The application of a very weak perturbing radiohyphen;frequency field in nuclear magnetic doublehyphen;resonance experiments in such a way as to irradiate just a single resonance transition is shown to give information about the relative signs of the spin coupling constants in systems of three or more spins. The method is not restricted to near firsthyphen;order spectra and may be used even when several multiplets overlap. It has been applied to the determination of the relative signs of the couplings in 1,2hyphen;dibromopropane, and the results agree with those of Finegold, who used conventional analysis methods. The sign of a longhyphen;range coupling is reported.
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