$$\pi $$ -Electron Long-Range Spin–Spin Coupling in the Full Configuration Interaction Method

Abstract

A technique for calculating spin perturbation effects in the framework of the previously developed matrix version of the full configuration interaction method is presented. The method is adapted to calculations of \(\pi \)-electron atom–atomic spin polarizability defined as the \(\pi \)-contribution to the indirect spin–spin nuclear coupling constant. Particular calculations show that pronounced long-range spin–spin coupling is actually possible in some characteristic conjugated systems such as polymethine dye carbocations or some nonalternant hydrocarbons. At the same time, spin–spin correlators appearing in McKonnel's theory are inapplicable in describing such long-range effects.