Coulomb repulsion in multiply charged ions: a computational study of the effective dielectric constants of organic spacer groups

Using dication and dianion models, effective dielectric constants have been calculated for several organic spacer groups (alkyl, poly-ether, and poly-ketone). Specifically, ab initio calculations at the MP2/6-31+G(d,p)//HF/6-31+G(d) level were used to estimate the Delta H(acid) and the proton affinity (PA) values of a series of diammonium and dialkoxide ions, respectively. The variation in these values as a function of the distance between charges leads to the effective dielectric constant (epsilon) of the organic spacer group. For all the spacer groups, effective dielectric constants near unity were obtained indicating that the spacer group does not reduce the Coulomb repulsion in the doubly charged ion (relative to a vacuum). The values obtained in this study are all slightly less than unity (similar to 0.85) because the definition of the charge separation as the distance between the formal charge centers (N or O) neglects delocalization and overestimates the true charge separation. For similar reasons, epsilon values slightly less than unity are also appropriate for zwitterionic systems. (C) 1999 Elsevier Science B.V.