The first sample was a chiral molecule modified by attachment of a 2-propynyl group. To be consistent with the presence of the propynyl group the apparent quartet of doublets in the spectrum below was assigned as two doublets of doublets (at 4.37 and 4.32 ppm) due to the two protons attached to the carbon adjacent to the oxygen.
In this assignment, coupling between the geminal protons gives the large splitting of 15.8 Hz, long range coupling to the proton on the far side of the triple bond produces the small 2.2 Hz splitting, and a chemical shift difference of 25 Hz for the two geminal protons leads to strong coupling that distorts the multiplet intensities so that the "outer" lines are reduced in intensity while the "inner" lines are increased.
Two other samples showing strong coupling came from a series of compounds with conjugated double and triple bonds. In one compound the protons on either side of a double bond produced what looked like a quartet of triplets, but was actually two doublets of triplets. Close examination of the multiplets shows that the small coupling on the two right lines is less than the coupling on the two left lines, a hint that this is two resonances showing strong coupling to each other.
The allylic resonances in another compound from this series produced a quite different spectrum. Here a modification in another part of the molecule reduced the chemical shift difference of the allylic protons, enhancing the strong coupling. Now the outer lines have nearly disappeared and the inner lines are nearly overlapping.
Strong coupling even more extreme than the example above is why a methyl group produces what looks like a singlet. Since the chemical shifts of all three protons in a methyl group are identical the scalar coupling between them is much larger than the coupling. The intensity of the outer lines is then reduced to virtually zero, while that of the inner lines is increased. Since the inner lines all overlap, a singlet is obtained.
As strong coupling appears when the chemical shift difference approaches the scalar coupling it is more often seen at lower fields, where the chemical shift range in hertz is less. Still, these examples show that even at 600 MHz strong coupling can occur and should be taken into account when you are trying to assign your spectra.
Thanks to Sam Kantonen of the Gilson lab and Xiao Wang of the Molinski lab for use of their spectra.
1. Reich, Hans J. "5.9 Second order effects in coupled systems" Structure Determination Using Spectroscopic Methods, University of Wisconsin http://www.chem.wisc.edu/areas/reich/nmr/05-hmr-09-2ndorder.htm
2. Jacobsen, Neil J. "NMR spectroscopy explained", Wiley Interscience 2007
(examples pp 63-69, density matrix explanation of origin pp 481-484)