Valence bond theory, modem

T orbital for benzene obtained from spin-coupled valence bond theory. (Figure redrawn from Gerratt ], D L oer, P B Karadakov and M Raimondi 1997. Modem valence bond theory. Chemical Society Reviews 87 100.) figure also shows the two Kekule and three Dewar benzene forms which contribute to the overall wavefunction Kekuleform contributes approximately 40.5% and each Dewar form approximately 6.4%. 

Gerratt J, D L Cooper, P B Karadakov and M Raimondi 1997. Modem Valence Bond Theory. Chemical Society Reviews pp. 87-100. 

Several methods of quantitative description of molecular structure based on the concepts of valence bond theory have been developed. These methods employ orbitals similar to localized valence bond orbitals, but permitting modest delocalization. These orbitals allow many fewer structures to be considered and remove the need for incorporating many ionic structures, in agreement with chemical intuition. To date, these methods have not been as widely applied in organic chemistry as MO calculations. They have, however, been successfully applied to fundamental structural issues. For example, successful quantitative treatments of the structure and energy of benzene and its heterocyclic analogs have been developed. It remains to be seen whether computations based on DFT and modem valence bond theory will come to rival the widely used MO programs in analysis and interpretation of stmcture and reactivity. 

Cooper, D.L., Gerratt, J. and Raimondi, M. (1987) Modem valence bond theory, Advances Chem. 

The combination of modem valence bond theory, in its spin-coupled (SC) form, and intrinsic reaction coordinate calculations utilizing a complete-active-space self-consistent field (CASSCF) wavefunction, is demonstrated to provide quantitative and yet very easy-to-visualize models for the electronic mechanisms of three gas-phase six-electron pericyclic reactions, namely the Diels-Alder reaction between butadiene and ethene, the 1,3-dipolar cycloaddition of fulminic acid to ethyne, and the disrotatory electrocyclic ringopening of cyclohexadiene. 

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. 

D.L. Cooper, J. Gerratt and M. Raimondi,Modem Valence Bond Theory, in Ab Initio Methods in Quantum Chemistry, Vol. II, ed. K. P. Lawley (John Wiley Sons Ltd., Chichester, New York, Brisbane Toronto, Singapore, 1987). 

Ab initio modem valence bond theory, in its spin-coupled valence bond (SCVB) form, has proved very successful for accurate computations on ground and excited states of molecular systems. The compactness of the resulting wavefunctions allows direct and clear interpretation of correlated electronic structure. We concentrate in the present account on recent developments, typically involving the optimization of virtual orbitals via an approximate energy expression. These virtuals lead to higher accuracy for the final variational wavefunctions, but with even more compact functions. Particular attention is paid here to applications of the methodology to studies of intermolecular forces. 

Raimondi M, Cooper DL (1999) Ab Initio Modem Valence Bond Theory. 203 105 -120... 

Being firmly based in modem valence bond theory, the very compact descriptions of the bonding which have emerged from our many spin-coupled calculations facilitate a direct and, hopefully, very convincing description of... 

Ponthieux S, Paulmier C (2000) Selenium-Stabilized Carbanions. 208 113-142 Raimondi M, Cooper EL (1999) Ab Initio Modem Valence Bond Theory. 203 105-120 Reinhoudt DN,see van ManenH-J (2001) 217 121-162 Renaud P (2000) Radical Reactions Using Selenium Precursors. 208 81 -112 Rigaut S, see Astruc D (2000) 210 229 - 259... 

R. McWeeny, Valence bond theory. A re-examination of concepts and methodology, in Z.B. Maksic, W.J. Orville-Thomas (Eds.), Pauling s legacy. Modem modeUing of the chemical bond, Elsevier, Amsterdam, 1999, pp. 365-401. 

Examples of some of the types of information that may be obtained from such modem VB wave functions have been illustrated here by means of applications to the ground state of benzene, to the and a A state of FeH, and to the two lowest Ag states of various model polyene systems. It has to be hoped that widespread use of the CASVB procedures via packages such as molpro [8] could diminish the traditional barriers between molecular orbital and valence bond theory. 

