Spectrum comparison 298 substitutable

A comparison of the relative basicities of pyrrole, furan and thiophene may be made by comparing the pK values of their 2,5-di-t-butyl derivatives, which were found to be -1.01, —10.01 and —10.16, respectively. In each case protonation was shown by NMR to occur at position 2. The base-strengthening effect of alkyl substitution is clearly apparent by comparison of pyrrole and its alkyl derivatives, e.g. A-methylpyrrole has a pKa. for a-protonation of -2.9 and 2,3,4,5-tetramethylpyrrole has a pK of 4-3.7. In general, protonation of a-alkylpyrroles occurs at the a -position whereas /3-alkylpyrroles are protonated at the adjacent a-position. As expected, electron-withdrawing groups are base-weakening thus A-phenylpyrrole is reported to have a p/sTa of -5.8. The IR spectrum of the hydrochloride of 2-formylpyrrole indicates that protonation occurs mainly at the carbonyl oxygen atom and only to a limited extent at C-5. 

The electronic spectrum of a compound arises from its 7r-electron system which, to a first approximation, is unaffected by substitution of an alkyl group for a hydrogen atom. Thus, comparison of the ultraviolet spectrum of a potentially tautomeric compound with the spectra of both alkylated forms often indicates which tautomer predominates. For example, Fig. 1 shows that 4-mercaptopyridine exists predominantly as pyrid-4-thione. In favorable cases, i.e., when the spectra of the two alkylated forms are very different and/or there are appreciable amounts of both forms present at equilibrium, the tautomeric constant can be evaluated. By using this method, it was shown, for example, that 6-hydroxyquinoline exists essentially as such in ethanol but that it is in equilibrium with about 1% of the zwitterion form in aqueous solution (Fig. 2). 

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. ... 

The 2-amino-selenazoles are crystalline compounds, and sometimes unstable, for example, the parent compound on heating in water undergoes complete decomposition. A few of these selenazoles which are substituted in the amino group are oily liquids. The basic character is more pronounced than in the alkyl- and aryl-selenazoles. The hydrochloride salts are, therefore, not so easily hydrolyzed in aqueous solution. 2-Amino- and 2-methylamino-4-methylselenazole have been considered to exist partly in the tautomeric selenazol-2-one imine form from comparison of their ultraviolet spectrum with that of 2-diethylamino-4-methylselenazole. ... 

Based both on the determined isotopic shifts and the comparison of the radical IR spectrum with the spectra of various substituted benzenes, the bands have been assigned to the normal modes and the force field of the benzyl radical calculated (Table 8). 

The quantitation of products that form in low yields requires special care with HPLC analyses. In cases where the product yield is <1%, it is generally not feasible to obtain sufficient material for a detailed physical characterization of the product. Therefore, the product identification is restricted to a comparison of the UV-vis spectrum and HPLC retention time with those for an authentic standard. However, if a minor reaction product forms with a UV spectrum and HPLC chromatographic properties similar to those for the putative substitution or elimination reaction, this may lead to errors in structural assignments. Our practice is to treat rate constant ratios determined from very low product yields as limits, until additional evidence can be obtained that our experimental value for this ratio provides a chemically reasonable description of the partitioning of the carbocation intermediate. For example, verification of the structure of an alkene that is proposed to form in low yields by deprotonation of the carbocation by solvent can be obtained from a detailed analysis of the increase in the yield of this product due to general base catalysis of carbocation deprotonation.14,16... 

The third member, trimethylenemethane (3), had some relevance to our studies on carbenes, since besides methylene and its simply substituted derivatives trimethylenemethane 3 is one of the few molecules having a triplet ground state.22 Also the experience with 3 could be of help in order to deal with the singlet/triplet differentiation in matrix-isolated carbenes. We learned that, if the calculated IR spectra of the singlet and triplet molecule are sufficiently different, it might be possible to determine the multiplicity of the matrix-isolated species by comparison with the experimental IR spectrum. In this context it is also worth mentioning that we were able to measure the matrix IR spectrum of 3, but a special technique (irradiation in halogen-doped xenon matrices) had to be developed in order to achieve a concentration of 3 sufficient for its IR detection.23... 

Compounds 1 and 2 were identified by FTIR and 13C-NMR. The 13C proton decoupled spectra for 1 and 2 are dominated by signals ranging from 62 to 195 ppm. The 13C chemical shift assignments were made based on comparisons with 4,4 -(hexafluoroisopropylidene)diphenol and from calculations based on substituted benzenes and naphthalenes.15 The 13C-NMR spectrum clearly showed that the Friedel-Crafts acylation of 1 by 4-fluorobenzoyl chloride yielded the 1,4-addition product exclusively. The 13C chemical shifts for 2 are listed in Table 8.1. The key structural features in the FTIR spectrum of2 include the following absorptions aromatic C-H, 3074 cnr1, ketone C=0, 1658 cm-1, aromatic ether Ar—0—Ar, 1245 cm-1, and C—F, 1175 cm-1. 

Electronic spectroscopy has been employed to study substitution reactions of sulfoxide complexes. An interesting example (104) is the reaction of [Fe(0-Me2S0) P+ with chloride ion. Addition of one equivalent of chloride ion to a Me2SO solution of [Fe(0-Me2S0)6P+ causes a change in spectrum, but further additions have no effect. Comparisons with known compounds indicate that [Fe(0-Me2S0)5Cip+ is the major species in solution. 

Observation of displacement effects resulting from O-substitution, and use of standards, led to the assignment of the chemical structure of the polysaccharide of Neisseria meningitidis serogroup 29e, which consists of 2-acetamido-2-deoxy-D-galactosyl and partly acetylated 3-deoxy-D-manno-octulosylonic acid (KDO) residues.166 The 13C-n.m.r. spectrum of the O-deacetylated polymer (see Fig. 36,B) contained 15 signals out of the possible 16 expected from an alternating structure. Comparison of these resonances with those of the anomers of 2-aceta-... 

In Table 13 the ionization potentials of some more C-nitroso compounds are collected. The spectrum of monomeric t-nitrosobutane126 exhibits a well separated band at 9.05 eV. The following ionizations show maxima at 11.85 and 12.46 eV. The spectrum is dominated by a strong composite band from 12.9-14.5 eV. The spectrum can be assigned by comparison with nitrosomethane. The substitution of Me by t-Bu lowers the n ionization energy of the nitroso group by 0.7 eV, whereas the n+ and it ionization energies are lowered by 1.8 and 1.7 eV, respectively. 

Pseudopterosin X (1) was isolated as a yellow colored gum. The UV spectrum of 1 showed maximum absorption at 280 nm due to the presence of a highly substituted benzene chromophore [10], Its IR spectmm displayed intense absorption bands at 3,470 (OH), 2,904 (CH), 1,705 (C = O), 1,595 (C = C) and 1,100 (C-0) cm . The high-resolution electron-impact mass spectmm (HREIMS) of 1 showed M+ at m/z 474.2622, and this mass provided molecular formula indicating the presence of nine double bond equivalents in 1. The C-NMR chemical shift assignments of 1 are shown around stracture 1. On the basis of the detailed NMR studies and comparison with the reported pseudopterosins in the literature and L-xylose [3-5], stmcture 1 was proposed for this new natural product. 

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