Ultraviolet conjugation

Saturated aldehydes and ketones absorb weakly in the near ultraviolet. Conjugation moves this weak band (the R band) to longer wavelengths (why ) and, more important, moves a very intense band (the K band) from the far ultraviolet to the near ultraviolet. 

The former exhibits absorption tjrpical of an isolated keto group, whereas the latter shows a high intensity -band associated with the conjugated system HO—C=C—C=0. The proportions of the two forms under various conditions are readily determined from the ultraviolet spectra. The ultraviolet spectra in various solvents are shown in Fig. A, 7, 2. Since the absorption of the keto form is negligible, the percentage of enol present is 100(em/e ), where e is the observed extinction at 245 mp. and that of the pure enol. It was shown that in alcoholic solution is 1900 and the percentage of enol is 12. Thus e is ca. 16000, and use of this value permits the approximate evaluation of the enol content in different solvents. The results are collected in Table XII. 

Determination of structural features. The ultraviolet spectrum has been of value in the determination of the structure of several vitamins. Thus the presence of an a-naphthoquinone system in vitamin K was first detected by this means. Also the 4-methylthiazole and the 2 5-dimethyl-6-aminopyridine system was first identified in vitamin Bj (thiamine), a- and /3-Ionones can be distinguished since the former contains two conjugated chromophores and the latter three conjugated chromophores. 

Ultraviolet visible (UV VIS) spectroscopy, which probes the electron distribution especially m molecules that have conjugated n electron systems Mass spectrometry (MS), which gives the molecular weight and formula both of the molecule itself and various structural units within it... 

Ultraviolet visible (UV VIS) spectroscopy (Section 13 21) An alytical method based on transitions between electronic en ergy states in molecules Useful in studying conjugated systems such as polyenes... 

This synthesis method can be utilised by any alkene or alkyne, but steric hindrance on internal double bonds can cause these reactions to be quite slow. Conjugated dienes and aromatic alkenes are not suited for the ultraviolet light-initiated process. The use of other free-radical initiators is required in free-radical-initiated reactions involving these species. 

The infrared spectrum of the l-methoxy-l,4-cyclohexadiene shows the absence of strong aromatic absorption at 1600 cm.the ultraviolet spectrum shows absence of absorption at 270 nm., indicating absence of the conjugated isomer. 

Ultraviolet The determination of organic compounds, expecially aromatic and heterocyclic substances or compounds with conjugated bonds. ... 

Dehydrotestosterone acetate (174) in nonprotic solvents (dioxane, benzene) undergoes a rearrangement to the isomer (175). This product is photolabile and isomerizes readily to new cross-conjugated dienones. Thus, ultraviolet irradiation of (174), its 1-, 2- and 4-methyl homologs, and its lOa-stereoisomer (188) in dioxane solution causes, in each case, a series of rearrangements as summarized on page 331 for (174) and (188). ... 

The ultraviolet spectra were also used for determination of the pyrroline structure (1,158-160). They exhibit a bathochromic shift to 225-235 m, caused by the auxochromic action of the nitrogen-free electron pair which is in conjugation with n electrons of the enamine double bond (161,162). 

Pyrazole and its 3,5-dimethyl and 3,4,5-trimethyl derivatives combined with two moles of dimethyl acetylenedicarboxylate giving products of similar ultraviolet absorption spectra to the parent pyrazoles. These products [e.g., (69)] do not possess the strong broad absorption at ca. 3.20 /u, characteristic of the bonded N—H group which is present in the parent pyrazoles and are formed by two successive Michael addition reactions. In the case of 3,5-dimethylpyra-zole, the initial fumarate (68) has been isolated and possessed a more conjugated type of absorption spectrum to those of the dipyrazolyl-... 

The analogous structures 309, 310, and 311 have been suggested for the pyrophthalones. Early work favored structure 309 (see reference 385 and references therein). Later, ultraviolet spectral data showed the presence of an extended conjugated system, and comparison with the spectra of the A -methyl compounds suggested structure 310. ... 

The isoindole-isoindolenine equilibrium has been studied quantitatively only in the case of certain 1-arylisoindoles. Although two structurally different isoindolenines are possible, only that with the carbon-nitrogen double bond conjugated with both aromatic rings was observed. Investigation of the isoindole-isoindolenine ratios for three compounds by NMR and ultraviolet spectroscopy indicated a... 

The ultraviolet absorption spectra of carboline derivatives have been repeatedly recorded. Since the basic jpyr-N in the carbohnes and in 3,4-dihydro-jS-carbolines is part of a conjugated system, protonation affects the electronic absorption spectra significantly. It is unfortunate therefore that the spectra of the protonated, as well as those of the unprotonated, species have not been reported in all instances. Protonation leads to a bathochromic shift of 20-30 mp,. This is illustrated by the absorption of j3-carboline, 1-methyl-jS-carboline, 7-methoxy-l-methyl-jS-carbohne, and the salts of these compounds. 

Addition of water across a C=N bond in a conjugated system breaks the conjugation and alters the electronic transitions. The ultraviolet and visible spectra of anhydrous and hydrated species are therefore usually dissimilar, and such differences have been used as the basis for much of the qualitative and quantitative work done on covalent hydration. 

The cation of 4,4 -biquinazolinyl and its 2,2 -dimethyl derivative readily add water across the 3,4- and 3, 4 -double bonds, but the cation of 2,2 -biquinazolinyl is not hydrated. Hydration in the 4,4 -isomers has been attributed to restricted rotation about the 4,4 -bond, a steric effect which is relieved by hydration. The ultraviolet spectrum of 2,2 -biquinazolinyl (neutral species and cation) shows that there is considerable conjugation between the quinazoline groups. Covalent hydration is absent from the latter compound because it would otherwise destroy the extended conjugation present. 

Ultraviolet spectroscopy Is a conjugated it electron system present ... 

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