Picosecond IR Study of UV-Induced Peroxide Decomposition: Formation and Vibrational Relaxation of CO2 in CH2Cl2 Solution

Abstract

Picosecond UV pump (248.5 nm)/IR probe spectroscopy has been applied to the study of the decomposition of several aromatic diacyl peroxides, peroxycarbonates, and of a peroxyester dissolved in CH 2 Cl 2 . Measuring the IR transient absorbance in the 2100 to 2450 cm −1 range allows to monitor the formation and vibrational relaxation of the photoproduct CO 2 via the asymmetric stretching mode (ν 3 ) with a time resolution of 1.8 ps. With each of the six peroxides a CO 2 molecule is released at delay times below 10 ps. The energy relaxation of the initially formed vibrationally hot CO 2 is followed over the time range up to 500 ps. Analysis of the transient IR spectra, via an anharmonic oscillator model proposed by Hamm et al. [1], shows a monoexponential decay of internal energy. Irrespective of the type of peroxide a single relaxation time of 67 ± 5 ps is found to adequately represent the cooling behavior of CO 2 in liquid CH 2 Cl 2 . The "initial" temperatures of vibrationally hot CO 2 at a delay time of 20 ps after applying the pump pulse differ considerably, between 1400 and 2700 K for the decompositions of tert-butyl benzoyl peroxide and tert-butyl benzoyl carbonate, respectively. These remarkably high temperatures are assumed to originate from energy release associated with structural relaxation of the bent OCO moiety to form the linear CO 2 molecule.

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

“…[125][126][127] From this point of view it appears important to investigate the mechanism of initiator dissociation, which releases CO 2 either in a prompt or delayed manner, and can thus be correlated with different time-dependent concentrations of carbon-or oxygen-centered free radicals. [11,128] A large body of literature is focused on the thermal decomposition of peroxides. [11,126,128] Until recently, the photochemical decomposition of peroxides has been investigated almost exclusively in the nano-and microsecond time domain by flash photolysis and visible absorption or EPR spectroscopy.…”

“…[11,128] A large body of literature is focused on the thermal decomposition of peroxides. [11,126,128] Until recently, the photochemical decomposition of peroxides has been investigated almost exclusively in the nano-and microsecond time domain by flash photolysis and visible absorption or EPR spectroscopy. [129][130][131] These studies revealed the principal reaction channels and some structural aspects of the peroxide decomposition, however the mechanistic details of the photodissociation were not understood very well.…”

“…[131,132] Whereas in early ps studies short-lived species could be detected, [133,134] the formation of CO 2 and its vibrational cooling in solution has been monitored recently on a ps timescale with transient IR spectroscopy. [9][10][11] These studies revealed that an improvement of the time resolution is still necessary, because many of the peroxides decompose on an even shorter timescale. Just recently, a study on decarboxylation of carbonyloxyl radicals, intermediates in the decomposition of organic peroxides, was published with fs-time resolution.…”

“…[135] Unfortunately, the wealth of experimental data about the thermal and photochemical decomposition of peroxides did not lead to a model that is able to explain the dependence of the dissociation dynamics on the molecular structure, internal energy, and molecular environment. [11,128] …”

“…The experimental setup for the transient IR absorption spectroscopy with ps time resolution was described recently. [11,138] . Figure 15.…”