Valence bond analysis

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. 

S. Humbel, Short strong hydrogen bonds a valence bond analysis, J. Phys. Chem. A 106, 5517-5520 (2002). 

The purpose of this review is to demonstrate that considerable chemical insight about the nature of conical intersections, that have proven to be a central feature of organic photochemistry, can be obtained from a simple valence bond analysis. The electronic structure, the molecular structure, and the nuclear motions that lift the degeneracy at the apex of the conical intersection can all be rationalized a posteriori and predicted a priori using such methods. The ideas are illustrated with case studies including 2 + 2 cycloaddition, the ring opening of cyclohexadiene and diarylethenes, benzene photophysics, the cycloaddition of ethylene and benzene, etc. 

Mike Robb is a leading exponent of the computational chemistry of electronically excited states and their intersections with ground states of molecules undergoing chemical reaction, as occur during the course of photochemical reactions. His chapter. In this molecule there must be a conical intersection, distils the underlying theory and practical experience of h h-level numerical computation on several instructive examples in order to demonstrate that considerable chemical insight about the nature of conical intersections can be obtained from a simple valence-bond analysis. The emphasis is to present a way of thinking that can be used not only to rationalize the... 

Valence Bond Analysis of the Lone Pair Bond Weakening Effect for the X—H Bonds in the Series XH = CH4, NH3, OH2, FH. 

Several methods of quantitative description of molecular structure based on the concepts of valence bond theory have been developed. These methods employ orbitals similar to localized valence bond orbitals, but permitting modest delocalization. These orbitals allow many fewer structures to be considered and remove the need for incorporating many ionic structures, in agreement with chemical intuition. To date, these methods have not been as widely applied in organic chemistry as MO calculations. They have, however, been successfully applied to fundamental structural issues. For example, successful quantitative treatments of the structure and energy of benzene and its heterocyclic analogs have been developed. It remains to be seen whether computations based on DFT and modem valence bond theory will come to rival the widely used MO programs in analysis and interpretation of stmcture and reactivity. 

Finally, the use of simple valence bond theory has led recently to a significant discovery concerning the nature of metals. Many years ago one of us noticed, based on an analysis of the experimental values of the saturation ferromagnetic moment per atom of the metals of the iron group and their alloys, that for a substance to have metallic properties, 0.72 orbital per atom, the metallic orbital, must be available to permit the unsynchronized resonance that confers metallic properties on a substance.34 38 Using lithium as an example, unsynchronized resonance refers to such structures as follows. 

An alternative stream came from the valence bond (VB) theory. Ovchinnikov judged the ground-state spin for the alternant diradicals by half the difference between the number of starred and unstarred ir-sites, i.e., S = (n -n)l2 [72]. It is the simplest way to predict the spin preference of ground states just on the basis of the molecular graph theory, and in many cases its results are parallel to those obtained from the NBMO analysis and from the sophisticated MO or DFT (density functional theory) calculations. However, this simple VB rule cannot be applied to the non-alternate diradicals. The exact solutions of semi-empirical VB, Hubbard, and PPP models shed light on the nature of spin correlation [37, 73-77]. 

Experimental information on the complexes between dihalogens and methylated amines is still comparatively scarce. Gas-phase investigations are available only for the complexes of trimethylamine with F2 [41 ] and with C1F [42], So far only a few theoretical investigations on XY- amine complexes have been presented [16,17,22,24,28,32,34,43,44]. On the basis of rotational spectroscopic analysis, the N(CH3)3 C1F complex was described as being dominated by a significant contribution of an ionic [(CH3)NC1]+- -F valence bond structure [41]. For the (CH3)3N F2 complex an even stronger tendency toward an ionic [(CH3)NF]+- F structure was reported [40]. 

Hiickel s application of this approach to the aromatic compounds gave new confidence to those physicists and chemists following up on the Hund-Mulliken analysis. It was regarded by many people as the simplest of the quantum mechanical valence-bond methods based on the Schrodinger equation. 66 Hiickel s was part of a series of applications of the method of linear combination of atom wave functions (atomic orbitals), a method that Felix Bloch had extended from H2+ to metals in 1928 and that Fowler s student, Lennard-Jones, had further developed for diatomic molecules in 1929. Now Hiickel extended the method to polyatomic molecules.67... 

We focus in this Section on particular aspects relating to the direct interpretation of valence bond wavefunctions. Important features of a description in terms of modem valence bond concepts include the orbital shapes (including their overlap integrals) and estimates of the relative importance of the different stmctures (and modes of spin coupling) in the VB wavefunction. We address here the particular question of defining nonorthogonal weights, as well as certain aspects of spin correlation analysis. 

An empirical method for correlating the oxidation state of a metal ion with the coordination geometry and the bond lengths Bond valence sum analysis has been used in characterizing the structural features of vanadium-dependent haloperoxidases . ... 

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