Carbon-13 nuclear magnetic resonance

times less sensitive than 1H NMR spectroscopy, 13C NMR spectroscopy is widely used for characterizing organic and organometallic compounds, primarily because 1H-decoupled 13C-NMR spectra are more simple, have a greater sensitivity to differences in the chemical structure, and, thus, are better suited for identifying molecules in complex mixtures.

[2] On the other hand, the sensitivity of 13C NMR spectroscopy benefits to some extent from nuclear Overhauser effect, which enhances signal intensity for non-quaternary 13C atoms.

This dispersion reflects the fact that non-1H nuclei are strongly influenced by excited states ("paramagnetic" contribution to shielding tensor.

In cryoprobes, the RF generating and receiving electronics are maintained at ~ 25K using helium gas, which greatly enhances their sensitivity.

Another potential complication results from the presence of large one bond J-coupling constants between carbon and hydrogen (typically from 100 to 250 Hz).

[9] With proton-noise decoupling, in which most spectra are run, a noise decoupler strongly irradiates the sample with a broad (approximately 1000 Hz) range of radio frequencies covering the range (such as 100 MHz for a 23,486 gauss field) at which protons change their nuclear spin.

The rapid changes in proton spin create an effective heteronuclear decoupling, increasing carbon signal strength on account of the nuclear Overhauser effect (NOE) and simplifying the spectrum so that each non-equivalent carbon produces a singlet peak.

[10] The relative intensities are unreliable because some carbons have a larger spin-lattice relaxation time and others have weaker NOE enhancement.

This largely prevents NOE enhancement, allowing the strength of individual 13C peaks to be meaningfully compared by integration, at a cost of half to two-thirds of the overall sensitivity.

[9] Distortionless enhancement by polarization transfer (DEPT)[11] is an NMR method used for determining the presence of primary, secondary and tertiary carbon atoms.

The polarization transfer from 1H to 13C has the secondary advantage of increasing the sensitivity over the normal 13C spectrum (which has a modest enhancement from the nuclear overhauser effect (NOE) due to the 1H decoupling).

Even though this technique does not distinguish fully between CHn groups, it is so easy and reliable that it is frequently employed as a first attempt to assign peaks in the spectrum and elucidate the structure.

It is, however, sometimes possible that a CH and CH2 signal have coincidentally equivalent chemical shifts resulting in annulment in the APT spectrum due to the opposite phases.