Theoretical studies of the low-lying states of ScO, ScS, VO, and VS

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Abstract
A theoretical study of the lowest three (X 2Σ+, A′ 2Δ, A 2Π) states of ScO and ScS is presented. The A′ 2Δ and A 2Π states lie considerably closer to the ground state in ScS than in ScO, and the unobserved A′ 2Δ state of ScS is predicted to lie at 9000±1000 cm−1. An analogous comparison is presented for the low-lying X 4Σ−, A′ 4Φ, A 4Π, and (1)2Δ states of VO and VS. Extended one-particle Gaussian basis sets are employed, and electron correlation is included using the singles-plus-doubles configuration interaction and the coupled pair functional formalisms. For the ground states of all four systems there is some triple-bond character arising from 3dσ and 3dπ bonds, combined with the donation of the Oπ lone pair into an empty 3dπ orbital. The ionic component of the bonding is reduced for the sulfides, owing to the smaller electronegativity of S. The low-lying states generally arise from excitation of the metal 4s electron into the nonbonding δ or π orbitals. However, the A′ 4Φ and A 4Π states of VO arise from an excitation of the nonbonding 3dδ electron into a π orbital. From the trends in the theoretical dissociation energies for ScO, ScS, and VO, we suspect that the new experimental values for the D0 of VS may be too high, and that the older value of 4.67±0.15 eV is more accurate.

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