Carbonyl frequency

The large differences between the infrared carbonyl frequencies of... 

This carbonyl frequency is, in the opinion of the present authors, unexpectedly low. Measured in 50% methanol. 

Furaldehyde is a classical example of such thermodynamically unfavoured monomers . Its strong conjugation with the ring is well represented by a carbonyl frequency at about 1670 cm-1 and the best indication of its reluctance to polymerize is simply the fact that, despite many attempts and some claims of success, no one has in reality been able to prepare a polyacetal with the structure given below ... 

The situation with 7V-acyloxy-/V-alkoxyureas and carbamates is similar although infrared data were mostly determined by liquid film or condensed phase (KBr/nujol mull).52,131 However, the limited data for V-acyloxy-TV-alkoxyureas (Table 2, entries 69-72) give amide carbonyl frequencies ca. 1730 cm-1 that are raised by some 37-40 cm-1 by acyloxylation. Values for carbamates (Table 2, entries 73-77) are higher (mostly 1780 cm-1) but are raised to a lesser extent (10-20 cm-1) relative to their parent carbamates. Clearly, carbonyl vibrational frequencies will be influenced strongly by the adjacent amino or alkoxyl group in both analogues. 

The infrared carbonyl frequencies of the monoketone (6), like those of 5a and 5b, appear at about 1700 cm-1 17h... 

In an effort to assign the bands to ee and ae isomers the (thixantphos)Rh(CO)2D complex was measured for comparison. Upon H/D exchange, only Vi and v3 shift to lower wavenumbers (respectively 18 and 14 cm-1), and therefore, these two bands are assigned to the carbonyl frequencies of the ee complex. The two bands that do not shift, v2 and v4, belong to the ae complex. From the disappearance of a low-frequency shoulder upon H/D exchange, it can be concluded that one of the rhodium hydride vibrations is partly hidden under v4. 

Figure 2. The correlation of plfBH Values of aromatic carbonyl compounds RCOX with carbonyl frequencies.

Figure 3. The correlation of carbonyl frequencies of compounds RCOX with om constants of groups X A, acetyl compounds B, benzoyl compounds (Liler, 1967 reproduced with permission from Liler, 1971c, p. 90).

The ambiguity of infrared criteria for the identification of protonation sites in imidazoles and 2-pyrazolines has been discussed by Elguero et al. (1967a), who have also provided new information on the infrared spectra of the hydrochloride of 3,5,5-trimethyl-2-pyrazoline in chloroform solution. There is a shift of the stretching vibration of the C=N bond from 1624 cm in the base to 1649 cm" in the hydrochloride salt in chloroform, i.e., in the N-1 protonated cation. This is analogous to similar shifts of the carbonyl frequency in some amide salts (see page 338). 

The infrared findings of Gompper cuid Altreuther (1959) on benzamide salts, claimed to indicate O-protonation in perchlorates, are equally inconclusive. The carbonyl frequency at 1656 cm in solid benzamide (in KBr) appears in the perchlorate in almost the... 

It may also be mentioned at this point that the carbonyl absorption of the cations of 2,2-dimethylquinuclidone-6, which is certainly N-protonated, is found at the very high frequency of 1799 cm (Pracejus, 1959). This bicyclic system is strained, however, and strain is known to increase the carbonyl frequency in cyclic ketones (Cross, 1964, p. 59). 

Strohmeier showed that the IR carbonyl frequencies of metal complexes could be used as a measure of the electronic properties of the ligands [8]. Tolman introduced a systematic approach to describe electronic and steric ligand effects [9]. The electronic parameter / is based on the difference in the IR frequencies of Ni(CO)3L and the reference compound Ni(CO)3(P Bu3), similar to the method introduced by Strohmeier. For phosphorus ligands the cone angle 6 is defined as the apex angle of a cylindrical cone, centered at 2.28 A from the center of the P atom, which touches the outermost atoms of the model. 

The exponential decay of the strongest carbonyl absorption is presented in Figure 6.16A. The natural logarithm of the relative absorption of the carbonyl frequency at 2020 cm versus time is presented in Figure 6.16B, indicative of first-order kinetics in the rhodium concentration and the pressure of FI2 and the H/D exchange rate was 1140 h Although the H/D exchange reaction was very... 

Branched iV-chlorohydroxamic esters exhibit much lower carbonyl frequencies in their IR spectra. Series of Ai-(phenylethyloxy)amides (Table 2, entries 1-7) and Af-butoxy-amides (Table 2, entries 12-16) show a clear movement to lower carbonyl stretch frequencies with branching alpha to the carbonyl, in accord with greater inductive stabilization of the polar resonance form III of the carbonyl (Figure la). Neopentyl (entry 17) is a special case. While the group should contribute much more inductive stabilization than ethyl, its carbonyl stretch frequency is higher. Similar changes have been noted in the IR spectra of branched ketones and have been ascribed to a degree of steric hindrance to solvation and therefore destabilization of the polar resonance form Dl". ... 

Amide carbonyl vibrational frequencies for the series behave similarly while those of the hydroxamic esters in this subset vary between 1678 and 1698 cm , the V-acyloxy-V-alkoxyamide carbonyl frequencies span only six wave numbers. 

Although the valence bond description of pyran-2-one could include a zwitterionic contribution involving the carbonyl group, thus making the ring aromatic, there is little evidence to support this. For example, the IR carbonyl frequency (1740 cm ) is typical of an unsaturated lactone, and the chemical shifts of the ring protons in the H NMR spectrum indicate that there is no ring current. 

Trimethylsilyl derivatives of ten hydroxy- and methoxyhydroxyflavonoids have been studied by the GC-FTIR technique." " The correlation found between retention and gas-phase IR data was used in structural identification of compounds having very similar chromatographic behavior. The shift of the carbonyl frequency gave information on the presence of substitution. Some hydroxy- and methoxy-substituted flavones have been studied following carbon dioxide supercritical fluid chromatography on polymethylsiloxane capillary columns using flame ionization and FTIR detection." " " ... 

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