High-Level Ab Initio Predictions for the Ionization Energy, Bond Dissociation Energies, and Heats of Formation of Vanadium Methylidyne Radical and Its Cation (VCH/VCH⁺)

Abstract

The ionization energy (IE) of VCH, the 0 K V−CH/VC−H bond dissociation energies (D 0 s), and the heats of formation at 0 K (ΔH f0 °) and 298 K (ΔH f298 °) for VCH/ VCH + are predicted by the wave function-based CCSDTQ/CBS approach. This compositecoupled cluster method includes full quadruple excitations in conjunction with the approximation to the complete basis set (CBS) limit. The contributions of zero-point vibrational energy, core−valence (CV) correlation, spin−orbit coupling, and scalar relativistic corrections are taken into account. The present calculations show that adiabatic IE(VCH) = 6.785 eV and demonstrate excellent agreement with an IE value of 6.774 7 ± 0.000 1 eV measured with two-color laser-pulsed field ionization-photoelectron spectroscopy. The CCSDT and MRCI+Q methods which include CV correlations give the best predictions of harmonic frequencies: ω 2 (ω 2 + ) (bending) = 534 (650) and 564 (641) cm −1 and the V−CH stretching ω 3 (ω 3 + ) = 835 (827) and 856 (857) cm −1 compared with the experimental values. In this work, we offer a streamlined CCSDTQ/CBS approach which shows an error limit (≤20 meV) matching with previous benchmarking efforts for reliable IE and D 0 predictions for VCH/VCH + . The CCSDTQ/CBS D 0 (V + −CH) − D 0 (V−CH) = −0.012 eV and D 0 (VC + −H) − D 0 (VC−H) = 0.345 eV are in good accord with the experimentally derived values of −0.028 4 ± 0.000 1 and 0.355 9 ± 0.000 1 eV, respectively. The present study has demonstrated that the CCSDTQ/CBS protocol can be readily extended to investigate triatomic molecules containing 3d-metals.

High-Level ab Initio Predictions for the Ionization Energies, Bond Dissociation Energies, and Heats of Formation of Vanadium Methylidene, Vanadium Methyl Species, and Their Cations (VCH2/VCH2+, VCH3/VCH3+)

“…All singly occupied molecular orbitals (SOMOs), along with π-bonding MOs of VCH, VCH 2 , and VCH 3 are, respectively, shown in Figure a,b,c. Recalling the bonding structure of VCH, the VC bond that is chosen to lie along the z -axis consists of a σ bond composited by V­(3d z 2 ) and C­(2p z ) orbitals, and a pair of π bonds formed between V­(3d xz , 3d yz ) and C­(2p x , 2p y ) orbitals as recapped in Figure a. The VCH 2 and VCH 3 species can be formed via successive addition of a single H atom to the π bonds of the VCH molecule.…”

“…All the individual components for corrections (Δ E extrapolated CBS , Δ E ZPVE , Δ E SO , Δ E HOC‑T , and Δ E HOC‑Q ) accounted for CCSDTQ/CBS IE­(VCH 3 ) and IE­(VCH 2 ) are listed in Table . The theoretical , and experimental value of IE­(V), which is necessary for subsequent derivation of D 0 and Δ H ° values, is tabulated as well. The respective average of the Δ E extrapolated CBS utilizing the two variations of the extrapolation scheme for IE­(VCH 3 ) [IE­(VCH 2 )] is 6.752 [6.817] eV.…”

“…The current CCSDTQ/CBS procedure has been attuned to reduce the number of single-point calculations so that it is more amenable for small organometallic molecules . The CBS limit is achieved at the CCSD­(T) level which adopts the partially unrestricted implementation ROH-UCCSD­(T). , The assorted corrections in our protocol are handled in a similar manner as in previous calculations on VCH 0,+ …”

“…Given the consistency of both schemes in previous computations on MX/MX + , we have taken the average of the two extrapolated values in the current protocol. , Since there is no marked improvement achieved by geometry optimization with the basis set beyond aug-cc-pwCVTZ-DK (see section ), all the subsequent single-point calculations have been simply performed with CCSD­(T)/aug-cc-pwCVTZ-DK structures.…”

“…As outlined in our calculations on VCH 0,+ , the collective higher-order correction (HOC) which accounts the full triple and iterative quadruple excitations is approximated as follows …”