Properties of 13C NMR
13C NMR uses the same property, nuclear magnetic resonance (NMR), as 1H NMR, but the NMR is tuned to carbon atom and not protons. The carbon-13 nucleus has nuclear spin because it has an uneven number of nucleons (6 protons and 7 neutrons). The carbon-12 nucleus has an even number of protons and neutrons and does not have a nuclear spin. Just as in 1H NMR, tetramethylsilane is used as a reference in the sample tube.
There are four main chemical shifts in 13C NMR spectra:
1. alkane carbons (0–50 ppm)
2. alkane carbons bonded to heteroatoms or alkyne carbons (50–100 ppm)
3. alkene or aromatic carbons (100–150 ppm)
4. carbonyl carbons (150–220 ppm)
In 13C NMR, it is even possible to differentiate which carbonyl functional group is in a compound by the chemical shift. Carboxylic acids and esters have a shift of 160 ppm to 185 ppm. Aldehydes have a shift of 190 ppm to 200 ppm. Ketones have a shift of 205 ppm to 220 ppm.
Chemical Shift in 13C NMR Spectra
|Carbon bond||Chemical Shift (in ppm)|
|Alkane carbons||Zero to 50|
|Alkane carbons to heteroatoms or alkyne carbons||50 to 100|
|Alkene carbons or aromatic carbons||100 to 150|
|Carboxylic acids, esters, and amides||160 to 185|
|Aldehydes||190 to 200|
|Ketones||205 to 220|
Each peak in the 13C NMR spectra indicates a nonequivalent carbon atom, meaning that each type of carbon is in a different chemical environment (signal). Consider the 13C NMR spectra for ethyl acetate (). There are four peaks, indicating that there are four unique carbon atoms:
- Two carbons have a shift less than 50, indicating they are alkane carbons.
- One carbon has a shift between 50 and 100, indicating it is attached to a hetero (noncarbon, nonhydrogen) atom, such as X, O, N, … .
- One carbon has a shift greater than 150, indicating it is a carbonyl carbon.
Carbon-13 NMR Spectra for Ethyl Acetate
When considering unknown NMR spectra, the approach is similar (but not exactly the same) to that of 1H NMR. 13C NMR spectra give the number of unique signals and details about the functional group or environments to which the carbons are connected, i.e., their shifts. But 13C NMR spectra do not provide splitting or integration information. This information can be used to narrow the range of potential compounds that could identify an unknown sample.
Distortionless enhancement of polarization transfer C-NMR (DEPT C-NMR) is a technique that provides the number of protons bonded to each carbon as well as the chemical shift provided by a regular 13C NMR. In a DEPT C-NMR spectra, any signals that are CH2 will appear to have a negative absorbance. The CH3 and CH signals still appear in the positive portion of the spectra. Carbons that have no hydrogen atoms attached are not usually observed in DEPT spectra.
A 13C NMR spectrum can be overlaid with the other four spectra to isolate the location of the quaternary carbon. The spectra are collected in a process where certain parameters vary, specifically the pulse angle. There are three different angles of importance in the DEPT experiments:
- DEPT 135 shows CH and CH3 as positive signals and CH2 as negative signals.
- DEPT 90 shows only CH, and they are positive signals.
- DEPT 45 shows CH, CH2 and CH3 as positive signals.
13C and 1H NMR are one dimensional and present data in the form of a chemical shift on one axis. It is possible to construct two-dimensional (2-D) spectra, where both the - and -axes contain chemical shifts.One type of 2-D NMR is correlation spectroscopy. Correlation spectroscopy(COSY) is a plot where both axes are from proton NMR, and they are plotted against one another to show which protons are coupled to each other. In the spectra, peaks on the diagonal represent the same signal when comparing one spectra to another. Off-diagonal signals (also known as cross peaks) indicate which protons are coupled to each other by connecting the two peaks that produce the off-diagonal signal.
COSY NMR Spectrum for Progesterone
3-D nuclear magnetic resonance (3-D NMR) plots the signals from three different nuclei against one another in three-dimensional space. A 3-D NMR takes much longer than its counterparts and is used in biochemistry in the analysis of protein structures.