1. Purpose

Dynamic NMR experiments such as that described in Experiment 5.3 can detect chemical exchange only if the exchange is fast enough to alter the NMR line-shape. However, slower chemical exchange processes can be detected by the saturation transfer experiment. One signal is irradiated and a change is observed in the intensity of another signal that is connected with the irradiated one by chemical exchange. A similar extension of the NMR time-scale is provided by the 2D EXSY experiment (Exp. 10.24). The FT modification [3] of the original Forsén–Hoffman method shown here requires modern instruments where the transmitter power can be attenuated; on older instruments a third frequency source is necessary.

2. Literature

S. Forsén, R. A. Hoffman, Acta Chem. Scand. 1963, 17, 1787–1788.
R. A. Hoffman, S. Forsén, Prog. NMR Spectrosc. 1966, 1, 15–204.
B. E. Mann, J. Magn. Reson. 1976, 21, 17–23.
J. J. Led, H. Gesmar, J. Magn. Reson. 1982, 49, 444–463.
M. L. Martin, J.-J. Delpuech, G. J. Martin, Practical NMR Spectroscopy, Heyden, London, 1980, 315–321.

3. Pulse Scheme and Phase Cycle

Carbon-13 saturation transfer sequence with proton composite-pulse decoupling

4. Acquisition

Time requirement: 1 h

Sample: 5% dimethylformamide in C₂D₂Cl₄.