The relative signs of N.M.R. spin coupling constants from double irradiation experiments

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Abstract
The method of double irradiation in high resolution proton magnetic resonance has been used to confirm that the spin coupling constant between the methyl group and the proton in the cis position in both trans-crotonaldehyde and trans-crotonic acid differs in sign from the other coupling constants. Since this is a particularly searching test of the double irradiation method, it is suggested that relative signs of coupling constants could be determined in this way for most molecules containing three coupled hydrogen nuclei provided that the chemical shifts are large compared with the coupling constants.