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.
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 Bon 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)
HETCOR spectra
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)
