Conjugative model

The stability of 2-methylpropene relative to trani-2-butene (Scheme 25c) can be understood by the analogy of the cross and linear ct conjugation model. The central... 

The flat model requires the introduction of an energy either of physical adsorption or of chemisorption by the formation of a a bond. The other models require the introduction of the resonance of the 77 electrons. Thus we obtain a conjugated model if the metal atoms are in the plane of the ethylene molecule. We obtain... 

The "end on" conjugated models must be excluded. For 6 1, values of the heat of chemisorption are found which do not exceed 20kcal/mole while the experimental values are between 20 and 100 kcal/mole, i.e. five times greater. Figures 13 and 14... 

In the first monolayer of conjugated model material, a model molecular solid or a polymer adsorbate, assume that no chemistry (covalent bonding) occurs, since, in the absence of, for example, mechanical rupturing, the bonds at the surface of the molecular film are completely satisfied. This assumption is supported by the fact that, at least for condensed molecular solids, vapor-deposited films may be re-evaporated (removed) from the surface by gentle heating in UHV. 

It is one of the standard results of theoretical organic chemistry that benzenoid hydrocarbons (and polycyclic conjugated molecules in general) have no unique classical structural formula. The conjugation models of their 7r-electrons must be visualized by means of several so-called Kekule structures [22, 25, 32, 33], none of which correctly describes the actual situation. An attempt to overcome this century-long problem was proposed by Clar in the 1950 s and eventually elaborated in due detail in his seminal book [18]. 

If an intramolecular inductive effect were responsible for the diminished nucleophilicity of the second nitrogen atom, a significant effect should be observed in the conjugated model compound,... 

Use the parameters in Table 8-3 to construct the HMO determinant for molecule (XXXI). Use the conjugative model for the methyl group. 

Gutman L Furtula B, Durdevic J, Kovacevic R, Stankovic S (2005) Annelated perylenes benzenoid molecules violating the Kekule-stiucture-based cyclic conjugation models. J Stah Chem Soe 70 1023-1031... 

Fig. 28. Conjugative model for the pseudo-tt orbitals in the Sq and S, states of thioacetaldehyde...

Conversion in both directions needs heuristic information about conjugation. It would therefore be more sensible to input molecules directly into the RAMSES notation. Ultimately, we hope that the chemist s perception of bonding will abandon the connection table representation of a single VB structure and switch to one accounting for the problems addressed in this section in a manner such as that laid down in the RAMSES model. 

The underlying principle of the PEOE method is that the electronic polarization within the tr-bond skeleton as measured by the inductive effect is attenuated with each intervening o -bond. The electronic polarization within /r-bond systems as measured by the resonance or mesomeric effect, on the other hand, extends across an entire nr-system without any attenuation. The simple model of an electron in a box expresses this fact. Thus, in calculating the charge distribution in conjugated i -systems an approach different from the PEOE method has to be taken. 

The Huckel method and is one of the earliest and simplest semiempirical methods. A Huckel calculation models only the 7t valence electrons in a planar conjugated hydrocarbon. A parameter is used to describe the interaction between bonded atoms. There are no second atom affects. Huckel calculations do reflect orbital symmetry and qualitatively predict orbital coefficients. Huckel calculations can give crude quantitative information or qualitative insight into conjugated compounds, but are seldom used today. The primary use of Huckel calculations now is as a class exercise because it is a calculation that can be done by hand. 

Later there was an attempt by ab initio calculation to fit the electron structure of diazirine into the Walsh model of cyclopropane (69MI50800). According to these SCF-LCAO-MO calculations three MOs add to the description of the lone electron pairs, all of which also contribute to some extent to ring bonding. As to strain, 7r-character and conjugative effect, the term pseudo-rr-character was used. 

McIntosh, A. Fitting Linear Models An Application of Conjugate Gradient Algorithms, Springer-Verlag, New York (1982). 

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