GAS-PHASE STRUCTURE OF THE BICYCLO[2.2.1]HEPTANE (NORBORNANE) CATION-RADICAL - A COMBINED AB-INITIO MO AND DENSITY-FUNCTIONAL STUDY

Unrestricted open-shell Hartree-Fock (HF), Moller-Plesset (MP), and density functional theory (DFT) MO methods, using the 6-31G(d) and 6-31G(d,p) basis sets, were used to explore the potential energy surface for the norbornane cation radical. Both MP2 and DFT theory predict that the lowest energy structure of the isolated norbornane cation radical is 1(C-2v), possessing C-2v symmetry. At the MP2 level, two additional minimum energy structures were located, one having C,symmetry and the other C-1 symmetry namely 2(C-s) and 4(C-1). However only one structure, 1(C-2v), was located using either local or nonlocal DFT methods. The isotropic proton hyperfine coupling constants (hfc's) for 2(C-s) are consistent with the experimental hfc values observed by Okazaki and Toriyama for the norbornane cation radical in frozen halocarbon matrices. There is no experimental evidence to support the existence of structure 3(C-1). The potential energy surfaces of the methane and ethane cation radicals were explored using the DFT methods. From the study of CH4.+ and C2H6.+ it is concluded that nonlocal DFT methods give geometries and isotropic proton hfc's in general agreement with the MP2 calculations. However, all DFT theoretical models used incorrectly predict the relative energies of the stationary points on the CH4.+ potential energy surface. Thus, all DFT models predict that the global energy minimum of CH4.+ is a structure which possesses D-2d symmetry, whereas both experiment and high-level ab initio calculations show the global minimum possesses C-2v symmetry.