The structure, energy parameters, and spectral manifestations of quasi-ion pairs (uncharged A⋯H⋯B complexes with incomplete proton transfer from the acid molecule (HA) to the base molecule (B)) in solutions of hydrochloric and methanesulfonic (MSA) acids in N,N-dimethylformamide, N-methylformamide, and N,N-dimethylacetamide are determined by IR spectroscopy and quantum chemical methods. It is shown that quasi-ion pairs, which are usually formed in solutions with a solvent excess, are fragments of stable hydrogen-bonded 2A·2B cyclic complexes whose structure changes little under the influence of solvating B molecules. It is established that the position of protons in the Cl⋯H⋯O bridges of 2HCl·2B tetramers and in O⋯H⋯O bridges of 2MSA·2B tetramers is determined by the following factors: acid strength, proton affinity of the amide molecule, mutual cooperative influence of molecules via additional hydrogen bonds, and density of the molecular packing in the complex.
Maiorov et al. (Sun,) studied this question.