Potential Energy Surfaces for Mo + CO and W + CO

Abstract

Potential energy surfaces for MoCO and WCO have been studied using the complete active space multiconfiguration self-consistent field (CAS-MCSCF) followed by multireference singles + doubles configuration interaction (MRSDCI). Spin−orbit effects are included through the relativistic configuration interaction (RCI) method for W−CO. Although at the CASSCF level the lowest 7Σ+ state does not form a very stable minimum for WCO relative to W(7S) + CO(1Σ+), at the higher MRSDCI level not only does this state form a minimum but the 5Φ and 5Σ+ states become nearly-degenerate with 7Σ+ and form stable minima for WCO. In contrast, the 7Σ+ state of MoCO does not form a minimum at all levels of theory, although the quintet states have minima in the potential surfaces and are above Mo(7S) + CO(1Σ+). Relativistic effects are found to stabilize WCO in contrast to MoCO. The lowest spin−orbit states arising from W(5D) and W(7S) are reversed in energy when spin−orbit effects were included consistent with the experimental atomic energy separations. The 0+ state of WCO was found to undergo an avoided crossing due to spin−orbit coupling. The nature of bonding is discussed using the wave function composition and the Mulliken population analysis.