Valence bond theory reaction path

In the last few years, the polarizable continuum model for the study of solvation has been extended to consider multideterminantal wavefunctions. Such novel techniques allow the study of the most important solvent effects on chemical reactions. In this context, the valence bond theory provides a way to analyze such effects through the transcription of the, generally, complicated multiconfigurational wavefunctions into sums of few selected classical structures, which are, in fact, more useful to understand the electron distribution rearrangement along a reaction path. In this chapter, the valence bond analysis of CASSCF wavefunctions calculated for chemical reactions in solution is discussed in details. By way of example, the results for some basic chemical processes are also reported. 

Either the molecular orbital or the valence bond method may be used to analyse the interaction of substituents on the reactant, products, and transition states. Substituents which stabilise the transition state more than reactants will accelerate the reaction, while those which stabilise the reactant more than transition state will slow down the reaction. In valence bond theory, the reactants are represented hy a nucleophile lone pair as an anion (Fig. 6-21, path a) and a neutral nucleophile (path b), and the carhonyl by the covalent and ionic re.sonance structures [70]. 

An alternative to the MO method for the quantum mechanical treatment of molecular systems is the so-called Valence-Bond (VB) theory where molecular wavef unctions Eire obtained as linear combinations of covalent and ionic structures. It was shown long ago 181> that for distances larger than equilibrium distances, VB approximate wave functions should be better than MO functions of the same level, and hence VB theory should find its most profitable application in the evaluation of potential surfaces and reaction paths. Although true in principle, this statement has little influence in practice this is mostly because VB theory has only recently been formulated in a nonempirical form 182-184) so that applications are only just beginning to appear. 

ACES II Anharmonic Molecular Force Fields Bench-mark Studies on Small Molecules Complete Active Space Self-consistent Field (CASSCF) Second-order Perturbation Theory (CASPT2) Configuration Interaction Core-Valence Correlation Effects Coupled-cluster Theory Density Functional Theory (DFT), Hartree-Fock (HF), and the Self-consistent Field G2 Theory Heats of Formation Hybrid Methods Hydrogen Bonding 1 M0ller-Plesset Perturbation Theory NMR Data Correlation with Chemical Structure Photochemistry Proton Affinities r 2 Dependent Wave-functions Rates of Chemical Reactions Reaction Path Following Reaction Path Hamiltonian and its Use for Investigating Reaction Mechanisms Spectroscopy Computational... 

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