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

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 = 14). 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 JenkinsPassmore volume-based thermodynamic model. BornHaber 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, ?rH degrees and entropy, ?rS degrees 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) angstrom, b = 12.3790(19) angstrom, c = 11.3851(17) angstrom, beta = 107.952(2)degrees, V = 1225.6(3) angstrom 3). 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 SO 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.