Picosecond studies of free radical reactions in solution
Falvey, Daniel Edward
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Permalink
https://hdl.handle.net/2142/19350
Description
Title
Picosecond studies of free radical reactions in solution
Author(s)
Falvey, Daniel Edward
Issue Date
1989
Doctoral Committee Chair(s)
Schuster, Gary B.
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Organic
Language
eng
Abstract
A laser spectrometer capable of detecting molecules with lifetimes as short as 17 ps has been developed. The technique employed is flash photolysis with optical absorption detection (transient absorption spectroscopy). In this thesis is described the application of picosecond transient absorption spectroscopy and other techniques to the study of some free-radical reactions.
The photodissociation of peresters was studied using picosecond transient absorption spectroscopy. It was found that photolysis of a perester, tert-butyl 9-methyl-fluorene-9-percarboxylate (BPMF), results in formation of the 9-methyl-9-fluorenyl radical (9-MF$\cdot$). There is a 55 ps delay between absorption of a photon by BPMF and the formation of 9-MF$\cdot$. Consideration of the photophysical properties of BPMF leads to the conclusion that the species which precedes 9-MF$\cdot$ is an acyloxy radical, 9-methylfluorene-9-carboxy (9-MFCO$\sb2\cdot$).
The neophyl-like rearrangement of alkoxy radicals was examined by picosecond spectroscopy. The trityloxy radical (Ph$\sb3$CO$\cdot$) rearranges to form the $\alpha$-phenoxy diphenylmethyl radical. Picosecond transient absorption experiments show that this reaction occurs with a rate constant of $>$5 $\times$ 10$\sp{10}$ s$\sp{-1}$. The rearrangement of 1,1-diphenylethoxy radical was also examined. In this case, an intermediate is formed in less than 17 ps and then decays with a lifetime of 312 ns. It is proposed that this intermediate is a bridged spiro (2.6) octadiene-type radical. This is the first evidence for this type of intermediate.
The 2,7-dihalofluorenylidenes were examined. Electron paramagnetic resonance (EPR) spectroscopy shows that these carbenes are ground-state triplets. These carbenes react with methanol to form 9-methoxy ethers. The absence of a heavy atom effect in this reaction suggests that intersystem crossing is not involved in the rate-determining step, and that diaryl carbenes react with methanol via their singlet states.
The reactions of several excited-state sensitizers with 4-azido-$N,N$-dimethylbenzamide (DAA) were examined. Laser flash photolysis shows that the reaction of excited-state pyrene with DAA results in formation of pyrene radical cation. The sum of the evidence indicates that the sensitized decomposition of DAA can occur via two pathways: single-electron transfer, and singlet-singlet energy transfer.
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