Theoretical Study Of Cyclohexane Dehydrogenation Mechanism By Gas Phase Ni_~2+ Cationic Dimer

In recent years, alkanes dehydrogenation reaction catalyzed by gas-phase transition metal cationic dimers have attracted the interests of chemists. Metal dimers often show more higher reactivities compared to the atomic transition metals. For example, homonuclear Ni₂⁺ can catalyze the dehydrogenation of alkanes and cyclohexane more efficiently than atomic Ni+, what’s more, Ni₂⁺ also shows the most efficient reactivity in other 3d cluster dimers M2+. When Ni₂⁺ reacted with [all-cis^-1,2,3,4,5,6-D₆]-cyclohexane, the observed products can be divided into one-face dehydrogenation products of 2H₂, 3H₂, 2D₂, 3D₂ and two-face dehydrogenation products of [H₂, D₂], [H₄, D₂], [H₂, D₄], which means the reaction is operated by two competitive mechanisms. In order to explore these two competitive mechanisms, we study the reaction of Ni₂⁺/c-C₆H₁₂ system.In the paper, on the basis of the molecular orbital theory, the transition state theory as well as Schwarz experimental results, the Ni₂⁺/c-C₆H₁₂ system have been carefully investigated using Density Functional Theory（DFT） Methods. The structures of the reactants, products and transition states along the reaction paths have been obtained, which are applied to invesgate the potential energy surfaces, the thermodynamic data as well as the information of orbitals. Based on these data, the detailed reaction mechanisms have been invesgatied.The whole paper consists of four chapters. Chapter 1 mainly reviews the dehydrogenation reaction of alkanes and cyclohexane catalyzed by 3d transition metal cationic dimers and our main work in this paper. The second chapter summarizes the theory of quantum chemistry and calculation methods of this paper. The contents of two chapters were the basis and background of our studies and offer us with useful and reliable quantum methods.In chapter 3, the one-face dehydrogenation mechanism has been studied. The calculated results show that the frontier molecular orbits（FMO） of Ni₂⁺ and c-C₆H₁₂ can match each other completely. The formation of encounter complex contributes 46 kcal/mol heat to the closed system, which provides sufficient energy to surmount the reaction barriers. The first molecule of dehydrogenation performs quite facile with exothermic by 30 kcal/mol. the second and third dehydrogenations are calculated to be exothermic by 23 kcal/mol and 21 kcal/mol, respectively. The rate-limiting barrier is 21 kcal/mol. The one-face dehydrogenation contains two crosses of different potential energy surfaces（PESs）, the spin-orbit coupling（SOC） constants of these two minimum energy crossing points（MECPs） are calculated to be 268.74 cm^-1 and 306.67 cm^-1, respectively, which allow the flip of electrons in MECPs.In chapter 4, the flip mechanism leading to the two-face dehydrogenation has been investigated at the B3 LYP level of density functional theory. The flip mechanism presented here contains four steps.（A） insertion of Ni₂⁺ to C-C bond.（B） migration of H51 from C5 to C2.（C） rotation of C2-C3 σ bond and（D） formation of the “flipped” product 4^IM13. The one-face dehydrogenation has been confirmed as the dominated channel since the higher barrier of 33 kcal/mol of flip process. Possibilities for two-face dehydrogenation after Ni₂⁺ dissociating from 2^IM5（4^IM6）, 2^IM16（4^IM19） or H-D scrambling elimination from 2^IM11（4^IM12） have been excluded due to the large energy requirement. The flip process also contains two MECPs, the SOC constants of MECPs have been calculated to be 268.74cm^-1 and 306.67cm^-1, respectively, which allow the spin of electron near the MECPs.