Ultraviolet-visible phenols

Hostettmann, K. et al., On-line high-performance liquid chromatography ultraviolet-visible spectroscopy of phenolic compounds in plant extracts using post-column derivatization, J. Chromatogr., 283, 137, 1984. 

The vast literature associated with flavanoid chemistry precludes a discussion here but two valuable reviews have been published. The first reviews a number of spectroscopic techniques used for flavonoid analysis, with a strong emphasis on NMR spectroscopy (plus also mass spectrometry, vibrational spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, X-ray crystallography, and circular dichrosim (CD)) . The second review deals with NMR methods that have been successful in the characterization of phenolic acids and flavonoids from plant extracts that have not been separated or isolated as single components. The emphasis of the article is 2-D NMR methodology and a variety of experiments such as total correlated spectroscopy (TOCSY), COSY, nuclear Overhauser enhancement spectroscopy (NOESY) and heteronuclear multiple quantum correlation (HMQC) are discussed . 

Figure 3 (Top) Structures of drug A and the known thermal degradation product, referred to as phenol in this report (bottom) ultraviolet/visible absorption spectrum of 0.064 mM drug A in water (dashed line) compared to the combined spectral outputs of the International Conference on Harmonization ultraviolet and visible lamps (solid line).

Solvatochromic shifts in the ultraviolet-visible absorption spectra of p-nitro-phenol and p-nitroaniline have been taken as measures of relative solvent affinities [25]. 

Both evidence of the browning caused by the high-intensity absorption of conjugated chromophoric groups and upward shifts in the ultraviolet-visible spectral baselines indicate that higher hydroquinone concentrations produce more polymers. Free radical coupling is probably involved in the polymerization since the existence of free radicals is correlated to hydroquinone concentration. The ultraviolet-visible absorption profiles of these polymers formed in the systems that are not sterilized (Kung and McBride, 1988) appear to be similar to those of phenol-derived polymers in the systems free... 

The first application of this methodology was directed to the preparation of phenol-terminated polymers [148, 158]. The tert-butyidimethylsilyl-protecting group was chosen for the aromatic hydroxyl group based on the previous results reported by Nakahama and coworkers [155, 156, 159]. The addition of poly(styryl)lithium to l-(4-ferf-butyldimethylsiloxyphenyl)-l-phenylethylene (24) (0.2 molar excess) in benzene at room temperature was monitored by the appearance of a ultraviolet-visible absorption at 406 nm complete addition required three days at room temperature. 

Unsubstituted polysaccharides do not appreciably absorb ultraviolet and visible radiation, but they can be made to do so intensely by combining them with chromophores and chromogens (e.g., a-naphthol, dihydroxynaph-thalein, anthrone, carbazole, phenol-sulfuric acid, 2-thiobarbituric acid, tolu-idine blue, diphenylamine, Congo red, aniline blue, and methyl orange), usually in acidic or basic media. Coloration is normally preceded by depoly-... 

Figure 4 Photodegradation (decreasing drug A levels versus increasing phenol levels) induced by various International Conference on Flarmonization visible and ultraviolet light exposures for a solution containing 0.57mM drug A, lOmM citrate (pFI 6), and 136 mM sodium chloride. A = 0.3 x 10 Lux hour B = 0.6 x 10 Lux hour C = 0.9 x 10 Lux hour D = 1.2 x 10 Lux hour E = 1.2 x 10 Lux hour followed by 50 W hr/m F = 1.2 x 10 Lux hour followed by 100 W hr/m G = 1.2 x 10 Lux hour followed by 150 W hr/m FI = 1.2 x 10 Lux hour followed by 200 W hr/m. ...

Visible (10 Lux hr) Ultraviolet (Whr/m ) Drug A Phenol Total degradation products... 

Because every phenol exhibits a higher or lower absorption in ultraviolet (UV) or UV-visible (UV-VIS) light, given the intrinsic existence of conjugated double and aromatic bonds, UV detection is the ideal method to localize a phenol in the effluent of a column. When a UV... 

The photochromism of HNA seems to be simpler than that of SAA, since no cls-trans isomerization is involved (63), the cis-qulnone being the photoproduct. In this case, too, the emitting state which is produced by proton transfer from the excited phenol is similar but not identical with the excited cis-guinone. This Is demonstrated by somewhat different fluorescence spectra obtained in the presence of ultraviolet and visible exciting light. 

The chemical interpretation of the yellow color in white wines has always been a little-known field. Phenolic compounds are certainly involved, but concentrations are low and their contribution has never really been established. Many studies have investigated the oxidative browning of wines, independently of enzyme mechanisms. Other molecules are involved besides tannins (Sapis and Ribereau-Gayon, 1968), especially compounds that have a high absorption in the visible—and especially ultraviolet—spectrum (Somers and Ziemelis, 1972). Cafeic and coutaric acids are responsible for browning in white wines (Cheynier, 2001). 

Accepted methods of the first type comprise the hide-power method [77], the refrac-tometric method, and various visible, ultraviolet, and infrared spectrometric methods. Accepted methods of the second type include comparative methods such as the Stiasny-Orth method [78,79] and its modifications, all these being gravimetric methods largely obsolete today due to the lack of reliability consequent to coprecipitation of some carbohydrates together with the phenolic material of the tannin extract and to the results being expressed in an absolute value which is never reportable to a percentage of useful material in the extract, the Lemme [80] sodium bisulfite backtitration method, the ultraviolet spectrophotometric molybdate ion method [77,81], and infrared spectrophotometric methods [82]. 

The fluorescence of certain compounds as a function of pH has been used for the detection of end points in acid-base titrations. For example, fluorescence of the phenolic form of l-naphthol-4-.sulfonic acid is not detectable by the eye because it occurs in the ultraviolet region. When the compound is converted to the pheno-late ion by the addition of base, however, the emission band shifts to visible wavelengths, where it can readily be seen. It is significant that this change occurs at a different pH than would be predicted from the acid dissociation constant for the compound. The explanation of this discrepancy is that the acid dissociation constant for the excited molecule differs from that for the same species in its ground stale. Changes in acid or base dis-... 

In aqueous solution, diazonium salts show absorption maxima in the ultraviolet (UV) region benzenediazonium ion has 261 nm (log e 4.3) and 300 nm (log e 3.17). Both absorption bands are shifted toward the visible by electron-donating substituents 4-dimethylaminobenzenediazonium has 382 nm (log e 4.6). Photolysis in aqueous solution leads to phenol as the main product, according to Scheme 1. Some replacement of the diazonium group by an atom of chlorine or bromine is also found in solutions containing chloride or bromide ions. ... 

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