Ultraviolet absorption spectroscopy

The ultraviolet absorption spectra of most new thiazoles currently synthesized have been described and occasionally used for structural 

The ultraviolet absorption spectrum of thiazole was first determined in 1955 in ethanolic solution by Leandri et al. (172), then in 1957 by Sheinker et al. (173), and in 1967 by Coltbourne et al. (174). Albert in 1957 gave the spectrum in aqueous solution at pH 5 and in acidic solution (NHCl) (175). Nonhydroxylic solvents were employed (176, 177), and the vapor-phase spectrum was also determined (123). The results summarized in Table 1-15 are homogeneous except for the first data of Leandri (172). Both bands A and B have a red shift of about 3 nm when thiazole is dissolved in hydrocarbon solvents. This red shift of band A increases when the solvent is hydroxylic and, in the case of water, especially when the solution becomes acidic and the extinction coefficient increases simultaneously. 

This bathochromic shift is typical of 77 — tt transitions. The behavior of the water solution when acidified was attributed by Albert (175) absorption by the thiazolium cation, by analogy with pyridine. However, allowance is made for the very weak basicity of thiazole (pK = 2.52) compared with that of pyridine (pK = 5.2), Ellis and Griffiths (176) consider the differences between the spectrum of thiazole in water and in 

TABLE 1-15. ULTRAVIOLET ABSORPTION SPECTRUM OF THIAZOLE IN THE VAPOUR PHASE AND IN DIFFERENT SOLVENTS 

TABLE 1-16 ULTRAVIOLET ABSORPTION SPECTRA OF THIAZOLE AND ITS MONOMETHYL DERIVATIVES 

The naturally occurring amino acids and peptides are colorless, they do not absorb visible light. Rare examples containing a chromophore, for instance the actinomycins, or colored derivatives obtained by reactions with chromogenic reagents, will not be discussed here. 

Absorption studies of amino acids in the UV region go back to the mid-thirties [15]. An overview on this subject appeared as early as 1952 [16]. Only the aromatic amino acids, phenylalanine, tyrosine and tryptophan, absorb in a conveniently observed region (Fig. 6). The weakest of them is phenylalanine, revealing at least 6 absorption bands it has a molecular absorption coefBcient of fimoi 0.195 X 10 at 257 nm. The UV-spectrum of tyrosine is strongly dependent on pH in the presence of 0.1 N HCl (protonated hydroxyl group) Sj oi = 1-34 x 10 at the maximum, (l x 275 nm) in 0.1 N NaOH (phenolate) = 2.33 x 10 at 293 nm. The absorption of tryptophan, is nearly independent of the pH it shows a maximum at 280 nm with = 5.55 x 10 and a second maximum (shoulder) at 288 nm of = 4.55 x 10. These parameters have often been used in the quantitative analysis of peptides. The sulfur-containing side chains show weak absorption below 250 nm and are of interest only in rare special cases. 

The general estimation of the amino group, in the form of copper salts, by UV-absorption at 230 nm, was suggested by Spies and Chambers [17]. 

The UV absorption of the peptide bond, —NH—CO—, is centered around 210 nm. Its molar absorption coefficient, approx. 3 x 10, is much higher than that of the amide group of saturated fatty acids at the same wave length, a consequence of the mesomeric electron-distribution (partial double bond character) of the —NH—CO— moiety. 

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Quantum chemistry methods allow the prediction of the ultraviolet transitions in good agreement with the experimental values in the case of thiazole and its three methyl derivatives (Table 1-18). A very weak absorption has been indicated at 269.5 nm that could correspond to an n- TT transition given by calculation at 281.5 nm (133). Ultraviolet absorption spectroscopy has been investigated in connection with steric interactions in the A-4-thiazoline-2-thione (74) series (181). It was earlier demonstrated by NMR technique that 4-alkyl-3 isopropyl-A-4-thiazoline-2-thiones exist in solution as equilibrium mixtures of two conformers (75 and 76), the relative populations of which vary with the size of R4 (182) for R4 = rBu the population of rotamer A is 100%, whereas for R4 = Me it is only 28%. Starting from the observed absorption wavelength for... 

CBurgess and A Knowles, Techniquesin Visibleand Ultraviolet Absorption Spectroscopy, Chapman and Hall, London, 1981... 

Once the FBA has been identified, ultraviolet absorption spectroscopy affords a rapid and accurate method of quantitative analysis. Care must be taken when interpreting the spectra of stilbene-type compounds, since turns to cis isomerisation is promoted by ultraviolet radiation. Usually, however, a control spectrum of the turns isomer can be obtained before the compound undergoes any analytically significant isomerisation. FBAs are often marketed on the basis of strength comparisons determined by ultraviolet spectroscopy. 

A rate enhancement effect due to secondary nucleation has been identified in the solution-mediated transformation of the 7-phase of (i)-glutamic acid to its / -phase [82]. In this study, the kinetics of the polymorphic transition were studied using optical microscopy combined with Fourier transform infrared, Raman, and ultraviolet absorption spectroscopies. The crystallization process of n-hexatriacontane was investigated using micro-IR methodology, where it was confirmed that single... 

The importance of ultraviolet absorption spectroscopy as a tool for the elucidation of the chemical structure of organic compounds is well established. Although it has been extensively used in lignin research, as yet definite conclusions have not been attained. 

Ultraviolet Absorption and Photoelectron Spectroscopy Ultraviolet Absorption Spectroscopy... 

The spectroscopy experiments are further subdivided into atomic spectroscopy found in Table XII, infrared and Raman spectra found in table XIII, visible and ultraviolet absorption spectroscopy found in table XIV, and luminescence spectroscopies found in table XV. 

Table XIV. Visible/Ultraviolet Absorption Spectroscopy Experiments...

As in the case of polypropylene, the question is whether the complex spectrum recorded should be interpreted as arising from alkylic or allylic radicals—i.e., structures XVII and XXVI, respectively. This basic problem could probably be solved by using ultraviolet absorption spectroscopy at liquid nitrogen temperature (9, 10, 11, 12). 

Spectroscopic methods of analysis 3.3.1. Ultraviolet absorption spectroscopy... 

The spectra (absorption or emission) of atoms are much sharper than those of molecules, because every electronic energy level in a molecule has a rich complement of vibronic levels and rotational sublevels (Fig. 3.15). In the late nineteenth century these smaller features could not be resolved in visible-ultraviolet spectroscopy, so, in ignorance of all the quantum effects explained decades later, the sharper spectra of atoms were called "line spectra," while the broadened spectra of molecules were called "band spectra." Cooling the molecules to 77 K or 4.2 K does resolve some of the vibronic substructure, even in visible-ultraviolet absorption spectroscopy. 

Key Mechanism 15-3 The Diels-Alder Reaction 684 15-12 The Diels-Alder as an Example of a Pericyclic Reaction 692 15-13 Ultraviolet Absorption Spectroscopy 696 15-14 Colored Organic Compounds 701... 

Spectroscopic Techniques. In absorption spectroscopy, Vanadium(V), a d° system, shows no transitions in the visible region. The yellow color of some V complexes can be ascribed to the tail of an intense absorption in the ultraviolet. Absorption spectroscopy is more useful for systems (3d ), where electronic energies of transitions generally correlate well with ligand type and with electron spin resonance (ESR) parameters. 

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