Werner s coordination theory, with its concept of secondary valence, provides an adequate explanation for the existence of such complexes as [Co(NH3)6]Cl3-Some properties and the stereochemistry of these complexes are also explained by the theory, which remains the real foundation of coordination chemistry. Since Werner s work predated by about twenty years our present electronic concept of the atom, his theory does not describe in modem terms the nature of the secondary valence or, as it is now called, the coordinate bond. Three theories currently used to describe the nature of bonding in metal complexes are (1) valence bond theory (VBT), (2) crystal field theory (CFT), and (3) molecular orbital theory (MOT). We shall first describe the contributions of G. N. Lewis and N. V. Sidgwick to the theory of chemical bonding. 

Computer programs have been developed which can transform the results of molecular orbital calculations into NBOs. An optimal Lewis structure can be defined as that one with the maximum amount of electronic charge in Lewis orbitals (Lewis charge). A low amount of electronic charge in Lewis orbitals indicates strong effects of electron delocalisation [226-228]. In resonance structures, major and minor contributing structures may exist. These analyses provide results which are similar to modem valence bond theory methods. 

The remainder of this book will be devoted to the modem ideas of bonding in several important classes of molecules. The emphasis will be on the molecular-orbital theory, with comparisons made from time to time to the valence-bond theory. Of the many scientists involved in the development of these theories, the names of R. S. Mulliken (molecular-orbital theory) and Linus Pauling (valence-bond theory) are particularly outstanding. 

Although the VB method is not part of the standard arsenal of current quantum chemistry methods, its modern forms have their strong adherents, who point out that VB theory provides valuable conceptual insights and that the computational efficiency of VB methods is increasing. Some reviews of classical and modem valence-bond theory are S. Shaik and P. C. Hiberty in K. B. Lipkowitz et al. (eds.). Reviews in Computational Chemistry, vol. 20, Chapter 1, 2004, Wiley-VCH P. C. Hiberty and S. Shaik, J. Comput. Chem., 28, 137 (2007) A. Shurki, Theor. Chem. Acc., 116, 253 (2006) S. Shaik and P. C. Hiberty, A Chemist s Guide to Valence Bond Theory, Wiley, 2007 Shaik and Hiberty, WIREs Comput. Mol. ScL, 1,18 (2011) W. Wu et al., Chem. Rev., Ill, 7557 (2011) P. Su and W. Wu, WIRES Comput. Mol. ScL, 3, 56 (2013). 

For a classical presentation of resonance theory, see G. W. Wheland, Resonance Theory in Organic Chemistry, John Wiley Sons, New York, 1955. Models of molecular structure based on mathematical description of valence bond theory have been developed F. W. Bodrowicz and W. A. Goddard HI, in Modem Theoretical Chemistry, Methods of Electronic Structure Theory, Vol. 3, H. F. Schaefer III, ed.. Plenum Press, New York, 1977, Chapter 4 A. Voter and W. A. Goddard III, Chem. Phys. 57 253 (1981) N. D. Epiotis, Unified Valence Bond Theory of Electronic Structure, Springer-Verlag, Berlin, 1983 D. J. Klein andN. Trinajstic, eds.. Valence Bond Theory and Chemical Structure, Elsevier, Amsterdam, 1990 D. L. Cooper, J. Gerratt, and M. Raimondi, Chem. Rev. 91 929 (1991). 

R.D. Harcouit, (a) in Valence-bond Theory and Chemical Structme (Delete H2 postscript) (Elsevier Science BV editors D. Klein and N. Trinajstic) pp. 251-285 (1990). (b) in Pauling s Legacy Modem Modelling of the Chemical Bond (Elsevier Science B.V. editors Z.B. Maksic and W.J. Orville-Thomas) pp. 449-480 (1999). (c) in Quantum Chemical Methods in... 

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