Quantum mechanical characterisation of functional groups for molecular solution theories using Bader fragments
A method for characterising functional groups in organic molecules for use in group contribution molecular theories for liquid mixtures is proposed. It uses a quantum mechanical criterion for electronic charge distribution based on the theory of atoms in molecules developed by Bader, the distribution being established from ab initio quantum mechanical computations. The method was developed by studying the main conformers of linear alkanes from ethane to hexane, in addition to the following linear esters; methyl, ethyl, propyl and butyl formate; methyl, ethyl and propyl acetate; methyl, ethyl and propyl propanoate; and methyl and ethyl butanoate. The proposed criterion allows one not only to identify functional groups but also to account for proximity effects. It was validated by using the Nitta–Chao model to estimate thermodynamic properties of the pure liquids studied (esters and alkanes) and their mixtures. The results were significantly better than those provided by earlier calculations for ester–alkane systems.