High-level ab initio predictions for the ionization energy, bond dissociation energies, and heats of formation of cobalt carbide (CoC) and its cation (CoC⁺)

The ionization energy (IE) of CoC and the 0 K bond dissociation energies (D₀) and the heats of formation at 0 K (ΔH°_f0) and 298 K (ΔH°_f298) for CoC and CoC⁺ are predicted by the wavefunction based coupled-cluster theory with single, double, triple and quadruple excitations (CCSDTQ) and complete basis set (CBS) approach. The CCSDTQ/CBS calculations presented here involve the approximation to the CBS limit at the coupled cluster level up to full quadruple excitations along with the zero-point vibrational energy, high-order correlation, core-valence (CV) electronic, spin-orbit coupling, and scalar relativistic effect corrections. The present calculations provide the correct symmetry, ¹Σ⁺, for the ground state of CoC⁺. The CCSDTQ/CBS IE(CoC) = 7.740 eV is found in good agreement with the experimental IE value of 7.73467 ± 0.00007 eV, determined in a two-color laser photoion and pulsed field ionization-photoelectron study. This work together with the previous experimental and theoretical investigations support the conclusion that the CCSDTQ/CBS method is capable of providing reliable IE predictions for 3d-transition metal carbides, such as FeC, CoC, and NiC. Among the single-reference based coupled-cluster methods and multi-reference configuration interaction (MRCI) approach, the CCSDTQ and MRCI methods give the best predictions to the harmonic frequencies ω_e (ω_e⁺) = 956 (992) and 976 (1004) cm^-1 and the bond lengths r_e (r_e⁺) = 1.560 (1.528) and 1.550 (1.522) Å, respectively, for CoC (CoC⁺) in comparison with the experimental values. The CCSDTQ/CBS calculations give the prediction of D₀(Co⁺-C) - D₀(Co-C) = 0.175 eV, which is also consistent with the experimental determination of 0.14630 ± 0.00014 eV. The theoretical results show that the CV and valence-valence electronic correlations beyond CCSD(T) wavefunction and the relativistic effect make significant contributions to the calculated thermochemical properties of CoC/CoC⁺. For the experimental D₀ and ΔH°_f0 values of CoC/CoC⁺, which are not known experimentally, we recommend the following CCSDTQ/CBS predictions: ΔH°_f0(CoC) = 775.7 kJ/mol and ΔH°_f0(CoC⁺) = 1522.5 kJ/mol, ΔH°_f298(CoC) = 779.2 kJ/mol and ΔH°₂₉₈(CoC⁺) = 1526.0 kJ/mol. © 2013 American Institute of Physics.