Nuclear magnetic resonance spectroscopy. Kinetics of isomerization of p-substituted hexafluoroacetone N-phenylimines

The kinetics of the degenerate isomerization about the carbon-nitrogen double bond of hexafluoroacetone N-phenylimine and its p-chloro, p-fluoro, p-methoxy, p-methyl, and p-nitro derivatives have been studied by line-shape analysis of nmr spectra at a series of temperatures. The free energies of activation at 25º were found to be 15.45, 15.53, 15.35, 14.35, 15.14, and 14.90 kcal mol^(-1), respectively. With the exception of the p-nitro compound, the rate constants at 25º give a Hammett plot with a linear correlation coefficient of 0.989 when σ^+ substituent constants are used; the p value is -0.980. The results are interpreted as being the consequence of an inplane inversion about the imine nitrogen of the p-nitro compound and a rotation about the imine carbon-nitrogen bond of the others.