Spectral comparisons

Annular tautomerism (e.g. 133 134) involves the movement of a proton between two annular nitrogen atoms. For unsubstituted imidazole (133 R = H) and pyrazole (135 R = H) the two tautomers are identical, but this does not apply to substituted derivatives. For triazoles and tetrazoles, even the unsubstituted parent compounds show two distinct tautomers. Flowever, interconversion occurs readily and such tautomers cannot be separated. Sometimes one tautomeric form predominates. Thus the mesomerism of the benzene ring is greater in (136) than in (137), and UV spectral comparisons show that benzotriazole exists predominantly as (136). 

Quinazolin-4-one and its 6-nitro derivative were both shown to exist predominantly in the o-quinonoid form (cf. 96) by ultraviolet spectral comparisons of the parent compound with the 0-methyl derivative and with the two A -methyl analogs, The existence of quinazolin-2- and -4-ones in an oxo-type structure was demonstrated in 1952 by using solid-state infrared data and later confirmed by... 

Quinazoline-2,4-dione (108) has been assigned the dioxo formulation on the basis of its infrared spectrum. A dioxo structure has also been demonstrated for the derivative 108a by ultraviolet spectral comparisons and infrared data. ... 

Ultraviolet spectral comparisons also indicate that structure 174 represents the predominant form of pteridin-4-one (a similar conclusion was reached for 2-amino-6,7-dimethylpteridin-4-one ), and a value of 1.72 has been calculated for log K p between forms 174 and 175 from pKa data. " It is possible, however, that the hydroxy... 

Mercaptopyridine and 3-mercaptoquinoline have been shown to exist predominantly in the zwitterion form (e.g., 204) by ultraviolet spectral comparisons and pK measurements (cf. Table V). 

Aminopyridine 1-oxides are protonated on the oxygen atom to give mesomeric ions of type 245 as predicted theoretically and confirmed by ultraviolet spectral comparisons (reference 91 and references therein). 

Cytidine (252, R = ribityl) has been shown to exist in the amino form, and, using refined X-ray methods, the positions of all the hydrogen atoms in compound 253 were determined. Structures 252 (R = Me) and 254 were shown to exist by ultraviolet spectral comparisons,and structure 252 (R = Me) has been shown to predominate over 255 by a factor of ca. 10 by using the pK method. ... 

Brown and Jacobsen demonstrated by ultraviolet spectral comparisons that 2-amino-(3H)-pteridin-4-one was the predominant tautomeric form. The evidence in favor of 4-amino-(lH)-pteridin-2-one is less conclusive, but, nevertheless, convincing. 

Mesomerism involving sulfonyl groups is relatively weak, and in the case of the sulfonyliminopyridones (279) destabilization caused by the factors previously mentioned for the acyliminopyridones should be less important. Early ultraviolet spectral comparisons showed that acetylsulfapyridine and sulfapyridine (cf. reference 354) exist in aqueous solution as mixtures of comparable amounts of 279 and 280. A recent investigation of 2-, 3-, and 4-methanesulfonamido-... 

Tetronic acids exist predominantly in the dioxo form (32) in solvents of low polarity, while the existence of the monoenol form (33) has been established in other solvents by infrared " and ultraviolet spectral comparisons- and from dipole moment data. " Haynes and Pliramer " have recently reviewed the structure of these compounds [see also reference 28(a)], and the tautomerism of vitamin A (34), which has a related structure, has also been surveyed.- Analogous compounds carrying an amino group in the 3-position are also know n. ... 

Infrared and ultraviolet spectral comparisons by Grob and Hofer show that the position of the equilibrium 86 87 predominantly favors 87. 

Ultraviolet spectral comparisons indicate that structure 107 predominates over 108 when R = H or OH, but that 107 is the predominant form when R = aryl. - Similarly, 109 predominates over 110 by a large factor when R = H, OH, or Me, and by a smaller factor when R is a higher alkyl group, but 110 predominates when R is an aryl group. (For a discussion of guanidine derivatives corresponding to 110, see reference 110.)... 

Chemical evidence has been advanced for the formulation of p nitrosoindoles as either 115 or 116 (sec reference 119 and references therein), but ultraviolet spectral comparison with both methylated forms clearly indicated that 116 was favored. Infrared spectral data are also considered to support structure 116. °... 

The similarity of the ultraviolet spectrum of 4,5-diphenyloxazol-2-one (91) with those of both alternative methyl derivatives preclude application of the spectral comparison method to the elucidation of their structures, but the fluorescence spectra of these compounds indicate that 91 exists in the oxo form. ° Infrared data for a number of substituted oxazol-2-ones support this conclusion. ... 

Benzothiazole-2-thiones are associated,but the predominance of the thione forms (154, R = H, Me) has been demonstrated by ultraviolet spectral comparisons wdth the methylated derivatives of both possible tautomeric forms (cf. reference 182), comparative dipole moment data, and infrared spectral data. " Ultraviolet spectroscopy has also been used to demonstrate that benzoseIenazole-2-thione exists as such. The predominance of the thione form of nonaromatic 3,4-dihydrothiazole-2-ones (155) has also been established on the basis of ultraviolet spectral evidence. ... 

The predominance of the amino structure has been demonstrated for 176 (R = Ph, CIl2Ph) by ultraviolet spectral comparisons with both types of alkylated derivatives.Similarly, comparison of the ultraviolet spectra of 2-ainino-5-chlorobenzoxazole (177) and its... 

Park S, Tong YY, Wieckowski A, Weaver MJ. 2002a. Infrared spectral comparison of electrochemical carhon monoxide adlayers formed by direct chemisorption and methanol dissociation on carbon-supported platinum nanoparticles. Langmuir 18 3233-3240. 

The alternative structure of ferulic acid was excluded by spectral comparison with an authentic sample of ferulic acid. 

Electron-transfer activation. The observation of intense coloration upon mixing the solutions of hydroquinone ether MA and nitrogen dioxide at low temperature derives from the transient formation of MA+ cation radical, as confirmed by the spectral comparison with the authentic sample. The oxidation of MA to the corresponding cation radical is effected by the nitrosonium oxidant, which is spontaneously generated during the arene-induced disproportionation of nitrogen dioxide,239 i.e.,... 

The disproportionation of nitrogen dioxide has been independently confirmed by the direct observation of nitrosonium cation as the EDA complex of hexamethybenzene by spectroscopic means as well as by the spectral comparison with the authentic [HMB, NO+] complex,240 i.e.,... 

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