Theoretical Study Of Activation C-X (X=H And C) N-H Bond Of Small Organic Compound By VO₂⁺ And W In The Gas Phase

In the recent years, experimental chemists have found that transition-metal cations M+ and their oxide cations MO+ have peculiar activations to the C-H, C-F, C-C and C-O bonds of small organic compounds in the gas phase using an inductively-coupled plasma selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Recently, theoretical approaches to these reactions also indicated that often the reactants and products had ground states of different spin multiplicities, and the transformations of spin multiplicities occurs frequently in thermal reactions. Namely, the reactions did not obey"spin conservation law". Usually, more than one state is involved in reaction process, which ensures the whole reaction always proceeds on the low-energy potential energy surface (PES). Such a phenomenon is called"two-state reactivity (TSR)". However, the spin inversion itself was often neglected. The assumptions of either strict spin conservation or its complete neglect prevailed until 1994, and the necessity to explicitly consider surface hopping as a mechanistic step in organometallic chemistry was accepted after the violations of spin conservation along the reaction path were observed by high-tech experimental instruments. This was in part due to the restriction of experimental conditions in the early phases and, more importantly, the dominant status of spin conservation along the reaction path. Today, TSRs have attracted a great deal of interest in the world for the sake of exact reaction mechanisms.In the thesis, the spin-forbidden reaction of gas-phase molecular activation by trans-mental will be studied at the density functional theory using the high-level basis. Using the mathematical algorithm proposed by Harvey, we obtain the minimum energy crossing point (MECP) of different potential energy surfaces (PESs), and then calculate the spin-orbit coupling matrix at this point, aim at understanding the impact of vibration for the spin-orbit coupling. Based on non-adiabatic version of RRKM theory and the Landau-Zener transition probability equation, we calculate the probability of hopping at the MECP and the micro-canonical rate coefficients k(E). Meanwhile, we will discuss electronic charge transfer and the intersystem crossing at the MECP using natural bond orbital (NBO) analysis. Since the pioneering works done by Landau, Zener and Stueckelberg the semiclassical theory has been well developed for the one–dimensional two-state problem. Our focus of interest is attempt to take the latest theory of Zhu-Nakamura to replace the Landau-Zener transition probability equation, to raise the accuracy of k(E). This research may thus provide an useful guide for understanding the mechanism of the trans-mental catalytic of spin-forbidden reaction.The whole thesis consists of five chapters. Chapter 1 and Chapter 2 describe the progress and application of quantum chemistry as well as the development and the present situation of TSR, and briefly introduce elementary dynamic theory and quantum chemistry 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 Chapters 3 and 4, the gas phase reactions of W and VO2+ with NH3, C2H4 have been studied carefully using UB3LYP methods and the activation of N—H, C—H and C—C bonds have been emphasized. The involving potential energy surface crossing behaviors have been discussed in detail. Firstly, for each reaction system, all molecular geometries were fully optimized on respective ground state and the lowest excited state PESs by high-level quantum chemistry calculation methods. Vibrational frequency calculations and intrinsic reaction coordinate (IRC) methods were used to characterize the reaction path channels on two PESs. Whereafter, a series of crossing points (CPs) and minimum energy crossing points (MECPs) involving the structures and energy values have been located and the spin inversion behaviors by which the reaction system can hop from one PES to another by effective spin-orbital coupling have been inspected. Finally, the energetically more favorable channel was confirmed according to the probability of hopping at the MECP by the thermodynamic and dynamic data, and try our best to calculate the rate coefficients according to the usual Rice-Ramsperger-Kassel-Marcus (RRKM) formula.All of our calculated results in this thesis are in agreement with the early experimental findings.