Generalized valence bond model

Generalized Valence Bond Model, vertical excitation... 

Evaluated In a photoionization mass spectrometric study from the difference in the thresholds for PHJ from PH3 and PHJ from PHg [1 ]. This value is only 4 kJ/mol higher than the almost coincident values calculated with the 4th-order Moller-Plesset perturbation procedure [1, 8] and with the generalized valence bond model [9]. - From the electron impact ionization of PH3 and PHg. - From the fluorescence excitation of PH3 photolysis fragments. - From the appearance potential of PHJ in electron impact studies of PH3 and the ionization potential of PHg. - From the appearance potential of PH2 from PH3 (2.2,2.3 eV) and the electron affinity of the PH2 radical (1.25 eV, see p. 62) [5] for earlier results (D<326 kJ/mol), see [10]. - From the highest populated rotational-vibrational level of HF, which is produced in a hydrogen abstraction reaction of PH3 with F atoms in a flowing afterglow experiment [6] for earlier results, see [11]. - Literature value based on the upper limit D<326 kJ/mol. 

The NRT description of TS complexes is closely related to the general two-state valence bond model of Shaik and Pross.93 This model emphasizes the coupled changes in two adiabatic states that evolve from distinct diabatic valence-bond (VB) configurations r and states cross in energy at s = s, ... 

To test the generality of the jr-distortivity phenomenon and of the Valence Bond model for delocalization, it is of interest to apply the o-it partition to conjugated molecules other -than hydrocarbons, e.g. containing nitrogen, silicon or phosphorus atoms that we have kept in a constrained planar geometry. The total distortion energies, as well as their a and it components are displayed in Table 3, as calculated at the 6-31G/it-CI level. 

FIGURE 6.37 The electron density for the if/g and ifil wave functions in the simple valence bond model for H2. (a) The electron density pg for if/g and Pu for calculated analytically as described in the text, (b) Three-dimensional isosurface of the electron density for the ipg wave function, as calculated numerically by Generalized Valence Bond Theory (GVB). 

Intermediate between the spin-coupled expansion in N orthonormal sets and the MO expansion in a single set is an expansion in just two distinct sets. The first term of (17) now becomes the electron-pair wavefunction of Hurley, Lennard-Jones and Pople " , in which pairs of orbitals (i/ i,i/ 2X (i/ j, 1/ 4),..., overlap with each other, but different pairs are (strongly) orthogonal. Much work has been carried out on this model by Goddard and coworkers under the nomenclature generalized valence bond (GVB) wavefunction . ... 

The general theory outlined in Chapter 14 has been used in the ROHF model. It can also provide the basis for a whole class of multi-determinant models of molecular electronic structure which are constructed from sets o/doubly occupied orbitals. Two such models are outlined in this chapter Paired-Electron MCSCF (PEMCSCF) and the General Valence Bond (GVB). These pair expansion theories are based on the idea of natural orbitals which diagonalise the one-electron density matrix. 

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