Carbonyl carbon:

The ¹³C nmr spectra for acid derivatives will consist of a quaternary signal for the carbonyl carbon. For an aliphatic ester this emerges at 169-176 ppm while conjugated esters encompass a signal at lower chemical shift (164-169 ppm). For acid anhydrides the carbonyl signal takes place in the range 163-175 ppm. For amides it takes place at 162-179 ppm, and for acid chlorides it is exist at 167-172 ppm. If a mass spectrum and elemental analysis have been taken, the molecular weight and molecular formula will be recognized. This can establish whether a specific acid derivative is present or not present. For instance, the lack of chlorine or nitrogen in the formula obviously rules out the chance of an acid chloride or amide; the existence of only oxygen rules out the probability of an ester or acid anhydride. This might seem obvious but it is surprising how frequently this information is ignored while students attempt to interpret IR spectra. The chemical shifts of specific groups can give indirect proof of particular acid derivatives, both in the ¹H and ¹³C nmr spectra. For instance, the methyl group of a methyl alkanoate appears at 3.7 ppm in the ¹H spectrum and at 51-52 ppm in the carbon spectrum. In difference, the N-methyl group of an amide takes place at 2.9 ppm (¹H nmr) and 31-39 ppm (¹³C nmr).