Ultrafast decomposition of organic peroxides

The photochemical decomposition of large organic peroxides of general structure RC(O)OOR' starts with optical excitation into a repulsive electronic state. Within one vibrational period, scission of the weak O-O bond occurs and a carbonyloxy radical, RCO₂, and an alkoxy radical, R’O, are formed. Both intermediate species can undergo follow-up processes. E.g., the carbonyloxy radical dissociates into an aryl or alkyl radical R and carbon dioxide:

Large organic peroxides belong to a group of molecules of fundamental as well as high practical importance. They are widely used as initiators in free-radical polymerization. Investigations on initiator dissociation mechanisms to answer the question of whether CO₂ formation occurs promptly or with some time delay are of eminent importance. The kinetics of such processes determines the time evolution of different types of intermediate radicals, carbon-centered or oxygen-centered, which in turn strongly affects the initiator efficiency for free radical polymerization.

From different absorption spectroscopy experiments (UV-VIS, IR) with femtosecond time resolution we know the temporal evolution of the intermediates and the formed products. The experiments are carried out in the groups of Profs. Abel, Buback and Schroeder at IPC Göttingen.

The aim of our research is to quantitatively understand the experimental traces. A combination of quantum-chemical calculations, mainly at the DFT level of theory, and statistical unimolecular rate theory is employed for this purpose. The results are very encouraging and allow for a prediction of the decomposition dynamics of other peroxides.

A missing reaction step in dithiobenzoate-mediated RAFT polymerization,
M. Buback, O. Janssen, R. Oswald, S. Schmatz and P. Vana,
Macromol. Symp. 248, 158 (2007).

Decomposition of tertiary alkoxy radicals,

M. Buback, M. Kling and S. Schmatz,
Z. Phys. Chem. 219, 1205 (2005) [special issue in honor of Prof. Dr. E. U. Franck].

Photo-induced decomposition of organic peroxides: Ultrafast formation and decarboxylation of carbonyloxy radicals,
M. Buback, M. Kling, S. Schmatz and J. Schroeder,
Phys. Chem. Chem. Phys. 6, 5441 (2004) [invited article]. “Hot article”

Femtochemistry of organic peroxides: ultrafast formation and decarboyxlation of carbonyloxy radicals,
B. Abel, M. Buback, Ch. Grimm, M. Kling, S. Schmatz and J. Schroeder,
in: C. Hynes and M. Martin (Eds.) Ultrafast molecular events in chemistry and biology, Proceedings of the Femtochemistry VI, Elsevier, p. 287 (2004), ISBN 0444516565.

Ultrafast decarboxylation of carbonyloxy radicals: Influence of molecular structure,
B. Abel, J. Assmann, M. Buback, C. Grimm, M. Kling, S. Schmatz, J. Schroeder and T. Witte,
J. Phys. Chem. A 107, 3499 (2003).

A seemingly well understood light-induced peroxide decarboxylation reaction reinvestigated with femtosecond time resolution,
B. Abel, M. Buback, M. Kling, S. Schmatz and J. Schroeder,
J. Am. Chem. Soc. 125, 13274 (2003).

Decarboxylation of carbonyloxy radicals: A density functional study,
M. Kling and S. Schmatz,
Phys. Chem. Chem. Phys. 5 , 3891 (2003).

Experimental and theoretical investigations of the ultrafast photoinduced decomposition of organic peroxides in solution: Formation and decarboxylation of benzoyloxy radicals,
B. Abel, J. Assmann, P. Botschwina, M. Buback, M. Kling, R. Oswald, S. Schmatz, J. Schroeder and T. Witte,
J. Phys. Chem. A 107, 5157 (2003).

Ultrafast decarboxylation of organic peroxides in solution: Interplay of different experimental approaches and theoretical modelling,
B. Abel. J. Assmann, M. Buback, M. Kling, S. Schmatz and J. Schroeder,
Angew. Chem. 115, 311 (2003);
Angew. Chem. Int. Ed. 42, 299 (2003).