CONJUGATED PI-ELECTRON SYSTEMS IN REACTIVE MOLECULES - MULTIPLE MINIMA ON (2)A POTENTIAL SURFACES OF CARBON-CHAIN FREE-RADICALS

Favorable, nonequivalent canonical structures for a given free radical may correspond to local minima on the ab initio potential surface of the ground electronic state, The isomerization path between two such s tructures involves the relocation of an unpaired electron and coincide nt transformation of the donor and acceptor molecular orbital hybridiz ations. In ab initio calculations, this may cause convergence to a loc al minimum. In experiments, the relatively flat bending potential will complicate the vibrational energy level structure. Self-consistent fi eld (SCF) and configuration interaction (CISD) calculations were emplo yed in a search for multiple minima on the potential energy surfaces o f seven X(2)A' free radicals: NC2O, FC3O, ClC3O, HC3NH, HC4H2, HC2N2, and FC4H2. Minima were assumed to lie near the acetylenic, cumulenic, or allylic canonical forms. All of the three- and four-carbon compound s in this set exhibit at least two minima on the SCF potential surface . At the CISD level, two stable structures were identified for ClC3O a nd HC4H2, and three were found for FC4H2 In all cases, bending amplitu des of more than 40 degrees are classically allowed at energies less t han 3000 cm(-1) above the global minimum potential energy. Ramificatio ns of these potential surfaces are discussed, and a crude energy addit ivity comparison is presented. Predicted stationary point geometries, permanent dipole moments (mu), Fermi contact terms (alpha), and isomer ization barriers are given.