Quantum-mechanical studies of vanadium oxides

A variety of V-O systems were examined in order to model the catalytic site for VPO-catalyzed conversion of butane to maleic anhydride. All V-O systems examined could be described as a V-O triple bond, with a covalent σ-bond and two π-bonds, and a bond length of approximately 1.56-1.60 Å. The nature of the π-bonds varied. In VOCl_4^(3-) and VOCl_(2-), the π-bonds were both described as donor-acceptors. All other systems displayed one covalent bond and one donor-acceptor bond. Resonance between the two π-bonds was displayed for four-chloride C_(4v) VOCl_4^(2-/-). Optimized C_(2v) geometries for VOCl_4^(2-/-) appear to be the result of this resonance, in which the wavefunction and geometry represent one resonance state of the overall C_(4v) system, in which one π -bond is highly covalent, and the other highly donor-acceptor. The C_(4v) system is lower in energy than the C_(2v) system at all levels of theory (HF, CASSCF, MRCI) except GVB(3/6)-PP. VOCl_2^(0/+) calculations show strong similarities to C_(2v) VOCl_4^(2-/-) in both V-O bond description and geometry, suggesting that these systems can be described as C_(2v) VOCl_4^(2-/-) systems without the axial chlorides. VO+ and VOCl_3 results, for which experimental data exists, support the descriptions of geometry and bonding given. V(V) systems display a strong degree of covalent character to the chlorides (as does VO_3 to the bound oxygen molecule), but this effect decreases significantly as the V(3dδ )-orbitals are occupied. Snap bond calculations indicate that the V(V)-O bond is approximately 28 kcal/mol weaker than the V(lV)-O bond - this may be due to the increased V-C1 interaction in the products of the V(V) bondbreaking.