Conformation and tautomers, vibrational spectra used

Infrared spectroscopy is widely used for the structural determination of tautomers, isomers conformers of various nitroimidazoles [42, 1043], Vibration spectra of different 1-alkyl [362]-, l-(trialkylsilylalkyl)-2-methyl-4-nitroimidazoles [363], allylated 4-nitroimidazoles [364], dinitroimidazoles [428] have been studied. The vibration frequencies of some medicinal compounds on the base nitroimidazoles, for example, diasteriomeric nido-carboranyl misonidazole congeners [389], antiviral agents [452], and adrenergic-receptor agonists [454] are analyzed. In the literature the number of publications devoted to vibration spectra is rather limited and, as a rule, the absorption band frequencies of nitroimidazoles are considered in synthetic works concerned with structure identification such as, for example, [354, 429, 461-464,468-471, 1044-1047],... 

At low enough temperatures vibrational fine structure of aromatic chromophores may be well resolved, especially if they are embedded in a suitable matrix such as argon or N2, which is deposited on a transparent surface at 15 K. This matrix isolation spectroscopy77166 may reveal differences in spectra of conformers or, as in Fig. 23-16, of tautomers. In the latter example the IR spectra of the well-known amino-oxo and amino-hydroxy tautomers of cytosine can both be seen in the matrix isolation IR spectrum. Figure 23-16 is an IR spectrum, but at low temperatures electronic absorption spectra may display sharp vibrational structure. For example, aromatic hydrocarbons dissolved in n-heptane or n-octane and frozen often have absorption spectra, and therefore fluorescence excitation spectra, which often consist of very narrow lines. A laser can be tuned to excite only one line in the absorption spectrum. For example, in the spectrum of the carcinogen ll-methylbenz(a)anthrene in frozen octane three major transitions arise because there are three different environments for the molecule. Excitation of these lines separately yields distinctly different emission spectra.77 Likewise, in complex mixtures of different hydrocarbons emission can be excited from each one at will and can be used for estimation of amounts. Other related methods of energy-... 

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