Interpreting the Carbon-13 13C NMR spectrum of methyl 2-hydroxybenzoate

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 methyl 2-hydroxybenzoate (methyl salicylate)

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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 - methyl 2-hydroxybenzoate here.

For more see The molecular structure and naming of aromatic compounds

Interpreting the C-13 NMR spectrum of methyl 2-hydroxybenzoate (methyl salicylate)

As you can see from the diagram above there are 8 different chemical shift lines in the C-13 NMR spectrum of methyl 2-hydroxybenzoate indicating 8 different chemical environments of the 8 carbon atoms of methyl 2-hydroxybenzoate.

¹³C chemical shifts δ (a) to (h) on the C-13 NMR spectrum diagram for methyl 2-hydroxybenzoate (methyl salicylate).

(a) gives the smallest ¹³C NMR chemical shift δ where the carbon atom is neither attached to, or part of, the benzene ring - though being attached to an electronegative oxygen atom increases it.

(b) gives the largest ¹³C NMR chemical shift δ where the carbon atom is attached to two electronegative oxygen atoms.

(c), (e)  and (g) give similar ¹³C NMR chemical shifts δ for carbon atoms 1, 3 and 5 of the benzene ring (clockwise numbering for minimum substituent numbers)

(d) gives a relatively large ¹³C NMR chemical shift δ where the carbon atom is linked to an oxygen atom.

(f) and (h) give similar ¹³C NMR chemical shifts δ for carbon atoms 4 and 6 of the benzene ring (larger than c, e and g).

The carbon-13 NMR spectra provides direct evidence of 8 different carbon atom environments for the 8 carbon atoms in the methyl 2-hydroxybenzoate molecule, deduced from the presence of 8 different 13C NMR chemical shifts (ppm).