Valence-bond state model

Figure 3.39 The minimal PSB model, cis-C5/75M7)7 for the PSB photoisomerization. The vertical line indicates the carbon-carbon bond about which the excited state isomerization occurs and defines the two sides used in the definition of the valence bond states.

THE VALENCE BOND STATE CORRELATION DIAGRAM MODEL 117... 

Some Guidelines for Quantitative Applications of the Valence Bond State Correlation Diagram Model... 

In our theoretical formulation for PCET [26, 27], the electronic structure of the solute is described in the framework of a four-state valence bond (VB) model [41]. The most basic PCET reaction involving the transfer of one electron and one proton may be described in terms of the following four diabatic electronic basis... 

MD simulations based on empirical interaction functions are able to overcome some of the statistical Hmitations of the ab initio MD scheme. It is possible, at least for single pore environments to calculate proton mobihties in a statistically accurate way. The chemical natiue of proton transfer, i.e., the structural diffusion from one hydrated cluster to the next, is efficiently taken into account by the use of empirical valence bond (EVB) models, which have been introduced by Warshel [ 134] and later extensively used for aqueous proton transport in bulk water by Vuilleuimier and Borgis [92,135,136] and the group of Voth [93,137-141]. hi the simplest version of such a model, a two-state EVB model [102], the proton can be regarded as being in a superposition state between two different valence bond states, the first one corresponding... 

In the empirical valence bond (EVB) model developed by Warshel s group, the protonation states of the system are described by valence bond type functions and the matrix representing the Hamiltonian is calculated with force-field like functions accounting for the interactions with the protein and the solvent. These functions are calibrated to reproduce experimental or ab initio data. The smallest eigenvalue of the matrix gives the ground state PES g( )."... 

The valence bond state correlation diagram (VBSCD) serves as a model for understanding the status of electronic delocalization in isoelectronic series. Consider, for example, the following exchange process between monovalent atoms, which exchange a bond while passing through an X cluster in which three electrons are delocalized over three centers. 

An extension of this approximation to the case of chemical reactions has been proposed by Warshel within the framework of discrete solvent models, and by Hynes and co-workers within the continuum ones. In a proton transfer reaction, the diabatic states are either taken to be the neutral and ionic forms of valence bond states for a hydrogen bonded complex or are replaced by hypothetical charge distributions in which the charges appropriate to the reactant or product... 

Valence bond and molecular orbital theory both incorporate the wave description of an atom s electrons into this picture of H2 but m somewhat different ways Both assume that electron waves behave like more familiar waves such as sound and light waves One important property of waves is called interference m physics Constructive interference occurs when two waves combine so as to reinforce each other (m phase) destructive interference occurs when they oppose each other (out of phase) (Figure 2 2) Recall from Section 1 1 that electron waves m atoms are characterized by their wave function which is the same as an orbital For an electron m the most stable state of a hydrogen atom for example this state is defined by the Is wave function and is often called the Is orbital The valence bond model bases the connection between two atoms on the overlap between half filled orbifals of fhe fwo afoms The molecular orbital model assembles a sef of molecular orbifals by combining fhe afomic orbifals of all of fhe atoms m fhe molecule... 

An alternative stream came from the valence bond (VB) theory. Ovchinnikov judged the ground-state spin for the alternant diradicals by half the difference between the number of starred and unstarred ir-sites, i.e., S = (n -n)l2 [72]. It is the simplest way to predict the spin preference of ground states just on the basis of the molecular graph theory, and in many cases its results are parallel to those obtained from the NBMO analysis and from the sophisticated MO or DFT (density functional theory) calculations. However, this simple VB rule cannot be applied to the non-alternate diradicals. The exact solutions of semi-empirical VB, Hubbard, and PPP models shed light on the nature of spin correlation [37, 73-77]. 

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