|
A conventional NMR spectrum is composed of two dimensions: the frequency axis (x-axis) which corresponds to chemical shifts and the intensity axis (y-axis). A 2D NMR spectrum is three-dimensional, consisting of two frequency axes in the same plane and a third dimension corresponding to intensity. There are two types of spectra for 2D NMR: a two-dimensional J-resolved spectrum and a two-dimensional (shift)-correlated NMR spectrum. The first refers to a spectrum in which chemical shifts are plotted along one frequency axis and coupling constants are plotted on the other, while the second refers to a spectrum with chemical shift plotted on both frequency axes (Friebolin 223).
Data from 2D NMR may represented as either a stacked plot or a contour plot. In a stacked plot, each row of the Fourier-transformed spectrum's matrix is plotted, producing an image of the peaks in a three-dimensional manner. (Bruch 148). The 2D NMR stacked plot has an advantage over the 1D NMR plot in that it becomes clear that a spectrum of overlapping peaks may be composed of quartets, triplets, doublets, and singlets (Skoog, Holler, & Nieman 486). The stacked plot is not as useful as the contour plot for data analysis, however (Bruch 148).
The contour plot represents the peaks in the stacked plot when viewed from above. (see figures below) In many cases, interpretation of the contour plot is easier than interpretation of the stacked plot (Friebolin 229). In a contour plot, points of equal intensity are connected by lines. A cross-section through the plot at a specific chemical shift is sometimes employed (Bruch 148).
| A stacked plot |
|
|
| Nikolai V. Shokhirev, F. Ann Walker, 2001 |
| A contour plot |
|
|
| Nikolai V. Shokhirev, F. Ann Walker 2001 |
|
