Decarboxylation of carbonyloxy radicals: a density functional study

We present a thorough theoretical investigation on the decarboxylation process of carbonyloxy radicals of general structure RC(O)O which play a key role in the decomposition of organic peroxides that are widely used as initiators in free-radical polymerizations. The effect of variation in the molecular structure (R = PhO, PhCH2 and NphO) (Nph = naphthyl) is studied systematically. Geometries, energies, and vibrational frequencies of the carbonyloxy radicals, the dissociation products, and the first-order saddle points pertinent to decarboxylation have been calculated employing density functional theory. The results are compared with recent data for the benzoyloxy radical (R = Ph). Insertion of an oxygen atom or a CH2 group between the chromophore and the CO2 moiety yields a significant reduction in the stability of the carbonyloxy radicals. Despite the structural similarity of the three compounds, the reaction coordinates are markedly different. The structural effects are discussed in terms of changes in thermochemistry, barrier heights and transition modes. The calculated data serve as essential input parameters for statistical unimolecular rate theory calculations and are crucial for the modeling of recent femtosecond pump/probe spectroscopy experiments on the carbonyloxy radical decomposition mechanism.