Structures of proton solvates in the HCl-H2O-(CH3)2NCHO system as determined by IR spectroscopy and quantum-chemical calculations

Abstract

The structures of proton solvates in the HCl-H2O-(CH3)2NCHO (DMFA) system at H2O: DMFA ratios ranging from 1: 1 to 21: 1 are studied by the IR spectroscopy method. It is demonstrated that H2O⋯H+⋯OH2 ions and (CH3)2NCHO⋯H+⋯OH2 mixed solvates with a strong quasi-symmetrical hydrogen bond are formed in solutions. With an increase in the DMFA concentration, the fraction of H5O2+ ions decreases. At HCl: H2O ≥ 1: 3 and arbitrary DMFA concentrations, only mixed proton solvates are formed. The continuous absorption coefficients for the (CH3)2NCHO⋯H+⋯OH2 ions are determined. The results obtained are compared with the results of quantum-chemical calculations of the structure and relative stability of the (DMFA)mH+(H2O)n (m = 0–2, n = 0–3) positively charged complexes which were performed by the B3LYP/6-31++G(d,p) DFT method. We identified 19 stable configurations with chain, cyclic, and branched structures. Most of these configurations contain the (CH3)2NCHO⋯H+⋯OH2 fragment. The parameters of the O⋯H+⋯O bridge show that some configurations have a strong quasi-symmetrical hydrogen bond. In some cases, the proton is located between two DMFA molecules. The H2O⋯H+⋯OH2 bridge is observed in none of the stable configurations of the (DMFA)mH+(H2O)n (m ≠ 0) complexes.