Interpreting the C-13 NMR spectrum of 2-methylpropanoic acid (isobutyric acid)

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 2-methylpropanoic acid

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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 resonances, called chemical shifts, are measured with respect to the TMS, and depend on the individual (electronic) chemical environment of the ¹³C atoms in an organic molecule - 2-methylpropanoic acid here.

Interpreting the C-13 NMR spectrum of 2-methylpropanoic acid

As you can see from the diagram above there are 3 different chemical shift lines in the C-13 NMR spectrum of 2-methylpropanoic acid indicating 3 different chemical environments of the 4 carbon atoms of 2-methylpropanoic acid.

CH₃CH(CH₃)COOH  or  (CH₃)₂CHCOOH

(Note the 3 different colours indicating the 3 different chemical environments of the 4 carbon atoms in 2-methylpropanoic acid).

¹³C chemical shifts (a) to (c) on the C-13 NMR spectrum diagram for 2-methylpropanoic acid.

The two methyl group carbon atoms are equivalent to each other, due to the symmetry of the 2-methylpropanoic acid molecule, and therefore have the same 13C NMR chemical shift.

Note the decreasing effect on the ¹³C chemical shift as the carbon atom is further from the more electronegative oxygen atoms in 2-methylpropanoic acid.

The carbon-13 NMR spectra provides direct evidence of 3 different carbon atom environments for the 4 carbon atoms in the 2-methylpropanoic acid molecule, deduced from the presence of 3 different 13C NMR chemical shifts (ppm).