Theoretical Study On The Mechanisms For Important C-H Activation And Hydrogen Atom Transfer Reactions

C–H activation （CHA） and hydrogen atom transfer （HAT） reactions have attracted much attention inchemistry. The development of selective and efficient methods to activate inert C–H bonds is one of the mostfundamental and challenging questions. In this thesis, density functional theory （DFT） has been employed toinvestigate the mechanisms for several important CHA and HAT reactions in detail. The main contents aresummarized as follows:（1） Theoretical study of the mechanism for C–H amination of alkanes, alkenes, alkynes and arenes byan electrophonic diruthenium nitrideThe reaction mechanism of C–H bond activation for various hydrocarbons （methane, ethane, ethene,ethyne and benzene） by electrophilic diruthenium nitride were comprehensively investigated by densityfunctional theory method at B3LYP level of theory. The polarized continuum model （PCM） andconductor–like screen model （COSMO） were used to mimic the role of solvent effect to the reactivity. ThisDFT computational investigation probed three possible reaction mechanisms. The first is the intermolecularH–atom abstraction mechanism in a concerted fashion, which undergoes N–atom insertion into a proximalC–H bond and creates the requisite C–N bond and N–H bond as well. The second is the intermolecularN–atom insertion mechanism which involves intermolecular C–N bond formation ensuing from C–H bondactivation and leads to a nitride intermediate. The third is the intramolecular C–H bond activation mechanismthat proceeds via intramolecular C–N forming and intramolecular hydrogen–atom （expect for C₂H₆, theintramolecular C–H activation mechanism is only one step, that is, the intramolecular N–H bond forming andC–H bond activation are concerted）. DFT calculations demonstrated that:（1） The relatively prohibitive highactivation energy barrier rules out the intermolecular H–atom abstraction mechanism;（2） The intermolecularN–atom insertion mechanism is the effective reaction pathway for C–H bond activation of C₂H₂. Whereas forthe C₂H₄and C₆H₆, the intramolecular C–H activation mechanism could preferentially occur.（2） Theoretical insights into the mechanism of benzonitrile hydrogenation into benzylamine anddihydrogen activation catalyzed by the ruthenium（II） complex RuH₂（H₂）₂（PCyp₃）₂The reaction mechanism of the RuH₂（H₂）₂（PCyp₃）₂–catalyzed hydrogenation of benzonitriles werecomprehensively explored by density functional theory method. The possible routes proposed in the literature,namely, the η²â€“mechanism （side–on coordinated） and η¹â€“mechanism （end–on coordinated） have beenconsidered in our study. Our calculations show that the reaction is initiated by the dissociation of H₂fromcomplex RuH₂（H₂）₂（PCyp₃）₂, then followed by the coordination of the substrate benzonitrile （PhCN） to themetal ruthenium centre, after that, a multi–step hydride transfer takes place. The overall process can beviewed as the―borrowing hydrogen‖methodology or―hydrogen autotransfer‖.9o probe the solvent effects, we make a comparison with the gas–phase results. It is shown that the solvent does not affect the reactionmechanism but does result in a considerable decrease in the barrier height. Theoretical calculations clearlydemonstrate that the η1–mechanism is not operating under the reaction conditions because of the high barriersand it can be discarded.<hile the η2–mechanism might be kinetically possible and thermodynamicallyfavored experimentally. or the case of the η2–mechanism, however, the reaction pathways take place viaseveral alternatives. In a typical hydrogenation of PhCN, H2can be activated by coordinating to metal. Thecalculations also prove that the intramolecular C–H bond can be activated and benzylimine can be formed inthe process of RuH2（H2）2（PCyp3）2–catalyzed hydrogenation of benzonitriles. This theoretical study expectedto provide fundamental element and deep insight into the nature of the hydrogenation mechanism.（3） Direct ab initio dynamics study of the reaction of C32（AΠu） with CH4The reaction of C2（A3Πu） with CH4has been investigated over a wide temperature range200～3000K bydirect ab initio dynamics method at the BMC-CCSD//BB1K/6-311+G（2d,2p） level of theory. The optimizedgeometries and frequencies of the stationary points are calculated at the BB1K/6-311+G（2d,2p） level, andthen the energy profiles of the reactions are refined using the BMC-CCSD method. The activation barrierheight for H-abstraction reaction was calculated to be4.44kcal/mol in temperature range （337～605K）, andthe electron transfer behavior was also analyzed by quasi-restricted molecular orbital method in detail. Thecanonical variational transition-state theory（CVT） with the small curvature tunneling（SCT） correction methodis used to calculate the rate constants over a wide temperature range200～3000K. The theoretical resultsshows that variational effect is to some extent large in lower temperature range, and small curvature andtunneling effect play important roles to the H-atom abstraction only at lower temperatures. The CVT/SCT rateconstants are in good agreement with the available experimental.（4） Direct ab initio study on the rate constants of radical C2（A3Πu）+C3H8reactionThe mechanism and kinetics of the radical3C2+C3H8reaction have been investigated theoretically bydirect ab initio kinetics over a wide temperature range. The potential energy surfaces have been constructed atthe CCSD（T）/B3//UMP2/B1levels of theory. The electron transfer was also analyzed by quasi–restrictedorbital （QRO） in detail. It was shown that all these channels proceed exclusively via hydrogen abstraction.The overall ICVT/SCT rate constants are in agreement with the available experimental results. The predictionshows that the secondary hydrogen of C3H8abstraction by3C2radical is the major pathway at lowtemperatures （below700K）, while as the temperature increases, the primary hydrogen of C3H8abstractionbecomes more important and more favorable. A negative temperature dependence of the rate constants for thereaction of3C2+C3H8was observed. The three–（k3） and four–parameter （k4） rate-temperature expressionswere also provided within243～2000K to facilitate future experimental studies（5） Direct ab initio dynamics study of rate constants and kinetic isotope effects for C32（AΠu）+CH3OHreactionThe hydrogen abstraction of CH33OH by C2（AΠu） has been investigated by direct ab initio dynamicsover a wide temperature range200～3000K. The potential energy surfaces have been constructed at the UCCSD（T）/aug-cc-pVTZ//UMP2/6-311++G（d,p） levels of theory. Two different hydrogen abstractions on themethyl and hydroxyl sites of methanol are considered. For the methyl H-abstraction, it is essentially ahydrogen atom transfer （HAT）, whereas the hydroxyl site H-abstraction is better described as a protoncoupled electron transfer （PCET） according to the Natural Bond Orbital analysis. The results suggest that themethyl site reaction is dominant, and the calculated rate constants are roughly consistent with availableexperimental values. On the other hand, the temperature dependence of deuterium kinetic isotope effects（KIEs） analysis reveals a substantial normal isotope effect in the methyl H-abstraction process, while normaland inverse KIEs coexist in the hydroxyl H-abstraction channel. Furthermore, the three and four–parameterexpressions of Arrhenius rate constants are also provided within200～3000K.