Methods for calculating solvent enthalpy of vaporisation values by a molecular modelling technique — effect of charge and method

Abstract

The enthalpy of vaporisation (Δ H vap) of the solvents methane, ethane, butane, chloromethane, chloroform, carbon tetrachloride, carbon disulphide, cyclohexane, and ethyl acetate have been calculated by a free space molecular modelling method. This technique is based on evaluating contact energies and co-ordination numbers using two commercial software packages (Polygraf™ and Cerius 2™). The total potential energy of the solvent is calculated by the use of the Dreiding II semi-empirical force field where non-bonded van der Waals interactions are modelled using a Lennard-Jone 12-6 potential. A bulk solvent (an isolated solvent shell with a co-ordination number of approximately 12) surrounding a central solvent molecule is created and the total energy is calculated. Re-calculating without the central molecule yields the solvent–solvent interaction energy and hence Δ H vap. The contact energy is pair-wise and additive and can be compared with literature values for Δ H vap. Three methods for generating solvent shells and a non-shell method are compared. For one method, the effect of charge generation is studied. The Δ H vap values calculated, show large discrepancies between the different shell methods used. However, values obtained by different charge generation methods are relatively similar. The coulombic term is therefore considered unimportant in the energy terms for the solvents studied here. Rather, the results show that the enthalpy of vaporisation is very sensitive to the co-ordination number, accompanying contact energy and spatial separation. The conclusion is, that because of the uncertainty in the co-ordination numbers, Δ H vap can only be estimated up to an accuracy of ∼20%.