The 1D NMR spectrum of large and complex molecules like natural products has complicated splitting patterns and overlapping signals, which can be easily interpreted using 2-dimensional (2D) NMR. Unlike 1D NMR, 2D NMR has two frequency axes that provide the coupling information between the nucleus A and nucleus B in a molecule. The process from which 2D spectra are obtained has four steps.
The first step is the preparation period, during which nucleus A is excited with a radiofrequency pulse. The second step is the evolution or wait period t1, which is incremented systematically, and no data is observed during this period. The next step is the mixing period, in which a second radiofrequency pulse is introduced, which transfers the magnetization to the spins of nucleus B. In the last step, the magnetization of nuclear spins from B is detected during the acquisition time t2 for each t1. Time domains t1 and t2 are Fourier transformed into frequency domains f1 and f2, and these correspond to two dimensions of 2D spectra, while the third dimension is the intensity. In 2D experiments, coupling between the same kind of nucleus is called homonuclear 2D experiments, whereas coupling between two types of nucleus is called heteronuclear 2D experiments. 2D spectra can be displayed as stacked or contour plots. When presented as stacked plots, 2D spectra are difficult to interpret. The contour plot is the horizontal cross-section of a stacked plot where the size of the circles shows the signal intensity. Its interpretation is straightforward and it is preferred for data analysis.
In 2-dimensional or 2D NMR, introducing a second frequency axis enables the identification of coupled spins, yielding connectivity information in a molecule.
First, one of the coupled nuclei is excited during the preparation period.
This is followed by a systematically incremented evolution or wait period, t1, during which no data is observed.
During the mixing period, a second pulse transfers the magnetization from the excited spins to the coupled nuclei.
Finally, the magnetization of the coupled spins is detected during the acquisition time t2 for each t1.
Time domains, t1 and t2, are Fourier transformed to yield the frequency domains F1 and F2, where F1 is usually a chemical shift scale.
Coupling between the same kind of nucleus is studied by homonuclear 2D experiments, whereas heteronuclear 2D experiments investigate the correlation between different nuclei.
2D spectra are generally represented as contour plots where the size of the circles is proportional to the signal intensity.
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