13C nmr spectrum of cyclobutane C4H8 analysis of chemical shifts ppm interpretation of C-13 chemical shifts ppm of cyclobutane C13 13-C nmr doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: Carbon-13 NMR spectrum of cyclobutane cyclo-C₄H₈

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Interpreting and explaining the Carbon-13 NMR spectrum of cyclobutane

[Author © Dr Phil Brown GRIC, PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy analysis of cyclobutane [spectra page updated RE-EDIT]

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C-13 NMR spectroscopy - spectra index

Isomers of molecular formula C₄H₈ (M_r = 56)

Introductory note on the 13C NMR spectrum of cyclobutane

Students and teachers please note that my explanation of the carbon-13 NMR spectrum of cyclobutane is designed for advanced, but pre-university, chemistry courses.

The description does not involve the chemical shift δ spin-spin coupling effects for cyclobutane and the relative size of the carbon-13 NMR shifts does not give the ratio of the carbon atoms in the different non-equivalent chemical environments of the cyclobutane molecule.

The most common solvent used for investigating the C13 NMR spectrum of compounds like cyclobutane, is CDCl₃ and other deuterated solvents.

13C nmr spectrum of cyclobutane C4H8 analysis of chemical shifts ppm interpretation of C-13 chemical shifts ppm of cyclobutane C13 13-C nmr doc brown's advanced organic chemistry revision notes

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.

Cyclobutane, C₄H₈ , , it is not planar, it is bent

The molecular structure and naming of alkanes

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 unique chemical environments of the 4 carbon atoms of cyclobutane (on a time averaged basis).

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.

All four carbon atoms in cyclobutane are equivalent to each other and their ¹³C fields cannot cause spin-spin coupling splitting effects in the carbon-13 NMR spectrum.

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.

Adding D₂O to the sample under investigation, dissolved in CDCl₃, makes no difference to the 13C NMR spectrum of cyclobutane because 13C NMR does not detect proton resonances!

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Isomers of molecular formula C₄H₈ (M_r = 56)

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The mass spectrum of cyclobutane

The H-1 NMR spectrum of cyclobutane

C-13 NMR spectroscopy index

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