Theoretical Study Of Activation CH₃X（X=OH,CN） By VO And Ta In The Gas Phase

In chemical reactions, bonding electrons get rearrangement, in which chemicalbonds is formed. It is always the pursuit of the chemists to realize the cleavage andsynthesis of chemical bonds with high efficiency and selectivity. Theoretical chemistshave committed to explore the theory of reaction mechanism, tries to better simulatereal chemical reaction process. However the chemistry of transition metals and theircompounds is strongly influenced by the availability of multiple low-lying electronicstates in these species. This means that even thermal chemical reactions of transitionmetal compounds are often spin-forbidden in that the reactants and products involvedhave a different overall electronic spin states. During the process of a chemicalreaction, the ability of the metal center to access these states and adapt to differentbonding situations may enable the system to find low-energy reaction pathways thatwould not be accessible otherwise. Thus, a reaction possibly occurs on two or morepotential energy surfaces (PESs) under thermal conditions, and therefore it has toinvolve the electronic process of radiationless transition from one potential energysurface to another surface. In this case, the changes between two or more differentelectronic states are known to accompany the progress of the reaction and can modifythe efficiency and/or selectivity of a given chemical rearrangement and can alsoinfluence a reaction from a process without change in spin state to a situation wherethe spin change completely determines the rate. Therefore the activation of these inertchemical bonds aroused wide concern.In this thesis, the gas phase reactions of transition metal oxides transition metalatoms and molecules with CH3OH and CH3CN which were selected as representativesystems of the activation of C H, O H and C CN bonds have been examined usingdensity functional theory (DFT)、CCSD and MP2methods with corresponding basissets to explore the reaction mechanisms of TSR. The Gaussian03, Gamess, and NBOprogram package were performed.The whole thesis consists of four chapters. Chapter1and Chapter2describe theprogress and application of quantum chemistry as well as the development and thepresent situation of TSR, and briefly introduce elementary theory and quantumchemistry computation methods. The contents of the two chapters are the basis and background of our studies and offered us with useful and reliable quantum methods.In Chapter3and4, the reactions of VO and Ta with CH3OH and CH3CN whichare selected as a representative system of the reactions of the transition-metal ions andthe activation of O-H, C-H and C-CN bonds by transition-metal oxide ions have beenemphasized. The crossing of the potential energy surface has been discusseddetailedly. Firstly, for each reaction system, all molecular geometries were fullyoptimized on respective ground state and the lowest excited state PESs by high-levelquantum chemistry calculation methods. Vibrational frequency calculations andintrinsic reaction coordinate (IRC) methods were used to characterize the reactionpath channels on two PESs. Whereafter, on the basis of the Hammond postulate, thecrossing points (CPs) between the different spin states have been selected by means ofthe procedure used by Yoshizawa et al. For the sake of comparison, the mathematicalalgorithm to MECPs developed by Harvey et al. had also been employed.Finally, the spin-orbit coupling is calculated between electronic states ofdifferent multiplicities at the crossing points (MECPs) to estimate the intersystemcrossing probabilities, and the probability of hopping from one surface to the other inthe vicinity of the crossing region is calculated by the Landau-Zener type model. Theenergetically more favorable channel was confirmed according to thermodynamic anddynamic date. Our motivation is to facilitate an understanding of the role of electronicstructures on mechanistic details at a molecular level, to clarify the dominant productchannels, and to support mechanistic proposals based on experimental results.