RESUMO
The reactivity of the two diatomic congeneric systems [CO](·+) and [SiO](·+) towards methane has been investigated by means of mass spectrometry and quantum-chemical calculations. While [CO](·+) gives rise to three different reaction channels, [SiO](·+) reacts only by hydrogen-atom transfer (HAT) from methane under thermal conditions. A theoretical analysis of the respective HAT processes reveals two distinctly different mechanistic pathways for [CO](·+) and [SiO](·+), and a comparison to the higher metal oxides of Group 14 emphasizes the particular role of carbon as a second-row p element.
Assuntos
Monóxido de Carbono/química , Metano/química , Modelos Químicos , Óxidos/química , Compostos de Silício/química , Gases/química , Hidrogênio/química , TermodinâmicaRESUMO
The heteronuclear oxo-cluster [VPO4](â¢+) is generated via electrospray ionization and investigated with respect to both its electronic structure as well as its gas-phase reactivity toward small hydrocarbons, thus permitting a comparison to the well-known vanadium-oxide cation [V2O4](â¢+). As described in previous studies, the latter oxide exhibits no or just minor reactivity toward small hydrocarbons, such as CH4, C2H6, C3H8, n-C4H10, and C2H4, while substitution of one vanadium by a phosphorus atom yields the reactive [VPO4](â¢+) ion; the latter brings about oxidative dehydrogenation (ODH) of saturated hydrocarbons, e.g., propane and butane as well as oxygen-atom transfer (OAT) to unsaturated hydrocarbons, e.g. ethene, at thermal conditions. Further, the gas-phase structure of [VPO4](â¢+) is determined by IR photodissociation spectroscopy and compared to that of [V2O4](â¢+). DFT calculations help to elucidate the reaction mechanism. The results underline the crucial role of phosphorus in terms of C-H bond activation of hydrocarbons by mixed VPO clusters.
Assuntos
Hidrocarbonetos/química , Fosfatos/química , Teoria Quântica , Vanádio/química , Catálise , Gases/química , Estrutura Molecular , OxirreduçãoRESUMO
The reactivities of the adamantane-like heteronuclear vanadium-phosphorus oxygen cluster ions [V(x)P(4-x)O(10)](.+) (x=0, 2-4) towards hydrocarbons strongly depend on the V/P ratio of the clusters. Possible mechanisms for the gas-phase reactions of these heteronuclear cations with ethene and ethane have been elucidated by means of DFT-based calculations; homolytic C-H bond activation constitutes the initial step, and for all systems the P-O(.) unit of the clusters serves as the reactive site. More complex oxidation processes, such as oxygen-atom transfer to, or oxidative dehydrogenation of the hydrocarbons require the presence of a vanadium atom to provide the electronic prerequisites which are necessary to bring about the 2e(-) reduction of the cationic clusters.
Assuntos
Cátions/química , Fósforo/química , Vanádio/química , Simulação por Computador , Estrutura Molecular , OxirreduçãoRESUMO
Hydrogen-atom transfer (HAT), as one of the fundamental reactions in chemistry, is investigated with state-of-the-art gas-phase experiments in conjunction with computational studies. The focus of this Minireview concerns the role that the intrinsic properties of gaseous oxo-clusters play to permit HAT reactivity from saturated hydrocarbons at ambient conditions. In addition, mechanistic implications are discussed which pertain to heterogeneous catalysis. From these combined experimental/computational studies, the crucial role of unpaired spin density at the abstracting atom becomes clear, in distinct contrast to recent conclusions derived from solution-phase experiments.
RESUMO
Why so different? The comparison of the reaction of "bare" [P4O10](·+) and [V4O10](·+) with ethene by mass-spectrometric and computational studies permits insight into mechanistic aspects of the competition between C-H bond activation and oxygen-atom and electron transfers. Whereas [P4O10](·+) reacts by homolytic C-H bond cleavage and electron transfer, the isostructural [V4O10](·+) shows only oxygen-atom transfer (see picture).
RESUMO
No need for a metal: A combination of mass spectrometry and computational studies (density functional theory and coupled-cluster methods) shows that [P(4)O(10)](.+) is the first polynuclear nonmetal oxide cation that is capable of activating the C--H bond of methane at room temperature (see picture). This process represents a further example in the reactivity of oxygen-centered radicals.