Interpreting the Carbon-13 NMR spectrum of cyclobutane

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 cyclobutane

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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 - cyclobutane here.

Interpreting the C-13 NMR spectrum of cyclobutane

As you can see from the diagram above there is only one chemical shift line in the C-13 NMR spectrum of cyclobutane indicating just one chemical environments of the 4 carbon atoms of cyclobutane.

¹³C chemical shift (a) 22.4 ppm on the C-13 NMR spectrum diagram for cyclobutane.

The carbon-13 NMR spectra provides direct evidence of only one carbon atom environment for the 4 carbon atoms in the cyclobutane molecule, deduced from the presence of just one 13C NMR chemical shift of 22.4 ppm.

Cyclobutane is NOT a true planar molecular arrangement in terms of the four carbon atoms because the square is slightly folded.

However all 4 hydrogen atoms of cyclobutane appear to be in the same chemical environment and give the same chemical shift i.e. there are 4 equivalent carbon atoms in the cyclobutane molecule and just one single 13C NMR resonance signal.