THEORETICAL INVESTIGATION OF THE ROTATIONAL BARRIER IN ALLYL AND 1,1,3,3-TETRAMETHYLALLYL IONS

Optimized equilibrium geometries and rotational transition structures for allyl and methyl-substituted allyl ions are obtained by using Hartree-Fock (HF) and second-order Moller-Plesset perturbation (MP2) theory. For the parent allyl cation, gas-phase stationary points are also found using the quadratic configuration interaction (QCISD) method. At the levels of theory beyond HF, the lowest energy path for exchange of two hydrogens on one terminal carbon in allyl cation is predicted to involve migration of the central hydrogen to the terminal carbon, whereas the anion transforms via the usual perpendicular transition structure. Allyl cation energies are also computed using the highly accurate CCSD(T) and QCISD(T) methods at the MP2 stationary points. Methyl substitution at the terminal carbon which is involved in the rotation stabilizes the perpendicular structure for the cation, and all levels of theory predict the usual transition structure. Finally, solvation effects on the rotational barrier are investigated using an Onsager reaction field model. The gas-phase rotational barriers of 19.4 and 8.6 kcal/mol calculated for the tetramethylallyl cation and anion, respectively, are reduced to 17.8 and 7.0 in a medium of dielectric constant 78.5.