Influence of Aprotic Solvents on the Structure of 3,5-Dimethylpyrazole Complexes with Methanesulfonic Acid in Solutions

The paper presents an IR spectroscopy and a quantum chemical study of the composition, structure, and energy parameters of acid-base complexes formed in three-component solutions 3,5-dimethylpyrazole (DMP) - methanesulfonic acid (MSA) - N,N-dimethylformamide (DMF) and DMP–MSA–acetonitrile (AN) for the DMP:MSA ratios of 1:1 and 1:2. It is shown that the DMP·MSA·DMF trimer performs a structure forming function in DMP–MSA–DMF solutions at DMP:MSA = 1:1, while the DMP·2MSA· ·DMF tetramer with alternating acid and base molecules performs such function at DMP:MSA = 1:2. Both complexes have a cyclic structure. In the former, a DMF molecule forms a O⋯H–N bridge with a DMP molecule, and in the latter a quasi-ionic O⋯H⋯O bond is formed due to the participation of a carbonyl group of the DMF molecule and a hydroxyl group of the MSA molecule. In DMP–MSA–AN solutions, the added acetonitrile is a neutral component of the system: it does not affect the composition and structure of 2DMP·2MSA and DMP·2MSA complexes, but forms their molecular environment. The obtained results allow predicting the change of acid-catalytic activity of solutions at high reagent or product concentrations.