Carbon-13 NMR

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Carbon-13 NMR is the application of nuclear magnetic resonance in spectroscopy with respect to carbon. It is analogous to proton NMR and allows the identification of carbon atoms in an organic molecule just as proton NMR identifies hydrogen atoms. As such carbon NMR is an important tool in structure elucidation in organic chemistry.

However, carbon NMR has a number of complications that are not encountered in proton NMR. Carbon NMR is much less sensitive than proton NMR since the major isotope of carbon, the ¹²C isotope, has a spin quantum number of zero and is not magnetically active. Only the less common ¹³C isotope present naturally at 1.1% abundance is magnetically active with a spin quantum number of 1/2 much like a proton. Therefore, only the few carbon-13 nuclei present resonate in the magnetic field, resulting in reduced sensitivity.^[1]. Also the gyromagnetic ratio (6.728284 10⁷ rad T^-1 s^-1) is only 1/4 of the proton one, yielding a lower sensitivity.

Another complication results from the complexity of spectra due to the large one bond J-coupling constants between carbon and hydrogen (typically from 100 to 250 Hz). In addition, because carbon-13 resonates at 75.47 MHz in a 7 T magnetic field (compared to 300 MHz for a proton), the splitting patterns often overlap and become too complicated to interpret easily. In order to remove this complexity, carbon NMR spectra are proton decoupled to remove the signal splitting. In contrast to a typical proton NMR spectrum with multiplets for each proton position, carbon NMR spectra show a single peak for each chemically nonequivalent carbon atom.

Chemical shifts of ¹³C atoms follow the same principles as those of ¹H, with the difference of ¹H having greater relative sensitivity (that is, it is more easily detected). In addition to the former fact, in the most common ¹³C-NMR experiments, the intensity of the signal is not directionally proportional to the number of equivalent ¹³C atoms (unlike in ¹H), but is a function of instrumental parameters, length, and delay of the pulse. The typical range of chemical shifts of ¹³C signals is larger than for ¹H. There are a number of modifications in ¹³C spectroscopy:

• completely decoupled (no couplings whatsoever)
• off-resonance decoupled (only ¹³C - ¹³C couplings)
• completely coupled (¹³C - ¹H couplings)

[edit] DEPT spectra

DEPT stands for Distortionless Enhancement by Polarization Transfer. It is a very useful method for determining the presence of primary, secondary and tertiary carbon atoms. The DEPT experiment basically differentiates between CH, CH₂ and CH₃ groups by variation of the selection angle parameter - that is the tip angle of the final ¹H pulse. The technique suppresses all quaternary carbons and carbons with no attached protons (as in deuterated solvents). The DEPT experiment basically uses polarization transfer from ¹H to ¹³C, in order to increase the sensitivity over the normal NOE (Nuclear Overhauser Effect) enhancement. The selection angle varies: it can be 45°, 90° or 135°. The value chosen dictates the result (as said before, quaternary and deprotonated carbons are always suppressed!):

• 45° angle gives all carbons with attached protons (regardless of number of the latter) in phase
• 90° angle gives only CH groups, the others being suppressed
• 135° angle gives all CH and CH₃ in a phase opposite to CH₂

[edit] References

1. ^ R. M. Silverstein, G. C. Bassler and T. C. Morrill (1991). in Wiley: Spectrometric Identification of Organic Compounds, 5. 

[edit] External links

• To see carbon NMR spectra look, check out this link, where there are three spectra of ethyl phthalate, ethyl ester of orthophthalic acid: completely coupled, completely decoupled and off-resonance decoupled (in this order).