A ¹³C{¹H} double resonance study of the signs of ¹H¹⁹F and ¹³C¹⁹F spin coupling constants in fluorobenzenes and 2‐fluoropyridine

The signs of all 13 C 19 F and 1 H– 19 F coupling constants in fluorobenzene, some substituted derivatives, and in 2‐fluoropyridine have been related using single‐frequency 13 C{ 1 H} double resonance techniques. All 13 C 19 F couplings in these compounds are shown to be positive. Direct experimental proof is given for a positive sign for the small five‐bond 1 H 19 F coupling constant (=0.2—0.3 Hz) in fluorobenzene. The positive sign for 4 J (C‐4, F) in fluorobenzene is in accord with a positive π‐electron contribution to this coupling.