Some regularities of vibrational spectra of a weak hydrogen bond: cooperative and `anticooperativeʹ effects within the framework of an electrostatic model Original Research Article

The empirical regularities of spectra of a weak hydrogen bond were studied within the framework of an electrostatic model. For a number of trimers of a L⋯AH⋯B (I) (where L is a positively charged ligand) type, arranged in the order of increasing of B basicity, it was revealed that the A–H stretching low-frequency shift Δν12 exceeds the Δν11 shift in dimers of a A–H⋯B type and grows linearly with its increase. The effect was stipulated by a trimolecular cooperative interaction. A semi-quantitative interpretation of this effect in terms of electrostatics was proposed earlier. For trimers of the other type, such as L⋯DAH⋯B (II) with L=B and negatively charged L (DAH is a semi-deuterated molecule, e.g., DOH), the Δν12 AH frequency shift is, in contrast, less than Δν11 and, also, increases linearly with the Δν11 growth. By analogy with the previous case, this effect was named `anticooperativeʹ in the literature. In the current work, it is treated in terms of the earlier proposed model. It was shown that, actually, the effect in complex II is caused by a pairwise (Coulomb) interaction of residual atomic charges. The repulsion between the ligand L and the atom A is of a great importance. The cooperative effect is insufficient and may be neglected. The asymmetrical trimers of a B⋯HAD⋯L (L≠B) type have been also considered. The semi-empirical procedure to define the Δν12 AH frequency shift for any arbitrary asymmetrical trimer through the known value of the Δν11 shift for respective dimers was suggested. The molecules of a solvent are considered as potential quasi-ligands.