The influence of halogens and calcogens on four carbons normal chains as simple models for the perfluorooctanoate and related ions

Abstract

Quantum mechanical calculations at HF/6-31+G level of theory were done to investigate the influence of the substitution of hydrogen by halogen atoms in amphiphilic molecules constituted by four-carbon normal chain ions as well as to investigate the effects of replacing the oxygen atoms at the polar head of the ions by sulfur and selenium atoms. In particular this work considers the following nine four-carbon chain ions: C 3X 7CO 2 −, C 3X 7CS 2 − (with X=H, F, Cl, Br) and C 3H 7CSe 2 −. While for the hydrogenate compounds the chain is almost linear and flexible others substitution lead a helicity of the carbonic chain and the ions becomes more and more stiff on going from fluorine to bromine. The calculated energy barrier for internal rotations of the polar head group and the internal rotation of the carbonic chain (about the central C–C bond) confirm these results. The influence of the length of the carbon chain was investigated at the same level of theory by calculating the optimized structure for the following six eight-carbon chains ions: C 7X 15CO 2 −, C 7Y 15CS 2 − (with X=H, F, Cl and Y=H, F) and C 7H 15CSe 2 −. The results show that the compounds with four- and eight-carbon chain are very similar having only small differences in the geometry and charge distribution at the polar head group of the ions.