Relative signs of three-bond fluorine-carbon and four-bond fluorine-hydrogen couplings in 2-substituted fluorobenzenes

Abstract

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