Ultrafast decarboxylation of carbonyloxy radicals: Influence of molecular structure

Experimental and theoretical investigations on the ultrafast photoinduced decomposition of three tert-butyl peroxides of general structure R-C(O)O-O-tert-butyl with R = phenyloxy, benzyl, or naphthyloxy in solution are presented. Photoinduced O-O bond scission occurs within the time resolution (200 fs) of the pump-probe experiment. The subsequent dissociation of photochemically excited carbonyloxy radicals, R-CO2, has been monitored on a picosecond time scale by transient absorption at wavelengths between 290 and 1000 nm. The measured decay of R-CO2 is simulated via statistical unimolecular rate theory using molecular energies, geometries, and vibrational frequencies obtained from density functional theory (DFT) calculations. The results are compared with recent data for tert-butyl peroxybenzoate (R = phenyl). While benzoyloxy radicals exhibit nanosecond to microsecond lifetimes at ambient temperature, insertion of an oxygen atom or a methylene group between the phenyl or naphthyl chromophore and the CO2 moiety significantly decreases the stability and thus lowers the lifetime of the carbonyloxy radicals in solution to picoseconds. The reasons behind this structural effect on decomposition rate are discussed in terms of barrier heights for decarboxylation on the ground-state potential energy surface and of a fast reaction channel via electronically excited states of carbonyloxy radicals. Arrhenius parameters are reported for thermal rate constants, k(T), of R-CO2 decarboxylation as deduced from modeling of the time-resolved experimental data in conjunction with the DFT calculations.