Interpreting 13C NMR Spectra

To work out a structure from a ¹³C NMR spectrum, use two pieces of information: the number of peaks and the chemical shift of each peak.Step 1: count the peaks, ignoring TMS at 0 ppm. This gives the number of carbon environments. If there are fewer peaks than carbon atoms in the formula, the molecule must have some symmetry.Step 2: match each shift to a type of carbon using the data booklet. Useful approximate regions:
• 5 to 40 ppm: C in an alkyl chain (C-C)
• 50 to 90 ppm: C bonded to oxygen (C-O), as in alcohols, ethers and esters
• 90 to 150 ppm: C in an alkene (C=C); aromatic carbons appear around 110 to 160 ppm
• 160 to 185 ppm: C=O in carboxylic acids and esters
• 190 to 220 ppm: C=O in aldehydes and ketonesStep 3: suggest a structure that fits the molecular formula, the number of environments and the shifts.Worked example: a compound C₃H₆O gives peaks at 30 and 205 ppm. Two peaks for three carbons means two carbons are equivalent. The peak at 205 ppm is C=O in an aldehyde or ketone, and 30 ppm is an alkyl carbon next to the C=O. The compound is propanone, CH₃COCH₃, in which the two CH₃ carbons are equivalent. Its isomer propanal, CH₃CH₂CHO, would give three peaks.Symmetry is also important in rings: benzene gives just 1 peak, because all six carbons are equivalent.