Chameleon effect

Bustamante, P, Ochoa, R., Reillo, A., Escalera, J.-B., 1994. Chameleonic effect of sulfanilamide and sulfamethazine in solvent mixtures—solubility curves with 2 maxima. Chem. Pharm. Bull. 42, 1129-1133. 

Escalera, J.B., Bustamante, P, Martin, A., 1994. Predicting the solubility of drugs in solvent mixtures - multiple solubility maxima and the chameleonic effect. J. Pharm. Pharmacol. 46, 172-176. 

The phenomena enumerated in Section 2.4 do not, of course, fully describe all the differences between chemical and electrode processes of ion radical formation. From time to time, effects are found that cannot be clearly interpreted and categorized. For instance, one paper should be mentioned. It bears the symbolic title ir- and a-Diazo Radical Cations Electronic and Molecular Structure of a Chemical Chameleon (Bally et al. 1999). In this work, diphenyldiazomethane and its 15N2, 13C, and Di0 isotopomers, as well as the CH2-CH2 bridged derivative, 5-diazo-10,ll-dihydro-5H-dibenzo[a,d]cycloheptene, were ionized via one-electron electrolytic or chemical oxidation. Both reactions were performed in the same solvent (dichloromethane). Tetra-n-butylammonium tetrafluoroborate served as the supporting salt in the electrolysis. The chemical oxidation was carried out with tris(4-bromophenyl)-or tris(2,4-dibromophenyl)ammoniumyl hexachloroantimonates. Two distinct cation radicals that corresponded to it- and a-types were observed in both types of one-electron oxidation. These electromers are depicted in Scheme 2-28 for the case of diphenyldiazomethane. 

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... 

Hayase, S. Hrovat, D. A. Borden, W. T. A B3LYP study of the effects of phenyl substituents on 1,5-hydrogen shifts in 3-(Z)-l,3-pentadiene provides evidence against a chameleonic transition structure, J. Am. Chem. Soc. 2004,126,10028-10034. 

One can consider that the Halpern mechanism and the cis mechanism are two extremes. The Halpern mechanism is most widely accepted and our ab initio MO calculations support this from the point of view of intrinsic electronic energy. However, there may be cases where the steric effect overshadows the electronic effect. There may also be cases where both effects are important. It may be, as Collman et al. said, that "this multistep reaction is very complicated. Like a chameleon, the dominant reaction mechanism changes when the nature of the catalyst, the ligands, or the substrate is altered" (8). 

Gajewski, J. J., Conrad, N. D., Emrani, J., Gilbert, K. E. Substituent effects on the Cope rearrangement, Neither centaurs nor chameleons can characterize them. ARKIVOC (Gainesville, FL, United States) [online computer file] 2002, 18-29. 

The ability of radical stabilizing substituents to have a large effect on R in the TS geometry is what makes the Cope TS chameleonic and the chameleonic nature of the TS is responsible for the cooperative and competitive phenyl substituent effects that have been both calculated [27,28] and observed [5,29,30]. Multiple phenyl substituents, attached where they either all stabilize structure A or all stabilize structure C, distort the TS geometry toward one of these two diradical extremes, thus enhancing the ability of all the substituents to stabilize the TS. In contrast, when one set of substituents stabilizes one of these resonance structures and another substituent stabilizes the other, the result is a compromise TS geometry, with a value of R that is optimal for neither set of substituents. 

Figure 17.20 summarizes schematically the effects of polymers on an idealized colloid. The colloid is idealized to the extent that its surface, chameleon like, can change from being inert to strongly anchoring as is required for heuristic purposes. 

In the area of interior architecture the properties of available fabrics and the diversity of their application are expanding rapidly. For example, products such as ETTLIN lux (ETTLIN, 2009) create fascinating optical effects when back-Ut, as a result of its tailor-made weave properties flexible, temperature-sensitive, colour-changing films such as ChroMyx by Chameleon International (2009) can be laminated onto a variety of fabrics for yet unforeseen purposes. 

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