MECHANISM ACID-BASE INTERACTIONS AND STRUCTURES OF COMPLEXES IN 3,5-DIMETHYLPYRAZOLE SOLUTIONS IN METHANESULFONIC AND TRIFLUOROACETIC ACIDS

Abstract: Structural and energy parameters of hydrogen-bonded 3,5-dimethylpyrazole (DMP) complexes in methanesulfonic acid (MSA) and trifluoroacetic acid (TFAA) solutions of 1:1 and 1:2 compositions are determined by IR spectroscopy and quantum chemistry methods. It is found that DMP complexation patterns with strong acids in DMP–MSA and DMP–TFAA systems are practically identical. The key stage of the mechanism of acid-base interactions in them is the proton transfer of the acid molecule (HA) to the nitrogen atom of the base molecule. The А anions contained in the composition of hydrogen-bonded ionic pairs containing N–H⋯O bridges are formed here. Complexes consisting of two ionic pairs are shown to be present in DMP–MSA and DMP–TFAA 1:1 solutions while the 1:2 solutions contain complexes representing an ionic pair solvated by the acid molecules. Structural features of DMP complexes with strong acids are due to the presence of the proton-donor NH group in the base molecule, which results in the formation of additional N–H⋯O bridges. The conditions are determined under which either only homoconjugated (А⋯Н⋯А) anions with strong quasi-symmetric Н-bonds or only А anions are formed in the solutions, which clarifies and supplements the general scheme of acid-base interactions in the solutions. [Figure not available: see fulltext.].