On the NMR spectra of pentafluorobenzenes

Abstract

The AA′BB′C NMR spin systems of the pentafluorobenzene derivatives with the substituents -Cl, -Br, -I, -OH, ⊖-0, -NH 2, -NH 3 and -CN have been carefully recorded and analyzed completely using in every case iterative fitting to the experimental transitions. Relative to the vicinal fluorine-fluorine NMR coupling as negative, it was possible to establish the signs of all the couplings. Contrary to some previous investigations, most of the relevant coupling constant sign information for these spin systems can be extracted from high quality spectra via close scrutiny of the line positions and relative line intensities. For the most weakly coupled systems, double resonance techniques were also employed to establish the correct spectral assignments. The coupling constant sign results provide further verification of the predictions of Abraham's recent treatment of the substituent effects on the 4 J FF and 5 J FF couplings in fluorine-substituted aromatic molecules. The coupling parameters, 3 J FF, 4 J FF and 5 J FF, in the pentafluorobenzenes are compared with similar couplings in fluoro-olefins and fluoro-1,3-diene systems as regards the important mechanisms of fluorine-fluorine couplings.