INDEX Cotton effect

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. 

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...

Both the pitch and handedness of the cholesteric helices can be detected by chiroptical techniques, optical rotatory dispersion (O.R.D.) and circular dichroism (C.D.), by characterizing the selective reflection band. A right-handed helix (P-helix) originates a negative reflection Cotton-effect and vice versa (A = np where is the wavelength of the reflection band, n the main refractive index and p the pitch). 

FIGURE 2.1. Simplified presentation of a dichroic band (Cotton effect), (a) Positive Cotton effect (b) negative Cotton effect. Molar extinction coefficients for ieft- and right-hand polarized iight components and (2) Refraction index of left- and right-hand polarized iight components and Circular dichroism Optical rotatory dispersion . 

Spanish Hemp Workers. Data collected on hemp workers have been offered in support of chronic respiratory effects associated with cotton dust exposure. Historically, hemp workers constituted the first group to call medical attention to their respiratory effects (27). More recent studies done on hemp workers, however, suggest similar limitations of methodology as mentioned above. The recruitment of subjects leaves much perplexity about the representative character of the group, when the source population is not accounted or even remotely described (28, 29, 30). The exposure length is not assessed (some of the workers have less than two years tenure) and/or the status of "retirement" is used uncritically as an index of exposure (29,... 

This approach is based on the introduction of molecular effective polarizabilities, i.e. molecular properties which have been modified by the combination of the two different environment effects represented in terms of cavity and reaction fields. In terms of these properties the outcome of quantum mechanical calculations can be directly compared with the outcome of the experimental measurements of the various NLO processes. The explicit expressions reported here refer to the first-order refractometric measurements and to the third-order EFISH processes, but the PCM methodology maps all the other NLO processes such as the electro-optical Kerr effect (OKE), intensity-dependent refractive index (IDRI), and others. More recently, the approach has been extended to the case of linear birefringences such as the Cotton-Mouton [21] and the Kerr effects [22] (see also the contribution to this book specifically devoted to birefringences). 

The Kerr and Cotton-Mouton experiments stand apart from the others in the sense that it is the electric-field-induced anisotropy in the refractive index which is measured (i.e. the difference in two quantities) rather than a single quantity such as an intensity. This means that, e.g., for the Cotton-Mouton effect, the experimentally-useful quantity is... 

The ability of anisotropic and anisometric particles to assume some co-orientation in external force fields is not only responsible for significant changes in scattering properties but also causes birefringence (double refraction), i.e., the average refractive indexes of two beams polarized in perpendicular planes happen to be different. The specific orientation of particles and birefringecne may be caused by the action of electric field (Kerr effect), magnetic field (Cotton-Mouton effect), or in the case of anisotropic particles by flow of medium (Maxwell effect) [25]. 

BTCA), SMPU finishing does not markedly affect the mechanical strength and whiteness index of the fabric. To some extent SMPU finishing on cotton increases the mechanical strength of fabrics. Repeated washing experiments showed that the wrinkle-free effect of SMPU emulsion treated fabric can last for hundreds of laundering cycles. ... 

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