On the question of a planar tetravalent carbon centre in the ethylene- and acetylene-benzenium ion - a semiempirical SCF–LCAO-study

Optimized equilibrium geometries of the acetylene- and ethylene-benzenium ion are computed by the semiempirical MINDO/2 method for a tetrahedral [(1) and (2)] and planar arrangement [(4) and (5)] of the spiro atom; comparison of the tetrahedral conformations with their planar counterparts reveals an energy difference of 22.3 kcal mol–1 for (1)vs.(4) and 15.5 kcal mol–1 for (2)vs.(5).