Date of Award

6-1-2013

Document Type

Campus Access Thesis

Degree Name

Master of Science (MS)

Department

Chemistry

First Advisor

Timothy J. Dransfield

Second Advisor

Jason J. Evans

Third Advisor

Michelle C. Foster

Abstract

Tropospheric ozone is formed from a series of reactions, and the rate-limiting reaction is that of Hydroxyl Radical (OH) with Volatile Organic Compounds (VOCs). Because scientists have looked to reduce the formation of this pollutant, the kinetics of the rate-limiting step have been studied for the reactions of OH with several VOCs. This thesis describes the experimental determination of the absolute temperature dependence of the reactions of OH with cyclopentane and OH with cycloheptane over the temperature range of 233 K to 351 K using the Harvard High Pressure Flow Reactor (HPFS).

A recent study using our instrument, which was the first absolute rate study for cycloalkanes encompassing low temperatures, yielded surprising results in terms of activation barriers of OH within the homologous series of cycloalkanes. The observed barrier for the reaction of OH with cyclohexane was considerably greater than that for OH with cyclo-octane. Structure-Activity Relationships had predicted that the activation barrier should be more or less equal throughout the homologous series of cycloalkanes. The reactions of OH with cyclopentane and cycloheptane had not been included in that study; hence the motivation for this work.

We have continued to use a modified Arrhenius functional form, as used with the previous study, based on transition state theory. Our determination of absolute rate constants for these compounds also included low-temperature data. This range of temperatures allowed us to further evaluate the accuracy of the modified Arrhenius fit for the temperature dependence of the reactions of OH and cycloalkanes.

Our experimentally-determined activation barrier for the reaction of OH with cyclopentane is considerably greater than that for the reaction of OH with cycloheptane. The barriers for the reactions of OH with cyclopentane and with cycloheptane are also much greater than the barrier previously determined for the reaction of OH with cyclo-octane using the same instrument. These results are not expected based on the models most commonly utilized to predict activation barriers. The thesis has also investigated the potential causes of this surprising behavior.

Comments

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