Ultrafast Decarboxylation of Carbonyloxy Radicals:  Influence of Molecular Structure

Abstract

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 pumpprobe 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.

Vinylidene fluoride telomers for piezoelectric devices

“…Figure 9 displays the D-E hysteresis loop of the Al/C 6 F 13 (VDF) 23 I/Al capacitor, and Table 8 summarizes the remanent polarization values (P r ), which were estimated from the D-E hysteresis loop of the samples. The results indicated that the P r values of all of the samples were similar (B100 mC m À2 ); however, P r values obtained in this study were lower than those observed in a previous study on CF 3 (VDF) 17 I (130 mC m À2 ). 6 The relatively low P r values observed in the current investigation may be due to the differences in the terminal chain length of R F (C n F 2n+1 ) groups, which are bulkier than CF 3 and do not contribute to polarization reversal.…”

“…In the initiation step, radicals are generated by the thermal decomposition [17][18][19] of a conventional initiator, such as bis(4-tert-butylcyclohexyl) peroxydicarbonate (step a). For a temperature below 90 1C (in this article), the carbonyloxy intermediate radical does not undergo any decarboxylation.…”

“…For a temperature below 90 1C (in this article), the carbonyloxy intermediate radical does not undergo any decarboxylation. [17][18][19] Next, the generated radicals react with the CTA to form R F 1+ Am-I (step b), which can undergo propagation, as shown in step b. The exchange of iodine from the transfer agent (R F I) to the propagating radical (P n 1) results in the formation of polymeric alkyl iodides with an oligo(VDF) chain (P n -I) and a new initiating radical (R1, step c).…”

“…Thus, a thin film can be prepared by vacuum evaporation, 6 which is an effective method for controlling the crystalline phases and the molecular orientation of VDF telomers. Among organic ferroelectrics, CF 3 (VDF) 17 I possesses a relatively high remanent polarization. 9 Furthermore, previous experiments on CF 3 (VDF) 13.4 C 2 H 5 revealed that the piezoelectric coefficient of VDF telomers was greater than that of oligo(VDF-co-TrFE) copolymers, and a thinner and more uniform film could be obtained.…”

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

“…75 In the case of the benzoyloxy radical, the search was less time-consuming because of the highly symmetric structure of the saddle-point geometry. However, C 2V symmetry was not used in the calculations, but C 1 symmetry was adopted to account for the following situation: According to the first Hohenberg-Kohn theorem, 76 the Hamiltonian (i.e., the external potential) is uniquely defined by the electron density of the ground electronic state.…”