D3d Ground-State Structure of V(CO)6: A Combined Matrix Isolation and ab Initio Study of the Jahn−Teller Effect

Abstract

Structures, relative energies, vibrational spectra, and ESR parameters of the 17e- molecule V(CO)6 in Oh, D3d, D4h, and D2h symmetry have been computed with density functional and high-level ab initio methods. At all theoretical levels applied, the same energetic order is obtained, D3d < D2h < D4h < Oh, with the D2h structure as a transition state connecting equivalent D3d species. At the RCCSD(T)/AE2 level using UBP86/AE2 geometries, the energies of the D2h, D4h and Oh species relative to that of D3d minimum are predicted to be 210, 535, and 731 cm-1, respectively. According to molecular dynamics simulations on the UBP86/AE1 potential energy surface, the D3d minimum is preserved at very low temperatures (around 16 K), whereas at 300 K the molecule is highly fluxional with an averaged structure indistinguishable from that of [V(CO)6]- with its Oh symmetric ground state. Nearly complete IR and Raman spectra of V(CO)6 and V(13CO)6 have been recorded at 300 K for the first time in the gas phase, in solution, and at cryogenic temperatures in Ne and Ar matrices. The spectra show a pronounced temperature dependence, especially for the Jahn−Teller active modes, Eg and T2g. The observed infrared matrix spectra generally agree well with the calculated spectrum (BP86/AE2 level) for the D3d structure and much less with that of the D4h isomer. The A1g modes in the Raman spectra are reasonably well reproduced computationally in the harmonic approximation, whereas this approximation breaks down for the Eg and T2g bands, as expected. Further evidence for the D3d symmetry of V(CO)6 is obtained from a reanalysis of the experimental ESR spectrum that is reported in the literature. The observed ordering of the hyperfine coupling constants A∥ and A⊥ is reproduced theoretically only when distortion to D3d symmetry is assumed. In addition, the bonding properties of V(CO)6 are compared to those of [V(CO)6]- and Cr(CO)6.