Rovibrational Quartic Force Fields of Metal Dicarbides and Tricarbides

For gas phase organic molecules, ab initio composite methods exist that are able to predict accurate rovibrational spectra. Typically, fundamental vibrational frequencies can be obtained to within 1 cm^-1 of known experimental values. However, the question remains as to whether a similar methodology can be developed for metal-bearing molecules due to issues like scalar relativistic considerations, static (or strong/multireference) correlation, inner/outer core correlation, and other considerations not typically considered in “black box” quartic force field (QFF) computations. Rovibrational spectroscopic properties have been computed for scandium and titanium dicarbide (ScC₂ and TiC₂) and magnesium tricarbide (MgC₃). Overall, we find that complete basis set CCSD(T) QFFs may not always provide the same level of agreement with experiment for inorganic molecules that it does for organic molecules. While agreement between theory and experiment is exceptional for ScC₂ and its isotopologues, the ground and low-lying excited electronic states of TiC₂ exhibit vibronic coupling and experimental characterization of its pure rotational spectrum is incomplete. Extremely small relative energies between various singlet and triplet isomers of MgC₃ dramatically increase the computational cost of the project. Overall, our findings will greatly enhance the knowledge of metal-carbon bonding for laboratory groups working in molecular astrochemistry and laser spectroscopy.