Date of Award

2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Pharmaceutical and Chemical Sciences

First Advisor

Bálint Sztáray

First Committee Member

Skylar Carlson

Second Committee Member

Liang Xue

Third Committee Member

Joseph Harrison

Fourth Committee Member

Kieran Holland

Abstract

Photoelectron Photoion Coincidence (PEPICO) spectroscopy is a robust tool for elucidating complex unimolecular dissociation mechanisms and for determining thermochemical and kinetic data of gas-phase ion dissociations with high accuracy. In this work, the dissociative photoionization of two sets of isomeric systems were analyzed with PEPICO: 1) C7H7+ ions of toluene (Tol) and 1,3,5-cycloheptatriene (CHT), and 2) two butyl alcohol isomers, 1-butanol and isobutanol. Threshold dissociative photoionization data on these four molecules of interest were collected on the imaging PEPICO apparatus at the VUV beamline of the Swiss Light Source. Data analysis was aided by ab initio calculations and Rice-Ramsperger-Kassel-Marcus (RRKM) statistical rate theory was employed to model the complex dissociation pathways of each system. Finally, thermochemical, reaction mechanism, and dissociation kinetics data were extracted from the modeled data and are reported here.

In the first project, the dissociation of energy-selected 1,3,5-cycloheptatriene (CHT) and toluene (Tol) cations was investigated by imaging photoelectron photoion coincidence spectroscopy. In the measured energy ranges of 10.30−11.75 eV for CHT and 11.45−12.55 eV for Tol, only the hydrogen atom loss channels open up, leading to C7H7+ from both molecular ions, which are both metastable at the H-loss threshold. Our quantum chemical calculations showed that these ions can interconvert below their dissociation thresholds. Therefore, we constructed a single statistical model to describe both systems simultaneously. We determined 0 K appearance energies (E0) for the tropylium and benzyl fragment ions from CHT to be 9.520 ± 0.060 eV and 9.738 ± 0.082 eV, and from Tol to be 10.978 ± 0.063 eV and 11.196 ± 0.080 eV, respectively. Using the experimentally determined benzyl ion appearance energy, its 0 K heat of formation was calculated to be 937.9 ± 7.7 kJ mol–1. On the basis of this value and the recently determined benzyl ionization energy, we point out discrepancies concerning the benzyl radical thermochemistry.

For the second project, the fragmentation processes of two internal energy-selected C4H10O+• cations, 1-butanol and isobutanol, were investigated. For both isomers, the first dissociation channel leads to the formation of C4H8+• ions (m/z = 56) by a water loss. Using statistical energy distribution and rate models, which include isomerization of the molecular ions, the 0 K appearance energies (E0) were determined to be 10.347 ± 0.015 eV and 10.566 ± 0.050 eV, for 1-butanol and isobutanol, respectively. The second dissociation channel, the formation of CH3OH2+, quickly overtakes the water-loss channel in isobutanol, with an E0 of 10.612 ± 0.020 eV, but appears only as a minor channel in 1-butanol with an E0 of 10.738 ± 0.080 eV. The methanol-loss channel, forming propylene ion, opens up at E0 = 10.942 ± 0.040 eV and 10.723 ± 0.020 eV in 1-butanol and isobutanol, respectively. The next two fragmentation pathways correspond to a complementary pair of C3H7+ through the loss of CH2OH, and CH2OH+ through the loss of C3H7. From both isomers, C3H7+ is the isopropyl ion, which is readily formed in isobutanol via a simple bond cleavage at E0 = 10.970 ± 0.050 eV and its pair, CH2OH+, at E0 = 11.11 ± 0.20 eV. However, there is an internal hydrogen shift necessary in 1-butanol and, therefore, the complementary ions appear at the same E0 of 11.104 ± 0.030 eV, which most likely corresponds to their common transition state. Finally, C3H5+, a product of sequential dissociation from m/z = 56, appears above 11.6 eV as a minor channel for both isomers.

Pages

126

Available for download on Thursday, July 29, 2021

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