The Carbon-13 NMR spectrum of propylbenzene

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing C-13 NMR spectra of propylbenzene

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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 - propylbenzene here.

Interpreting the C-13 NMR spectrum of propylbenzene

As you can see from the diagram above there are 7 different chemical shift lines in the C-13 NMR spectrum of propylbenzene indicating 7 different chemical environments of the carbon atoms.

On the C-13 NMR spectrum diagram for propylbenzene you can see four chemical shifts (a) to (d) are for the benzene ring carbon atoms and three chemical shifts (e) to (g) for the side-chain carbon atoms of the alkyl propyl group CH₂CH₂CH₃.

The carbon-13 NMR spectra a provides direct evidence of 7 different carbon atom environments in the propylbenzene molecule from 7 different 13C chemical shifts (ppm).