Falloff curves for the unimolecular decomposition of two acyl radicals: RCO (+M) → R + CO (+M) by pulsed laser photolysis coupled to time‐resolved infrared diode laser absorption

Abstract

The rate constants k d for the unimolecular decomposition of two acyl (RĊO) radicals: 2,2-dimethylpropionyl (CH 3 ) 3 CĊO and 2-methylpropionyl (CH 3 ) 2 CHĊO (pivaloyl and isobutyryl, respectively in what follows) have been directly measured via the time-resolved kinetics of CO formation as a function of pressure and temperature. The acyl radicals are generated by H-atom abstraction of the aldehydic hydrogen from the corresponding substituted propanals using pulsed laser photolysis of aldehyde/Cl 2 /N 2 mixtures. The buildup of CO concentration is monitored by time-resolved infrared absorption at ∼ =2177 cm −1 . The extent of the chain reactions triggered by the reactions of Cl 2 with alkyl and acyl radicals (thus generating further Cl atoms after the initial laser pulse) has been limited by adding variable known concentrations of O 2 , a radical scavenger which is sequestering all radicals to form less reactive RO 2 radicals. Besides k d , we have also determined for the first time the rate constants of the reactions of the two acyl radicals with O 2 ; since the latter do not exhibit any clear trend with either temperature or pressure, we thus adopt the average of all experimental values: i.e. for pivaloyl : k O 2 = (3.5 ± 1.0) × 10 −12 cm 3 molecule −1 s −1 , and for isobutyryl : k O 2 = (3.2 ± 1.0) × 10 −12 cm 3 molecule −1 s −1 . Our values of k d , which are in the falloff range, are compared to those obtained from theoretical calculations using the Troe formalism [5]. For pivaloyl at 298 K, we obtain the following limiting rate constants : k 0 = 2.1 × 10 −13 cm 3 molecule −1 s −1 , k ∞ = 4.3 × 10 5 s −1 , F C = 0.515, which are at variance with the theoretical predictions of Tomas et al. (Phys Chem Chem Phys 2000, 2, 1165-1174

Strategies to Reduce Oxygen Inhibition in Photoinduced Polymerization

“…Oxygen is reported to react very rapidly ( k ox > 5 × 10 8 L·mol –1 ·s –1 ) with carbon-centered radicals to form peroxyl radicals (R • + O 2 → ROO • ) that do not readily react with the alkenic double bonds of the acrylate monomers; hence, chain polymerization is inhibited. In a typical UV-curable formulation, the reaction of carbon-centered radicals with predissolved oxygen has been shown to be from 2 to several orders of magnitude faster than acrylate initiation ( k i ≈ 10 6 –10 7 L·mol –1 ·s –1 ) or propagation ( k p ≈ 10 3 L·mol –1 ·s –1 ). Table provides a list of representative carbon- and phosphorus-centered radicals (stemming from commonly used photoinitiators) and the rates at which they react with acrylate monomer and with oxygen.…”

Photolysis of CF3CH2CHO in the Presence of O2 at 248 and 266 nm: Quantum Yields, Products, and Mechanism

“…The experimental setup has already been partly described in previous papers. − It combines pulsed laser photolysis with time-resolved detection by diode laser infrared absorption spectroscopy.…”