, show some variation between the three titrations, with an average value of K 2 = 66 ± 8 M −1 .…”