¹⁹F contact interaction shifts: Effects of fluorine conjugation

19F contact interaction constants for a series of ortho-, meta- and para- substituted fluorobenzenes have been derived from isotropic hyperfine contact interaction shifts on appropriate N-substituted nickel (II) aminotroponeimineates. It is shown that the simple proportionality between proton contact interaction constants, ai , and pπ spin densities of adjacent carbon atoms, ρi , of aromatic Ċ-H fragments (ai = Qρi ) does not hold for aromatic Ċ-F fragments. Instead, there is an important ππ contribution to 19F contact interaction constants in fluoroaromatics which results from π-bonding between fluorine and the aromatic system. A self-consistent analysis of 19F contact interaction constants in fluoroaromatics is given in terms of C-F π-bond orders.