Theoretical study of the cyclopropenyl radical

Abstract

Comprehensive ab initio (MCSCF) and (CI) calculations are performed to determine properties of the cyclopropenyl radical, which has not yet been directly observed experimentally. Jahn-Teller distortion is responsible for a dramatic change from the high-symmetry D/sub 3h/ ..pi.. radical parent structure to a low-symmetry nonplanar C/sub s/ sigma radical equilibrium structure. The equilibrium structure is found to be an ethylenic form that can exist in three equivalent conformations. These can interconvert (via a pseudorotation of the carbon-carbon double bond around the ring) by passing through a nonplanar allylic transition state. Planar geometries turn out to be of such high energy as to play no significant role in the dynamics of this system. Large-scale CI calculations, with corrections for differences in zero-point vibrational energies, indicate the pseudorotation barrier height to be about 3-4 kcal/mol. Qualitative results are obtained for the energy and geometry changes that occur along the interconversion path. A number of one-electron properties are also reported for the equilibrium form. Notable among these are the spin-density predictions, which should be of use in experimental identification of the cyclopropenyl radical by ESR spectroscopy. A qualitative survey of the energies and geometries of other C/sub 3/H/sub 3/ isomers is also presented.