2 research outputs found
Low-Pressure Photolysis of 2,3-Pentanedione in Air: Quantum Yields and Reaction Mechanism
Dicarbonyls in the atmosphere mainly
arise from secondary sources
as reaction products in the degradation of a large number of volatile
organic compounds (VOC). Because of their sensitivity to solar radiation,
photodissociation of dicarbonyls can dominate the fate of these VOC
and impact the atmospheric radical budget. The photolysis of 2,3-pentanedione
(PTD) has been investigated for the first time as a function of pressure
in a static reactor equipped with continuous wave cavity ring-down
spectroscopy to measure the HO<sub>2</sub> radical photostationary
concentrations along with stable species. We showed that (i) SternāVolmer
plots are consistent with low OH-radical formation yields in RCO +
O<sub>2</sub> reactions, (ii) the decrease of the photodissociation
rate due to pressure increase from 26 to 1000 mbar is of about 30%,
(iii) similarly to other dicarbonyls, the SternāVolmer analysis
shows a curvature at the lower pressure investigated, which may be
assigned to the existence of excited singlet and triplet PTD states,
(iv) PTD photolysis at 66 mbar leads to CO<sub>2</sub>, CH<sub>2</sub>O and CO with yields of (1.16 Ā± 0.04), (0.33 Ā± 0.02) and
(0.070 Ā± 0.005), respectively, with CH<sub>2</sub>O yield independent
of pressure up to 132 mbar and CO yield in agreement with that obtained
at atmospheric pressure by Bouzidi et al. (2014), and (v) the PTD
photolysis mechanism remains unchanged between atmospheric pressure
and 66 mbar. As a part of this work, the O<sub>2</sub> broadening
coefficient for the absorption line of HO<sub>2</sub> radicals at
6638.21 cm<sup>ā1</sup> has been determined (Ī³<sub>O2</sub> = 0.0289 cm<sup>ā1</sup> atm<sup>ā1</sup>)
Gas-Phase Reaction of Hydroxyl Radical with Hexamethylbenzene
Aromatic hydrocarbons are important components of polluted
ambient air. The reaction of OH radicals with hexamethylbenzene (HMB)
is a prototype system to study ipso addition leading eventually to
dealkylation. We have investigated the OH + HMB and OD + HMB reactions
between 323 and 433 K using a discharge fast-flow reactor coupled
to a time-of-flight mass spectrometer with single-photon VUV photoionization
(10.54 eV). The H atom abstraction channel has been found to be equal
to (13.7 Ā± 4.4) % at 330 K leading to (11.1 Ā± 3.6) % at
298 K, higher than predicted by commonly used structureāreactivity
relationships. The back dissociation rate constant has also been measured
and has been found to be smaller than the rate of other aromatic hydrocarbons,
in good agreement with density functional theoretical calculations.
The dealkylation channel, leading to pentamethylphenol (PMP) + CH<sub>3</sub>, is always found to be the minor channel, estimated inferior
to 2% at 298 K