Carbonyl stretch

Infrared IR spectroscopy is quite useful in identifying carboxylic acid derivatives The, carbonyl stretching vibration is very strong and its position is sensitive to the nature of IKT the carbonyl group In general electron donation from the substituent decreases the double bond character of the bond between carbon and oxygen and decreases the stretch mg frequency Two distinct absorptions are observed for the symmetric and antisym metric stretching vibrations of the anhydride function... 

Hydrogen bonding to a carbonyl group causes a shift to lower frequency of 40 to 60 cm k Acids, amides, enolized /3-keto carbonyl systems, and o-hydroxyphenol and o-aminophenyl carbonyl compounds show this effect. All carbonyl compounds tend to give slightly lower values for the carbonyl stretching frequency in the solid state compared with the value for dilute solutions. 

Carbonyl stretching frequency. Aldehyde proton, relative to TMS. Carbonyl carbon, relative to TMS. 

Infrared spectra of fats and oils are similar regardless of their composition. The principal absorption seen is the carbonyl stretching peak which is virtually identical for all triglyceride oils. The most common appHcation of infrared spectroscopy is the determination of trans fatty acids occurring in a partially hydrogenated fat (58,59). Absorption at 965 - 975 cm is unique to the trans functionaHty. Near infrared spectroscopy has been utilized for simultaneous quantitation of fat, protein, and moisture in grain samples (60). The technique has also been reported to be useful for instmmental determination of iodine value (61). 

The infrared carbonyl stretching frequencies of n- and isobutyraldehyde in the condensed phase occur at 1727.6 and 1738.0 cm , respectively (38). The proton nmr spectra of both aldehydes are weU-known (39). 

Figure 4 IR frequencies (cm ) for keto heterocycles (carbonyl stretching frequencies bracketed frequencies are for C=C stretches)...

IR spectroscopy has been particularly helpful in detecting the presence of keto tautomers of the hydroxy heterocycles discussed in Section 3.01.6. Some typical frequencies for such compounds are indicated in Figure 4. Here again the doublets observed for some of the carbonyl stretching frequencies have been ascribed to Fermi resonance. 

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. 

This mode corresponds to the IR peak associated with carbonyl stretch, used to identify the C-O double bond. Its predicted frequency is about 1810 (after scaling). This is in reasonable agreement with the experimental value of 1746. Using a larger basis set will improve this value. We ll discuss basis set effects in the next chapter. 

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.)... 

After scaling, the predicted frequencies are generally within the expected range for carbonyl stretch (-1750 cm ). The table below reproduces our values, published theoretical values using the 6-31+G(d) basis set (this basis set includes diffuse functions), and the experimental values, arranged in order of ascending experimental frequency ... 

This table provides an introduction to the basis set effects we U discuss in the next chapter. Adding diffuse functions lowers the frequency by about 20-30 cm. However, both sets of numbers are in reasonable agreement with the observed values, with the better theoretical values producing quite good agreement. However, even using the smaUer basis set, we can successfully identify the carbonyl stretch. 

The normal modes associated with these frequencies are characterized by motion limited to the hydrogen atom in question. The values of the frequencies are in reasonable agreement with observations which place this peak in the range 2745-2710 cm S given our knowledge of basis set effects from the carbonyl stretch frequencies. ... 

Compute the frequency associated with carbonyl stretch in solution with acetonitrUe for the carbonyl systems we looked at in the gas phase in Chapter 4. Run your calculations using RHF/6-31+G(d) with the Onsager SCRF model. Discuss the substituent effect on the predicted solvent effects. 

The following table lists recommended values of og for the v.irunij jy, ] includes the published frequency (scaled) associated with carbonyl stretch in the gas phase for each compound that we gave in Chapter 4 ... 

Solution A geometry optimiiation and frequency calculation (both in solution) are needed for each system (we ran the formaldehyde calculations earlier in this chapter). Here are the resulting scaled frequencies associated with carbonyl stretch for each system, along with the corresponding experimental values ... 

C=C stretch 80 C-13 chemical shifts 22, 53 C60 31,32 C60O isomers 54 carbon dioxide 120, 182 carbon monoxide 175,191 carbonyl series 84 carbonyl stretch 84, 220 in solution 244 Carmichael 136 Carpenter 152, 196 Cartesian coordinates 52, 286, 287 CASSCF keyword 228 CASSCF method 228,229,230,231, 232,233, 234,235 state-averaged 233... 

Carbonyl stretching frequency in 2-acetyl-5-R-thiophenes CCI4 0.0075 0.001 0.002 0.951 6 k... 

Because of the frequent mutual interference of electronic, inductive, and steric effects, and because of the influence of ring strain, the carbonyl stretching frequency is naturally not an absolute criterion for the methylation course. The heterocyclic systems in question are too diverse for this to hold. Careful inspection of Table I discloses certain deviations from the relationships mentioned. These deviations will now be discussed. 

Run infrared spectra of pure acetone and of pure propan-2-ol. From them select an absorption band for acetone which does not overlap significantly with any of those for the propan-2-ol. The best band is most probably that at 1718 cm-1, the carbonyl stretching frequency. 

Adduct formation by IrCl(CO)(PPh3)2 and similar compounds results in a shift in the IR carbonyl stretching frequency (Table 2.8). 

The carbonyl stretching frequencies in the ir spectra of cis-3-substituted methyl acrylates (set 12-18) were also correlated with eq. (24) and eq. (2). Barely significant correlations were obtained with both equations. The value of i// obtained in the correlation with eq. (24) was not significant. We may therefore probably exclude cases (a) and (b). As the hcaic is not significantly different from hobs > we may exclude case (c). This set is therefore probably an example of case (d) that is, there is no meaningful steric effect. 

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