In practice, the routine ¹³C nmr spectrum are recoding by the pulsed FT method, with sensitive advanced by summation of several spectra.
¹H DECOUPLING OR NOISE DECOUPLING OR BROAD BAND DECOUPLING :-
Both ¹³C and ¹H have I = 1/2, so that we can expect to see couplings in the spectrum between (a) ¹³C − ¹³C and (b) ¹³C − ¹H. The probability of the two ¹³C isotopes being together in the same molecule is so low that ¹³C− ¹³C couplings are not usually observed. Couplings from ¹³C − ¹H should be observe in the ¹³C spectra. However, this couplings make the ¹³C spectra is very extremely complex and they has been eliminated by decoupling.
To eliminating the complicating affect of the proton coupling in the ¹³C spectra, we can must decoupling the ¹H nuclei by double irradiation at their resonance frequency is 60MHz at 1.4T etc.. The example of heteronuclear decoupling. But we don't wish merely to decouple a specific protons, rather we wish to double irritated the all protons simultaneously while recording the ¹³C spectrum. A decoupling signal is used that's all the ¹H frequencies spreading around 60MHz, and is therefore a form of radiofrequency noice: spectra derived thus are ¹H − decoupling or noise - doubled or broad - band decoupling. Most of the ¹³C spectra the recording in this way. In such proton decoupling ¹³C spectra the individual resonance for carbon atoms in different environments appearing as a singlet, making these spectra very simple to interpret.
Since decoupling it can interfere with and thereby shorten relaxation times, the nuclear overhauser effect may operate and leading to signal enhancement of certain ¹³C peaks. It turning out that the major relaxation route for ¹³C nucleus involving dipolar transferring of it's excitation energy to the protons directly attached to it, there's a corollary that maximum nuclear overhauser effect operating on CH₃, CH₂, and CH carbons, whereas no enhancement arising for quaternary carbons. It happens also that this non- proton bearing carbons have a long relaxation times and also end to give low- intensity signals for this reason. Hence in ¹³C nmr the relative intensities of the signals don't give reliable information about the relative numbers of the carbon atoms in different environments.
OFF RESONANCE DECOUPLING :-
A common mode of obtaining ¹³C spectra constant of carrying out the proton decoupling by irradation of the sample with radiofrequency which is not quite exactly that of protons but it's few hundred hertz, displayed. This is usually referred to as off resonance decoupling or SFORD. Single frequency off resonance decoupling. This consequence of this is an incomplete collapse of the multiplicity, and vestigial quartets remains from methyl carbon, with triplets from CH₂, doublets from CH and singlets from fully substituted carbons. It must also being stressed that only the multiplicity of the signals and not the actual spacings within the multiplets is of significantly in the SFORD spectra. The space are governing principally by the numerically values of the difference between the decoupling frequency and the resonance frequency of the protons involved and are thus dependent not only on molecular structure but on experimental parameters.
DEUTERIUM COUPLING:-
Deuterinated solvents such as CDCl₃ gives rise to carbon - 13 signals which are split by decoupling to deuterium. The multiplicity is calculable from the general formula (2nI +1) and deuterium has I =1, so that in molecules with one deuteron attaching to each carbon as in CDCl₃ the carbon - 13 isotopes signal from the solvent is a 1: 1 : 1 triplet. For CD₃ groups in DMSO-d₆ and in acetone - d₆ the solvent gives rise to a "septet" with line intensition is
1 : 3: 6: 7: 6: 3: 1 this type of ratio
Chemical shift values for the carbon in ¹³C nmr.
Compound chemical shift (ppm)
CH₄ −2.1
CH₃CH₃ 7.3
CH₃ 11.5
CH₂ 26.5
CH₂ 46.7
Cl
CH₂Cl₂ 52.9
CHCl₃ 77.3
CH₃−CHO 200.5
CH₂= CH₂ 122.1
benzene 128
Aldehyde and ketone ≈ 200
Acid derivatives ≈ 175
Good
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