Experimental and Theoretical Investigations of the Ultrafast Photoinduced Decomposition of Organic Peroxides in Solution:  Formation and Decarboxylation of Benzoyloxy Radicals

Abstract

The light-induced (266 nm) ultrafast decarboxylation of two peroxides R 1 -C(O)O-OR 2 , with R 1 ) phenyl and R 2 ) benzoyl or tert-butyl, in solution has been studied on the picosecond time scale by absorption spectroscopy with a time resolution typically of 100 to 200 fs. The reaction was investigated in various solvents of different polarity and viscosity to elucidate the influence of the solvent environment on the decarboxylation rate. Transient intermediate benzoyloxy radicals, R 1 -CO 2 , were monitored at wavelengths between 300 and 1000 nm. While the primary dissociation of the peroxide is too fast to be resolved, the dissociation of intermediate benzoyloxy radicals is clearly detected on the picosecond time scale. The mechanism of light-induced two-step dissociation is discussed, as is the dependence of reaction dynamics on the type of substituent R 2 as well as the branching ratio between prompt and delayed CO 2 formation. A model for the decarboxylation process is presented that is based on molecular structure parameters and energies. The latter quantities, which are obtained from density functional theory calculations, serve as input data for calculations of the specific decomposition rate coefficients of benzoyloxy intermediates via statistical unimolecular rate theory. The predicted benzoyloxy radical decay data are compared with corresponding experimental concentration versus time traces.

Watching Photoinduced Chemistry and Molecular Energy Flow in Solution in Real Time

“…[147] This was also proved in our own calculations for the significantly larger benzoyloxyl radical, which is formed in the decay of dibenzoyl peroxide, Ph-C(O)O-O(O)C-Ph. [137] We therefore assume that DFT calculations also give results with reasonable accuracy for the decomposition reactions of peroxides highlighted in this article. The microcanonical (statistical) specific rate constant for a molecule reacting over a distinct barrier to the products is defined in Equation (9) [146] (W # (EÀE 0 ) = number of energetically accessible transition state energy levels, 1(E) = density of states at energy E).…”

“…[136,137] Briefly, in the present experiments the pump wavelength was tuned between 250 and 300 nm to a broad electronic absorption of the S 1 ! S 0 transition of the peroxides and the probe wavelength was varied between 290 and 1000 nm.…”

“…[136] Transient spectra between 290 and 1000 nm, which mapped out dominant spectral changes caused by solvation and VET, helped us to identify these spectral windows. [137] Transient absorption traces after the decomposition of di(4-methoxybenzoyl)peroxide (DMBP) in the wavelengths region 290-1000 nm after excitation at 266 nm in propylene carbonate are shown in Figure 17. In contrast to the case with DNPO, we found large variations in the kinetic traces, and evidently the wavelength dependence of the transients is more complicated.…”

Ultrafast Decarboxylation of Carbonyloxy Radicals: Influence of Molecular Structure

“…In particular, the dependence of the dynamics on molecular structure, internal energy distribution, and solvent environment is not adequately understood so far. 4,5 In recent studies with picosecond to femtosecond time resolution, we directly observed the formation and decarboxylation of intermediate aroyloxy radicals 13,29 and the formation of aryl radicals and CO 2 upon photoinduced decomposition of tert-butyl peroxyesters and diaroyl peroxides. 5,30,31 The results of these experiments clearly show that peroxide photofragmentation occurs as a fast sequential process, in which the primary step, that is, the breaking of the O-O single bond, takes place on a subpicosecond time scale.…”

“…The formation of intermediate radicals and their decarboxylation is monitored with femtosecond time resolution. The decarboxylation of carbonyloxy radicals will be analyzed with a model that has already been applied for the quantitative description of decarboxylation of benzoyloxy and naphthoyloxy radicals from photolysis of TBPB 13 and of di(1-naphthoyl) peroxide, 29 respectively. The application of experimental and theoretical methods in conjunction with the kinetic modeling of highly time-resolved concentration versus time profiles is the key toward the understanding of reaction mechanisms and time scales.…”