Cotton effect definition

Figure 13. Definition of exciton chirality. Summation of the two Cotton effects (broken lines) separated by Davydov splitting A). gives the curves shown in solid lines. Adapted from N. Harada, K. Nakanishi. Circular Dichroic Spectroscopy - Exciton Coupling in Organic Stereochemistry, University Science Books. Mill Valley, California. 1983...

Palytoxin is a white, amorphous, hydroscopic solid that has not yet been crystallized. It is insoluble in nonpolar solvents such as chlorophorm, ether, and acetone sparingly soluble in methanol and ethanol and soluble in pyridine, dimethyl sulfoxide, and water. The partition coefficient for the distribution of palytoxin between 1-butanol and water is 0.21 at 25°C based on comparison of the absorbance at 263 nm for the two layers. In aqueous solutions, palytoxin foams on agitation, like a steroidal saponin, probably because of its amphipathic nature. The toxin shows no definite melting point and is resistant to heat but chars at 300°C. It is an optically active compound, having a specific rotation of -i-26° 2° in water. The optical rotatory dispersion curve of palytoxin exhibits a positive Cotton effect with [a]25o being -i-700° and [a]2,j being +600° (Moore and Scheuer 1971 Tan and Lau 2000). 

Another equilibrium partner of the form 102a is the cyclic Schiff base 101a, formed by dehydration. The C=N chromophore in 101a exhibits a weakly positive Cotton effect at 250 nm, by which the proportion of 101a present can be demonstrated. The Cotton effect disappears at pH values below 6. By comparison with 5-amino-5-deoxy-D-xylopyranose (17), 102a is definitely the more stable toward acids. Neither an Amadori rearrangement product, nor aromatization to a p)nTole derivative, is observed down to pH 1.0. 

Determinations of absolute configurations of trans-cyclopropane-l,2-dicarboxylic acids on the basis of a definite Cotton effect seem to be possible for carboxylic acid thioamides. In these molecules, such as 123a and 123b, there is a Cotton effect near 330 nm which results from the (n, n ) excitation of the thioamide chromophore . In the... 

The qualitative definition of exciton chirality is very simple (1) if two transition moments constitute a clockwise screw sense, CD shows positive first and negative second Cotton effects. On the other hand, (2) if they describe a counterclockwise screw sense, negative first and positive second Cotton effects are observed. In most cases, intense exciton-coupled CD Cotton effects are observed at the long-axis-polarized transition, and therefore the above results are rephrased as follows ... 

Qualitative definition Quantitative definition Cotton effects... 

The /(-value of exciton split CD is the function of the dihedral angle between two transition moments. In the case of vicinal glycol dibenzoates, the sign of the exciton split Cotton effects remains unchanged from 0 to 180°. Therefore, the qualitative definition shown in Table 1 is applicable to a dibenzoate with the dihedral angle of more than 90°. The maximum /(-value is around 70° 8... 

The absolute configuration of the epoxide group of insect JHs was determined by spectroscopy and circular dichroism [29], In the first method, perchloric acid was used to catalyze the hydrolysis of the epoxide in the presence of H2180. The resulting v/c-diol was analyzed by mass spectroscopy and definitively demonstrated the 10/ , 115 chirality of the epoxide [30]. The chirality was confirmed by circular dichroism. The epoxide was hydrolyzed by sulfuric acid and tetrahydrofuran and the circular dichroism cotton effect of the resulting acyclic glycol was measured in the presence of tris(dipivaloylmethanato)praseodymium [31]. 

ABA is a C, organic acid with one asymmetric carbon atom at C-1 (Fig. 1). The naturally occurring form is 5-( + )-ABA the side chain of ABA is by definition 2-cis,4-trans [2]. The optical rotatory dispersion (ORD) spectrum of ABA exhibits an intense Cotton effect. This property was initially exploited to identify and measure ABA in purified extracts of a number of plants [11]. ABA in organic solvents can be photo-isomerized by ultraviolet radiation to give a mixture of approximately 50% ABA and 50% 2-trans,4 trans-ABA (f-ABA) (Fig. 1). The latter compound is biologically inactive. 

In spite of the dramatic changes in the CD we cannot definitely assume a change in conformation in this case. Yamaguchi [15] prepared optically active polyamides, by polycondensation of (+)-camphoryldichloride with hexamethylenediamine, which were also chlorinated in the carbonamide group [16] (III). A negative Cotton effect occurs at 218 nm... 

Repellents on Cloth. Each candidate repellent is appHed to a knit cotton stocking or cloth patch at 3.3 g/m cloth, usually as a 1% solution of active ingredient (AI) ia acetone. Two hours later, the stock or cloth patch is placed over an untreated nylon stocking on the arm of a subject, the hand covered, and the arm exposed to 1500 female mosquitoes for one minute. If fewer than five bites are counted, the test is repeated at 24 h, then weekly until failure, which is, by definition, five bites per minute. The standard mosquitoes used are Piedes aegppti Anopheles quadrimaculatus or M. albimanus. Candidate repellents ia cloth tests are ia one of the foUowiag classes class 1, effective 0 d class 2, 1—5 d class 3, 6—10 d class 4, 11—21 d and class 5, >21 d. 

It is difficult to find any definitive information that indicates ozone damage as extensive as that suggested in the mri report. The latter identified cotton, nylon, and rayon as particularly susceptible to ozone. The oxidation of cellulose fibers by ozone was the subject of a study in 1952 that showed that dry cotton was not seriously degraded by ambient ozone. In more recent studies, these conclusions were confirmed. The only information available on fibers other than cotton addressed the effect of ozone on modacrylic, acrylic. Nylon 66, and polyester fabrics.The results indicated minimal effects on these fibers. 

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