Bond dissociation energies of TiC, ZrC, HfC, ThC, NbC, and TaC

Sharp predissociation thresholds have been observed in the resonant two-photon ionization spectra of TiC, ZrC, HfC, ThC, NbC, and TaC. Because of the large density of states in these species, particularly near the ground separated atom limit, we argue that the sharp predissociation threshold occurs at the thermochemical bond dissociation energy. The bond dissociation energies, D₀(MC), measured are 3.857(4) eV (TiC), 4.892(10) eV (ZrC), 4.426(3) eV (HfC), 5.060(3) eV (ThC), 5.620(4) eV (NbC), and 4.975(3) eV (TaC). Using atomic enthalpies of formation, the diatomic enthalpies of formation, Δ_f,0KH^○(MC(g)), were also calculated as 810.0(16.7) kJ mol^-1 (TiC), 847.9(8.5) kJ mol^-1 (ZrC), 902.1(6.3) kJ mol^-1 (HfC), 825.0(6.0) kJ mol^-1 (ThC), 898.8(8.0) kJ mol^-1 (NbC), and 1012.6(2.2) kJ mol^-1 (TaC). Combining our D₀(MC) values with accurate values of the ionization energies of MC and M, we also report precise values of D₀(Ti⁺-C) = 4.089(4) eV, D₀(V⁺-C) = 3.724(3) eV, and D₀(Nb⁺-C) = 5.390(4) eV. Combining the present D₀(MC) results with guided ion beam measurements of cationic bond dissociation energies, we report MC ionization energies of IE(ZrC) = 6.91(16) eV, IE(HfC) = 8.06(3) eV, IE(ThC) = 6.55(29) eV, and IE(TaC) = 8.73(4) eV. Trends in the transition metal MC bond energies and a comparison to MSi bond energies are also presented.