Signs of two- and three-bond coupling constants in pentafluorocyclopropanes and their possible origins

Abstract

The signs and magnitudes of 2 J FF and 3 J FF in pentafluorocyclopropane and its chloro-, bromo-, and iodo-derivatives, obtained by double resonance, are discussed in terms of the theoretical Fermi contact, orbital and spin dipolar contributions to J FF. Semi-empirical correlations of 2 J FF and 3 J FF with the electric field term < E 2> provide some rationalization for the signs of the coupling constants within the CF 2CF 2 fragment, where the different types of < E 2> dependence suggest that while cis- 3 J FF is dominated by the orbital term, the variation in trans- 3 J FF is attributable to changes in the spin dipolar term. The orbital term also appears to dominate 3 J FF within the CF 2CF fragment in that it is the most substituent sensitive and it possibly provides the major contribution to the coupling mechanism.