Density functional investigations of carboxyl free radicals: Formyloxyl, acetyloxyl, and benzoyloxyl radicals

摘要

The structure of the lowest electronic states of HCOO. in C-2v and C-s symmetries were optimized employing density functional theory (DFT) methods 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 employed for studying HCOO., as well as the isomer HOCO. (trans), the dissociation limit H-.+ CO2, and the transition state for the decomposition. 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) state is only about 4 kJ/mol above (without inclusion of zero-point energies). 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/ANO calculations on this system. Therefore, it is concluded that some DFT models can be used safely for the study of larger radicals of the same type (despite several drawbacks discussed at length in this study). B3PW91, using several basis sets, is subsequently applied to the study of the possible reaction mechanisms of acetyloxyl radical, which exhibits a much more complicated path than formyloxyl, due to the presence of the methyl group. The optimum structures of isomers with coplanar or perpendicular CH and CO bonds were obtained for CH3COO. and two saddle 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 CH3O.+ CO investigated. Finally, the structure of the benzoyloxyl radical (C6H5COO.) and its possible decomposition products were investigated along the same lines. (C) 1998 John Wiley & Sons, Inc. [References: 44]