Epiotis, Shaik, Pross, Hiberty, Harcourt, etc. The general VB model method has been developed, which enables it to study chemical reactions, aromaticity and anti-aromaticity, transition metal bonds, etc., and inspired VB, QM(VB)/MM and other methods have been developed to deal with the influence of macromolecules and solvents. The establishment of a new VB qualitative model lays the foundation for the emergence of non-semi-empirical VB calculation methods.
1972, W.A. Goddard put forward the generalized valence bond theory (GVB). The wave function of multi-electron system is expressed by VB wave function. For the linear combination of trial wave functions, the coefficients are obtained from the duration equation, and the symmetry factor is considered. GVB is a variational method.
Balint-Kurti and Vanrens proposed the valence bond self-consistent field (VBSCF), which introduced the basic self-consistent field method of molecular orbital theory into valence bond theory. However, unlike the HF method of molecular orbital theory, VBSCF only calculates the wave function, and optimizes the molecular structure coefficient and VB orbit to minimize the energy. VBSCF considers the static correlation energy, which ensures the correctness of qualitative conclusions, but the quantification is still not accurate enough.
In the early 1980s, this problem was solved. On the basis of MO-VB wave function expansion method -Hiberty semi-determinant method and corresponding theoretical model -CASVB, P. C. Hiberty of the University of Paris-Sud, France, improved VBSCF, established the valence bond of respiratory orbit (BOVB), and introduced the dynamic correlation through the freedom of movement, which made this method achieve satisfactory quantitative accuracy. Although BOVB is difficult to converge and cross-bond, it is still a theory that can really be compared with Hartree-Fock self-consistent field method in molecular orbital theory, and it is a milestone in the development of valence bond theory.
Later, similar to the development process of self-consistent field method after molecular orbital theory, various improved theories of BOVB appeared. At the same time, because the valence bond theory has been quite accurate, it is possible to deal with the thorny problems of molecular orbital theory from a new angle.
The most basic viewpoint of VB and MO is the combination of tracks, and the tracks are combined into configurations. We can't give it an intuitive explanation, but VB can be understood as a link. However, the combination of configurations can tend to a real wave function. The configuration of MO used for combination is mostly difficult to analyze, and it is also difficult to intuitively explain the configuration-related effects. The configuration of VB used for combination is the wave function of anion and cation, the form of classical valence bond and free radical, and their combination can be understood as * * * vibration. Theoretically, the combination of all configurations-casscf (completely active space self-consistent field) of ——MO and CASVB of VB is the real wave function. This proves that MO and VB actually tend to the same limit, but they have different characteristics by adopting different approximation methods. It is absurd to arbitrarily say that valence bond theory is only a qualitative theory that is easy for chemists to understand.
Molecular orbital theory is good at dealing with molecular structure problems and designing orbital symmetry, while modern valence bond theory shows its advantages in reaction mechanism and reaction energy barrier height on the basis of keeping the traditional easy-to-understand characteristics: through VB state correlation diagram, the energy barrier of SN2 reaction is only related to the ionization potential of nucleophilic reagent and the electron affinity of substrate, while the energy barrier of free radical reaction is only related to the energy difference of bond-broken triplet and singlet, which is convenient to understand and calculate, but it must be achieved. However, the explanations given by Hiberty and S. Shaik of Hebrew University in Israel on the properties, aromaticity and anti-aromaticity of * * * yoke (benzene ring substituted by alkyl in Siegel molecule presents the structure and properties of cyclohexene) are far behind.