Chameleonic model

The 1,4-disubstituted cases are degenerate, so no averaging is needed. The chameleonic model suggests that the substituent effects should be additive, or a predicted value of A/ = 31.6 kcal mol for l,4-dicyauo-l,5-hexadiene and AH = 31.4 kcal mol for l,4-diphenyl-l,5-hexadiene. The computed barriers are actually lower thau these values. The barrier for the 1,3,4,6-tetrasubstituted... 

These results suggest a competitive interaction between the active and nodal substituents. The geometries of these transition states support this competition their values are quite similar to the distance found in the parent 1,5-hexadiene. Computational examinations of the substituent effects on the Cope rearrangement conclude that the centauric model does not apply. The chameleonic model makes a better accounting of the cooperative and competitive ways the substituents affect the Cope rearrangement. Borden has proposed a simple mathematical model that allows for the prediction of the stabilization of the transition state by substituents solely on the change in... 

Mining, 32 (8), 68 (1999). Chameleon A Hierarchical Clustering Algorithm Using Dynamic Modeling. 

Although, as stated above, olefin epoxidation is commonly referred to as an electrophilic oxidation, recent theoretical calculations suggest that the electronic character of the oxygen transfer step needs to be considered to fully understand the mechanism [451]. The electronic character, that is, whether the oxidant acts as an electrophile or a nucleophile is studied by charge decomposition analysis (CDA) [452,453]. This analysis is a quantitative interpretation of the Dewar-Chatt-Dimcanson model and evaluates the relative importance of the orbital interactions between the olefin (donor) and the oxidant (acceptor) and vice versa [451]. For example, dimethyldioxirane (DMD) is described as a chameleon oxidant because in the oxidations of acrolein and acrylonitrile, it acts as a nucleophile [454]. In most cases though, epoxidation with peroxides occurs predominantly by electron donation from the 7t orbital of the olefin into the a orbital of the 0-0 bond in the transition state [455,456] (Fig. 1.10), so the oxidation is justifiably called an electrophilic process. 

Karypis G, Han J, Kumar V. CHAMELEON A hierarchical clustering algorithm using dynamic modeling. Computer 1999 32 68-75. 

Such models would be structurally compatible with the other constituents of petroleum and are able to represent the thermal chemistry and other process operations. It must also be recognized that although such models can have the large dimensions that have been proposed for asphaltenes, they can also be molecular chameleons insofar as they can vary in dimensions depending on the angle of rotation around an axis (Figure 9) or the freedom of rotation around one, or more, of the bonds. 

A simple valence bond (VB) model was used to account for this chameleon nature of Cpd I, as shown in Figure 2.12 - The electronic structure of Cpd I can be constructed from two resonance structures, a> and b> a> describes a thiolyl radical and a closed-shell iron-oxo porphyrin, while ]b> is an ion pair composed of a thiolate anion and an iron-oxo porphyrin cation... 

Polo V, Andres J (2005) A joint study based on the electron localization function and catastrophe theory of the chameleonic and centauric models for the cope rearrangement of 1,5-hexadiene and its cyano derivatives. J Comput Chem 26 1427-1437... 

Kane, M.X Gough, A., 1996. Database to prioritize mains rehabiUtation. Proc. of IWSA Spec. Conf on New Developments in Modelling, Monitoring and Control of Water Supply Systems, Amsterdam, The Chameleon Press Ltd. London, p. 195-213. 

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