Determination of the relative signs of spin coupling constants from the temperature dependence of N.M.R. spectra.J(H-C-C≡C-C-H)

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Abstract
A method of determining the relative signs of J AB and J BX in nuclear spin systems of the type AB2X q is described and applied to the AB2X3 proton spin system of 2-butyn-1-ol. Variation of the relative chemical shift v A—v B (through changes in temperature or in the composition of the solvent) causes certain pairs of ‘labile’ transitions in the X spectrum to coalesce on one side or other of the X spectrum. The information about the relative signs of the coupling constants is obtained by noting the two different critical values of v A-v B at which these coalescences occur. Since the method remains applicable even if the third coupling constant J AX is vanishingly small, it is particularly useful in the determination of the signs of long-range spin coupling constants where the usual double resonance methods often break down. It is concluded that the long-range coupling J(H-C-C≡C-C-H) of 2-butyn-1-ol has a positive sign.