Valence bonds, types

It is readily apparent that the three a bonds are capable of holding the six bonding electrons in the a t and e molecular orbitals. The possibility of some 7r-bonding is seen in the molecular orbital diagram as a result of the availability of the a2" orbital, and in fact there is some experimental evidence for this type of interaction. The sum of the covalent radii of boron and fluorine atoms is about 152 pm (1.52 A), but the experimental B-F bond distance in BF3 is about 129.5 pm (1.295 A). Part of this "bond shortening" may be due to partial double bonds resulting from the 7r-bonding. A way to show this is by means of the three resonance structures of the valence bond type that can be shown as... 

Molecular-orbital- versus valence-bond-type descriptions Let us now examine the connection between localized and delocalized descriptions in somewhat more formal terms, using the actual numerical energy levels of PtH42 to illustrate the formal connections. Inevitably, this discussion entails somewhat more technical detail concerning computational MO methodology than required elsewhere in this book, and we assume a general familiarity with Sections 1.4 and 1.5, Appendix A, and other sections mentioned below. 

LEPS London—Evans—Polanyi—Saito. A valence bond type method based on HLVB and used to construct potential energy surfaces for molecular dynamics. 

Early quantum mechanical calculations treating chemisorption employed a valence bond-type method and a few atoms to simulate the surface. Sherman and Eyring (54) used a 4-atom model to simulate H2 dissociation on charcoal. The calculations were unable to duplicate the low activation energy observed experimentally for this reaction. Later calculations by Sherman and Eyring (55) showed that H2 dissociation on Ni2 has a low activation energy, suggesting that quantum mechanics is a useful tool for such studies. 

As the chemist has gained his sea legs in the use of wave mechanics he has attempted to define covalent and ionic character in terms of wave functions. The new point of view is presented, for example, by Coulson, who maintains, there are two distinct definitions of a covalent bond (447, p. 145). He proceeds to describe the bond wave function, first by a molecular orbital type approximation, and then by a valence bond type approximation. In either case the approximate wave functions consist of linear combinations of atomic orbitals. Partial ionic bonding is revealed by the magnitudes of coefficients which imply asymmetry of electron distribution. 

A much more realistic set of CMD calculations is presently under way at Penn [46] which involves the dynamics of excess proton migration in water. To this end, a valence bond type of model has been developed for the stable states in the dimer. This approach has proven... 

In the empirical valence bond (EVB) model developed by Warshel s group, the protonation states of the system are described by valence bond type functions and the matrix representing the Hamiltonian is calculated with force-field like functions accounting for the interactions with the protein and the solvent. These functions are calibrated to reproduce experimental or ab initio data. The smallest eigenvalue of the matrix gives the ground state PES g( )."... 

Magnetic measurements find intensive applications in the study of transitional metal 7r-complexes. Such data can give information that is useful for the elucidation of valency, bond type, and stereochemistry of such compounds. 

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