Matrix Isolation Infrared Spectroscopic And Theoretical Studies On The Reactions Of Transitional Metal Mono- And Dioxides With Methane

The reactions of transitional metal monoxides (TiO, ZrO, NbO, TaO, MnO, and FeO) and dioxides (VO₂, NbO₂, and TaO₂) with methane were investigated by matrix isolation infrared spectroscopic and quantum chemical calculations. The metal monoxide and dioxide molecules were prepared by laser-evaporation of bulk metal oxide targets. The reaction intermediates and products were identified by isotopic substitution and comparison to quantum chemical frequency calculations. The reaction mechanism and periodic trends are discussed.The reactions of metal monoxides with methane are summarized in Scheme 1:The initial step of the MO + CH₄ reaction is the formation of the MO(CH₄) complex. From the complex, one hydrogen atom of methane transferred to the metal center to form the CH₃M(O)H metallo acetaldehyde molecule upon UV or visible light irradiation for early transition metals. The later transition metal monoxide-methane complexes underwent photochemical rearrangement to form the CH₃MOH molecules with hydrogen being transferred from methane to O atom of MO. The different reactivity can be rationalized in terms of changes in the strength of the M-O bond and the electron count. The bonds of early transition metal monoxides are stronger than those of later transition metal monoxides. The valence d electrons of early transition metals are inclined to participate in bonding to form high valent compounds, whereas the late transition metals prefer to form divalent molecules due to the stability of the d electrons.In the case of Ti, the titano-acetaldehyde molecule underwent further photochemical rearrange to the CH₂Ti(H)OH titano-vinylalcohol molecule, whichreacted with a second methane to form the (CH₃)₂Ti(H)OH titano-isopropyl alcohol molecule with negligible activation energy. The (CH₃)₂Ti(H)OH molecule also can be produced via ultraviolet photon induced rearrangement of the CH₃M(O)H(CH₄)complex (Scheme 2):The reactions of metal dioxides with methane are summarized in Scheme 3:The metal dioxide molecules (MO₂, M=V, Nb and Ta) reacted with methane to form the O₂M(CH₄) complex. The binding energies of O₂M(CH₄) are larger than those of the corresponding OM(CH₄). The O₂M(CH₄) complex rearranged to the CH₃M(O)OH isomer under UV irradiation.