Doublet carboxylic acids

In comparison, no structural modification of model B was seen before 120 h of aging (80 °C). However, after 120 h two small doublets appeared in the NMR spectrum and several additional peaks became noticeable in the NMR spectrum. It was determined by NMR and IR spectroscopy that the hydrolysis products were an imide/carboxylic acid and an imide/anhydride. Model B was then aged for 1200 h at 80 °C to quantitatively determine the amount of hydrolysis products as a function of time. The relative intensity of the peaks due to carboxylic acid is constant after some time. The authors suggest that an equilibrium occurs between model B and the products formed during hydrolysis, and therefore, the conversion to hydrolysis products is limited to about 12%. This critical fraction is probably enough to cause some degradation of polymeric materials, but research on six-membered polyimides has remained active. 

That compound B is a carboxylic acid is evidenced by the singlet (area = 1) at 8 12.1 ppm. The remaining H NMR signals—a quartet at S 3.9 ppm (1H) and a doublet at S 1.6 ppm (3H)—suggest the fragment CH—CH3. All that remains of the molecular formula is —N02. Combining this information identifies compound B as 2-(4-nitrophenyl)propanoic acid. 

Amides Simple amides have much lower carbonyl stretching frequencies than the other carboxylic acid derivatives, absorbing around 1640 to 1680 cm-1 (often a close doublet). This low-frequency absorption agrees with the resonance picture of the amide. The C=0 bond of the amide carbonyl is somewhat less than a full double bond. Because it is not as strong as the C=0 bond in a simple ketone or carboxylic acid, the amide C=0 has a lower stretching frequency. 

The present results confirm that the doublets observed in the e.s.r. spectra of several carboxylic acids immediately after X- or y-irradiation at 77°K are due to the radical anions of the acid. Further it is reasonable to assume that a similar sequence of reactions to that postulated above occurs in the formation of radicals in carboxylic acids by high energy irradiation. Thus the radical anion is the primary product and the more stable free radicals are formed subsequently by hydrogen abstraction by the anion from a neighbouring acid molecule. 

Amides Simple amides have much lower carbonyl stretching frequencies than the other carboxylic acid derivatives, absorbing around 1640 to 1680 cm (often a close doublet). This low-frequency absorption agrees with the resonance picture of the amide. 

Anhydrides (-CO-0-CO-) may be identified by a distinctive carbonyl band region. A strong doublet is observed in this region with components in the 1840-1800 and 1780-1740 cm ranges. Anhydrides also show a strong C-O stretching band near 1150 cm for open-chain anhydrides and at higher wavenumbers for cyclic structures. Table 4.3 summarizes the major infrared bands observed for carboxylic acids and anhydrides. 

---