First spectroscopic investigation of the 4d transition metal monocarbide MoC

The first optical spectroscopic investigation of MoC has revealed a complicated vibronic spectrum consisting of about 35 bands between 17 700 and 24 000 cm−1. Analysis has shown the ground state to be the Ω=0+ spinorbit component of a Σ3− state that derives from a 10σ211σ25π42δ2 configuration. The X 3Σ0+− rotational constant for Mo9812C was determined to be B0=0.553 640±0.000 055 cm−1, giving r0=1.687 719±0.000 084 Å. Consideration of spin-uncoupling effects in the X 3Σ− state requires that this value be revised to r0=1.6760 Å, which represents our best estimate of the true Mo–C bond length. Spectroscopic constants were also extracted for six other major isotopic modifications of MoC in this mass resolved experiment. All rotationally resolved transitions were found to originate from the ground state and terminate in electronic states with Ω=1. An attempt is made to classify the observed transitions into band systems, to rationalize the complexity of the spectrum, and to understand the bonding from a molecular orbital point of view.