Ultraviolet spectroscopy model compounds

The presence of iminium salts can be detected by chemical means or by spectroscopic methods. The chemical means of detecting iminium salts are reactions with nucleophiles and are the subject of this review. The spectroscopic methods are more useful for rapid identification because with the large number of model compounds available now the spectroscopic methods are fast and reliable. The two methods that are used primarily are infrared and nuclear magnetic resonance spectroscopy. Some attempts have been made to determine the presence of iminium salts by ultraviolet spectroscopy, but these are not definitive as yet (14,25). 

A-2-Thiazoline-5-one may exist in three tautomeric forms (Scheme 108). The tautomeric equilibrium has been studied by H NMR (446. 453. 457. 464), infrared (453. 464-466). and ultraviolet (453, 464) spectroscopies 13C NMR data would be very instructive since model compounds for the three tautomers are now available. Polar solvents favor the enolic (211b) and the mesoionic (211c) forms (Table VII-12), the later being even more favored by high dilution (464). The percentage of the enol... 

Second derivative ultraviolet spectroscopy is a simple yet powerful technique for highlighting the fine structure of spectral curves. It involves calculating the second derivative of a spectrum with respect to wavelength and plotting the derivative rather than the spectrum itself. Second derivative absorbance spectra of lignin model compounds, milled wood lignins and lignosulfonates have been published (Lin 1982). 

Although, the ultraviolet spectroscopy of softwood lignin and guaiacyl model compounds has been thoroughly studied, the same cannot be said about hardwood lignins. It has been shown that the absorptivity of hardwood milled wood lignins at 280 nm decreases linearly with the methoxyl/carbon ratio (Sarkanen et al. 1967), but the effects of side chain structures on the ultraviolet absorption properties of hardwood lignins have yet to be determined. 

SY Lin. Derivative Ultraviolet Spectroscopy of Lignin and Lignin Model Compounds A New Analytical Technique. Svensk. Papperstidn. 85 R162-R171, 1982. 

The photo-Fries reaction occurs readily in solid polymers and is observable in phenyl esters, particularly in poly(phenyl acrylate) and poly (phenyl methacrylate) and their derivatives. The course of the reaction can be followed very easily by ultraviolet spectroscopy, since the product hydroxy ketones have strong absorbance at 260 and 320 nm (Fig. 10). Reaction occurs with equal efficiency in small model compounds in solution and in the polymers in the solid phase (44). An Arrhenius plot of the quantum yield for the para product (Fig. 11) shows a linear increase up to 294 K, above which no further change in quantum efficiency is observed, either above or below the glass transition temperature. 

This approach— the use of model compounds— is one of the best ways to put the technique of ultraviolet spectroscopy to work. By comparing the UV spectrum of an unknown substance with that of a similar but less highly substituted compound, you can determine whether or not they contain the same chromophore. Many of the books listed in the references at the end of this chapter contain large collections of spectra of suitable model compounds, and with their help you can estabHsh the general structure of the part of the molecule that contains the K electrons. You can then utilize infrared or NMR spectroscopy to determine the detailed structure. 

Smith and Aplin (1966) have irradiated a solution of uracil (2.8 x 10" M) and cysteine HCl (10" M) with ultraviolet light (2537 A). The irradiation produced a heterodimer, 5-S-cysteine-6-hydrouracil, the structure of which was determined by ultraviolet, infrared, NMR, and mass spectroscopy. This compound may serve as a model for the mechanism by which DNA and protein are cross-linked in vivo by... 

N. Sato, H. Inokuchi, K. Seki, J. Aoki, and S. Iwashima, Ultraviolet Photoemission Spectroscopic Studies of Six Nanocyclic Aromatic Hydrocarbons in the Gaseous and Solid States, J. Chem. Soc. Faraday Trans. 2 78, 1929-1936 (1982) N. Sato, K. Seki, and H. Inokuchi, Polarization Energies of Organic Solids Determined by Ultraviolet Photoelectron Spectroscopy, J. Chem. Soc. Faraday Trans., 2 77, 1621-1633 (1981) N. Sato, K. Seki, and H. Inokuchi, Ultraviolet Photoelectron Spectra of Tetrahalo-P-Benzo-quinones and Hexahalobenzenes in the Solid State, J. Chem. Soc. Faraday Trans. 2 77, 47-54 (1981) I. Ikemoto, Y. Sato, T. Sugano, N. Kosugi, H. Kuroda, K. Ishii, N. Sato, K. Seki, and H. Inokuchi, Photoelectron Spectroscopy of the Molecule and Solid of 11,11,12,12-Tetracyanonaphthoquinodimethane (TNAP), Chem. Phys. Lett. 61, 50-53 (1979) K. Seki, S. Hashimoto, N. Sato, Y. Harada, K. Ishii, H. Inokuchi, and J. Kanbe, Vacuum-Ultraviolet Photoelectron Spectroscopy of Hexatricontane (N-C36-H74) Polycrystal A Model Compound of Polyethylene, J. Chem. Phys. 66, 3644-3649 (1977). 

JC Austin, T Jordan, TG Spiro, Ultraviolet resonance Raman studies of proteins and related model compounds. In RJH Clarke, RE Hester, eds. Biomedical Spectroscopy. New York Wiley, 1993, pp 55-127. 

The pH of a synthetic solution made of reagent grade transition metals and sodium chlorides does not have the same pH as the crevice solution of the same composition (Figure 10.19). In addition, this pH spontaneously decreases with time [33,40] (Figures 10.18 and 10.20) for periods of time of several weeks which should not be neglected when modeling the crevice evolution. Indeed, Zhen [41] showed, by ultraviolet (UV) spectroscopy, the presence of at least two different Cr compounds in stainless steel crevice solutions and observed a spontaneous evolution with time toward one of these species. 

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