Structure of the Complexes of Hydrochloric and Methanesulfonic Acids with N,N-Dimethylformamide, N-Methylformamide, and N,N-Dimethylacetamide

Abstract 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.