Sulfur dioxide–benzene association. Second virial cross coefficients for sulfur dioxide–benzene and sulfur dioxide–cyclohexane derived from gas phase excess enthalpy measurements

Abstract

A flow mixing calorimeter has been used to measure the excess molar enthalpy H^E_m of gaseous (sulfur dioxide + benzene) and (sulfur dioxide + cyclohexane) at the mole fraction y = 0.5, at standard atmospheric pressure, and over the temperature range 363.15 K to 473.15 K. The measurements were analysed in terms of the virial equation of state. The cross coefficient B₁₂ was calculated using the arithmetic mean rule for collision diameters and the rule ε₁₂ = (1 − k₁₂)(ε₁₁ε₂₂)^1/2 for the depth of the potential well. The H^E_m measurements on (sulfur dioxide + cyclohexane) were fitted to within experimental error by the choice (1 − k₁₂) = 1.01 but this value did not fit the measurements on (sulfur dioxide + benzene). The difference was attributed to a specific interaction which was analysed in terms of a quasi-chemical association model in which B₁₂ = B^ns₁₂ − (RTK₁₂)/2. The non-specific term B^ns₁₂ was calculated using (1 − k₁₂) = 1.01, and values of the equilibrium constant K₁₂ were determined from the difference between the calculated and experimental excess enthalpies. A plot of ln K₁₂ against reciprocal temperature yielded an enthalpy of formation ΔH₁₂ of the sulfur dioxide–benzene complex and this was found to be −(8.7 ± 2) kJ mol⁻¹. The sum of the specific and non-specific contributions is −(13.8 ± 3) kJ mol⁻¹. As this energy is an average taken over all orientations it should be less than the binding energy of the van der Waals complex in the minimum energy configuration computed by ab initio methods. Taleb-Bendiab et al. (A. Taleb-Bendiab, K. W. Hillig and R. L. Kuczkowski, J. Chem. Phys., 1992, 97, 2996) computed an ab initio value of −12 kJ mol⁻¹, which is smaller than our value. Grover et al. (J. R. Grover, E. A. Walters, J. K. Newman and M. G. White, J. Am. Chem. Soc, 1985, 107, 7329) used photoionisation spectroscopy and obtained −(16.7 ± 0.8) kJ mol⁻¹, which is in better agreement.