| Popis: |
Low-temperature fluorine magnetic resonance spectroscopy has been used to investigate the conformational equilibration of four cyclic gem-fluorides. Ring inversion in 1,1-difluorocyclodecane was stopped on the nmr time scale by -164°, and an activation energy of 7.0 kcal/mole was determined for this process. The spectra were interpreted in terms of the boat-chair-boat conformation. Changes in the spectra of 3,3-difluoro-trans-cyclodecene occurred continuously over a wide range of temperature, from above ambient temperature to about -140°. The free energy of activation for the first process, which is nearly stopped on the nmr time scale by -30°, was 12.4 kcal/mole at 2.8°. Activation parameters were not calculated for the changes which took place at lower temperatures. At -148°, two conformations were present in a ratio of 85:15, and other conformations may be present in smaller amounts. The activation energy for inversion of γ,γ-difluoro-ε-caprolactone was 12.4 kcal/mole, and the free energy of activation for this process was 10.0 kcal/mole at -53°. The spectra indicate that the compound exists in the chair conformation, and the activation parameters provide information about the barrier to cis-trans isomerization of esters. The free energy of activation for γ,γ difluoro-ε-caprolactam was 10.4 kcal/mole at -53°. The lactam also prefers the chair conformation. |
