Interpreting the carbon-13 NMR spectrum of Methylbenzene (Toluene)

TMS is the acronym for tetramethylsilane, formula Si(CH₃)₄, whose ¹³C atoms are arbitrarily given a chemical shift of 0.0 ppm. This is the 'standard' in ¹³C NMR spectroscopy and all other ¹³C shifts, called chemical shifts, depend on the individual (electronic) chemical environment of the ¹³C atoms in an organic molecule: Methylbenzene  C₇H₈, C₆H₅CH₃ , ,

The molecular structure and naming of aromatic compounds

Interpreting the C-13 NMR spectrum of Methylbenzene (Toluene)

As you can see from the diagram above there are only five ¹³C chemical shift lines in the C-13 NMR spectrum of methylbenzene indicating 5 different chemical environments of carbon atoms.

Apart from the side-chain methyl group carbon atom 7, the ring carbons can occupy 4 different positions on benzene ring of methylbenzene (carbon atoms unique 1, 2 = 6, 3 = 5 and unique 4 on the diagram above).

Therefore you see 5 chemical shift lines in the C-13 NMR spectrum of methylbenzene.

C₆H₅CH₃

Apart from carbon atom 1 next to, and influenced by, the other ring carbon atoms show a similar chemical shift ~125 to 129 ppm, perhaps not surprising given the symmetry and electron distribution of the benzene ring.

The side-chain 'aliphatic' carbon atom of methylbenzene has a very different and typically much lower chemical shift than the aromatic benzene ring carbon atoms.