Unusually low 13C chemical shift sensitivity to charge in cyclic 4nπ anions. Potential relation to anisotropic ring current effects

Abstract

For various pairs of neutral π-systems and their corresponding di- or tetra-anions, a relationship between the ¹³C n.m.r. chemical shift/charge ratio (K_C) and the altered ¹H ring current anisotropy is observed. Thus for a 4nπ anion system, the ¹³C shift sensitivity to charge is significantly lower than the K_C value obtained for a diatropic anion. This is explained in terms of the average electronic excitation energy, which influences both the ¹³C chemical shifts and the ring current intensities. An empirical model which describes the ¹³C shifts on the basis of a charge term and an anisotropy term is postulated. The anisotropy term is shown to be linearly related to the ¹H ring current anisotropy.