Cotton effects positive effect

These compounds show typical s+rans-1,3-butadiene absorption bands between 230 and 235 nm, with emax 30000. In correspondence, the CD spectra show an intense (Ae 3-5) Cotton effect, positive for the (S) absolute configuration of the chiral centres. It is noteworthy that if one considered (E)-(S)-6 as a half of (E, E)-(3S,8S)-7 a value of Ae of about 0.4 would be predicted the actual Ae of +3 is one order of magnitude larger ... 

Negative Cotton effect Positive Cotton effect... 

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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. 

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. 

The first report of the chiroptical properties of a planar s-fraws-diene chromophore is due to Di Corato12, who described the CD data of (+)-(S)-6, which shows a positive weak (Ae +0.2) Cotton effect at about 220 nm, in both the E and Z isomers. Lardicci and coworkers13 described in 1978 the absorption and CD spectra of the planar s-fraws-diene derivatives 7 and 41. 

Figure 2 Comparison of positive and negative Cotton effects.

As for the homopolymers, dilute solution (1 x 10-4 M in toluene, isooctane or THF) spectroscopic studies were carried out over the temperature range of —70 to +80°C. The CD spectrum of 54 (Figure 4.39), containing 50% p-(S)-2-methylbutylphenyl-substituted monomer units, exhibits a positively signed Cotton effect (389 nm), almost coincident with the lowest energy main-chain electronic transition (or-or ) in the 20°C UV spectrum (394 nm). The CD spectra of 53 and 55 show similar features to that of 54, and the similarity... 

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. 

Trzaska and co-workers showed a similar propeller mechanism for the formation of helical columns from disclike metallomesogens (29-31).34 These metallomesogens also have C3 symmetry and 30 and 31 are provided with chiral side chains. In the hexagonal columnar mesophase these chiral side chains induce a Cotton effect in the chromophore of the helically arranged core. Heating the mesophase to the isotropic liquid results in the disappearance of the Cotton effect because of the loss of helical order. This effect illustrates the need for the molecules to be positionally ordered in order for the side-chain chirality to be transferred to the supramolecular column. 

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... 

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