Pulsed Spin Decoupling in Nuclear Magnetic Resonance

group. The spectra, as shown in Figure 2 , can be semiquantitatively reproduced using the LAOCOON computer program with reduced values for the carbon-proton coupling constant^.^ These calculations are approximate because they do not include the asymmetry introduced by the decoupling field or any relaxation effects. We have performed additional experiments which show that it is not necessary for the protons to be equivalent to observe the additional splittings in a CWSD experiment, but the unusual effects shown in the figures are diminished as the difference between the chemical shifts of the vicinal protons increases or the vicinal proton-proton coupling constant decreases. For identical reduced J C H , the “extra” middle peak in 1,1,2,2-tetrachloroethane is less than 20 % that observed in fumaric acid because J H H is 16 Hz in the latter but only 4 Hz in the former. A more detailed explanation of the conditions required for virtual coupling is given by Musher and C ~ r e y . ~ Our main point is t o stress that caution must be exercised in interpreting CWSD experiments when there is strong coupling between two or more protons with similar chemical shifts on different carbon atoms in a molecule.