1. Purpose

Very often organic molecules have complex spin systems comprised of four or even more different types of protons. Standard homonuclear decoupling as described in Experiment 4.4 simplifies the residual multiplet, but the extraction of the relevant spin-coupling constant may still be difficult. In principle it is possible to decouple more than one proton type at the same time and by more recent developments even an area of other protons [3]. In the experiment described here we have chosen the four-spin AMXY system of strychnine, demonstrating simultaneous decoupling of protons M and X to observe the spin coupling between A and Y.

2. Literature

J. P. Jesson, P. Meakin, G. Kneissel, J. Am. Chem. Soc. 1973, 95, 618–620.
M. L. Martin, J.-J. Delpuech, G. J. Martin, Practical NMR Spectroscopy, Heyden, London, 1980, 203–206.
A. Hammarström, G. Otting, J. Am. Chem. Soc. 1994, 116, 8847–8848.

3. Pulse Scheme and Phase Cycle

Homonuclear decoupling sequence with proton irradiation at two frequencies

4. Acquisition

Time requirement: 10 min

Sample: 3% strychnine in CDCl₃.

Load standard ¹H parameters, record a normal ¹H NMR spectrum, and note the offsets of the signals for the protons 11α at δ_H = 3.13 and 11β at δ_H = 2.67. Change the setup for the spectrometer so that two proton channels are available. Load a pulse program for homonuclear decoupling and obtain first two spectra with single frequency decoupling for protons 11α and 11β. Optimize the decoupling conditions as discussed in