| Light alkanes are major constituents of natural gas. However, only limited practical industrial processes are available for their conversion into more valuable organic products, due to the relative inertness of their C-H bonds and the lack of selectivity in alkane transformations. Transition-metal oxoes (M=O) are well-known active oxidants for the oxidation of hydrocarbons, but the underlying mechanisms of C-H bond activation by M=O are yet unclear.In this dissertation we present a systematic DFT exploration of C-H bond oxidation by metal oxoes. The main conclusions are summarized as follows:1. Our calculations reveal that H abstraction (H-Ab) mechanism is a dominant pathway in a wide range of oxidation processes involving high-valence tansition-metal oxoes as oxidants. In some cases other pathways, such as [3+2], may become important.2. Although recent kinetic studies showed that metal oxo behaves like oxyradicals in the activation of C-H bond, our theoretical work discloses that there are intrinsic differences between these two: (i) H-Ab by oxyradical involves three electrons and three active orbitals (3e,3o); whereas H-Ab by metal-oxo is a (4e,4o) process; (ii) the unpaired spin density at the terminal oxygen in metal oxoes is not a prerequisite for accepting a H atom, hence even a closed-shell d?metal-oxo can act as a H abstractor; (iii) not only the reaction enthalpy but also the coupling between PT (proton-transfer) and ET (electron-transfer) processes must be taken into account for oxo abstraction; (iv) the alkyl radical formed upon H-Ab from alkane by metal oxo can be quickly rebounded to the nearby oxygen, greatly prohibiting the unselective radical-chain reactions.3. A closed-shell metal oxo can adopt a radical pathway by opening its M=O bond and, accordingly, the reactivity of a metal oxo depends on the character of 7M=o, This finding is of great importance to the design of effective catalysts for the oxidative activation of light alkanes.
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