Chapter 13: Spectroscopy

• So far the NMR spectral methods we have discussed have been one dimensional (since they have a single chemical shift x coordinate axis).
• With the development of more advanced spectroscopic methods as computational power has increased, it has become possible to obtain two dimensional spectra.
• In two dimensional experiments, both the x and the y axes have chemical shift scales and the 2D spectra are plotted as a grid like a map.
• Information is obtained from the spectra by looking at the peaks in the grid and matching them to the x and y axes.

 
• COSY - Correlation Spectroscopy
•  both axes correspond to the proton nmr spectra.
•  the COSY spectra indicates which H atoms are coupling with each other.
•  An example of a COSY is provided below.

• HETCOR - Heteronuclear Correlation Spectroscopy
• proton nmr spectra on one axis and the 13C nmr spectra on the other.
• the HETCOR spectra matches the H to the appropriate C.

COSY spectra

• The information on the H that are coupling with each other is obtained by looking at the peaks inside the grid.  These peaks are usually shown in a contour type format, like height intervals on a map.
• In order to see where this information comes from, let's consider an example shown below, the COSY of ethyl 2-butenoate 
• First look at the peak marked A in the top left corner.  This peak indicates a coupling interaction between the H at 6.9 ppm and the H at 1.8 ppm.  This corresponds to the coupling of the CH₃ group and the adjacent H on the alkene.
• Similarly, the peak marked B indicates a coupling interaction between the H at 4.15 ppm and the H at 1.25 ppm.  This corresponds to the coupling of the CH₂ and the CH₃ in the ethyl group.
• Notice that there are a second set of equivalent peaks, also marked A and B on the other side of the diagonal.

(COSY spectra recorded by D. Fox, Dept of Chemistry, University of Calgary on a Bruker Advance DRX-400 spectrometer)

• The information on how the H are C are matched is obtained by looking at the peaks inside the grid.  Again, these peaks are usually shown in a contour type format, like height intervals on a map.
• In order to see where this information comes from, let's consider an example shown below, the HETCOR of ethyl 2-butenoate.
• First look at the peak marked A near the middle of the grid.  This peak indicates that the H at 4.1 ppm is attached to the C at 60 ppm.  This corresponds to the -OCH₂- group.
• Similarly, the peak marked B towards the top right in the grid indicates that the H at 1.85 ppm is attached to the C at17 ppm.  Since the H is a singlet, we know that this corresponds to the CH₃- group attached to the carbonyl in the acid part of the ester and not the CH₃- group attached to the -CH₂- in the alcohol part of the ester.
• Notice that the carbonyl group from the ester has no "match" since it has no H attached in this example.

(HETCOR spectra recorded by D. Fox, Dept of Chemistry, University of Calgary on a Bruker Advance DRX-400 spectrometer)

---