Cotton effect, absorption-

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. 

The behaviours of CD and ORD curves in the vicinity of an absorption band are collectively known as the Cotton effect after the French physicist A. Colton who discovered them in 189S. Their importance in the present context is that molecules with the same absolute conhguration will exhibit the same Cotton effect for the same d-d absorption and, if the configuration of one compound is known, that of closeiy similar ones can be established by comparison. 

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. 

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 ... 

It has been shown that CD measurement is a proper tool to determine the absolute configuration of the C-3 stereo center in corynantheine and yohimbine alkaloids (300). The chiroptical properties of stereoisomeric yohimbanes and 17-ketoyohimbanes also have been studied. Cotton effects due to aromatic and ketone absorptions have been considered in terms of the appropriate sector and... 

It was found that these showed UV absorption maxima at the same wavelength (ca. 395 nm) as their non-chiral-substituted analog, poly(bis- -butylphenylsilylene), as shown in Figure 37, from which it was concluded that the dihedral angles of these polymers were the same.282 The CD spectra showed Cotton effects, thus indicating that the polymers adopted PSS helical conformations and that they do not take all-anti conformations. 

Another interesting chiral chain end effect is exhibited by the helical polymer block co-polymer, poly(l,l-dimethyl-2,2-di-/z-hexylsilylene)- -poly(triphenylmethyl methacrylate), reported by Sanji and Sakurai (see Scheme 7) and prepared by the anionic polymerization of a masked disilene.333 The helical poly(triphenylmethyl methacrylate) block (PTrMA) is reported to induce a PSS of the same sign in the poly(di- -propylsilylene) block in THF below — 20 °C, and also in the solid state, by helicity transfer, as evidenced by the positive Cotton effect at 340 nm, coincident with a fairly narrow polysilane backbone UV absorption characteristic of an all-transoid-conformation. This phenomenon was termed helical programming. Above 20°C, the polysilane block loses its optical activity and the UV absorption shifts to 310 nm in a reversible, temperature-dependent effect, due to the disordering of the chain, as shown in Figure 45. 

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. 

Often the UV absorption tails into the visible due to light scattering by the microparticles, although the Amax is usually very close to that in solution. The aggregate CD spectrum, however, is markedly different from the solution-state spectrum, showing an intense bisigned Cotton effect coincident with the UV due the Davidov coupling. 

Methyl alkyl sulfoxides that do not contain other strongly perturbing groups have negative Cotton effects centered at the strong absorption band near 200 nm and have the (i > configuration. 

Enantiomers can be distinguished by their rotation of plane-polarized light at a specific wavelength, or over a range of wavelengths (optical rotatory dispersion, ORD), as well as by the difference in absorption of right and left circularly polarized light (circular dichroism. Cotton effect, CD). 

The absolute configuration of chiral nonracemic compounds can be established by forming a complex which can bind to the axial position of [Rh2(02CCF3)4] and thus induce circular dichroism within its electronic absorption bands. This method works well for chiral secondary alcohols and monoolefins. For these compounds tentative rules were proposed, which correlate the configuration of the starting materials with the sign of certain Cotton effects of their complexes178. 

In /(,y-unsaturated ketones overlap of the olefinic rc-orbital with the orbitals of the carbonyl group leads to enhancement of both the isotropic absorption and the Cotton effect of the n-71 band. The generalized" octant rule predicts a large positive n-71 Cotton effect when the C = C bond in /j.. -imsatijrated ketones of cisoidgeometry appears in a positive octant [Figure 6 (a)]43. 

The CD spectrum of the 1,2-dibenzoate 6 in 9 1 MeOH/dioxane showed a pair of typical exciton-split Cotton effects with opposite signs centred upon the UV absorption (227 nm) of the benzoate chromophore AE235 5 -15.9 and A 221 5 +6.66. The negative longer wavelength Cotton effect clearly defines the negative chirality between the two electric transition dipoles of the benzoate chromophores... 

Similar curves are obtained with other synthetic polypeptides, and in most cases they are reasonably independent of the nature of the amino acid side chains. In synthetic polypeptides and proteins the observed Cotton effects do not arise from isolated chromophores but are composite curves resulting from several transitions assigned to the amide bonds in the 200-m/x region. The a-helical curve, for example, results from three optically active absorption bands. One around 222 m/ arises from an n — 7T transition of nonbonding electrons, and the other two at 208 and 191 m/ji are attributed to w — tt transitions parallel and perpendicular to the axis of the helix. These transitions of the a-helix and the resulting Cotton effects characteristic of the a-helix are at present of great interest in interpreting ORD curves of membranes. 

Circular dichroism arises from the same optically active transitions responsible for the Cotton effects observed in ORD curves, but unlike ORD it is an absorption, not a dispersion, phenomenon. Hence, the CD effect is restricted to the region of the transition and can be interpreted more straightforwardly. Both ORD and CD can best be understood if one imagines the incident plane-polarized beam resolved into two in-phase circularly polarized beams whose vectors rotate in opposite directions. A difference in index of refraction between the left and right circularly polarized beams results in rotation of the transmitted plane polarized beam while differential absorption of the two circularly polarized beams results in depolarization of the transmitted beam, so that an incident plane-polarized beam whose frequency is within that of an optically active absorption band becomes both rotated and elliptically polarized upon passage through the sample. This depolarization effect is CD, and the measured parameter is (et — er), the difference in extinction coefficient between the left and right circularly polarized beams. The data is usually recorded as the specific ellipticity, defined as ... 

Fig. 12.25 The Cotton effect (a) positive Cotton effect (b) negative Cotton effect. The absorption band is not shown it would be a positive Gaussian curve centered on Am, but o scale. The dashed line represents the ORD curve (and relates to the refractive index scale on left. The solid line represents the CD curve ( ( — scale on right). The maximum absorption, zero values of DRD. and maxima and minima of CD values occur at The...

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