General Second-Order Perturbation Treatment of σ Spin Densities in Planar, Conjugated π Radicals

Abstract

We calculate σ spin density in π planar radicals by a full second‐order perturbation treatment, the correlation potential being taken as the perturbation Hamiltonian. This paper allows a clear discussion of the way π inner‐shell spin densities contribute to a_H splittings and of the type of approximations implied in the use of the simple McConnell relationship with improved π spin densities. We show that second‐order contributions to Q constants are at least about one‐fourth of the first‐order contribution and come from σσ diexcited states. Some new contributions to the ``charge effect'' appear, which are of the same order of magnitude as the previous Colpa—Bolton effect.