C-13-(H-1) DOUBLE-RESONANCE STUDY OF THE SIGNS OF H-1-F-19 AND C-13-F-19 SPIN COUPLING-CONSTANTS IN FLUOROBENZENES AND 2-FLUOROPYRIDINE

The signs of all 13C19F and 1H–19F coupling constants in fluorobenzene, some substituted derivatives, and in 2‐fluoropyridine have been related using single‐frequency 13C{1H} double resonance techniques. All 13C19F couplings in these compounds are shown to be positive. Direct experimental proof is given for a positive sign for the small five‐bond 1H19F coupling constant (=0.2—0.3 Hz) in fluorobenzene. The positive sign for 4J(C‐4, F) in fluorobenzene is in accord with a positive π‐electron contribution to this coupling. Copyright © 1979 Heyden & Son Ltd.