Reversible SO2 Uptake by Tetraalkylammonium Halides: Energetics and Structural Aspects of Adduct Formation Between SO2 and Halide Ions

Abstract

Contact with SO2 causes almost immediate dissolution of tetraalkylammonium halides, R4NX, (R = CH3 (Me), X = I; R = C2H5 (Et), X = Cl, Br, I; R = C4H9 (nBu), X = Cl, Br), with the formation of an adduct, [R4N]+[(SO2)nX]– (n = 1–4). Vapor pressure measurements indicate the proclivity for SO2 uptake follows the order N(CH3)4+ < N(C2H5)4+ < N(C4H9)4+. This trend is in accord with the Jenkins–Passmore volume-based thermodynamic model. Born–Haber cycles, incorporating the lattice energy and gas phase energy terms, are used to evaluate the energetic feasibility of reactions. Density functional theory calculations (B3PW91; 6-311+G(3df)) have been used to calculate the energetics of (SO2)nX– (X = Cl and Br) anions in the gas phase. The experimental studies show that tetraalkylammonium halides are feasible sorbents for SO2. In order to correlate the theoretical model, experimental enthalpy, ΔrH° and entropy, ΔrS° changes have been determined by the van't Hoff method for the binding of one SO2 molecule to (C2H5)4NCl, resulting in the liquid adduct (C2H5)4NCl·SO2. The structure of the analogous 1:1 bromide adduct, (C2H5)4NBr·SO2, has been determined by single-crystal X-ray diffraction (monoclinic, P21/c, a = 9.1409(14) Å, b = 12.3790(19) Å, c = 11.3851(17) Å, β = 107.952(2)°, V = 1225.6(3) Å3). The structure consists of discrete alkylammonium cations, bromide anions and SO2 molecules with short contacts between the anion and SO2 molecules. The (C2H5)4N+ cationadopts a transoid conformation with D2d symmetry, and represents a rare example of a well-ordered (C2H5)4N+ cation in a crystal structure. The Br– anions and SO2 molecules forms a chain, (SO2Br–)n, with bifurcated contacts. Non-bonding electron pairs on the halide anions engage in electrostatic interactions with the sulfur atoms and charge-transfer interactions with the antibonding S–O orbitals of the bound SO2 moiety. Raman and 17O NMR spectra provide compelling evidence for a charge-transfer interaction between SO2 molecules and the halide ions.