When we talk about a two-dimensional spectrum (3-dimensional graph) what does the third dimension show?
1 Answer
In a two-dimensional NMR spectrum, the x and y axes show the frequencies. The third dimension shows the intensity of the peaks.
A two-dimensional spectrum is a plot of two frequencies against each other. They can be either two chemical shifts or a chemical shift plus a coupling constant.
The plot can be displayed in two ways — as a stacked plot or as a contour plot.
A stacked plot contains a large number of 1D NMR spectra. Each spectrum is shifted slightly relative to the preceding one.
(a) above shows a stacked plot of a 2D NMR COSY spectrum. Two frequencies are plotted in the x,y plane, and intensity is plotted in the third dimension.
On the right is a contour plot. It shows the peak heights as a series of cross sections through the signals above the x,y plane projected onto that plane. They resemble the contours on a topographic map.
(b) shows a COSY 2D NMR spectrum of the disaccharide xylobiose. The off-diagonal or "cross peaks" appear at the intersection of the two frequency values. They show which groups in the molecule are coupled to each other.
