ROTATIONAL-DYNAMICS OF STABLE CARBOCATIONS IN SUPERACIDS - INVESTIGATION OF CARBOCATION MEDIUM INTERACTIONS

The correlation times for rotational reorientation have been determined from the 13C dipole-dipole spin-lattice relaxation rates of 1-adamantyl (1), 2-methyl-2-norbornyl (2), 3-methyl-3-nortricyclyl (3), 2-phenyl-2-norbornyl (4), 1-methyl-1-phenylethyl (5), and 1-cyclopropyl-1-phenylethyl (6) cations in appropriate superacids. Measurement of the viscosity and temperature dependencies (η/T) of the rotational correlation times (τ) of 1-5 have provided the data necessary for the calculation of their effective volumes by use of hydrodynamic relationships. For cations 2-5 these volumes were consistent with charge-induced interactions between the cations and the supporting media. However, rotational reorientation of the 1-adamantyl cation (1) does not conform to classical hydrodynamic theory despite a linear dependence of the correlation times upon η/T. The anisotropy of rotational reorientation as measured by the ratio of the diffusion coefficients (σ = Dt/D) was consistent with restriction of rotation of cations 2 and 3 by interaction of the media, FSO3H/SO2CIF and SbF5/FSO3H/SO2CIF, respectively, at the cationic site. For the phenyl-substituted cations 4-6, the interactions were not localized to specific sites. Application of two criteria, volumes calculated from hydrodynamic relationships and anisotropy of rotational reorientation of cations, showed that rotation of these cations 1-6 is restricted in superacids by electrostatic interactions with the supporting medium, but the cations do not rotate as ion pairs. © 1990, American Chemical Society. All rights reserved.