Structure-Forming Complexes in Solutions: 3,5-Dimethylpyrazole–Trifluoroacetic Acid –N,N-Dimethylformamide

IR spectroscopy and quantum chemical methods are applied to study the compositions, structures, and energy parameters of acid-basic complexes formed in three-component solutions of 3,5-dimethylpyrazole (DMP)–trifluoroacetic acid (TFAA)–N,N-dimethylformamide (DMF) at DMP:TFAA molar ratios of 1:1 and 1:2. The DMP·TFAA·DMF trimer is found to be the structure-forming fragment in the 1:1:1 solution, and in 1:1:≥2 solutions, it is the DMP·TFAA·2DMF tetramer. Both complexes have the cyclic structures and contain quasi-ionic pairs in which protons of TFAA molecules move away from them but do not transfer to DMP molecules. Cyclic tetramer DMP·2TFAA·DMF with alternating acid and base molecules is the structure-forming fragment in the DMP–TFAA–DMF triple system with DMP:TFAA = 1:2 in the entire concentration range. It consists of two quasi-ionic pairs with O⋯H⋯O and N⋯H⋯O hydrogen bridges (the TFAA molecule proton passes to the DMP molecule in it). The obtained results can be used to predict the catalytic activity of multicomponent acid solutions.