Nuclear Magnetic Resonance Spectroscopy

Carbon-13 NMR

Properties of 13C NMR

Carbon nuclear magnetic resonance (NMR) shifts are larger and provide less information than proton NMR. Carbon NMR can distinguish four types of carbons based on their shifts: carbons connected to other carbons, carbons connected to heteroatoms, alkene and aromatic carbons, and carbonyl carbons.

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

A partial list of common carbon chemical shifts used in determining the structure from a spectrum

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 (CH3COOCH2CH3{\rm{CH_3{-}COO{-}CH_2{-}CH_3}}). 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

The 13C NMR spectrum of ethyl acetate shows four different types of carbon atoms, with one of them having a chemical shift characteristic of a carboxylic acid or ester C=O {\rm {C{=}O}} group.
In the 13C NMR spectra for ethyl acetate, the signal on the far left (171 ppm) is a C=O{\rm{C{=}O}}, specifically the carbonyl of the ester functional group in ethyl acetate. The signal at 60 ppm is a carbon bonded to a heteroatom, specifically the OCH2{-}{\rm{OCH}}_{2^-} group in ethyl acetate. The signals at 20 and 15 ppm are the CH3 connected to the carbonyl and the CH3 connected to the OCH2{-}{\rm{OCH}}_{2^-} group, respectively.

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.

DEPT Spectra

A DEPT spectrum shows the types of carbons in a compound. In the DEPT spectrum of propyl benzoate (C10H12O2) at 135°, CH2 is shown as a negative absorbance. The DEPT spectrum of propyl benzoate at 90° only shows CH signals. The DEPT spectrum of propyl benzoate at 45° shows CH, CH2, and CH3 signals. Carbons without an H do not show in any of the three DEPT spectra.

Multidimensional NMR

COSY, HETCOR, and 3-D NMR are types of multidimensional NMR techniques that can correlate to other types of NMR spectra to provide even more information about structures.

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 xx- and yy-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

A correlation spectroscopy (COSY) is a two-dimensional nuclear magnetic resonance spectrum. In a COSY, two 1H NMR plots are superimposed to provide more information about which protons are coupled.
Another type of 2-D NMR is heteronuclear correlation spectroscopy. In heteronuclear correlation spectroscopy (HETCOR), 1H NMR spectra are plotted on one axis with 13C spectra plotted on the other axis. A correlation map shows which carbon is attached to which proton.

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.