Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Benzene valence bond calculation

Qualitatively, the resonance picture is often used to describe the structure of molecules, but quantitative valence-bond calculations become much more difficult as the structures become more complicated (e.g., naphthalene, pyridine, etc.). Therefore the molecular-orbital method is used much more often for the solution of wave equations.5 If we look at benzene by this method (qualitatively), we see that each carbon atom, being connected to three other atoms, uses sp1 orbitals to form a bonds, so that all 12 atoms are in one plane. Each carbon has a p orbital (containing one electron) remaining and each of these can overlap equally with the two adjacent p orbitals. This overlap of six orbitals (see Figure 2.1) produces six new orbitals, three of which (shown) are bonding. These three (called it orbitals) all occupy approximately the same space.6 One of the three is of lower energy than... [Pg.27]

According to the relationship between three-membered rings and 7r-complexes, cyclopropane can be considered as a resonance hybrid of three equivalent methylene-ethene n-complexes Of course, such Ti-complexes do not exist but this is also true in the case of the two cyclohexatriene resonance structures normally used to present benzene. Spin-coupled valence bond calculations of Karadakov and coworkers reveal that there is a small but significant contribution of 3.7% to the electronic structure of 1 resulting from 7r-com-plex structures (see Section V. E). This indicates that the Ti-complex description of is not totally unreasonable and, although seldom used, helps to unravel some of the peculiarities of bonding in 1 ... [Pg.73]

The question of the benzene structure was taken up again by Lennard-Jones and Turkevich [113]. They showed, using molecular orbital arguments, that the x-system of CjnHjn is unstable with respect to bond localisation. Wheland [114] questioned their result on the basis of an (unpublished) valence bond calculation. It would take almost sixty years before this seeming discrepancy was finally settled [115]. [Pg.12]

This determinant size is a strict minimum. Even for benzene, determinants can be quite large. For example, Norbeck and Gallup used 175 resonance structures in an ab initio valence bond calculation for benzene and found that the ionic structures are major contributors to the calculated structure Norbeck, J. M. Gallup, G. A. /. Am. Chem. Soc. 1973, 95, 4460. [Pg.239]

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. [Pg.65]

Since a calculation of the resonance energy of benzene by the valence bond method shows that the greater part of it is a result of the resonance between the two Kekule structures shown, we might suppose that its homologs would also have significant resonance stabilization energies. Such conclusions are at variance with experimental fact, however, since cyclobutadiene appears to be too unstable to have any permanent existence, and cyclooctatetraene exists as a nonplanar tetraolefin, having no resonance stabilization of the sort considered. [Pg.158]

In summary, the radical cations of the benzene valence isomers show several interesting structures. Although most of their structural features can be rationalized by considering the HOMOs of the precursor molecules, some of the species show substantial changes in individual bond lengths. Accordingly, species such as the 2B2 and 2A1 Dewar benzene radical cations, the 2Bj and 2At benzvalene radical cations, or the 2Bt prismane radical cation cannot be expected to qualify as Koopmans radical cations. To date, most of the information available in this series is based on CIDNP results and ab initio calculations. It is safe to predict increasing involvement of ESR spectroscopy in this area. [Pg.221]

In benzene it is found by superposition of the five canonical configurations mentioned that each bond has a double bond character of 0.389 + 0.073 = 0.462. In view of the single o-bond always present one also speaks of total bond character equal to 1 -j- the double bond character. Though this bond character ( indice de liaison double , Daudel and Pullman) from the calculations based on the Valence Bond method is different from the bond order (Penney, Coulson), e.g. in the Molecular Orbital Method, both terms are frequently used without discrimination. In Pauling s earlier very simple concept (p. 210) the bond character of benzene is just 0.5 since there only the Kekule configurations are taken into account. [Pg.269]

Both the valence-bond and molecular-orbital methods show that there is delocalization in benzene. For example, each predicts that the six carbon-carbon bonds should have equal lengths, which is true. Since each method is useful for certain purposes, we will use one or the other as appropriate. Recent ab initio, SCF calculations confirms that the delocalization effect acts to strongly stabilize symmetric benzene, consistent with the concepts of classical resonance theory. ... [Pg.36]

In 1927, Heitler and London used valence bond theory to treat the H2 molecule but to treat larger molecules, further simplifications were needed. In 1931, Erich Hiickel introduced an extremely simple approximation which could be used to treat the 7i-electrons in flat organic molecules such as benzene, napthaline, and so on. This approximation yielded matrices to be diagonalized, and it is a measure of the state of computers at that time to remember that during World War II, Alberte Pullman sat in a basement room in Paris diagonalizing Hiickel matrices with a mechanical desk calculator, while her husband-to-be Bernard drove a tank with the Free French Forces in North Africa. Alberte s hand-work led to the publication of the Pullmans early book Quantum Biochemistry. ... [Pg.55]

When a compound is so constituted that more than one normal valency formula may be used to represent its molecule, as with benzene where the two Kekule forms and the three Dewar forms are possible, the energy of formation usually proves to be greater than that calculated for any of the possible individual formulae. The molecule thus appears to exist in a state which is more stable than that corresponding to any of the conventional valency-bond representations. Detailed evidence from many somces, indeed, suggests that these formulae are quite often inadequate. Benzene, for example, is best regarded not as possessing three single and three double carbon-carbon bonds, but six equal bonds of order approximately 1 -S. [Pg.266]


See other pages where Benzene valence bond calculation is mentioned: [Pg.41]    [Pg.73]    [Pg.267]    [Pg.264]    [Pg.33]    [Pg.3]    [Pg.156]    [Pg.588]    [Pg.733]    [Pg.257]    [Pg.284]    [Pg.12]    [Pg.523]    [Pg.223]    [Pg.656]    [Pg.51]    [Pg.96]    [Pg.69]    [Pg.244]    [Pg.204]    [Pg.466]    [Pg.974]    [Pg.241]    [Pg.49]    [Pg.504]    [Pg.510]    [Pg.515]    [Pg.312]    [Pg.466]    [Pg.226]    [Pg.202]    [Pg.435]    [Pg.23]    [Pg.32]    [Pg.450]    [Pg.670]    [Pg.736]    [Pg.101]   
See also in sourсe #XX -- [ Pg.238 ]




SEARCH



Benzene calculations

Benzene valence bonds

Bond calculated

Bond calculations

Bonds benzene

Valence bond calculations

© 2024 chempedia.info