13C-NMR spectroscopy is nuclear magnetic resonance spectroscopy performed with respect to the C-13 isotope of carbon in organic molecules. There are three main isotopes of the carbon element. These include the most abundant C-12 isotope, the radioactive C-14 isotope, and the isotope of interest for us today i.e., the C-13 isotope. The 13C isotope is only 1.11% present in naturally occurring carbon compounds. Nonetheless, this isotope is important because it behaves as a tiny magnet and can be used to gain structural information about a compound using NMR spectroscopy.
What is the basic working principle of 13C-NMR spectroscopy
13C-NMR spectroscopy is a type of nuclear magnetic resonance spectroscopy. The C-13 isotope of carbon has an odd mass number and thus an odd number of neutrons i.e., 7 so it possesses a specific angular momentum value. The 13C isotope present in different organic compounds is always spinning about its fixed axis.
Under the influence of an external magnetic field (B0), Zeeman splitting occurs and the 13C nuclei occupy two different spin orientations. The nuclei flip their spin orientation by absorbing energy equal to the energy gap between the two spin states. This energy is called resonance energy and it is provided by the radio wave frequency (25-100 MHz) of the electromagnetic spectrum. The amount of energy absorbed is ultimately used to determine the chemical environment and structural arrangement of 13C nuclei in the targeted sample molecules.
The sample consists of a large number of molecules of the same chemical compound. The 13C-NMR picks signals only from the molecules or different positions in the molecule at which C-13 nuclei are present. It is due to a bulk amount of substance present that the tiny signals accumulate and give us sufficient structural information.
TMS short for tetramethyl silane (CH3)4Si is added as an internal reference standard in the sample solution while performing 13C-NMR spectroscopy. It has a total of 4 carbon atoms attached to identical functional groups on each side. The C atoms in C-Si bonds are electronically shielded thus TMS gives a single strong peak on the NMR spectrum. This peak is considered a reference peak and is assigned an arbitrary value of zero (0).
The chemical shift (symbol δ) is measured in ppm for the tested compounds relative to the TMS peak, ongoing from right to left on the NMR spectrum. The chemical shift value for a particular NMR signal can be calculated using the formula given below.
where ѵ= radiofrequency. It is related to resonance energy by the equation ∆ E=hѵ where ∆ E= resonance energy= energy gap between the two spin orientations and h=Planck’s constant (6.626 x 10-34).
What does 13C-NMR tell you
The signals or peaks obtained at different positions on the NMR spectrum give us a hint about the different types of carbon atoms i.e., different chemical environments present in a sample. The peak intensity signifies how many carbon atoms are present at a particular point in the molecule’s structure. All the data combined is then used to identify the different functional groups present in the molecule ultimately revealing a compound’s complete structural profile.
Strongly shielded 13C-nuclei resonate at a lower frequency. This is called upfield NMR shift. To accommodate the strong shielding effect offered by neighboring electrons, a stronger magnetic field needs to be applied. In the presence of an electronegative atom in the vicinity of targeted 13C nuclei, the nuclei get de-shielded. So, it resonates at a higher frequency and the phenomenon is known as downfield NMR shift. The NMR signal exhibits a stronger chemical shift by shifting to the left of the spectrum.
Hybridization is another important factor that influences the resonance energy in 13C-NMR spectroscopy.sp3 hybridized carbon nuclei experience the smallest chemical shift followed by sp hybridized carbon nuclei. Conversely, the sp2 hybridized carbon nuclei experience the strongest chemical shift by resonating at the highest radio frequency as opposed to the other two.
The presence and/or absence of symmetry in a molecule also affects the results obtained on the 13C-NMR spectra.
How to read a 13C-NMR spectrum
Example # 1:
Previously we studied the proton NMR spectrum of ethanol so following the same lead, we have shown below the 13C-NMR spectrum of ethanol.
There are two main peaks in this NMR spectrum. Ethanol (CH3-CH2-OH) has two different types of carbon atoms. There is a type 1 C-atom (C from CH3) that is bonded to another carbon atom (i.e., CH2) on one side, on the other side it is singly bonded to three hydrogen (H) atoms.
In contrast to that, the type 2 C-atom (C from CH2) is bonded to a carbon (i.e., CH3 ) on one side and to a highly electronegative oxygen (O) atom on the other side. The electronegative atom strongly attracts the shared electron cloud from the C-O bond so the CH2 carbon is more deshielded than the CH3 carbon.
Additionally, any alkyl functional group attached in the vicinity of the targeted carbon acts as an electron donor. So, the CH3 carbon experiences a stronger shielding effect as the CH2 group push electrons towards it. Consequently, these two carbon atoms are not chemically equivalent. The 13C-NMR signal on the left represents CH3 carbon while the CH2 carbon gives a signal at a higher chemical shift i.e., the signal recorded on the left of the spectrum (further away from the reference peak).
