Conformational properties of peroxyacids, peroxyesters and of structurally related radicals: a theoretical ab initio MO approach

Abstract

The geometrical features relative to the lowest conformational minimum, and absolute and relative energy contents of a number of peroxyacids RC(O)OOH (R = H, CH3, CF3, C6H5) and methyl esters (R = H, CH3, CF3) have been calculated at the ab initio MO level with the 6-31G* basis set. For the peroxyacids with R = H and CH3, the most stable conformation obtained from this calculation is of cis type (referring to internal rotation around the O-O bond) and agrees with that found experimentally. This conformation is the most stable one also for the other peroxyacids examined and for the peroxyesters. Poor agreement is, however, found between calculated and experimental geometrical parameters relative to the O-O and O-H bond lengths, and to the OOC and OOH bond angles. Comparison of calculated structural features of peroxyacids with those of the corresponding acids shows that stabilization of the planar cis conformation of the former compounds is due to intramolecular hydrogen bonding. This property influences the energy difference between cis and trans forms, ΔE, which, for the same R, is larger in the peroxyacid than in the peroxyester. The ΔE values increase approximately with the bulk of R. The energy content, conformational features and geometrical parameters of the more stable conformation have been analysed within the same theoretical approach for the radicals RC(O)O, RC(O)OO and C(O)OOR' (R' = H, CH3), formally derived from homolytic bond dissociation of the peroxyacids and peroxyesters examined. The σ(RC(O)O and C(O)OOR') and π (RC(O)OO) characters, atom spin densities and relative stabilities of the conformers of these radicals are discussed. © 1994.