AN INTRODUCTION : C-13 NMR
Middha Akanksha*, Kataria Sahil, Sandhu Premjeet, Arora Praveen
Seth G. L. Bihani S.D. College of Technical Education,
Institute of Pharmaceutical Sciences and Drug Research,
Sri Ganganagar, Rajasthan,
Nuclear Magnetic Resonance (NMR) Spectroscopy is not limited to the study of protons. Any element with a nuclear spin (13C, 17O, 19F, 31P and many others) will give rise to an NMR signal.Carbon-13 NMR (13C NMR or referred to as carbon NMR) is the application of nuclear magnetic resonance (NMR) spectroscopy applicable to carbon. It is similar to proton NMR (1 H NMR) and allows the identification of carbon atoms whereas in other identification of H. As such 13C NMR is an important tool in chemical structure elucidation in organic chemistry. 13C NMR detects only the 13C isotope of carbon, whose natural abundance is only 1.1%, because the main carbon isotope, 12 C, is not detectable by NMR since it has zero net spin.
Reference Id: PHARMATUTOR-ART-1203
It is useful to compare and contrast H-NMR and C-NMR as there are certain differences and similarities:
- 13C has only about 1.1% natural abundance (of carbon atoms)
- 12C does not exhibit NMR behaviour (I=0)
- 13C nucleus is also a spin 1/2 nucleus
- 13C nucleus is about 400 times less sensitive than H nucleus to the NMR phenomena
- Due to the low abundance, we do not usually see 13C-13C coupling
- Chemical shift range is normally 0 to 220 ppm
- Chemical shifts are also measured with respect to tetramethylsilane, (CH3)4Si (i.e. TMS)
- Similar factors affect the chemical shifts in 13C as seen for H-NMR
- Long relaxation times (excited state to ground state) mean no integrations
- "Normal" 13C spectra are "broadband, proton decoupled" so the peaks show as single lines
- Number of peaks indicates the number of types of C
The general implications of these points are that 13C-NMR spectra take longer to acquire than H-NMR, though they tend to look simpler. Accidental overlap of peaks is much less common than for H-NMR which makes it easier to determine how many types of C are present6
In comparison to 1H NMR spectroscopy, 13C NMR spectra are more easily interpreted and give following information:
1. The common range of energy absorption for 13C is wide δ 0 – 200 relative to TMS, contrasted with δ 0-15 for 1H NMR. Thus fewer peaks overlap in 13C NMR spectra
2. Because only 1.1% of carbon in a compound is 13C, 13C- 13C coupling is negligible and thus is not observed. Therefore, in one type of 13C NMR Spectra( proton decoupled) each magnetically non equivalent carbon gives a single unsplit peak
The presence of plane of symmetry in an organic molecule may render some carbons chemically equivalent to others. This may lead to discrepancy between the apparent number of peaks and the actual number of carbon atoms present in the molecule
3. The area under the peaks in 13C NMR Spectra are not necessarily proportional to the number of carbons giving rise to the signals.
4. The proton coupled spectra, the signal for each carbon (or a group of magnetically equivalent carbons) is split by the protons bonded directly to that carbon and n+1 rule is followed.3
Carbon -13 NMR spectroscopy is similar to proton NMR in that the no. of the peaks in the spectrum normally corresponds to the number of different carbon enviornments and the chemical shifts of carbon signals provide some indication of nature of each enviornment.
Carbon -13 NMR differs from proton NMR in that integration is normally
Table 1: Comparison of Different Nuclear Characteristics8
There are three short-comings of 13C-NMR spectra, namely :
(1) Only 1% of the carbon in the molecule is carbon-13,
(2) Sensitivity is consequently low, and
(3) Recording the NMR-spectra is a tedious and time consuming process. However, with the advent of recent developments in NMR-spectroscopy it is quite possible to eliminate some of these short comings adequately. They are :
(a) Development of powerful magnets (‘supercon’ magnets) has ultimately resulted in relatively stronger NMR-signals from the same number of atoms,
(b) Improved hardware in NMR-spectroscopy has gainfully accomplished higher sensitivity, and
(c) Development of more sensitive strategies has made it possible to record these C—H correlation spectra in a much easier manner.8
Whereas carbon-carbon signal splitting does not occur in 13C NMR spectra, hydrogen atoms attached to carbon can split 13C NMR signals into multiplet peaks. However it is useful to simplify the appearance of 13C NMR spectra by initially eliminating signal splitting for 1H -13C coupling. This can be done by choosing instrumental parameters that decouple the proton-carbon interactions and such a spectra is said to be broadband(BB) proton decoupled, thus in a broadband proton decoupled 13C NMR spectrum each carbon atom in a unique environment gives a signal consisting of only one peak. Most 13C NMR spectra are obtained in the simplified broadband mode first and then in modes that provide information from the 1H-13C couplings.4
The background to C-13 NMR spectroscopy
Nuclear magnetic resonance is concerned with the magnetic properties of certain nuclei. On this page we are focussing on the magnetic behaviour of carbon-13 nuclei.
Carbon-13 nuclei as little magnets
About 1% of all carbon atoms are the C-13 isotope; the rest (apart from tiny amounts of the radioactive C-14) is C-12. C-13 NMR relies on the magnetic properties of the C-13 nuclei.
The effect of this is that a C-13 nucleus can behave as a little magnet. C-12 nuclei don't have this property.
If we have a compass needle, it normally lines up with the Earth's magnetic field with the north-seeking end pointing north. Provided it isn't sealed in some sort of container, we could twist the needle around with your fingers so that it pointed south - lining it up opposed to the Earth's magnetic field.
It is very unstable opposed to the Earth's field, and as soon as you let it go again, it will flip back to its more stable state.
Fig. 1: Allignment in Earth’s Magnetic Field
Because a C-13 nucleus behaves like a little magnet, it means that it can also be aligned with an external magnetic field or opposed to it.
Again, the alignment where it is opposed to the field is less stable (at a higher energy). It is possible to make it flip from the more stable alignment to the less stable one by supplying exactly the right amount of energy.
Fig. 2: Carbon 13 nucleus in External Magnetic Field
The energy needed to make this flip depends on the strength of the external magnetic field used, but is usually in the range of energies found in radio waves - at frequencies of about 25 - 100 MHz.
It's possible to detect this interaction between the radio waves of just the right frequency and the carbon-13 nucleus as it flips from one orientation to the other as a peak on a graph. This flipping of the carbon-13 nucleus from one magnetic alignment to the other by the radio waves is known as the resonance condition.5
Among the atoms that, like the protons, give rise to NMR Spectra is one of the isotopes of carbon 13C
The CMR spectrum gives information about the carbon skeleton as
· The number of signals tell us how many different carbons – or different sets of equivalent carbons – there in a molecule
· The splitting of a signal tells us how many hydrogens are attached to each carbon
· The chemical shift tells us the hybridization (sp3, sp2, sp ) of each carbon
· The chemical shift tells us about the electronic environment of each carbon with respect to other, nearby carbon or functional groups7
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