SOLVENT EFFECTS ON STRONG CHARGE-TRANSFER COMPLEXES .I. TRIMETHYLAMINE AND SULFUR DIOXIDE IN GAS AND IN HEPTANE

Ultraviolet spectra and thermodynamic data have been obtained for the 1:1 donor-acceptor complex of trimethylamine and sulfur dioxide in the gas phase and in heptane solutions. The prominent spectral feature is the charge-transfer band of the complex, with maximum absorbance at 276 mµ in gas, 273 mµ in heptane, and 258 mµ dichloromethane. Values are given for the equilibrium constant, enthalpy, and entropy of the formation reaction in gas and in heptane, and for individual energies and free energies of interaction of donor, acceptor, and complex with the solvent heptane. For the first time, the energy and free energy of sublimation of a charge-transfer complex (without dissociation) are reported. The complex is stabilized by transfer from gas to heptane to dichloromethane, as evidenced by increasing values of the equilibrium constant and – ΔH° for the formation reaction (K298 = 340 1. mole-1 in the gas and 2550 1. mole-1 in heptane; -ΔH° = 9.7 kcal/mole in gas and 11.0 kcal/mole in heptane). Explanations are offered for the unusual effects of solvent on the stability of the complex and the frequency of the ultraviolet absorption band. A general method is proposed for predicting the effect of variation in solvent properties on the thermodynamic functions for charge-transfer formation reactions. © 1968, American Chemical Society. All rights reserved.