Carbonyl group, infrared spectrum

The 4-Hydroxy-thiazoles are characterized by infrared absorption near 1610 cm (KBr) (3) or 1620 to 16.S0cm (CCI4) (8), indicating a strongly polarized carbonyl group. H-5 resonates near 5.6 ppm in the NMR spectrum like similar protons in other mesoionic compounds (3). Two fragmentations of the molecular ion are observed in the mass spectra. The first involves rupture of the 1,2 and 3,4 bonds with loss of C2R 0S . In the second, the 1,5 and 3,4 bonds are cleaved with elimination of C2R 0. ... 

Compound A undergoes hydrolysis of its acetal function in dilute sulfuric acid to yield 1,2-ethanediol and compound B (CgHg02), mp 54°C. Compound B exhibits a carbonyl stretching band in the infrared at 1690 cm and has two singlets in its H NMR spectrum, at 8 2.9 and 6.7, in the ratio 2 1. On standing in water or ethanol, compound B is converted cleanly to an isomeric substance, compound C, mp 172—173°C. Compound C has no peaks attributable to carbonyl groups in its infrared spectrum. Identify compounds B and C. 

These structural problems are also insoluble by physical methods alone. The infrared spectrum often gives an unambiguous decision about the structure in the solid state the characteristic bands of the carbonyl or the hydroxyl group decided whether the compound in question is a carbinolamine or an amino-aldehyde. However, tautomeric equilibria occur only in solution or in the liquid or gaseous states. Neither infrared nor ultraviolet spectroscopy are sufficiently sensitive to investigate equilibria in which the concentration of one of the isomers is very small but is still not negligible with respect to the chemical reaction. 

The existence of imidazole-4-aldehyde (232) in the enolic form 233 was postulated on the basis of chemical evidence," but the infrared spectrum indicates the presence of a carbonyl group and absence of a hydroxyl group, suggesting that structure 232 should... 

Except in simple cases, it is very difficult to predict the infrared absorption spectrum of a polyatomic molecule, because each of the modes has its characteristic absorption frequency rather than just the single frequency of a diatomic molecule. However, certain groups, such as a benzene ring or a carbonyl group, have characteristic frequencies, and their presence can often be detected in a spectrum. Thus, an infrared spectrum can be used to identify the species present in a sample by looking for the characteristic absorption bands associated with various groups. An example and its analysis is shown in Fig. 3. 

A second reason why AI is of value to scientists is that it offers powerful tools to cope with complexity. In favorable circumstances, the solutions to problems can be expressed by rules or by a well-defined, possibly trivial, model. If we want to know whether a compound contains a carbonyl group, we could record its infrared spectrum and check for a peak near 1760 cm1. The spectrum, paired with the rule that ketones generally show an absorption in this region, is all that we need. But other correlations are more difficult to express by rules or parametrically. What makes a good wine We may (or may not) be able to recognize a superior wine by its taste, but would have considerable difficulty in determining whether a wine is good, or even if it is palatable, if all we had to go on was a list of the chemicals of which it is comprised. 

The carbonyl stretching frequency in the infrared (IR) spectrum for the two carbonyl groups in the perhydro-6-oxopyrrolo[l,2- ]pyrazole 26 are 1772 and 1737 cm ... 

Suppose the input to the network is an infrared spectrum, from which the network must determine whether the molecule whose spectrum is being assessed contains a carbonyl group. We could require that the network output a value of one if it believes that a carbonyl group is present in the molecule, and zero otherwise. It is very unlikely that the untrained network will generate exactly the correct output when it is presented with the first sample, so the error in its prediction will be nonzero. In that case, the connection weights in the network are modified (see below) to reduce the error, and thus, they make it more likely that the network will provide the correct answer the next time it sees this spectrum. 

The relative abundance of each of these species can be followed for instance by infrared analysis (4, 5). Figure 3 shows the I.R. spectrum of acrylic acid in the 17 O O cm l region. The main peak at 1705 cm-1 is due to the vibration of the carbonyl group in the cyclodimeric form, whereas the shoulders at 1730 and 1740 cm-l correspond to "open dimers" and "linear oligomers". 

The C=0 in all carbonyl groups has a very intense band in the 1,700 cm region of its infrared spectrum. In many cases this band is the most prominent feature of the spectrum. Table 9-1 shows some typical carbonyl stretches. (See Chapter 5 for a discussion of IR stretches and peaks.)... 

One of the main routine uses of infrared spectroscopy is identification of specific functional groups present in an unknown molecule and, as a result, further characterization of the unknown. By far the most common example involves the carbonyl group. Location of a strong band in the infrared in the vicinity of 1730cm is almost certain proof that carbonyl functionality is present. This confidence is based on the fact that the characteristic frequency (the CO stretch in this case) is isolated, that is to say, it is sufficiently far removed from the other bands in the infrared spectrum to not be confused with them. It also assumes that carbonyl groups in different chemical environments will exhibit similar characteristic... 

Pentacarbonyl(diphenylmethylene)tungsten(0) is a moderately air-stable soild that is readily soluble in most organic solvents. The resulting solutions are air and light sensitive and decomposed thermally at about 50°. The infrared spectrum of a heptane solution shows bands in the metal carbonyl region at 2070 (m), 1971 (s), and 1963 (s) cm"1, characteristic of a group VI pentacar-bonyl species. The proton NMR spectrum in CS2 or acetone-d6 shows a complex multiplet at 5 7.2 relative to internal tetramethylsilane. 

The intermediate cyclooctene complex appears to be more reactive with respect to CS coordination and more sensitive to oxidation when the arene ring bears electron-withdrawing groups (e.g., C02CH3). Dicarbonyl(methyl rj6-benzoate)-thiocarbonyl)chromium is air stable in the solid state and reasonably stable in solution.9 The infrared spectrum exhibits metal carbonyl absorptions at 1980 and 1935 cm"1 and a metal thiocarbonyl stretch at 1215 cm"1 (Nujol) (these occur at 1978, 1932, and 1912 cm"1 in CH2C12 solution).10 Irradiation of the compound in the presence of phosphite or phosphine leads to slow substitution of CO by these ligands, whereas the CS ligand remains inert to substitution. The crystal structure has been published."... 

The spectral data provide information about the structure of a-acetoxydibenzylnitrosamine (VII). The high frequency (1780 cm ) for the carbonyl group in the infrared spectrum (Fig. 9) is consistent with this structure ( ) and the introduction of the chiral center at the benzylic position causes the methylene of the other benzyl ic substituent to be diastereotopic and appear in the nmr spectrum as an AB quartet. The center of the quartet... 

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