Structure of complexes in the H2SO4—2-pyrrolidone system as determined by IR-spectroscopy and quantum-chemical calculations

Abstract

Specific features of complexation in solutions of a strong dibasic acid in the H₂SO₄–2-pyrrolidone (Pyr) system (in the range of compositions of 0–100% H₂SO₄) are studied using multiple frustrated total internal reflection IR spectroscopy. The conclusions drawn on the structure of the complexes formed in such solutions are confirmed by quantum-chemical calculations of the mPyr · nH₂SO₄ (m, n = 1, 2) heteroassociates and by comparison of their calculated and measured vibrational spectra. It is found that, in the investigated solutions, four types of acid–base complexes, with various degrees of proton transfer in the OHO bridge, are formed: (AHA)^– anions with quasi-symmetric H-bonds, solvated by acid molecules, or entering into the composition of PyrH⁺ · (AHA)^– ion pairs; quasi-ion pairs with incomplete proton transfer to the base molecule of 1: 1 and 2: 2 compositions; and 2Pyr · H₂SO₄ complexes with two O–H···O bridges of molecular type. The main differences in the mechanisms of the acid–base interactions in the H₂SO₄–Pyr system as compared to the CH₃SO₃H–Pyr system result from the participation of two OH-groups of H₂SO₄ molecule in these interactions. Therefore, two types of quasi-ion pairs and complexes of 2Pyr · H₂SO₄ composition are formed.