RELATIVE SIGNS OF 3-BOND FLUORINE-CARBON AND 4-BOND FLUORINE-HYDROGENCOUPLINGS IN 2-SUBSTITUTED FLUOROBENZENES

In a series of 2-X-fluorobenzenes, the relative signs between (3)J(CF) and the corresponding (4)J(HF) couplings were determined, In most cas es like signs were found, However, for the coupling pathway involving the X group, unlike signs were found for X = CN, NCS and NO2. For othe r electronegative substituents, (4)J(H3F) couplings were found to have too small a magnitude to permit determination of their signs with res pect to (3)J(C3F), No adequate correlation between the algebraic value s of (3)J(C3F) couplings and the electronegativity of the substituent placed at ring position 2 was found, Some speculative arguments are ad vanced in order to rationalize the observed trend.