Cotton effects negative effect

The 260 nm band of chiral thiiranes is optically active and a Cotton effect is observed R) (+)-methylthiirane shows a negative Cotton effect at ca. 250 nm followed by a positive effect below 200 nm. An MO analysis indicates that charge transfer contributions are most important in determining the optical activity of the transition (81JCS(F2)503). The... 

The UV spectra of thiirane 1-oxide and (15,25)-(+)-2-methylthiirane 1-oxide show a broad maximum at about 205 nm (e —23 000). The latter shows a positive Cotton effect at low energy followed by a negative effect at high energy. The lowest excited states of thiirane 1-oxide involve excitations from the two lone pairs of the oxygen atom (79G19). 2,3-Diphenylthiirene 1-oxide and 1,1-dioxide show absorption due to the 1,2-diphenyl-ethylene chromophore. 

Figure A Diagrammatic representation of the Cotton effect (actually po-sitivc Cotton effect. The negative effect oceur.s when the CD curve. shows a minimum and the ORD curve is the reverse of the above).

Circular Dichroism. - The absolute configuration of the ot-hydr oxyphosphonat e (49) was established through its negative Cotton effect at 215 nm.144... 

Surfactants, not surprisingly, exert a highly significant influence on the fluorescence of FBAs in solution. This effect is associated with the critical micelle concentration of the surfactant and may be regarded as a special type of solvent effect. Anionic surfactants have almost no influence on the performance of anionic FBAs on cotton, but nonionic surfactants may exert either positive or negative effects on the whiteness of the treated substrate [33]. Cationic surfactants would be expected to have a negative influence, but this is not always so [34]. No general rule can be formulated and each case has to be considered separately. 

It is apparent that on increasing the polarizability of the allylic axial substituent, the Cotton effect becomes stronger. If we refer to the nearest double bond, however, the chirality defined by the C—X bond is negative, thus we should expect a decrease of Ae. Only by considering the diene as a whole (diene-picture), as depicted in the lower part of Figure 7(b), can one justify the reported trend. 

A few examples will illustrate the case. The parent trans-diene derivatives 31a and 3235 have nearly planar chromophores, but the Cotton effects are quite strong and opposite in sign (+15 and —27.9, respectively). This can be attributed mainly to the allylic axial C—CH3 bonds, which provide a positive contribution for compounds 31 and a negative for 32. Furthermore, the As values of P-chiral s-trans-31 are strongly dependent on the polarizability of the allylic C—X bond. 

Figure 2 Comparison of positive and negative Cotton effects.

In contrast, the CD spectra of 50, 51, and 52 exhibit negative Cotton effects, as shown in Figure 4.36 for 50. This implies a helical conformation, but of the opposite preferential screw sense to 48 and 49, even though the enantiopure chiral moiety in all cases is the (5 )-2-methylbutyl group. Similarly, the Cotton effect is of greatest magnitude (negative) at low temperature and almost very much reduced at 80°C. 

A temperature cycling experiment comprising three cycles of —70/50°C with CD and UV spectra recorded at each temperature for 56 indicated that the transition is reversible, with negative and positive Cotton effects being observed at the low and high temperatures, respectively, as is evident in Figure 4.44. 

For a system containing two chromophores i and j, the exciton chirality (positive or negative) governing the sign and amplitude of the split Cotton effect can be theoretically defined as below59 ... 

In the case of a positive chirality, a Cotton effect with positive first and negative second is observed, whereas a Cotton effect with negative first is found for negative chirality. As this method is based on theoretical calculations, the absolute configuration of organic compounds can be deduced unambiguously from their corresponding CD curves. 

Figure 14 shows the circular dichroism spectra for the LB films of p-CDNH C12-H25 including Naph-SOsNa molecules under the initial surface pressure of 30 mN/m. Different induced circular dichro-isms are clearly observed at1 Bbband of naphthalene, depending on the substituted position the negative and positive Cotton effects occur for 1 - and 2-Naph-SOaNa included in the cavity of the CD... 

From the positive Cotton effect observed in the amylose case and the negative Cotton effect in the SPG case, it is clear that the oligosilane adopts opposite screw sense helical conformations in the two cases.337 It now appears possible to associate an d/-screw sense helix with a positive sign Cotton effect (and vice versa), although other experimental confirmations of this are desirable. 

The UV and CD spectra of 117 and 121 (V) are shown in Figure 48. Considering 117, at —40°C a negative Cotton effect, coincident with the UV absorption, is evident, and at —5 °C a positive Cotton effect, coincident with the UV absorption (both of which are slightly red-shifted with respect to the —40°C profiles), is observed. It is thus apparent that 117 underwent a helix-helix transition at some temperature between —5 and —40°C. In contrast, the Cotton effects of 121(V) were positive at all temperatures, indicating that no helix-helix transition occurred. Similarly to 121, 88 did not undergo a helix-helix transition. These results are due to the different stiffness of the molecules, which is quantified by the viscosity index, a. 

With the same (.y)-2-methylbutyl side chain in each case, the solution CD was, as expected, almost identical, with positive Cotton effects. However, on addition of MeOH to form aggregates, while 127 showed a negative bisigned Cotton effect, 128 showed a positive bisigned Cotton effect, as shown in Figure 53. 

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