Structure of Diethylamine Complexes with Methanesulfonic Acid in Solutions According to IR Spectroscopy Data and Quantum Chemical Calculations

Abstract

Structural and energy parameters and existence regions of hydrogen-bonded complexes determining the properties of solutions in the diethylamine (DEA)–methanesulfonic acid (MSA) system are established by IR spectroscopy and quantum chemistry. Four concentration–structural zones can be distinguished in the component ratio ranging from 0:1 to 1:1. The first zone is pure MSA (which, as shown previously, contains 3MSA complexes and small amounts of 2MSA and 4MSA complexes). In the second zone (from 0:1 to 1:3), DEA·3MSA complexes are formed in addition to MSA self-associates. In the remaining zones, two types of structural solution species coexist. In the third zone (from 1:3 to 1:2), the observed complexes are DEA·3MSA and DEA·2MSA, and those in the fourth zone (from 1:2 to 1:1) are DEA·2MSA and 2DEA·2MSA. All mDEA·nMSA complexes (m = 1-2, n = 1-4) include \(\text{C}{{\text{H}}_{3}}\text{SO}_{3}^{-}\cdot {{({{\text{C}}_{2}}{{\text{H}}_{5}})}_{2}}\text{NH}_{2}^{+}\) contact ion pairs serving as structure-forming and stabilizing elements of DEA solutions in MSA. The most energetically favorable complex, 2DEA·2MSA, consists of two ion pairs interacting with each other in such a way that all four of its N–H⋯O bridges are equivalent. The DEA–MSA and TEA–MSA systems exhibit similarities in their complexation processes. The obtained results complement the general scheme of acid–base interactions in acid solutions and expand the database for predicting the catalytic activity of acid–base systems of arbitrary composition.