Carbon-13 nuclear magnetic resonance

Carbon-13 nuclear magnetic resonance most com-monly known as carbon-13NMR or 13CNMR or some-times simply referred to as carbon NMR is the applica-tion of nuclear magnetic resonance (NMR) spectroscopyto carbon. It is analogous to proton NMR (1H NMR) andallows the identification of carbon atoms in an organicmolecule just as proton NMR identifies hydrogen atoms.As such 13CNMR is an important tool in chemical struc-ture elucidation in organic chemistry. 13C NMR detectsonly the 13C isotope of carbon, whose natural abundanceis only 1.1%, because the main carbon isotope, 12C, isnot detectable by NMR since it has zero net spin.

1 Implementation13C NMR has a number of complications that are not en-countered in proton NMR. 13C NMR is much less sen-sitive to carbon than 1H NMR is to hydrogen since themajor isotope of carbon, the 12C isotope, has a spin quan-tum number of zero and so is not magnetically activeand therefore not detectable by NMR. Only the muchless common 13C isotope, present naturally at 1.1% nat-ural abundance, is magnetically active with a spin quan-tum number of 1/2 (like 1H) and therefore detectable byNMR. Therefore, only the few 13C nuclei present res-onate in the magnetic field, although this can be overcomeby isotopic enrichment of e.g. protein samples. In addi-tion, the gyromagnetic ratio (6.728284 107 rad T−1 s−1) isonly 1/4 that of 1H, further reducing the sensitivity. Theoverall receptivity of 13C is about 4 orders of magnitudelower than 1H.[1]

Another potential complication results from the presenceof large one bond J-coupling constants between carbonand hydrogen (typically from 100 to 250 Hz). In orderto suppress these couplings, which would otherwise com-plicate the spectra and further reduce sensitivity, carbonNMR spectra are proton decoupled to remove the signalsplitting. Couplings between carbons can be ignored dueto the low natural abundance of 13C. Hence in contrastto typical proton NMR spectra which show multiplets foreach proton position, carbon NMR spectra show a singlepeak for each chemically non-equivalent carbon atom.In further contrast to 1H NMR, the intensities of the sig-nals are not normally proportional to the number of equiv-alent 13C atoms and are instead strongly dependent on thenumber of surrounding spins (typically 1H). Spectra canbe made more quantitative if necessary by allowing suf-ficient time for the nuclei to relax between repeat scans.

High field magnets with internal bores capable of accept-ing larger sample tubes (typically 10 mm in diameter for13C NMR versus 5 mm for 1H NMR), the use of relax-ation reagents,[2] for example Cr(acac)3 (chromium (III)acetylacetonate, CAS number 21679-31-2), and appro-priate pulse sequences have reduced the time needed toacquire quantitative spectra and have made quantitativecarbon-13 NMR a commonly used technique in many in-dustrial labs. Applications range from quantification ofdrug purity to determination of the composition of highmolecular weight synthetic polymers.13C chemical shifts follow the same principles as thoseof 1H, although the typical range of chemical shifts ismuch larger than for 1H (by a factor of about 20). Thechemical shift reference standard for 13C is the carbonsin tetramethylsilane (TMS), [3] whose chemical shift isconsidered to be 0.0 ppm.

Typical chemical shifts in 13C-NMR

2 Distortionless enhancement bypolarization transfer spectra

Various DEPT spectra of propyl benzoateFrom top to bottom: 135°, 90° and 45°

Distortionless enhancement by polarization transfer(DEPT) is a NMRmethod used for determining the pres-ence of primary, secondary and tertiary carbon atoms.The DEPT experiment differentiates between CH, CH2

and CH3 groups by variation of the selection angle pa-rameter (the tip angle of the final 1H pulse): 135° anglegives all CH and CH3 in a phase opposite to CH2; 90°angle gives only CH groups, the others being suppressed;45° angle gives all carbons with attached protons (regard-less of number) in phase.

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Signals from quaternary carbons and other carbons withno attached protons are always absent (due to the lack ofattached protons).The polarization transfer from 1H to 13C has the sec-ondary advantage of increasing the sensitivity over thenormal 13C spectrum (which has a modest enhancementfrom the nuclear overhauser effect (NOE) due to the 1Hdecoupling).

3 Attached proton test spectra

Another useful way of determining how many protons acarbon in a molecule is bonded to is to use an attachedproton test (APT), which distinguishes between carbonatoms with even or odd number of attached hydrogens. Aproper spin-echo sequence is able to distinguish betweenS, I2S and I1S, I3S spin systems: the first will appear aspositive peaks in the spectrum, while the latter as nega-tive peaks (pointing downwards), while retaining relativesimplicity in the spectrum since it is still broadband pro-ton decoupled.Even though this technique does not distinguish fully be-tween CH groups, it is so easy and reliable that it is fre-quently employed as a first attempt to assign peaks in thespectrum and elucidate the structure.[4]

4 See also• Nuclear magnetic resonance

5 References[1] R. M. Silverstein, G. C. Bassler and T. C. Morrill (1991).

Spectrometric Identification of Organic Compounds. Wi-ley.

[2] Caytan, Elsa; Remaud, Gerald S.; Tenailleau, Eve;Akoka, Serge, GS; Tenailleau, E; Akoka, S (2007).“Precise and accurate quantitative 13C NMR with re-duced experimental time”. Talanta 71 (3): 1016–1021.doi:10.1016/j.talanta.2006.05.075. PMID 19071407.

[3] The Theory of NMR - Chemical Shift

[4] Keeler, James (2010). Understanding NMR Spectroscopy(2nd ed.). John Wiley & Sons. p. 457. ISBN 978-0-470-74608-0.

6 External links• Carbon NMR spectra, where there are three spectraof ethyl phthalate, ethyl ester of orthophthalic acid:completely coupled, completely decoupled and off-resonance decoupled (in this order).

• For an extended tabulation of 13C shifts andcoupling constants.

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