DENSITY-FUNCTIONAL INVESTIGATIONS OF CARBOXYL FREE-RADICALS - FORMYLOXYL, ACETYLOXYL, AND BENZOYLOXYL RADICALS

The structure of the lowest electronic states of HCOO. in C-2v and C-s symmetries were optimized employing density functional theory (DFT) m ethods with extended basis sets including up to f- (on C and O) and d- (on H) polarization functions. Generalized gradient functionals (BLYP ) and adiabatically connected functionals (B3LYP and B3PW91) were empl oyed for studying HCOO., as well as the isomer HOCO. (trans), the diss ociation limit H-.+ CO2, and the transition state for the decompositio n. At the best DFT levels employed, the ground state of HCOO. is (2)A( 1) (in C-2v) with equal C-O bond lengths, while the low-lying B-2(2) s tate is only about 4 kJ/mol above (without inclusion of zero-point ene rgies). The broken-symmetry (2)A' State (with unequal C-O bond lengths , i.e., C-s symmetry) is predicted to be about 13 kJ/mol above the (2) A(1) state and to be a transition state for the isomerization HCOO. (( 2)A(1)) --> HOCO. ((2)A'), with the trans-HOCO. isomer about 55 kJ/mol more stable. These facts agree closely with the most recent CASPT2/AN O calculations on this system. Therefore, it is concluded that some DF T models can be used safely for the study of larger radicals of the sa me type (despite several drawbacks discussed at length in this study). B3PW91, using several basis sets, is subsequently applied to the stud y of the possible reaction mechanisms of acetyloxyl radical, which exh ibits a much more complicated path than formyloxyl, due to the presenc e of the methyl group. The optimum structures of isomers with coplanar or perpendicular CH and CO bonds were obtained for CH3COO. and two sa ddle points identified on the path of decomposition into CH3. and CO2. On the other side, saddle points for isomerization into CH3OCO . and CH2COOH(.) were also located, and the decomposition of the former to C H3O.+ CO investigated. Finally, the structure of the benzoyloxyl radic al (C6H5COO.) and its possible decomposition products were investigate d along the same lines. (C) 1998 John Wiley & Sons, Inc.