| Owing to latent economic and environmental significance, the activated reactions of C-H(X) and C-C bonds of the hydrocarbons aroused by transition metals(TMs)with alkanes, halomethanes and other small organic molecules have become one important subject of experimental and theoretical research. The investments in both experimental and theoretical aspects have expressly harvested the organometallic species and the overall mechanism of TM catalyst. However, owing to the developed technologic measures, the violation of “spin-forbidden” phenomenon is detected in these category reactions partially. How to explain reasonably these reaction mechanisms is the very important theoretical matter in quantum chemistry area. This dissertation is to study a series of reactions of TMs with CH3X(X=CH3, CN) which involve the intersystem crossing from density functional theory(DFT) method and focus on the spin-flip mechanism under different spin multiplicity with the Fractional Orbital Occupation Number(FON)-DFT approach suggested by us.In Chapter 1, a brief presentation is for the fundamental principle of chemistry reaction and the spin conservation to follow. We introduce TMs catalytic reactions with small organic molecules which often violate the spin conservation. Then, we elaborate on the theoretical research and status on the spin-flip reaction. Lastly, we simply state our work on the reactions of TMs with CH3X(X=CH3, CN) containing the spin-flip process.In Chapter 2,we give a brief review to the progress and research methods of quantum chemistry firstly. Then, the theoretical basis of DFT on the Hohenberg-Kohn theorem, Koohn-Sham equation and exchange correlation functionals are expatiated.In addition, we introduce the relativistic effect and the natural bond orbital(NBO),which are often considered and used in transition metal complexes system. As last, we focus on one new theoretical methodology suggested by us for the search of the minimum energy crossing point(MECP) in the spin-flip reaction, namely, the fractional occupation number(FON) approach based on DFT.In Chapter 3, we investigate the spin-flip reactions of Zr + C2H6 by relativistic density functional theory. Firstly, the potential energy surfaces(PES) in triplet and singlet spin-states are determined and the activation processes of C-H bonds of ethane aroused by Zr atom are explored in detail. Then, the possible minimum energy reaction path is suggested and we propose the spin inversion mechanism between different spin multiplicities. The MECP is determined with the help of the DFT-FON approach. At last, frequency and NBO analysis for all intermediate and product species are also applied to confirm with the experimental observed data.In Chapter 4, the spin-flip reactions of the IV transition metal Ti + C2H6 and Hf +C2H6 are also researched. Firstly, we obtain the equilibrium geometries and energies of all intermediates, transition states and products in both of the triplet and singlet spin states. Then, PESs are analyzed in different spin-restricted state and both of the reactions would be searched for the MECPs through the spin inversion mechanism. At last, we study the trends of reactivity in detail of Ti, Zr, Hf interacting with ethane.In Chapter 5, we explore the integrated reaction mechanisms for Re + CH3 CN and Mn + CH3 CN theoretically which obtain the methylidyne(CH≡M(H)2NC) and methylidene(CH2=M(H)NC) isocyanide products, respectively. Firstly, we analyze the effect on the different theoretical methods and choose the best one. Then, the activation mechanisms of C-H and C-C bonds for both reactions are researched in detail, three adiabatic PESs in sextet, quartet and doublet spin-states are determined,each equilibrium and transition state are obtained. And, the detailed minimum energy reaction pathway is suggested. The MECPs around the intersystem crossing region have been determined by the DFT-FON approach. At last, to better understand the detailed reaction mechanism, the properties of stationary states on both sides of the spin-flip process are analyzed by the aspects of geometry, energy, harmonic frequency,NBO analysis, Wiberg bond order, Weinhold charges, etc.In Chapter 6, we investigate the reaction for acetonitrile with the VIII transition metal Os atom theoretically. Firstly, we research the three adiabatic PESs along quintuplet, triplet and singlet spin states and suggest the possible minimum energy reaction pathway. Then, the spin-flip transition structures for intersystem crossing areexplored and the integrated reaction mechanism is completed. In addition, the NBO analysis is applied for the equilibrium species. We also analyze the trends of reactivity of group VII-VIII metal atoms with acetonitrile in detail.Lastly, the summary is briefly extracted for this dissertation. |
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