Density Functional Study On The Reaction Mechanisms Of The Activation Of The Low Alkanes By The Selected Transition Metal Clusters And Oxide Ions

The activation of the low alkanes is of special significance,beacuase it can transform the raw material to the functional products.However,the transformation and activation of the alkanes require harsh conditions due to their solid stability.Therefore,looking for efficient industrial catalysts has been a research hotspot.An effective means is to research the activation of C-C and C-H bond by transition metal(or clusters)ions in gas phase since it can explore the intrinsic factor free of various condensed phase effects.Over the past few decades,the activation of the low alkanes by transition-metal cluster ions in the gas phase has attracted considerable attention both experimentally and theoretically,which provided important mechanistic information and models for analogous reactions.In this thesis,we have been performed the density functional studies to explore the mechanisms of the activation of methane,ethane and propane by the transition metal clusters Rhn+(n=2-4),transition metal oxide ions OsO3+ and transition metal ions Pt+ in gas phase.The structures of the reactants,the reaction products and transition structures along the reaction paths have been obtained,and then the potential energy surfaces of the reactions,thermodynamic data as well as the information of orbitals are obtained.The calculated results can explain the relevant experiment,and provide a theoretical basis for further research.Specific contents are as follows:Firstly,density functional calculations with UB3LYP,modified LanL2dz+f and 6-311G**for Rh and other elements,respectively,have been performed to investigate the mechanism of the reaction of the activation of methane by gas-phase Rhn+(n=2-4).The doublet,quartet,and sextet potential energy surfaces of the reactions of the activation of methane by the c112 and RrcN have been explored.The singlet,triplet,and quintet potential energy surfaces of the reaction of the activation of methane by the rcn3 have been explored.The results show that the reactions occur adiabatically on the two or more potential energy surfaces with different spin states.The reactions also involve crossings between the high-spin and low-spin potential energy surfaces.For the reaction of rcn2 with methane,the overall reaction is calculated to be exothermic by 71.5 kJ/mol.The barrier of the rate-determining step for is 91.6 kJ/mol.So it can take place at room temperature spontaneously.For the reaction of Rh3+ and Rh4+ with methane,the overall reactions are endothermic by 17.1 kJ/mol and 54.5 kJ/mol,respectively.The barriers for the rate-determining steps of these reactions are 82.1 kJ/mol and 65.9 kJ/mol,respectively.These two reactions are not likely to take place under room temperature.The results are in consistent with the findings from FT-ICR experimental studies.Secondly,the reaction mechanism of the activation of methane and ethane by Os03+ have been investigated using the density function theory method with UPBEO and 6-311GaG*basis set.The results show that the potential energy surfaces in sextet and quartet are located much higher in energy than the PES in doublet state.In other words,no spin inversion occurs in the title reactions,and the mnimium energy reaction pathway only involves the doublet PES.For the reaction of Os03+ with ethane,the overall reaction is exothermic by 190.8 kJ/mol.The rate-determining step is the activation of the second C-H bond,which its barrier is 59.6 kJ/mol.Hence it found to be thermodynamically and Kinetically preferred.For vsV3+ with methane,it is exothermic by 102.2 kJ/mol.The barriers for the rate-determining step are 177.4 kJ/mol and the Gibbs free energies of the transition state are above that of the reactants.Our calculations show that the channel is not kinetically preferred,despite the fact that the channel is thermodynamically more favorable.These results are in accord with the experimental observation.Thirdly,the potential energy surfaces(PESs)and the detailed mechanisms of the reaction of platinum cation with propane have been investigated by the UB3LYP method.Calculations show that the overall reactions are exothermically by 151.5 kJ/mol,199.8 kJ/mol,208.7 kJ/mol and 173.6 kJ/mol for 1,1-,1,2-,2,2-and 1,3-elimination of single dihydrogen,respectively.According to the negative ?G298,the reaction is thermodynamically favorable reaction channels.The barriers for the rate-determining step are 71.7 kJ/mol,48.7 kJ/mol,55.4 kJ/mol and.78.4 kJ/mol respectively.This indicates that the dehydrogenation of propane can occur through the four approaches.Only in 1,2-dehydrogenation path,every steps are exothermic reaction.So 1,2-dehydrogenation is the most easily to occur.The results could justify the conclusions from experimental studies.