Empirical valence bond models

Electrostatic stabilization, 181, 195,225-228 Empirical valence bond model, see Valence bond model, empirical Energy minimization methods, 114-117 computer programs for, 128-132 convergence of, 115 local vr. overall minima, 116-117 use in protein structure determination,... 

Molecular dynamics free-energy perturbation simulations utilizing the empirical valence bond model have been used to study the catalytic action of -cyclodextrin in ester hydrolysis. Reaction routes for nucleophilic attack on m-f-butylphenyl acetate (225) by the secondary alkoxide ions 0(2) and 0(3) of cyclodextrin giving the R and S stereoisomers of ester tetrahedral intermediate were examined. Only the reaction path leading to the S isomer at 0(2) shows an activation barrier that is lower (by about 3kcal mol ) than the barrier for the corresponding reference reaction in water. The calculated rate acceleration was in excellent agreement with experimental data. ... 

Advancement of MD Methods 2.4.1. Empirical Valence Bond Models... 

The Knudsen effusion method In conjunction with mass spectrometrlc analysis has been used to determine the bond energies and appearance potentials of diatomic metals and small metallic clusters. The experimental bond energies are reported and Interpreted In terms of various empirical models of bonding, such as the Pauling model of a polar single bond, the empirical valence bond model for certain multiply-bonded dlatomlcs, the atomic cell model, and bond additivity concepts. The stability of positive Ions of metal molecules Is also discussed. 

The use of empirical models of bonding has been Invaluable for the interpretation of the experimental dissociation energies of diatomrLc Intermetallic molecules as well as for the prediction of the bond energies of new molecules. In the course of our work, conducted for over a decade, we have extended the applicability of the Pauling model of a polar single bond (31) and have developed new models such as the empirical valence bond model for certain multiple bonded transition metal molecules (32,33) and the atomic cell model (34). 

The empirical valence bond model has shown good predicting power if one defines the bond multiplicity to be two for compounds such as CePd or BaPd and three for compounds such as LaRh, Ylr, or RhTh. The limitations of the model could also be shown (40) when applied to assumed quadruple bond formation, as in RuV or ThRu or to double bond formation as in CePt or ThPt. The case of ThRu (41) shows that quadruple bonding is approached where suitable valence states are available, but not fully achieved, apparently because of the directional requirements of the bonding in a diatomic molecule. The examples of CePt and ThPt (40) show the calculated values that have been based on an assumed double bond to be too low. Platinum has no suitable valence state for triple bond formation, but apparently forms triple bonds with other... 

A comparison of experimental values for intermetallic diatomic molecules with gold with the corresponding value calculated by the Pauling model and by the atomic cell model has been given in Table 6 of Reference ( ). Table 7 of Reference ( ) shows a comparison between experimental dissociation energies with values calculated by the atomic cell model and the empirical valence bond model. Table 9 of Reference ( ) takes Mledema s refinements (43) of the atomic cell model into account In these comparisons. 

D.E. Sagnella and M.E. Tuckerman, An empirical valence bond model for proton transfer in water, J. Chem. Phys., 108(1998), 2073-2083. 

Chang, Y.-T. and Miller, W.H. (1990) An Empirical Valence Bond Model for Constructing Global Potential Energy Surfaces for Chemical Reactions of Polyatomic Molecular Systems, J. Phys. Chem. 94, 5884-5888. 

Vuilleumier, R. and Borgis, D. (1998). An extended empirical valence bond model for describing proton transfer in H 1 (H20) clusters and liquid water. Chem. Phys. Lett. 284,... 

Schmitt, U.W. and Voth, G.A. (1998). Multistate empirical valence bond model for proton transport in water. J. Phys. Chem. B 102, 5547-5551... 

VuiLLEUMiER, R., Borgis, D., An Extended Empirical Valence Bond Model for Describing Proton Transfer in H" (H20) Clusters and Liquid Water, Chem. Phys. Lett. 1998, 284, 71-77. 

Schmitt, U. W., Voth, G. A., Multistate Empirical Valence Bond Model for Proton Transport in Water, f Phys. Chem. B 1998, 102, 5547-5551. 

G. A., A multi-state empirical valence bond model for weak acid dissociation in aqueous solution, J. Phys. Chem. A 2001, 105, (12), 2814-2823. 

Other approaches to model transition states with force fields are the empirical valence bond model (EVB). [336] the reactive force field (RFF) [337] and the multiconfigurational molecular mechanics (MCMM) method [338]. 

In the empirical valence bond model [31], the potential energy surface is typically non-linearly related to two model states, called effective diabatic states corresponding to the reactant and the product bonding characters, Eq. 9. Although additional diabatic states, important for describing the transition state, can be included and have often been discussed, the computational complexity makes it difficult in practical use for the study of enzyme reactions. 

Karmelin SCL, Warshel A (2011) The empirical valence bond model theory and applications. WIREs Comput Mol Sci 1 30... 

Recently, we studied the water dependence in mixtures of water and the protonated form of Nafion [53] using both standard force field models and an empirical valence bond model to account for the Grotthuss structural diffusion mechanism of aqueous proton transport. Results showed a transition of an irregularly shaped filamentous (cylindrical) structure in the case of low water (A = 5, where A is defined as the ratio of water molecules to sulfonate groups) to a structure more in accord with the above-discussed models of nano-separation, where larger clusters form which are connected by narrow bridges. A comparison of aqueous cluster sizes indicated, for a simulation time of 30 ns, no percolating clusters for A = 5, whereas at A = 10 most water molecules were located in a connected cluster (see [53]). Other structural... 

Computer simulations of excess proton conductivity in water have reached a powerful level [8,92,93,102]. Importantly, simulations extend to quantum-mechanical proton dynamic features, so that proton motion can be coupled to details of the molecular environmental dynamics. A recent feature article explored an analytical theory in order to rationalize these complex processes that involve interconversion of proton-bearing clusters and proton transfers [103]. With a simple two-state empirical valence bond model (see below for details), which implements in a classical way the above-mentioned idea of two limiting protonated structures, namely the 11502 and the H30 cluster, it was indeed observed that the two alternative sequences are equivalent with similar life times for both clusters, and that conversions between the two clusters are purely fluctuative. 

The unconstrained dynamics of anionic groups in the TAM solid enables the substantial local reorganizations by which the intermediate state could form. This indicates that an appropriate flexibility of anionic sidechains could be vital for high proton mobility in PEM under conditions of minimal hydration and high anion density. This was also demonstrated independently by MD simulations on the basis of an empirical valence bond model [83] (see below). 

The empirical valence-bond model has been used to compare the 5 2 identity reaction of methyl chloride in water, at the water-vapour interface, and in the gas phase. The rate of reaction decreases as one goes from bulk solution through the water-vapour interface, and increases only when the reaction occurs more than 10 A... 

Brancato G, Tucketman ME (2005) A polarizable multistate empirical valence bond model for proton transport in aqueous solution. J Chem Phys 122 224, 507... 

Ufimtsev IS, Kallnlchev AG, Todd MJ, Kirkpatrick RJ (2009) A multistate empirical valence bond model for solvation and transport simulations of OH in aqueous solutions. Phys Chem Chem Phys 11 9420... 

Wu Y, Chen H, Wang F, Paesani F, Voth GA (2008) An improved multistate empirical valence bond model fra- aqueous proton solvation and transport. J Phys Chem B 112 467... 

I. Benjamin,/. Chem. Phys., 129, 074508 (2008). Empirical Valence Bond Model of an S 2 Reaction in Polar and Non-Polar Solvents. 

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