Signed values of all intra-ring (2,3,4) J(C,C) couplings in nine monosubstituted benzenes (C6H5-X where X = F, Cl, Br, CH3, OCH3, Si(CH3)3, C≡N, NO, NO2) are experimentally determined as well as nine couplings to substituent carbons. It is confirmed that while all the vicinal intra-ring (3)J(C,C) are positive and all geminal (2)J(C2,C4) are negative, both signs are found for geminal (2)J(C1,C3) couplings. All the determined signs agree with those already predicted by theoretical calculations.

Institute of Chemical Process Fundamentals of the ASCR v v i Prague 6 Czech Republic

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