Quantum Chemical Study On Two-State Reactivity Of Small Molecules

Recently, theoretical approaches to gas-phase transition-metal chemistry indicated that often the reactants, possible intermediates, and products had ground states of different spin multiplicities, namely, do not obey "spin conservation law". Examples were the reduction of water by the early transition-metal cations Sc⁺V⁺, oxidations involving the cationic metal dioxides VO₂⁺ and CrO₂⁺. However, the spin inversion itself was often neglected, and it had even been argued that "the notion of 'spin-forbidden' of organometallic reactions in the condensed phase was inappropriate", Although the necessity to explicitly consider surface hopping as a mechanistic step in organometallic chemistry was pointed out more than 10 years ago, the assumptions of either strict spin conservation or its complete neglect prevailed until 1994. This was in part due to difficulties in the appropriate theoretical descriptions of spin crossover in polyatomic metal compounds and, more importantly, the lack of unambiguous experimental examples for a violation of spin conservation along the reaction path, which provoked the necessity to tackle "two-state reactivity( TSR)". Today, Chemists had become increasingly interested in TSR in the world. In this paper, we choosed several typical reactions(Ge Sn Pb, Au⁺ N₂O N and CS₂ etc.) which have been carefully studied using quantum methods, obtained some interesting results.In the paper, on the basis of the molecular orbital theory, the tradition transition state theory as well as quantum chemistry theory, the systems choosed have been investigated using Density Functional (DFT) Methods, the Moller-Plesset correlation energy correction MPn, the coupled cluster CCSD(T) calculations, the time-dependent DFT method and the Natural Bond Orbital analysis. The structures of the reagents, the reaction products and the transition states along the reaction paths have been obtained, then obtained the reaction surfaces, the spectrum datum, the thermodynamic datum as well as the information of orbitals. The reaction mechanism has been argued deeply using these datum.The whole paper consists of eight chapters. Chapter 1, described the theoretical basis, application, limits and error correcting of ab initio. At the same time, the development and the current situation of two-state reactivity( TSR) are explained. In Chapter 2, simply introduced elementary theory, mainly contained the reaction surface, crossing rules of the potential energy surfaces, tradition transition state theory, spin-orbit coupling mechanismand rules for intersystem crossing. The contents of the two chapters were the basis and background of our studies and offer us with useful and reliable quantum methods.In Chapter 3 and Chapter 4, Two-state reactions of N₂O with Ge Sn Pb and Au⁺ have been studied carefully using B3LYP and CCSD(T) methods. First, to monitor the electronic rearrangements in the coordination process, the atomic charges with orbital populations along the IRC path are calculated using NBO and molecular orbital theory. It turns out that electron transfer is an no essential factor in the reaction, and the coulombic interaction and exchange repulsion interaction have important role in the initial complexes. Second, Crossing points are locatized by means of the intrinsic reaction coordinate single point vertical excited approach in this paper. The mechanisms of spin inversion are discussed by TD-B3LYP method and rules for intersystem crossing, and obtained some important results. In Chapter 5 and Chapter 6, We choose another interesting reactions which are single-state reactions of N atom with CS₂ molecule, and XCOOH (X=F, Cl, Br),their reactions mechanisms have been discussed using molecular orbital symmetry theory, at the same time, the reactions activation thermodynamic data were calculated, based on the transition states theory, and the rate constants, frequency factors and reaction equilibrium constants were obtained under different temperature.In Chapter 7 and Chapter 8, While study the reaction mechanism, we also manage...