The MOVB Method

In the MOVB method, we use one Slater determinant with block-localized molecular orbitals to define individual VB configuration, called diabatic state. For example, the reactant state of the Sn2 reaction between HS- and CH3CI is defined as the Lewis bond structure of the substrate CH3CI  

The MOVB wave function for the adiabatic ground state is written as a linear combination of the diabatic states in Eqs. (4-11) and (4-12)  

The CDC-MOVB method is the appropriate computational approach for studying properties associated with the adiabatic ground state such as the reaction barrier for a chemical reaction and the solvent reorganization energy. 

While MOVB can yield reasonable energetic results and an excellent description of the overall potential energy surface on diabatic states and the adiabatic ground 

In the EH-MOVB model, the energy of the diabatic ground state is determined by using the modified secular equation  

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Abstract A mixed molecular orbital and valence bond (MOVB) method has been developed and applied to chemical reactions. In the MOVB method, a diabatic or valence bond (VB) state is defined with a block-localized wave function (BLW). Consequently, the adiabatic state can be described by the superposition of a set of critical adiabatic states. Test cases indicate the method is a viable alternative to the empirical valence bond (EVB) approach for defining solvent reaction coordinate in the combined quantum mechanical and molecular mechanical (QM/MM) simulations employing explicit molecular orbital methods. 

Solvent effects can significantly influence the function and reactivity of organic molecules.1 Because of the complexity and size of the molecular system, it presents a great challenge in theoretical chemistry to accurately calculate the rates for complex reactions in solution. Although continuum solvation models that treat the solvent as a structureless medium with a characteristic dielectric constant have been successfully used for studying solvent effects,2,3 these methods do not provide detailed information on specific intermolecular interactions. An alternative approach is to use statistical mechanical Monte Carlo and molecular dynamics simulation to model solute-solvent interactions explicitly.4 8 In this article, we review a combined quantum mechanical and molecular mechanical (QM/MM) method that couples molecular orbital and valence bond theories, called the MOVB method, to determine the free energy reaction profiles, or potentials of mean force (PMF), for chemical reactions in solution. We apply the combined QM-MOVB/MM method to... 

The method described above has been termed as the MOVB method,14 16 which represents a combined approach using MO and VB theories. The method perhaps is more conveniently illustrated by a specific example involving the SN2 reaction of... 

We illustrate the MOVB method by the SN2 reaction between Cl- and CH3C1, and apply this technique to model substitution reactions. We show that the MOVB method can yield reasonable results for the ground state potential energy surface of the Sn2 reaction both in the gas phase and in solution in comparison with MO and ab initio VB calculations. In all calculations, the standard Gaussian 6-31G(d) basis function is used to construct the MOVB wave function. 

In Fig. 2, the reaction coordinate XR is the difference between the two C-Cl distances, i.e., XR = Rr(C- Cl7) - Wp(C1 - C), where C-Cf is the carbon and leaving group distance and Cl-C is the nucleophilic chloride and carbon distance. The double well potential for an SN2 reaction is clearly characterized by the MOVB method with a binding energy of — 9.7 kcal/mol for the ion-dipole complex.51,52 This may be compared with values of —10.3 kcal/mol from HF/6-31G(d), — 10.5 kcal/mol from the G2(+) model,53 — 10.0 kcal/mol from ab... 

Figure 1 shows, in schematic form, a constellation of problems as treated by the MOVB method. The definition and solution of these and other problems constitutes the basis of a new electronic theory for chemistry. Some of the questions that arose in my mind about ten years ago and which led to the development of the ideas I will discuss in this monograph were ... 

With the introduction of two parameters in Eq. (4-15), the EH-MOVB method can be calibrated to reproduce exactly the experimental barrier height and the desired reaction energy. 

The VBSCF and EH-MOVB potential energy surfaces for the nucleophilic substitution reaction of HS and CH3CI are depicted in Figure 4-2. The energy contours determined using the EH-MOVB method (Figure 4-2A) are found to be in good accord... 

Figure 4-2. Computed potential energy surface from (A) ab initio valence-bond self-consistent field (VB-SCF) and (B) the effective Hamiltonian molecular-orbital and valence-bond (EH-MOVB) methods for the S 2 reaction between HS- and CH3CI...

In this article, we present an ab initio approach, suitable for condensed phase simulations, that combines Hartree-Fock molecular orbital theory and modem valence bond theory which is termed as MOVB to describe the potential energy surface (PES) for reactive systems. We first provide a briefreview of the block-localized wave function (BLW) method that is used to define diabatic electronic states. Then, the MOVB model is presented in association with combined QM/MM simulations. The method is demonstrated by model proton transfer reactions in the gas phase and solution as well as a model Sn2 reaction in water. 

Consequently, the present method is not limited to the MOVB potential energy surface. In our work, we have used both the MOVB and the HF energy as the ground state potential to compare the performance of the method.14,16... 

The three diabatic states used to construct the MOVB wave function have been described in equation (16) earlier to illustrate the computational method. The PMF for the reaction of methylsulfonium ion and ammonia in water have been determined both using the geometrical reaction coordinate (XR) and the solvent reaction... 

We have described a mixed MOVB model for describing the potential energy surface of reactive systems, and presented results from applications to SN2 reactions in aqueous solution. The MOVB model is based on a BLW method to define diabatic electronic state functions. Then, a configuration interaction Hamiltonian is constructed using these diabatic VB states as basis functions. The computed geometrical and energetic results for these systems are in accord with previous experimental and theoretical studies. These studies show that the MOVB model can be adequately used as a mapping potential to derive solvent reaction coordinates for... 

In Part One of this work, we will use the MOVB bond diagrammatic method in order to tackle problems,most of which lie "within" monodeterminantal MO theory, in order to demonstrate the conceptual power of MOVB theory. 

We are now prepared to outline a rational method for unlocking the secrets of C-Li bonding and their implications for the stereochemistry of lithiated hydrocarbons. As a first step, the general principles of MOVB theory are applied to the problem at hand, starting with application of the ID model and considering additional electronic factors, such as "classical" interaction effects and low lying vacant orbital participatipn. As a second step, quantum chemical computations are carried out in order to test the validity of the MOVB analysis. 

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