Theoretical study of the peroxy radicals RO2 self-reaction: Structures and stabilization energies of the intermediate RO4R for various R

Abstract

We have determined the lowest-energy structures of the RO2[RCH3, CH3CH2, CH3CH2CH2, CH3CH2CH2CH2, (CH3)2CH, (CH3)3C, (CH3)(C2H5)CH, (CH3)2CHCH2, CH2CH, CH3CHCH, and CH2CHCH2] peroxy radicals. Further, the self-reaction of these peroxyls may produce RO4R tetroxide adducts, for which the influence of the nature of R on their structure and stability was examined. These studies were done using theoretical calculations, of the all-electron type, performed at the MP2, DFT, and HF–DFT levels of theory. All calculations (optimization, frequencies, and total energies) were done using 6-31G** basis sets for each method. In all cases, we used HF frequencies on the MP2 geometry to compute Gibbs free energy from MP2 energy. DFT calculations were done using the BLYP functional, while for HF–DFT, the B3LYP scheme was used. Our main findings for RO2 are the following: The three methods provide rather similar distances and angles. The OO distance does not change significantly with increasing the size of R and with the branching, but it is more sensitive when a double bond is immediately neighboring. The CO distance is more sensitive to the kind of R. Concerning the RaO4Rb adducts, they were located as local minima, that is, they behave as intermediates. It was found that there is a dissymmetry between the structural parameters of the peroxy “a” and those of “b.” The OO distances of peroxy increase by more than 0.1 Å when going from the isolated peroxy to the intermediate. Similarly, the CO distances show a decrease of about 0.02–0.03 Å. In the searched RO4R intermediates, when R changes, the evolution of peroxy parameters is similar than that for the isolated ones. As for energetic aspects, we show the variation of the stabilization energy of RO4R compounds with the kind of R. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 671–682, 1999