Dienes, absorption maxima

Figure 13.37 shows the UV spectrum of the conjugated diene cis,trans-, 3-cyc o-octadiene, measured in ethanol as the solvent. As is typical of most UV spectra, the absorption is rather broad and is often spoken of as a band rather than a peak. The wavelength at an absorption maximum is refened to as the X ax of the band. There is only one band in the UV spectrum of 1,3-cyclooctadiene its X ax is 230 ran. In addition to UV-VIS bands are characterized by their- absorbance (A), which is a measure of how much of the radiation that passes through the sfflnple is absorbed. To correct for concentration and path length effects, absorbance is converted to molar absorptivity (e) by dividing it by the concentration c in moles per liter and the path length I in centimeters. 

Hydroperoxide formation by the ene reaction path may lead to formation of conjugated double bonds in polyunsaturated fatty acids (see Section V.A) this reaction is concurrent with POV increase. An increase of the CDV, as measured from the absorbance at 233 nm, therefore indicates oxidation of polyunsaturated lipids. A strong correlation exists between CDV predicted from the absorbance in the 1100 to 2200 nm NIR region and CDV determined by the Ti Ia-64 AOCS official method , by UV spectrophotometry at 233 nm. The method was applied to determine CDV for oxidized soybean oil. A secondary absorption maximum of lesser intensity appears in the 260-280 mn range, and is assigned to ketone dienes . 

The second compound looks like buta-1,3-diene (217 nm) with three additional alkyl substituents (circled). Its absorption maximum should be around (217 + 15) nm, and 232 nm must be the correct value. 

Richard followed the course of the reactions of Cr(CO)6 and Mo(CO)6 with hexamethylborazine by UV-vis spectroscopy but although he observed that the intensity of the absorption maximum of the hexacarbonyls at around 290 nm decreased and a new band at around 350 nm appeared, he was unable to identify the new product. Experiments with other starting materials such as norborna-diene chromium and molybdenum tetracarbonyl or tris(aniline) molybdenum tricarbonyl which readily react with arenes by ligand exchange, also failed. The key to success was to use tris(acetonitrile) chromium tricarbonyl as the precursor, which in dioxan under reduced pressure afforded the desired hexamethylborazine chromium tricarbonyl as a stable crystalline solid in 90% yield. This was the breakthrough and, after we had communicated the synthesis and spectroscopic data of the complex in the January issue 1967 of Angewandte Chemie, Richard finished his work and defended his Ph.D. thesis in June 1967. Six months before, in December 1966,1 defended my Habilitation thesis in front of the faculty and became Privatdozent (lecturer) on the 1st of January 1967. 

The 233-nm absorption maximum is reported to occur because of the presence of two chromophores, a conclusion deduced from the respective value of the extinction coefficient (Uemura et al. 1980a). H NMR studies suggest that both these chromophores are conjugated dienes (Moore et al. 1978). 

Reaction of 1,4—diene-3-ketosteroids with isonicotinic acid hydrazide (isoniazid) produces a yellow hydrazone with an absorption maximum of 404 mp38. This reaction can be adapted to triamcinolone for formulation assays. It also has been used as a spray reagent and for quantitation in paper and thin layer chromatography... 

Excitation from the HOMO (i/zi), the highest occupied molecular orbital, to the LUMO ( j/ i), the lowest unoccupied molecular orbital, of buta-1,3-diene (Figure 4.5) requires — 1.236/3 (/3 is a negative energy). We can use this result to obtain a first calibration of /3. The absorption maximum of the first 7t,7t transition of buta-1,3-diene lies at vnrdx = 4.6 jam hence /3/he 3.7 pm ... 

The necessity for these detailed examinations has. been emphasized by a recent paper of Muller and Velluz (348). Tetrahydrodemethoxy-colchicine was reported to form a cyanohydrin which could be dehydrated to diene CII which shows an absorption maximum at 251 m/, and the cyanodiene (CIV) derived from tetrahydrodemethoxyisocol-chicine (CIII) possesses a maximum at 268 m/u.. These spectral values are incompatible with the formulation of tetrahydrodemethoxycolchicine... 

An empirical method of calculating the position of the long wavelength absorption maximum of any steroid diene has been proposed by Fieser, Fieser and Rajagopalan17. The method is described in Table 7.1. 

The compound [Pd(dien)Cl]Cl separates as a light yellow microcrystalline solid. It is very soluble in water, but only slightly soluble in polar organic solvents such as acetone. The electronic spectrum depends on the chloride-ion concentration. Thus, the complex has an absorption maximum in water at 330 nm (e = 462 cm dm mol ) and in 0.1 M NaCl at 334 nm (e = 480 cm dm mol ). The Pd—Cl stretching vibration is situated as a strong band at 331 cm . ... 

There are essentially four organic molecular systems of interest. The principal parent chromophore systems are (1) conjugated dienes, (2) monosubstimted benzene rings, (3) disubstituted benzene rings, and (4) conjugated carbonyl systems. The method of calculation is to identify a parent system and assign an absorption maximum. The parent system is then modified by the presence of other systems within the molecule. From these modifications, the absorption maximum of a specific molecular structure can be calculated. 

Examples 5.3 and 5.4 are molecules with very similar formulas, but different structures. The double bonds are in different positions. The wavelength of the absorption maximum for Example 5.3 is 309 nm, which is 54 nm longer than in Example 5.4. The major difference between the two compounds is that the molecule in Example 5.3 has a two double bonds within one ring and a conjugated system of three double bonds, that is, the double bonds are each separated by one single bond. That results in the addition of 36 nm for the diene in a ring and 30 nm for the double bond extension. In Example... 

The only contributions to the absorption maximum in Example 5.4 are the exocyclic double bond (the diene bond on the right at the bottom of the ring containing the two double bonds) and the alkyl substituents. These two compounds can readily be distinguished by a simple UV absorption spectrum because of the large difference in... 

An example of the calculation of the wavelength of the absorption maximum using Table 5.7 is shown in Example 5.6. In this example, the calculations are similar to those used in the conjugated diene system. Note that in order to affect the absorption maximum, the substituents must... 

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