Relocalization in Floppy Free Radicals: Ab Initio Calculations of the C3H3O Isomers

Several isomers with molecular formula C 3 H 3 O may exhibit unusual properties by virtue of a configurational isomerization pathway that changes the localization of an unpaired electron. A series of ab initio calculations was undertaken at the UHF, B3LYP, MCSCF, and QCISD levels with the 6-311G(d,p) basis set to ascertain the geometries, relative energies, isomerization barriers, and relevant spectroscopic properties of the lowest energy structural and configurational C 3 H 3 O isomers. The most stable structures, with QCISD relative energies (kJ mol -l ) of the stable configurations given in parentheses, are H 2 C 2 HCO (0, 8, 8), H 3 C 3 O (47), H 2 C 3 HO (87, 100), and HC 2 HCHO (94, 95, 101, 107). The H 2 C 2 HCO and H 2 C 3 HO structures are found to have distinct isomers connected by relocalization pathways. The H 3 C 3 O structure possesses two favorable canonical geometries, but the single optimized structure is an average of these. Potential surfaces have been calculated along two vibrational bending coordinates for H 2 C 2 HCO and H 2 C 3 HO and along a single bending coordinate for H 3 C 3 O. Extremely large amplitude bending motions are predicted for these three lowest energy structures, and at typical combustion temperatures it is likely that these molecules exist effectively as admixtures of two or more canonical structures.