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Femtosecond to Microsecond Observation of the Photochemical Reaction of 1,2-di(quinolin-2-yl)disulfide with Methyl Methacrylate

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Original languageEnglish
Pages (from-to)12981-12991
Number of pages11
JournalPhysical Chemistry Chemical Physics
Volume19
Journal issue20
Early online date3 May 2017
DOIs
StatePublished - 28 May 2017

Abstract

The mechanism of the thiol-ene reaction induced by 330-nm ultraviolet excitation of 1,2-di(quinolin-2-yl)disulfide (QSSQ) in the presence of methyl methacrylate (MMA) is investigated by sub-picosecond to microsecond transient absorption spectroscopy. The measurements, spanning more than seven orders of magnitude of time, directly reveal multiple radical reaction steps. The ground state quinoliene-2-thiyl radical (QS) is formed with a time constant of ~200 fs by photolysis of QSSQ, followed by (64 ± 1)% decay of the initially formed QS radical because of solvent cage induced geminate recombination and QS dimer formation with a rate coefficient of (3.4 ± 0.2) × 1010 M-1 s-1 in methanol solution. In MMA solution, the carbon centered radical QS-MMA forms with a bimolecular reaction rate coefficient of (2.8 ± 0.2) × 107 M-1 s-1. The distinct infrared band at 1653 cm-1 assigned to the C=O stretch mode of the QS-MMA radical decays rapidly in aerated solution, in contrast to observations in a solution purged of O2 by N2 bubbling. This decay is attributed to reaction of the QS-MMA radicals with molecular oxygen, producing peroxy radicals. Kinetic analysis of the intensity of the band at 1653 cm-1 reveals a bimolecular reaction rate coefficient of (3.3 ± 0.3) × 109 M-1 s-1 for the reaction of the QS-MMA radicals with molecular oxygen, and indicates that this reaction step is reversible.

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