Question

What information do we get from analyzing 2D NMR spectra?

Answer 1

2D NMR spectra tell us which atoms are connected to one another.

In 2D NMR, both the x and the y axes have chemical shift scales.

In `"COSY"` (`"CO""rrelation ""S""pectroscop""Y"`) spectra, both axes correspond to `""^1"H"` NMR spectra.

In `"HETCOR "` (`"HET""eronuclear ""CORR""elation")` spectra, a `""^1"H"` spectrum is plotted on one axis against a `""^13"C"` spectrum on the other.

The 1D spectrum ethyl but-2-enoate, CH₃CH=CHCOOCH₂CH₃, is

COSY spectra

The COSY spectrum is

The peaks on the diagonal from lower left to upper right correspond to the peaks in the 1D spectrum.

The off-diagonal or cross peaks result from the coupling between two different nuclei.

To determine which couplings a cross peak represents, find the diagonal peaks that are directly above or below and to the left or right of the cross peak. The nuclei represented by those two diagonal peaks are coupled.

From high to low field on the horizontal axis, we assign the following couplings:

• 1.1 and 4.2 ppm: The CH₂CH₃ group.
• 5.7 and 6.9 ppm: The alkene hydrogens.
• 6.9 and 1.8 ppm: The CH₃CH= group.

HETCOR Spectra

The HETCOR spectrum is:

From high to low field on the `""^13"C"` axis, we assign the following couplings:

• C at 14 ppm and H at 1.1 ppm — the CH₃ of the ethyl group.
• C at 18 ppm and H at 1.8 ppm — the CH₃ on the alkene carbon.
• C at 60 ppm and H at 4.2 ppm — the CH₂ of the ethyl group.
• C at 123 ppm and H at 5.7 ppm — the alkene CH next to the C=O.
• C at 145 ppm and H at 6.9 ppm — the alkene CH next to the CH₃.