INTERPRETATION OF HUND RULE FOR 1ST-ROW HYDRIDES LIH, BH, NH, FH

Configuration interaction calculations have been carried out for low-lying singlet and triplet states of LiH(1,3Σ+), BH(1,3II), NH(1Δ,3Σ-), and FH(1,3II), with the aim of evaluating singlet-triplet energy component differences. These calculations represent the first systematic study of the quantum mechanical interpretation of Hund's rule in a series of molecules. While early molecular work on Hund's rule suggested that the electron-nuclear attraction may always be deeper in the triplet than in the singlet, exceptions are found in the present work. Also, in certain cases, where configuration mixing and molecular charge separation play a key role, the singlet electron-electron repulsion exceeds that of the triplet. It is also observed that changes in electron-electron repulsion tend to be in the same direction as changes in kinetic energy, whereas changes in electron-nuclear attraction are of the opposite sign, for a given geometry. © 1990 American Chemical Society.