Semi-empirical calculations of chemical shifts in partially paratropic anions

Abstract

A theoretical study is presented of the ring-current and excess-charge effects on the chemical shifts of the conjugate bases of fluorene, 9,10-dihydroanthracene, xanthene, thioxanthene, dibenzo[a,d]cycloheptene and dibenz[b,f]azepine. Several approaches (SCF methods, coupled and uncoupled perturbation theories and ring-current models) have been compared.

The use of coupled perturbation theory is necessary for those systems which exhibit paramagnetic ring currents. The chemical shifts are satisfactorily described if the PPP method and the so-called LA I approximation are invoked, and the various contributions are parametrized properly.