Infrared spectroscopic and theoretical studies of the OTiF2, OZrF2 and OHfF2 molecules with terminal oxo ligands

Abstract

The isolated group 4 metal oxydifluoride molecules OMF₂ (M = Ti, Zr, Hf) with terminal oxo groups are produced specifically on the spontaneous reactions of metal atoms with OF₂ through annealing in solid argon. The product structures and vibrational spectra are characterized using matrix isolation infrared spectroscopy as well as B3LYP density functional and CCSD(T) frequency calculations. OTiF₂ is predicted to have a planar structure while both OZrF₂ and OHfF₂ possess pyramidal structures, all with singlet ground states. Three infrared absorptions are observed for each product molecule, one M–O and two M–F stretching modes, and assignments of these molecules are further supported by the corresponding ¹⁸O shifts. The molecular orbitals of the group 4 OMF₂ molecules show triple bond character for the terminal oxo groups, which are also supported by an NBO analysis. These molecular orbitals include a σ bond (O_2p + Ti_{sd hybrid}), a normal electron pair π bond (O_2p + Ti_d), and a dative π bond arising from O lone pair donation to the overlapping Ti d orbital. The M–O bond dissociation energies for OMF₂ are comparable to those in the diatomic oxide molecules. The OTiF intermediate is also observed through two slightly lower frequency bond stretching modes, and its yield is increased in complementary TiO + F₂ experiments. Finally, the formation of group 4 OMF₂ molecules is highly exothermic due to the weak O–F bonds in OF₂ as well as the strong new MO and M–F bonds formed.