Electronic states and nature of bonding in the molecule MoC by all electron ab initio calculations

In the present work all electron ab initio multiconfiguration self-consistent-field (CASSCF) and multireference configuration interaction (MRCI) calculations have been carried out to determine the low-lying electronic states of the molecule MoC. The relativistic corrections for the one electron Darwin contact term and the relativistic mass-velocity correction have been determined in perturbation calculations. The electronic ground state is predicted as ${}^3{\Sigma }^{-}.$ The spectroscopic constants for the ${}^3{\Sigma }^{-}$ electronic ground state and eight low-lying excited states have been derived by solving the Schrödinger equation for the nuclear motion numerically. Based on the results of the CASSCF calculations the ${}^3{\Sigma }^{-}$ ground state of MoC is separated from the excited states ${}^3\mathrm{\Delta ,}$ ${}^5{\Sigma }^{-},$ ${}^1\mathrm{\Gamma ,}$ ${}^1\mathrm{\Delta ,}$ ${}^5\mathrm{\Pi ,}$ ${}^1{\Sigma }^{+},$ and ${}^3\Pi$ by transition energies of 4500, 6178, 7207, 9312, 10 228, 11 639, and $\text{16\hspace{0.17em}864\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}},$ respectively. The transition energy between the ${}^3{\Sigma }^{-}$ ground state and the ${}^3\Pi$ state as derived in the MRCI calculations is $\text{15\hspace{0.17em}484\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}.$ For the ${}^3{\Sigma }^{-}$ ground state the equilibrium distance has been determined as 1.688 Å, and the vibrational frequency as $\text{997\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}.$ The chemical bond in the ${}^3{\Sigma }^{-}$ electronic ground state has triple bond character due to the formation of delocalized bonding π and σ orbitals. The chemical bond in the MoC molecule is polar with charge transfer from Mo to C, giving rise to a dipole moment of 6.15 D at 3.15 a.u. in the ${}^3{\Sigma }^{-}$ ground state.