GROUP FREQUENCIES amides

Carbonyl group frequencies for quinuclidin-2-ones are on the average 80 cm-1 higher than for common lactams, and integral intensities of the same absorptions are nearly half those of quinuclidin-2-ones. Ultraviolet (UV) absorption maxima for quinuclidin-2-ones are midway between those for amides and ketones. On account of the lack of amide mesomerism quinuclidin-2-ones gain in reactivity. This is shown by the rather high rates of hydrolysis of such compounds... 

The carbonyl groups of amides absorb at particularly low IR frequencies about 1640 to 1680 cm-1 (Figure 12-13). The dipolar resonance structure (shown next) places part of the pi bond between carbon and nitrogen, leaving less than a full C=0 double bond. 

The nucleic acid sensor was prepared by first derivatizing the quartz surface with a 3 1 styrene-acrylic acid copolymer. Poly (A) was then covalently immobilized onto pendant carboxylic acid groups by amide bond formation with the amino groups on the adenine base. Hybridization occurred during incubation with poly(U). Following each of the three steps, the sensor was rinsed and dried and the resonant frequency of oscillation was measured. Prior to any treatment, the quartz crystals exhibited a resonant frequency of 9 MHz. Because each step in the surface treatment involved the addition of mass to the crystal surface, a frequency decrease was expected after each step. Figure 7.9 shows the actual frequency changes measured for a sensor prepared as described above, as well as for a sensor prepared with no poly(A) included in the second step (i.e., a control sensor). 

In the group of amide compounds, acetyl imidazole (1740 cm ) has a much greater free energy of hydrolysis (Stadtman, 1954) and a greater carbonyl frequency than ordinary am ides (1627 to 1673 cm ) the correlations between infrared carbonyl frequency and reactivity for the series acetylpyrrole, acetylimidazole, acetyltriazole, and acetyltetrazole and the analogous acetylbenzimidazole series have been presented by Otting (1956) and Staab (1956, 1957). 

The rearrangement product has incorporated a heteroatom into its structure, but the carbonyl group and the ring systems have survived. The infrared spectrum of benzanilide (Fig 10.7W) must be consistent with the formation of a secondary amide. We expect to observe a macro group frequency for the presence of monosubstituted phenyl groups, plus a second macro group frequency for a secondary aromatic amide. 

The second macro group frequency for the aromatic secondary amide uses the following bands 3350,3060,1659,1602,1587,1539,1322, and 680 (broad) cm . ... 

Figure 38 shows characteristic group frequencies of amides. In amides, the C=0 stretching vibration gives rise to an absorption at... 

The next step in assigning group frequencies is to establish the constancy of position of the group frequency in a series of related molecules. It has been reported [ ] that for amides a band near 1408 cm is indicative of the N—CH3 bend. Thus, it appears that for amides a band near 1410 cm is a group frequency of the N—CH3 group. 

The amide II band is a group frequency found for primary amides (RCONH2) secondary amides (RCONHR). The band occurs in the region from 1600 to 1500 cm Apparently the amide II band is due to the vibration of several structural units such as NH and CN. An extensive study of this band in N-methylacetamide has led to the suggestion that the band is 60% NH bend and 40% CN stretch... 

Figure 5-29 includes a number of group frequency assignments of amide II and III bands. 

They interact to give two bands 1550 cm (amide II) and 1300-1220 cm (amide III). The first is very useful in the IR. The latter is not a useful group frequency. It is usually somewhat better in the Raman than in the IR. 

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