Influence of the out-of-plane distortions of nuclear configurations of molecules with then-AO of the heteroatom directed along theC 2 symmetry axis on spin-orbit coupling between thenπ*- and ππ*-states

Abstract

This paper studies changes in the matrix elements (Ump) of spin-orbit coupling between the nπ*- and ππ*-states, which are induced by the (“chair,” “bath”) distortions of the nuclear configurations of molecules. The analysis is performed for acridine molecules in which the n-pz atomic orbital (AO) of the heteroatom is directed along the C2 symmetry axis. Earlier, for molecules with a planar nuclear configuration of C2v symmetry and with the heteroatom lying on the C2 axis, we established the dependence of Ump on the symmetry of ππ*-states [Γ(ππ*)=A1 or B2]. The values of Ump differ by more than one order of magnitude; this is in line with the difference between the interconversion rate constants (Kisc; two or three orders). In this work we have found that this contrast in Ump (and, accordingly, in Kisc) is retained when the nuclear configuration of the acridine molecule is distorted to the “chair” (AC-A) configuration, although the individuality of both molecular orbital types (nσ-MO and π-MO) and states nπ* and ππ* is annihilated to a certain extent. For the “bath” (AC-B) conformation the difference in Ump considerably diminishes. Reasons for the changes in the matrix elements of spin-orbit coupling and rate constants of the S-T conversion are analyzed. The available energy level diagram is critically analyzed, and a slightly different diagam as well as a scheme of nonradiating deactivation of acridine are suggested.