Cotton curve

Fig. XIII-15. Adsorption isotherm for sodium dodecylbenzenesulfonate on cotton. Curve I, 30°C curve II, 0°C. (From Ref. 64.)...

Comparison of (—)-(R)-( )-cyclooctene (50) and the (—)-(S)-anti-Bredt rule compound 51 would suggest their close structural relationship, and this is reflected in their respective absolute rotation values 5,16b) [otJoabs —458° (neat) and —725° (CHC13) as well as in their respective (- )-Cotton curves [0] —1.4x 10s at 196 nm (cyclohexane) and —13.6 x 10s at 213 nm (isooctane). 

Fig. 1. Kinetic curves of styrene absorption. Curve 1 viscose staple fibre, curve 2 cotton, curve 3 graft copolymer of viscose staple fibre and polystyrene, composition 86.1% 13.9%, curve 4 graft copolymer of cotton and polystyrene, composition 84.8% 15.2%, curve 5 grafted polystyrene isolated from the copolymer with staple fibre, curve 6 grafted polystyrene isolated from the copolymer with cotton absorption conditions [St] -1%, M-25, emulsifier concentration 0.2%, temperature - 65°...

Scott and Wrixon have developed a quadrant rule for the c.d. of platinum(ii)-olefin complexes that depends on d-d orbital transitions. Application of the rule to monoterpenes was considered, and generally conformed to expectations based on known absolute configurations, but in some cases (notably 8-pinene) the results were not satisfactory. The complex measured may be that of a-pinene, for which a Cotton curve of the opposite sign is predicted. Further work on the use... 

B. Alkaloids Aminated at C-20.— The i.r. spectrum of cyclomikuranine (204), isolated from B. microphylla, shows a six-membered-ring ketone, and the mass spectrum shows a 20-dimethylamino-group. The negative Cotton curve is typical of a 4,4-dimethyl-3-oxo-5a-steroid. N.m.r. indicates an NMc2 group as well as a 16a-OH group. 

The mechanical properties of acryUc and modacryUc fibers are retained very well under wet conditions. This makes these fibers well suited to the stresses of textile processing. Shape retention and maintenance of original bulk in home laundering cycles are also good. Typical stress—strain curves for acryhc and modacryUc fibers are compared with wool, cotton, and the other synthetic fibers in Figure 2. 

The abihty of a fiber to absorb energy during straining is measured by the area under the stress—strain curve. Within the proportional limit, ie, the linear region, this property is defined as toughness or work of mpture. For acetate and triacetate the work of mpture is essentially the same at 0.022 N/tex (0.25 gf/den). This is higher than for cotton (0.010 N/tex = 0.113 gf/den), similar to rayon and wool, but less than for nylon (0.076 N/tex = 0.86 gf/den) and silk (0.072 N/tex = 0.81 gf/den) (3). 

Eig. 1. Typical stress—strain curves for cotton and PET fibers. A, industrial B, high tenacity, staple C, regular tenacity, filament D, regular tenacity, staple ... 

The effect of temperature fluctuations on net carbon dioxide uptake is ikustrated by the curves in Figure 18. As the temperature increases, net photosynthesis increases for cotton and sorghum to a maximum value and then rapidly declines. Ideally, the biomass species grown in an area should have a maximum rate of net photosynthesis as close as possible to the average temperature during the growing season in that area. 

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).

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. 

Surprisingly, in contrast to a- and y9-peptides, CD spectra of y-peptides gave only a very hmited amount of stmctural information. Experiments conducted on heh-cal y" -hexapeptides did not reveal any characteristic CD signature (no Cotton effect) [200, 201]. Similarly, y -peptides built from 2,4-disubstituted y-amino acids of like configuration and shown to adopt a more stable 2.6-helical structure, do not display typical CD curves either [201]. However, CD spectra of the 2.6-helical -peptide 147 and its Boc-protected derivative recorded in MeOH and CD3CN present an intense maximum around 215 nm with a shoulder at ca. 200 nm [207]. 

The paramount importance of efficient preparation in producing goods of thorough and uniform absorbency for continuous dyeing cannot be over-emphasised [157,158] all continuous dyeing systems are heavily dependent on this prerequisite. Typical build-up curves on mercerised and unmercerised cotton fabrics in Figure 12.20 illustrate the... 

Figure 12.20 Typical dye build-up curves for pad-steam process variants A and B on mercerised and unmercerised cotton [158]...

A comparison of the thus calculated with the measured specific rotations of the 0th- to 4th-generation dendrimers of this kind gave a close resemblance, with a curve, approaching asymptotically a limiting value (Fig. 26). It was also shown that the shape of this curve was independent of solvent, concentration and temperature. This was not the case when CD spectra of these dendrimers were compared (Fig. 27) in solvents such as CH2C12 and f-butyl methyl ether a constant rise of the Cotton effect was observed, which correlates with the increasing amount of benzene chromophores in the dendrimers. However, in the... 

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. 

Addition of lithiated heterocycles to aldonolactones yields carbon-linked nucleosides (56). Thus, the reaction of 2,3 5,6-di-O-isopropylidene-L-gu-lono-1,4-lactone (9b) or 2,3-O-isopropylidene-D-ribono-l,4-lactone (16a) with various lithiated heterocycles gave gulofuranosyl derivatives 53a-g or ribofuranosyl derivatives 54b,c. Gulonolactols 53a-g and ribonolactols 54b,c were acetylated with acetic anhydride in pyridine to yield their acetyl derivatives. The stereochemistry of compounds 53a-g and 54b,c was discussed in terms of the Cotton effect of circular-dichroism curves of the ring-opened alcohols formed upon reduction by sodium borohydride. The configuration at C-l of 53g was proved by means of X-ray analysis (57,58). 

In Cotton effect curves, we use the terms peaks and troughs. The use of maxima and minima is not recommended because of the confusion that may arise. Since these terms are employed to describe ultra violet spectra, the sign of rotation is unimportant. Sometimes we also use the terminology amplitudes and widths of Cotton effects. The meanings of these terms are clear from Fig. (10.6). The plain curves are much less useful than Cotton effect curves. 

The focus of the earlier chapter was a brief outline of the sources of chiral amino compounds, application of Brewster s rules for the assignment of the absolute configurations to a few chiral amines, a discussion of the ORD and CD in the visible (380-780 nm) and near-ultraviolet (200-380 nm) spectral regions of chiral amino compounds and some of their derivatives, and how the observed Cotton effects (CEs) in their ORD curves and CD spectra relate to their conformational preferences and absolute configurations. 

Andersen (75,76), as well as Mislow (221), discovered that the ORD curves of alkyl aryl sulfoxides show a strong Cotton effect in the region below 250 nm. An extensive study by Mislow and his coworkers (47) led to the following empirical rules, correlating the sign of the Cotton effect with the absolute configurations of chiral dialkyl, alkyl aryl, and diaryl sulfoxides, as well as menthyl esters of aromatic sulfinic acids ... 

The ORD and CD curves of chiral sulfinamides derived from aromatic sulfinic acids show a strong Cotton effect centered at 240 to 256 nm (83). The position of this Cotton effect, its dependence on the solvent used, and the amplitude reveal a complete analogy with that observed in alkyl aryl sulfoxides and aromatic menthyl sulfin-ates. Thus, the positive sign of the Cotton effect observed in the ORD curves of dextrorotatory sulfinamides is indicative of the (5)-configuration around sulfur in these compounds. 

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