MOVB theory

The Type 3 SN2 reaction between Cl- + CH3SHf is interesting because it represents a formal anion-cation recombination through substitution. Because charges are annihilated in forming the transition state, polar solvents will significantly destabilize product formation. Fortunately, the loss in solvation of the two ions is compensated for by electrostatic attractions in bringing the two reactant species into contact. Therefore, the outcome of an SN2 reaction of Type 3 depends on the balance of Coulomb stabilization and solvent destabilization. The reactant and product diabatic states are defined as follows in MOVB theory ... 

According to the MOVB theory, anomeric interactions in lower row anomeric atoms must be accompanied by a decrease of A ,s (Eq. [19]). Such a statement stands in contrast to calculations performed by Reed and Schleyer (143), who found the strongest anomeric interactions at atoms of intermediate electronegativity (see Section III.B.V). Nevertheless Epiotis (203) was not... 

MOLECULAR ORBITAL - VALENCE BOND (MOVB) THEORY... 

With this background, we can now outline a general MOVB theory of chemical bonding which can be routinely implemented by following these steps ... 

In tailoring VB theory to an operationally useful tool for the practicing chemist, we are less interested in detail and more interested in immediate application which can inspire new experiments or computations. To this extent, one may alternatively use the compact MOVB theory of chemical bonding which can be implemented by following these steps ... 

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 now reexamine these concepts using MOVB theory. 

In a previous work f we applied MOVB theory to the problem of the stereochemistry of H2O and its derivatives. The salient features of our approach can be illustrated by reference to the conversion of Linear (L) to Bent (B) H2O. The appropriate bond diagrams are shown in Figure 1 and the subsystem convention is ... 

Let us recall the forms of the and matrix elements of MOVB theory ... 

MOVB theory is a rigorous qualitative theory of chemical bonding because none of these approximations has been made in developing the central concepts. 

N, 0, and F which in different combinations and numbers can define an infinite number of excited cores. The Periodic Table also provides us with weak and strong overbinding atoms which can serve as ligands of the aforementioned cores with a switch from the latter to the former nearly guaranteeing impressive stereochemical change, all predictable on the basis of MOVB theory,in a qualitative sense. Indeed, the research possibilities are endless and a systematic investigation is certain to be of great value. 

The expression of the total energy of a system as a sum of the energies of the individual subsystems is the direct result of the neglect of electron-electron interaction of these subsystems. This is done only for qualitative purposes. Indeed, the Independent Bond Model MOVB theory can be rigorously implemented in a quantitative ab initio sense without neglecting the subsystem bielectronic interaction. However, this is not one of the aims of this work. 

Replacement of hydrogen atoms by lithium atoms may radically alter the stereochemistry of the parent hydrocarbon". Thus can be summarized some of the most important findings of the imaginative computational work of Schleyer and his coworkers. What fundamental property of lithium is primarily responsible for the unexpected geometrical preferences of perlithio hydrocarbons Is there some way to predict the geometry of these molecules These and related questions can be dealt with within the frameWork of MOVB theory in a way which illustrates the basic utility of the Induced Deexcitation (ID) model presented in the previous... 

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. 

We also require an understanding of the equivalence relationships between MO and VB computational schemes. These have been discussed in a previous work and are displayed in Table 2. EHMO theory is equivalent to a brand of MOVB theory, termed EHMOVB theory, which involves the same integral approximations as EHMO theory. Because of these approximations, EHMOVB theory is an overdelocalized version of MOVB theory. Similarly, EHMO is an overdelocalized version of SCF-MO-Configuration Interaction(CI) theory. Also, SCF-MO theory is equivalent to a brand of MOVB theory which makes use of the so-called constraint approximation characteristic of monodeterminantal SCF-MO theory. Once again, this brand of MOVB theory is an overdelocalized version of MOVB theory. The same relationship... 

MOVB theory is a "perfect"theory of chemical bonding to the extent that all (nonrelativistic) electronic effects are explicitly included in the analysis though attention is paid exclusively to the electronic factors deemed most important in the case at hand. Because of this reason, i.e., due to the fact that MOVB theory does not make use of any of the standard approximations of Single Determinant (SD) MO theory, whether of the EHMO or SCF-MO type, the predictions of this brand of theory can formally be tested only by polydeterminantal computations. In this context, the computations cited in support of the LID model are valid indices of its success simply because the underlying principles of the LID model are "contained" within SD MO theory, these being overlap interaction principles. Furthermore, the prediction of... 

The complete MOVB theory of chemical bonding was first presented at the NATO Advanced Study Institute on "Topic in Theoretical Organic Chemistry" in Gargnano, Italy, in June, 1978. 

Orbital Valence Bond (MOVB) theory to the chemical community was entitled "Unified... 

With the background of the previous sections and the previous chapters, let us now consider how a conceptual unification of ground and excited state chemistry can be effected. First, we recognize that the conceptual poles of MOVB theory are two ... 

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