ZEKE Spectroscopy of the Organometallic Radicals MgCH3 and ZnCH3: Construction of a High-Resolution “Experimental” Molecular Orbital Diagram

Restricted open-shell time-dependent density functional theory with perturbative spin-orbit coupling

When using quantum chemical methods to study electronically excited states of open-shell molecules, it is often beneficial to start with wave functions that are spin eigenfunctions. For excited states of molecules containing heavy elements, spin-orbit coupling (SOC) is important and needs to be included as well. An efficient approach is to include SOC perturbatively on top of a restricted open-shell Kohn-Sham (ROKS) time-dependent density functional theory, which can be combined with the Tamm-Dancoff approximation (TDA) to suppress numerical instabilities. We implemented and assessed the potential of such a ROKS-TDA-SOC method, also featuring the possibility of calculating transition dipole moments between states to allow for full spectrum simulation. Our study shows that the ROKS-TDA-SOC formalism yields a clear and easy-to-use method to obtain electronically excited states of open-shell molecules that are of moderate size and contain heavy elements.

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+)

The ionization energies of VCH₂ and VCH₃, the various 0 K bond dissociation energies (D₀s) in their neutrals and cations, and their respective heats of formation at 0 and 298 K are computed by the single-reference, wave function-based CCSDTQ/CBS procedure. The core of the composite method is the approximation to the complete basis set (CBS) limit at the coupled cluster (CC) level which includes up to full quadruple excitations. The zero-point vibrational energy, core-valence correlation, spin-orbit coupling, and scalar relativistic effects have their contributions incorporated in an additive manner. For the species in the current study, this protocol requires geometry optimizations and harmonic frequency calculations practically no higher than the CCSD(T)/aug-cc-pwCVTZ and CCSD(T)/aug-cc-pVTZ levels, respectively. The present calculations successfully predict D₀(V⁺-CH₃) = 2.126 eV and D₀(V⁺-CH₂) = 3.298 eV in remarkable agreement with the data recently measured by a spin-orbit state selected V⁺ + CH₄ collision experiment (Phys. Chem. Chem. Phys. 2021, 23, 273-286). The good accord encourages the use of CCSDTQ/CBS protocol in thermochemical predictions of various feasible product channels identified in methane activation by transition metal species.

High-Resolution Spectroscopic Investigation of the B̃2A1−X̃2A1 Transitions of CaCH3 and SrCH3

High-resolution spectra of the ~B(2)A(1)-- ~X(2)A(1) transitions of CaCH(3) and SrCH(3) have been recorded in a molecular jet/laser ablation source using laser excitation spectroscopy. Transitions arising from the K = 0 and 1 sub-bands have been observed for both molecules. An analysis of the data using a (2)A(1) symmetric top Hamiltonian has determined rotational and spin-rotation constants for the ~B(2)A(1) state of each molecule. From the rotational constants, structures have been estimated for both CaCH(3) and SrCH(3). The spin-rotation constant, epsilon(bc) = (epsilon(bb) + epsilon(cc))/2, in the ~B(2)A(1) state for both molecules is in reasonable agreement with the value calculated using the pure precession approximation. For CaCH(3), the K' = 1 levels of the ~B(2)A(1) state exhibit a perturbation that interchanges the energy ordering of the spin-rotation components.

On the Nature of Metal−Carbon Bonding: AIM and ELF Analyses of MCHn (n = 1−3) Compounds Containing Early Transition Metals

Ab initio and DFT calculations have been performed on a series of organometallic compounds, according to the formula MCH(n), where M = K, Ca, Sc, Ti, V, Cr, or Mn and n = 1-3. Various theoretical methods are compared, the B3LYP level yielding the same agreement with the experimental geometries available as the correlated MP2 and CISD methods, with the 6-311++G(3df,2p) basis set for C and H and Wachter's (15s11p6d3f1g)/[10s7p4d3f1g] basis set for transition metals. The main geometric and electronic features of the molecules studied are described, analyzing the M-C bonding characteristics in terms of the atoms in molecules theory (AIM) and the electron localization function (ELF). Although multiple bonding is expected from the Lewis bonding scheme, the results indicate an almost pure ionic bond for all of the systems studied. The net charge transfer from the metal to the carbon atom ranges from 0.5 to 1 e(-), and the electronic structure of the CH(n)(-) moiety is unaltered after the interaction with the metal cation, showing little or no effect on the shape of the electron pairing. The bond paths corresponding to a possible alpha-agostic bond for these systems are not present.