Positive Cotton effect

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 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. 

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... 

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. 

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. 

Figure 4 shows CD spectra of (a) the DNA-lipid complex in organic solution containing a small amount of water (CHCl3/Et0H/Fl20 =4 1 0.07, 790 mM of H2O), and (b) native DNA in an aqueous buffer solution (20 mM NaCl, 10 mM Tris, pH 7.8). The DNA-lipid complex shows a positive Cotton effect at 270 nm and a negative Cotton effect at 245 nm similar to native DNA in aqueous solution, which indicates the B-form structure for the DNA strands [11]. Thus, the DNA-lipid complex forms a double helical B-form... 

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...

---