Valence bond theory defined

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. 

In classical Valence Bond theory, a bond is simply defined as a singlet coupled orbital (electron) pair. Thus, a single bond is obtained using ... 

Earlier in this chapter, you learned the definition of bond order in the valence bond theory. In molecular orbital theory, the bond order is defined as one-half the difference between the number of electrons in bonding orbitals and the number of electrons in antibonding orbitals. Mathematically, this can be expressed as... 

Valence bond theory, in the terms defined by Pauling, is not able to account for the 4n+2 rule, and the properties of cyclobutadiene and cyclooctatetraene. It has been suggested that the problem with these molecules is the strain associated with the bond angles in the planar structures.10 However, this was shown to be incorrect by the observation that the addition of two electrons to cyclooctatetraene leads to the planar dianion. It is only recently that it has been recognized that cyclic permutations must be included in order to properly treat cyclic systems via valence bond theory.11 One of Pauling s few failures in structural theory is his nonrecognition of the problems associated with the 4n molecules. 

The first electronic function to be defined in direct association with a formal chemical bond was, as expected, for the H2 molecule. It marked the beginning of the so called valence-bond theory, in 1927, and was suggested by the Austrian physicist Walter H. Heitler (1904—1981) and by the German physicist Fritz W. London (1900-1964). 

Equation (1) defines the valence bond theory for the ground state of the hydrogen molecule. McWeeny [51] describes how... 

The hybridization scheme in valence bond theory is a very useful concept for chemists since it permits a localized view of the bonding. The most general method for generating hybridized orbitals is based on defining a bond wavefunction (a linear combination of atomic orbitals) in a specific bond direction (usually the z-axis direction). Then the second and subsequent hybrids are obtained by a rotation transformation. Orthogonality conditions are then used to evaluate the hybrid coefficients. These bond wavefunctions are defined as equivalent because they differ from one another only by a rotation. Generally, the first bond wavefunction is... 

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. 

Valence Bond theory using localized orthogonal orbitals For small couplings, i.e. nearly degenerate 5 = 0 and 5=1 states. Valence Bond (VB) theory provides a more intuitive starting point than the previous molecular orbital reasoning. For the Ms = 0 wave functions we make use of the local orthonormal orbitals and tjrb as defined in Eq. 3.10a and use them to construct two neutral determinants and... 

How does each of the three major bonding theories (the Lewis model, valence bond theory, and molecular orbital theory) define a single chemical bond A double bond A triple bond How are these definitions similar How are they... 

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