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https://hdl.handle.net/2142/67226
Description
Title
Picosecond Studies of Unimolecular Reactions
Author(s)
Smith, Kevin Konrad
Issue Date
1980
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, Physical
Language
eng
Abstract
Several photochemical reactions occur on a very fast timescale. These ultrafast reactions may best be studied by the use of picosecond spectroscopy. These studies require advanced instrumental and data acquisition techniques. The construction of an apparatus which allows resolution of events on the picosecond timescale is discussed. The streak camera is used as the primary detector in these experiments. Video detection is used to detect signals from the streak camera. A two dimensional digitizer is used to acquire the data from the video detector. This data is then fed to a microcomputer which stores, analyzes and displays the digitized signals. Use of a computer interactive with the two dimensional digitizer allows large signal to noise enhancements.
This picosecond fluorescence apparatus is first used to study the photodissociation of 1,1,2,2 tetramethyldioxetane. Acetone fluorescence is observed in less than 10 picoseconds after excitation of the tetramethyldioxetane. This indicates that only intermediates with lifetimes shorter than 10 picoseconds may occur in this photochemical reaction. An energy level diagram which explains the fast dissociation and previous experimental results is proposed.
The rate of intramolecular proton transfer in methyl salicylate is found to be greater than 10('11) sec('-1) even at 4 K. The fluorescence lifetime of the zwitterion formed by the transfer of the proton is strongly dependent on temperature. An energy of activation of 3.7 kcal/mol is found for the nonradiative decay rate of the excited zwitterion. The barrier to non-radiative decay is strongly solvent dependent with high dielectric strength solvents causing a very fast nonradiative decay. The neutral excited state species has a lifetime at least three times larger than the excited zwitterion. Therefore the two excited state forms of methyl salicylate are not in equilibrium as was previously believed.
The logarithm of rates of proton ejection from 2-napthol-3,6-disulfonate and 1-hydroxy 3,6,8-pyrenetrisulfonate (HPS) are found to scale inversely with the excited state pK*. The rate of proton ejection for HPS is found to be only slightly dependent on temperature. The presence of a "cage" best fits the experimental data. This is supported by a biphasic deprotonation rate with (TURN) 10% of the excited HPS dissociating immediately. The rate of proton ejection for deuterated HPS is 4.7 x 10('9) sec('-1) which also scales inversely with its pK*. The rotational relaxation times of the dissociated and nondissociated forms of HPS are found to be identical. The rotational relaxation time of these species varies linearly with the viscosity of the solvent, and is not related to the deprotonation rate.
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