RELOCALIZATION IN FLOPPY FREE-RADICALS - AB-INITIO CALCULATIONS OF THE C3H3O ISOMERS

Several isomers with molecular formula C3H3O may exhibit unusual prope rties by virtue of a configurational isomerization pathway that change s the localization of an unpaired electron. A series of ab initio calc ulations was undertaken at the UHF, B3LYP, MCSCF, and QCISD levels wit h the 6-311G(d,p) basis set to ascertain the geometries, relative ener gies, isomerization barriers: and relevant spectroscopic properties of the lowest energy structural and configurational C3H3O isomers. The m ost stable structures, with QCISD relative energies (kJ mol(-1)) of th e stable configurations given in parentheses, are H2C2HCO (0, 8, 8), H 3C3O (47), H2C3HO (87, 100), and HC2HCHO (94, 95, 101, 107). The H2C2H CO and H2C3HO structures are found to have distinct isomers connected by relocalization pathways. The H3C3O structure possesses two favorabl e canonical geometries, but the single optimized structure is an avera ge of these. Potential surfaces have been calculated along two vibrati onal bending coordinates for H2C2HCO and H2C3HO and along a single ben ding coordinate for H3C3O Extremely large amplitude bending motions ar e predicted for these three lowest energy structures, and at typical c ombustion temperatures it is likely that these molecules exist effecti vely as admixtures of two or more canonical structures.