Example # 2:
Now see the 13C-NMR spectrum of pentane given below.
There are a total of 5 carbon atoms present in pentane (CH3-CH2-CH2-CH2-CH3), but the 13C-NMR spectrum displays 3 characteristic peaks only. This is because each CH3 on the terminals is equivalent. Both are chemically bonded to a C atom on one side and three hydrogen atoms on the other side. Thus, the CH3 carbons give a single peak 1. These two carbons are strongly shielded so they give an NMR peak nearest to the TMS peak.
The CH2 groups right next to CH3 are also chemically equivalent so they give a single peak 2. The third peak, peak 3 thus belongs to the central CH2 group. The intensity of peaks 1 and 2 are equal as both correspond to 2 C-atoms while the intensity of peak 3 is halved because it belongs to 1 C-atom.
An interesting thing is that the 13C-NMR peaks do not split due to spin-spin coupling between adjacent C atoms. This is because there is only a small chance that two adjacent carbons are both 13C in nature. However, the 13C peaks do split in the presence of directly bonded protons (1H-atoms).
If the proton bonded to the targeted C-atom align with the applied magnetic field, the effect of B0 increases on the spinning 13C nuclei. On the other hand, if the proton aligns against the field, the effect of B0 on 13C decreases. If there is more than one proton for example in CH2 or CH3 groups, the combined effect of all the protons significantly manipulates the external magnetic field experienced by 13C nuclei. Consequently, a single 13C-NMR peak splits into a doublet, a triplet, or a quartet following the n+1 splitting rule.
Splitting = n+1
where n= number of protons directly bonded to the resonating C-atom.
Example: The three singlet peaks witnessed in the 13C-NMR spectrum of pentane are split due to directly bonded protons as shown in the high-resolution spectrum below.
The C-H spin coupling sometimes complicates things and reduces the sensitivity of a 13C-NMR spectroscopic technique. Thus, 13C-NMR spectra are usually recorded post proton NMR decoupling, also called broadband decoupling.
Dive deeper into the world of 13C-NMR signals here.
What are the uses of 13C-NMR spectroscopy
- 13C-NMR spectroscopy is valuable in analytical research and development such as for studying newly synthesized organic compounds.
- It is used in the industrial sector to study organometallic compounds.
- 13C-NMR spectroscopy provides both qualitative as well as quantitative information about synthetic polymers, pharmaceutical drugs, and their closely related structural isomers.
- It can give complex information such as compound ratios, polymer composition, polymer end groups, etc.
- A fascinating application reported for 13C-NMR spectroscopy is in studying brain energy metabolism.
Find other interesting applications of NMR spectroscopy and some related techniques here.
What is the difference between proton NMR and 13C-NMR spectroscopy
13C-NMR spectroscopy and proton NMR spectroscopy are two principal NMR spectroscopic techniques. Both are analogous with respect to their basic principle and operation but let’s find out how are these two techniques different.
| 13C-NMR spectroscopy | Proton NMR spectroscopy |
| 13C-NMR spectroscopy is based on studying the magnetically spinning 13C nuclei present in a chemical molecule | Proton NMR spectroscopy is based on studying the magnetically spinning 1H nuclei present in a molecule |
| Only a small number (1 in 100) of 13C nuclei are present in an organic molecule | A large number of 1H nuclei are often present in an organic molecule |
| The peak splitting pattern reflects how many protons are directly attached to the targeted C-atom | The peak splitting pattern is characteristic of the protons attached to a C-atom adjacent to the carbon carrying the targeted protons |
| Stronger chemical shifts ( up to 200 ppm) |
Comparatively weaker chemical shifts (up to 12 ppm) |
| Greater spectral selectivity due to a large dispersion in chemical shift. So, it gives a more detailed structural characterization | Less spectral selectivity as compared to 13C-NMR spectroscopy. So, less detailed structural characterization as opposed to the other |
| Less sensitive as compared to proton NMR spectroscopy | More sensitive as compared to 13C-NMR spectroscopy |
Considering the significance and the relevance of the two sister spectroscopic techniques, the data from both is often combined to gain complete structural information about the tested compound.
Here is a detailed article on NMR spectroscopy if you want to go back to the basics.
You may also like: What are those 10 different spectroscopic techniques.
Also valuable for structural elucidation is an instrumental analytical technique called X-ray spectroscopy.
References
1.Karunakaran, C., P. Santharaman and M. Balamurugan (2018). Chapter Two – 1H and 13C Nuclear Magnetic Resonance Spectroscopy. Spin Resonance Spectroscopy. C. Karunakaran, Elsevier: 49-110.
2. Yadav, L. D. S. (2005). 13C NMR Spectroscopy. Organic Spectroscopy. Dordrecht, Springer Netherlands: 195-223.