Carbonyl compounds, characteristic frequencies

This exercise will investigate various carbonyl compounds. Examine the frequencies for the systems pictured below and determine the frequencies associated with carbonyl stretch in each case. In addition, locate the characteristic peak produced b) the single hydrogen attached to the carbonyl for the applicable systems. (We looked a this mode in formaldehyde in Example 4.1.)... 

The Infrared Region 515 12-4 Molecular Vibrations 516 12-5 IR-Active and IR-lnactive Vibrations 518 12-6 Measurement of the IR Spectrum 519 12-7 Infrared Spectroscopy of Hydrocarbons 522 12-8 Characteristic Absorptions of Alcohols and Amines 527 12-9 Characteristic Absorptions of Carbonyl Compounds 528 12-10 Characteristic Absorptions of C—N Bonds 533 12-11 Simplified Summary of IR Stretching Frequencies 535 12-12 Reading and Interpreting IR Spectra (Solved Problems) 537 12-13 Introduction to Mass Spectrometry 541 12-14 Determination of the Molecular Formula by Mass Spectrometry 545... 

The infrared (IR) spectra of these compounds were mostly studied in the solid state which showed all the basic peaks characteristic of various functionalities attached to such bicyclic heterocycles with bridgehead nitrogen atoms. The difference in the frequency of carbonyl and carbon nitrogen double bond in tautomers of compound 18 (R = H) has been discussed previously in CHEC-II(1996) <1996CHEC-II(8)713>. 

Different organic functional groups (i.e., methyl, methylene, phenyl, and the hydrogen atoms adjacent to the carbonyl carbon in aldehydes and organic add groups) absorb at different frequencies and thus can be easily identified. Similarly, different 13C environments result in different absorption characteristics. For instance, carbon atoms in aromatic compounds absorb different frequencies than do those in carbonyl groups. 

The use of infra-red or ultraviolet spectroscopy to examine the molecular groups present in a chemical compound is familiar to any chemist. One of the main uses of this technique is to apply a range of electromagnetic frequencies to a sample and thus identify the frequency at which a process occurs. This can be characteristic of, say, the stretch of a carbonyl group or an electronic transition in a metal complex. The frequency, wavelength or wavenumber at which an absorption occurs is of most interest to an analytical chemist. In order to use this information quantitatively, for example to establish the concentration of a molecule present in a sample, the Beer-Lambert law is used ... 

In addition to the characteristic CH stretching VCD, a number of molecules that are strocturally similar to 3-methylcyclohexanone exhibit characteristic ROA features in the skeletal region below 700 cm. Figure 6 (11). In particular, a bisignate couplet near 500 cm is observed in six-membered ring compounds with a carbonyl substituent [(-F)-pulegone(73), (-F)-camphor (74), (-t-)-3-brom-ocamphor (74), (+)-nopinone (75)] and a broad low frequency couplet is observed in ketones with a 3-methyl substituent (75). The ROA of 3-methylcyclohexanone exhibits a third characteristic couplet near 400 cm. In all cases the sense of the observed couplets correlates with the absolute configuration of the most stable chair conformation. 

H and NMR studies on 477-3,l-benzoxazin-4-one derivatives 12 and their acylanthranilic acid precursors 13 led to the conclusion that differentiation between these two series of compounds was best achieved through the characteristic coupling interactions in the high-frequency carbonyl region <2000SAA1079>. 

Recently from a comparison of the infra-red absorption spectrum of this compound with that of o-benzoquinone Glowiak [10] came to the conclusion that dinitro-benzenediazo-oxide has a quinonoid structure. Both substances show the presence of the strong absorption band of the carbonyl group 1666 cm-1 for dinitrobenzene-diazo-oxide and 1680 cm-1 for o-benzoquinone. In addition dinitrobenzenediazo-oxide gives a band with a frequency of 2190 cm-1, characteristic of a double bond between nitrogen atoms. (Some derivatives of this compound may also have the diazo structure (Ha), which is discussed later on.)... 

By comparisons among the spectra of large numbers of compounds of known structure, it lias been possible to recognize, at specific positions in the spectrum, bands which can be identified as characteristic group frequencies associated with the presence of localized units of molecular structure in the molecule, such as methyl, carbonyl, or hydroxyl groups. Many of these group frequencies differ in the Raman and infrared spectra. 

The carbonyl frequency in the infrared spectrum provides a fairly characteristic method for differentiating between 1,4- and 1,5-lactones of aldonic acids. With few exceptions, the absorptions are in the range 1790-1765 and 1760 to 1725 cm-1, respectively.69 Configurational and conformational conclusions have been drawn from H and 13C NMR spectroscopy of aldonic acids and aldonolactones, using different correlation methods, enriched compounds, and shift reagents. For example, the solution conformation of aldono-1,4-lactones enriched with 13C at C-l have been determined on the basis of the coupling constants (homo and heteronuclear). In general, 0-2 is oriented quasi-equatorially due to stereoelectronic factors.36 Similar conclusions were made by Horton and Walaszek, who described the conformation of pentono- 1,4-lactones as an equilibrium between the 3E and forms.70 Conformations of D-hexono-1,4-lactones in solution have also been studied by NMR spectroscopy.70a The solution equilibrium of protected derivatives and their conformations have been described.71... 

Correction factors improve the fit between semiempirically calculated and experimentally measured spectra, but the agreement does not become as good as does the fit of corrected ab initio to experimental spectra. This is because deviations from experiment are less systematic for semiempirical than for ab initio methods (a characteristic that has been noted for errors in semiempirical energies [98]). For AMI calculations, correction factors of 0.9235 [99] and 0.9532 [100], and for PM3, factors of 0.9451 [99] and 0.9761 [100], have been recommended. A factor of 0.86 has been recommended for SAMI for non-H stretches [101]. However, the variation of the correction factor with the kind of frequency is bigger for semiempirical than for ab initio calculations for example, for correcting carbonyl stretching frequencies, examination of a few molecules indicated (author s work) that (at least for C, H, O compounds) correction factors of 0.83 (AMI) and 0.86 (PM3) give a much better fit to experiment. 

The mesoionic 1,3-dithiolones of type (2) show substituent-dependent IR carbonyl stretching vibrations between 1612 and 1558 cm-1 (Table 7). These low frequencies are characteristic for this class of compound and are in agreement with the mesoionic structure. Most of the mesoionic 1,3-dithiolones containing p-substituted phenyl groups show split carbonyl absorption bands, presumably as the result of Fermi resonances (76CB740). 

Phenols (ArOH) also show both these bands, but the C 0 stretching appears at somewhat higher frequencies. Ethers show C—O stretching, but the 0-—H band is absent. Carboxylic acids and esters show C—O stretching, but give absorption characteristic of the carbonyl group, C O, as well. (For a comparison of certain oxygen compounds, see Table 20.3, p. 689.)... 

A singlet at 8 2.15 is H on carbon next to carbonyl, the only type of proton in the compound. The IR spectrum shows no OH, and shows two carbonyl absorptions at high frequency, characteristic of an anhydride. Mass of the molecular ion at 102 proves that the anhydride must be acetic anhydride, a reagent commonly used in aspirin synthesis. q q... 

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