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

Abstract Contact with SO 2 causes almost immediate dissolution of tetraalkylammonium halides, R 4 N X , ( R = CH 3 (Me), X = I; R = C 2 H 5 (Et), X = Cl, Br, I; R = C 4 H 9 ( n Bu), X = Cl, Br), with the formation of an adduct, [ R 4 N] + [(SO 2 ) n X ] – ( n = 1–4). Vapor pressure measurements indicate the proclivity for SO 2 uptake follows the order N(CH 3 ) 4 + < N(C 2 H 5 ) 4 + < N(C 4 H 9 ) 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 (SO 2 ) n X – ( X = Cl and Br) anions in the gas phase. The experimental studies show that tetraalkylammonium halides are feasible sorbents for SO 2 . In order to correlate the theoretical model, experimental enthalpy, Δ r H° and entropy, Δ r S° changes have been determined by the van't Hoff method for the binding of one SO 2 molecule to (C 2 H 5 ) 4 NCl, resulting in the liquid adduct (C 2 H 5 ) 4 NCl · SO 2 . The structure of the analogous 1:1 bromide adduct, (C 2 H 5 ) 4 NBr · SO 2 , has been determined by single‐crystal X‐ray diffraction (monoclinic, P 2 1 / c , a = 9.1409(14) Å, b = 12.3790(19) Å, c = 11.3851(17) Å, β = 107.952(2)°, V = 1225.6(3) Å 3 ). The structure consists of discrete alkylammonium cations, bromide anions and SO 2 molecules with short contacts between the anion and SO 2 molecules. The (C 2 H 5 ) 4 N + cationadopts a transoid conformation with D 2 d symmetry, and represents a rare example of a well‐ordered (C 2 H 5 ) 4 N + cation in a crystal structure. The Br – anions and SO 2 molecules forms a chain, (SO 2 Br – ) 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 SO 2 moiety. Raman and 17 O NMR spectra provide compelling evidence for a charge‐transfer interaction between SO 2 molecules and the halide ions.

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