Theoretical and Experimental Evidence for Unsymmetrical Bridging in the Cation Derived from 2-tert-Cumyl-2-adamantanol and 2-Isopropenyl-2-phenyladamantane in Magic Acid Solution

A previously published 400 MHz H-1 and 101 MHz C-13 NMR study on the ionization (to 8) of 2-tertcumyl-2-adamantanol (6) and 2-isopropenyl-2-phenyladamantane (7) in cold (-20 to -78 degrees C) Magic Acid/SO2ClF revealed spectroscopic features expected for a stable carbocation but did not allow for an unambiguous assignment of the exact structure for species 8. To clarify this situation, the present paper reports the deuterium isotope effects on the C-13 NMR spectrum of 8 formed by ionization of a 1:2 mixture of 2-tert-cumyl-2-adamantanol and its dimethyl-d(6) isotopomer. The differences between the chemical shifts of the protiated and deuteriated ions were found to be the same at -60 and -40 degrees C. In the deuteriated compound, the C+ center was shifted 1 ppm downfield, relative to the protium compound, whereas the quaternary C-13 atom was shifted 0.9 ppm upfield. This is strong evidence that 8 is a single phenyl-bridged ion and supports the assignment of the structure for 8 as a partially bridged 2-tert-cumyl-2-adamantyl cation. The experimental studies have been complemented with theoretical computations. We probed the potential energy surface (PES) of the C19H25+ ion in the region of the 2-tert-cumyl-2-adamantyl cation using geometry optimizations at the semiempirical AM1 and ab initio HF/STO-3G, HF/3-21G, and HF/6-31G(d) levels of theory. At the highest level of Hartree-Fock (HF) theory used, the HF/6-31G* level, the C-s 2-tert-cumyl-2-adamantyl cation (9) which is stabilized by the interaction of the C+ center with the phenyl p system is the predicted structure for 8. Phenyl stabilization is indicated by the phenylC-C+ bond angle of 92.63 degrees. Correlation effects on the relative energies for the C19H25+ ions were estimated through MP2/6-31G(d) single-point computations. The MP2 correlation-corrected energies again indicated that 8 is a partially bridged 9; however, the minimum in the PES profile for phenyl bridging is shifted to 75 degrees, a considerable increase in bridging and phenonium ion character. In contrast to the HF prediction and as seen in the previous paper with the C12H17+ ion, MP2/6-31G* calculations predict that there is a minimum rather than a maximum along the phenyl-bridging coordinate. This minimum at 75 degrees is about 10 degrees larger than the minimum found for the C12H17+ system and indicates the expected influence of the greater positive charge stabilization at the 2-adamantyl position versus the 2-propyl position. We have calibrated the theoretical method used by doing similar calculations on the C12H17+ homologues that occupy the 2,3-dimethyl-3-phenyl-butyl cation section of the PES. Our theoretical study of this cation shows how the MP3- and the MP4(SDQ)/6-31G(d) levels would affect the predicted relative energies of the C19H25+ ions and the structure of 8. Use of the numbers obtained from the latter study gave estimated MP3/6-31G* and MP4(SDQ)/6-31G* PESs which still showed the partially bridged 9 to be the predicted structure for species 8 but with larger phenyl-bridging angles of 80 and 82 degrees, respectively. We showed previously that solvent effects or theoretical error might cause the benzyl cation isomer to be stabilized by as much as 7 kcal/mol. However, even if the MP4(SDQ)/6-31G(d) energies are corrected so that the benzyl cation isomer of 9 is stabilized by an additional 7 kcal/mol, the partially phenyl-bridged 2-tert-cumyl-2-adamantyl cation is 3.3 kcal/mol more stable than it.