ARTICLES Ultrafast Decarboxylation of Carbonyloxy Radicals: Influence of Molecular Structure Bernd Abel, Jens Assmann, Michael Buback, Christian Grimm, Matthias Kling,* Stefan Schmatz, Jo 1 rg Schroeder, and Thomas Witte Institut fu ¨r Physikalische Chemie, UniVersita ¨t Go ¨ttingen, Tammannstrasse 6, 37077 Go ¨ttingen, Germany ReceiVed: April 22, 2003; In Final Form: August 27, 2003 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-CO 2 , has been monitored on a picosecond time scale by transient absorption at wavelengths between 290 and 1000 nm. The measured decay of R-CO 2 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 CO 2 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-CO 2 decarboxylation as deduced from modeling of the time-resolved experimental data in conjunction with the DFT calculations. I. Introduction Organic peroxides belong to a group of compounds of fundamental as well as of application-oriented interest due to their extensive use as initiators in free-radical polymerizations. 1-4 The time scale of peroxide decomposition in different solvents and the dynamics of active intermediates affect both initiation rate and efficiency. 4,5 To optimize the initiation efficiency, a thorough understanding of the influence of peroxide structure on fragmentation dynamics and kinetics is required. A large body of literature has accumulated on the thermal decomposition of organic peroxides, 2,4 among which tert-butyl peroxides and peroxyesters have been investigated in considerable detail. The mechanism of peroxide decarboxylation, that is, whether bond breaking in peroxyesters, diacyl peroxides, or peroxycar- bonates occurs in a stepwise (sequential) or a concerted fashion, is of particular interest. This aspect has been addressed by several groups. 6-12 The question about the mechanism is obviously related to the time scale of observation. On the basis of thermal experiments, concerted bond breaking and stepwise dissociation via intermediate radicals on a picosecond or nanosecond time scale cannot be safely distinguished. Therefore, highly time-resolved (subpicosecond) studies are required to understand the decarboxylation mechanism in more detail. 13,14 Studies on photochemical decomposition of organic peroxides by means of microsecond and nanosecond flash photolysis in conjunction with visible absorption or electron paramagnetic resonance (EPR) spectroscopy 15-22 have revealed that interme- diate aroyloxy radicals are involved in the decomposition of diaroyl peroxides. tert-Butyl peroxides such as tert-butyl per- oxybenzoate and tert-butyl peroxy-4-methoxy-benzoate were found to release the same reactive intermediates as the corre- sponding diaroyl peroxides, that is, benzoyloxy and 4-methoxy- benzoyloxy radicals, respectively. 15,22 Investigations on substi- tuted aroyloxy radicals revealed nanosecond to microsecond lifetimes, 17 in keeping with the observation of aroyloxy end groups in polymers produced in reactions with dibenzoyl peroxide being applied as initiator. 23 However, the reason behind the significantly different photodissociation quantum yields of aroyloxy radicals from aromatic tert-butyl peroxyesters and diaroyl peroxides 15,22 remained unclear. Intermediate radicals from light-induced decomposition of tert-butyl peroxyesters were also identified in experiments with nanosecond to picosecond time resolution. 15,22,24-26 Kochi and co-workers studied the decarboxylation of carbonyloxy radicals after photogeneration via electron donor-acceptor complexes by femtosecond visible spectroscopy. 27,28 However, insufficient time resolution or spectral overlap prevented a detailed insight into the dissociation mechanism in prior studies. In particular, the dependence of the dynamics on molecular structure, internal * Corresponding author. Present address: Department of Chemistry, University of California, Berkeley, D90 Hildebrand Hall, Berkeley, California 94720. E-mail: mkling@uclink.berkeley.edu. 9499 J. Phys. Chem. A 2003, 107, 9499-9510 10.1021/jp0350823 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/22/2003