MINDO/3 Study of 7-Norbornyl, 7-Norbornenyl, and 7-Norbornadienyl Cations

The geometries, charge distributions, and heats of formation for 7-norbornyl, 7-norbornenyl, and 7-norbornadienyl cations have been calculated by the use of the semi-empirical MINDO/3 SCF MO method. Each of these cations is symmetrical with respect to the plane which contains the C(7)–H(7) bond and bisects the C(1)–C(7)–C(4) angle. Both the 7-norbornenyl and the 7-norbornadienyl cations take the nonclassical bridge-structure: the C(2)–C(3) and C(2)–C(7) distances are about 1.43 and 1.67 Å respectively, and the C(2)–C(1)–C(4)–C(7) dihedral angle is about 75°. The classical 7-norbornyl cation also is quite unsymmetrical with regard to the C(1)–C(7)–C(4) plane: the C(2)–C(1)–C(4)–C(7) and C(7)–C(1)–C(4)–C(5) dihedral angles are 94° and 144°, respectively. The calculated barriers to the bridge flipping in the 7-norbornyl and 7-norbornadienyl cations are 3.2 and 33.0 kcal/mol respectively. Although the latter is much higher than its observed value of 19.6 kcal/mol, it seems possible that solvation considerably lowers the barrier height. The present article also touches upon the solvolyses of derivatives of these carbocations.