Analysis of the effect of spin-orbit coupling on the electronic structure and excitation spectrum of the Bi22- anion in (K-crypt)2Bi2 on the basis of relativistic electronic structure calculations
Résumé
The Bi22- anions that have been characterized in (K-crypt)2Bi2 are isoelectronic with O2 but are diamagnetic and EPR-silent, unlike O2. The UV-vis spectrum measured for (K-crypt)2Bi2 shows two broad absorption peaks located at 2.05 and 2.85 eV, but no absorption at lower energies down to 0.62 eV. To account for these observations, the electronic structures of the isoelectronic diatomic dianions Q22- (Q = N, P, As, Sb, Bi) were compared on the basis of relativistic density functional theory calculations, and the electronic excitations of Bi22- were analyzed on the basis of relativistic configuration interaction calculations. The extent of spin-orbit coupling, brought about by the relativistic effect, increases steadily in the order N < P < As < Sb < Bi such that the "closed-shell" state is more stable than the "open-shell" state for Bi22-, while the opposite is the case for N22-, P22-, As22-, and Sb22-. The nature of the electronic excitations of Bi22- was assigned and discussed from the viewpoint of molecular orbitals in the absence of spin-orbit coupling.