Carbon-13 NMR

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

In the ¹³C NMR spectra for ethyl acetate, the signal on the far left (171 ppm) is a ${\rm{C{=}O}}$, 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 ${-}{\rm{OCH}}_{2^-}$ group in ethyl acetate. The signals at 20 and 15 ppm are the CH₃ connected to the carbonyl and the CH₃ connected to the ${-}{\rm{OCH}}_{2^-}$ 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.

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 $x$- and $y$-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. 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.