Ultraviolet, absorption spectrum relation with

O-H bond. Among such properties a prominent one is the ultraviolet absorption spectrum and the theory may therefore be used for the examination of some of the spectroscopic shifts which accompany the lactam-lactim tautomerization. Much caution must, however, be exercised in this respect. Thus, in a recent paper Kwiatkowski135,137 performed Pariser-Parr-Pople-type calculations on the electronic structure of hydroxypurines, essentially to interpret their ultraviolet spectra. In these calculations he assumed that these compounds exist predominantly in their lactim form, and the results of his calculations, at least for 6- and 8-hydroxypurine, did not seem to contradict this assumption. It is only in the case of the 2-hydroxy isomer that a particularly striking disagreement between theory and experiment led him to admit that this last compound may exist in the lactam form. Calculations carried out for this form gave, in fact, a more satisfactory agreement with experiment.138 As we have seen, unambiguous infrared spectroscopy evidence clearly show s that all three isomers exist essentially in the lactam form. This shows that ultraviolet absorption may provide only very uncertain evidence about the lactam-lactim tautomerism in hydroxypurines and related compounds. 

To relate the wettability changes more firmly to the photooxidation processes and products, a detailed study was carried out with polystyrene. This polymer was selected because the formation of oxidation products in the hydrocarbon surface gave rise to large changes in wettability and because these products would be readily accessible to optical methods of analysis. The ultraviolet absorption spectrum of polystyrene shows a sharp cut-off, and the extinction coefficients for the radiation absorbed are sufficiently high that almost all of the photochemical reaction should be confined to the surface layers. 

It is reported that a purine-requiring mutant of Escherichia coli accumulated a substance related to 5-amino-iV-D-ribosyl-4-imidazolecarboxamide. Its ultraviolet absorption spectrum and its diazo chromogen spectrum differed from those of the known D-ribosyl derivative. Escherichia coli (strain B-96) converts it to 5-amino-4-imidazolecarboxamide, and it can be utilized by Escherichia coli B. It was suggested that it is an amino-(D-ribosyl)-imidazole. The accumulation of this substance was, however, somewhat surprising, since bacterial extracts of the purine-requiring mutant effected synthesis of 5-amino-4-imidazolecarboxamide from o-ribose 5-phosphate together with adenosine 5-triphosphoric acid and an energy source. 

A quinoxaline derivative was formed as was shown by analysis and the ultraviolet absorption spectrum, which was almost identical with that of a related compound prepared for comparison. The isolation of a quinoxaline and not of the dihydro derivative formed originally proved in addition that each of the carbons bearing the nitrogen atoms had at least one hydrogen atom as a substituent. This indicated that the grouping (6)... 

The following physico-chemical properties of the analyte(s) are important in method development considerations vapor pressure, ultraviolet (UV) absorption spectrum, solubility in water and in solvents, dissociation constant(s), n-octanol/water partition coefficient, stability vs hydrolysis and possible thermal, photo- or chemical degradation. These valuable data enable the analytical chemist to develop the most promising analytical approach, drawing from the literature and from his or her experience with related analytical problems, as exemplified below. Gas chromatography (GC) methods, for example, require a measurable vapor pressure and a certain thermal stability as the analytes move as vaporized molecules within the mobile phase. On the other hand, compounds that have a high vapor pressure will require careful extract concentration by evaporation of volatile solvents. 

In addition to a fluorescence perturbation, the Cd(II)-5d combination also uniquely yields aperturbation in the ultraviolet (UV) spectrum. A difference spectrum obtained by subtracting a fractional amount of an uncomplexed 5d spectrum from the perturbed spectrum is the mirror image of a fluorescence difference spectrum obtained by similar means. Moreover, excitation at 400 nm (where 1-4 are weakly absorbing but where moderate absorption is seen in the difference spectrum) gives rise to an emission spectrum with identical shape and Amax (456 nm) to that of the fluorescence difference spectrum. Thus, evidence points to the existence of two equilibrating ground state species as the physical basis for the chelatoselective emission. Bouas-Laurent has reported a related observation in methanol where a red-shifted CHEF was observed for a T1(I) 7r-complex.(14)... 

The value of Xmax in the ultraviolet spectrum of dienes becomes larger with increasing alkyl substitution. Since energy is inversely related to Xmax, the energy needed to produce ultraviolet absorption decreases with increasing substitution. 

Analytical absorption spectroscopy in the ultraviolet and visible regions of the elechomagnetic spectrum has been widely used in pharmaceutical and biomedical analysis for quantitative purposes and, with certain limitations, for the characterisation of drugs, impurities, metabolites, and related substances. By contrast, luminescence methods, and fluorescence spectroscopy in particular, have been less widely exploited, despite the undoubted advantages of greater specificity and sensitivity commonly observed for fluorescent species. However, the wider availability of spectrofluorimeters capable of presenting corrected excitation and emission spectra, coupled with the fact that reliable fluorogenic reactions now permit non-fluorescent species to be examined fluorimetrically, has led to a renaissance of interest in fluorimetric methods in biomedical analysis. 

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