Microsolvation of anions by molecules forming CH··X− hydrogen bonds

Abstract

Various anions were surrounded by n molecules of CF3H, which was used as a prototype CH donor solvent, and the structures and energies studied by M06-2X calculations with a 6-31+G∗∗ basis set. Anions considered included the halides F−, Cl−, Br− and I−, as well as those with multiple proton acceptor sites: CN−, NO3−, HCOO−, CH3COO−, HSO4−, H2PO4−, and anions with higher charges SO42−, HPO42− and PO43−. Well structured cages were formed and the average H-bond energy decreases steadily as the number of surrounding solvent molecules rises, even when n exceeds 6 and the CF3H molecules begin to interact with one another rather than with the central anion. Total binding energies are very nearly proportional to the magnitude of the negative charge on the anion. The free energy of complexation becomes more negative for larger n initially, but then reaches a minimum and begins to rise for larger values of n.