Nuclear Magnetic Resonance Spectroscopy. Conformations and Conformational Equilibration of Some Cyclooctane Derivatives

A study of the temperature dependence of the ^(19)F nuclear magnetic resonance spectra of 1,1-difluorocyclooctane and 1,1,4,4-tetrafluorocyclooctane has shown that these substances, at low temperatures, prefer to assume the boat-chair conformation. At higher temperatures, pseudorotation within the boat-chair series of conformations becomes rapid and, at still higher temperatures, equilibrations leading to ring inversion become rapid on the nmr time scale. 1,1,2,2-Tetrafluorocyclooctane, at low temperatures, appears to prefer the twist-boat (S_4) conformation and, at higher temperatures, pseudorotation through the boat-boat conformations becomes rapid. At still higher temperatures, ring inversion becomes rapid on the nmr time scale. At low temperatures, perfluorocyclooctane seems to prefer (i) the twist-boat or (ii) the boat-boat conformation. If i, then even at - 173º, pseudorotation through the boat-boat conformation must be rapid. At room temperature, what amounts to ring inversion of perfluorocyclooctane is rapid, apparently by way of equilibration with a conformation(s) outside the boat series. Activation parameters have been obtained for several of these pseudorotation and inversion processes.