Roaming Transition States and Highly Accurate Thermochemistry: A PEPICO Study of Two Small Combustion Systems

Author

Kyle John Covert, University of the PacificFollow

Date of Award

2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Pharmaceutical and Chemical Sciences

First Advisor

Balint Sztaray

First Committee Member

Anthony D. Dutoi

Second Committee Member

Charles M. McCallum

Third Committee Member

Kieran Holland

Abstract

Two small combustion systems, methyl hydroperoxide (CH₃OOH) and 2-propanol ((CH₃)₂CHOH), were studied using imaging photoelectron photoion coincidence spectroscopy (iPEPICO), which combines photoelectron spectroscopy and photoionization mass spectrometry to detect coincident photoelectron-photoion pairs. In the photon energy range of 11.4–14.0 eV, energy selected CH₃OOH⁺ ions dissociate into CH₂OOH⁺, HCO⁺, CH₃⁺, and H₃O⁺ ions. The lowest-energy dissociation channel is the formation of the cation of the smallest “QOOH” radical, CH₂OOH⁺. A statistical rate model fitted to the experimental data yields a 0 K appearance energy of 11.647 ± 0.005 eV for the CH₂OOH⁺ ion, and a 74.2 ± 2.6 kJ mol^–1 mixed experimental-theoretical 0 K heat of formation for the CH₂OOH radical. The proton affinity of the Criegee intermediate, CH₂OO, was also obtained from the heat of formation of CH₂OOH⁺ (792.8 ± 0.9 kJ mol^–1) to be 847.7 ± 1.1 kJ mol^–1, reducing the uncertainty of the previously available computational value by a factor of 4. RRKM modeling of the higher-energy fragmentation processes, supported by Born–Oppenheimer molecular dynamics simulations, found that the HCO⁺ fragment ion is produced through a roaming transition state; H₃O⁺ is formed in a consecutive process from the CH₂OOH⁺ fragment ion; and direct C–O fission of the molecular ion leads to the methyl cation. Experimentally, 2-propanol has been found to dissociate primarily into CH₂CHOH⁺, CH₃CHOH⁺, CH₃CHCH₃⁺, and, as a minor product, into (CH₃)₂COH+ ions within a photon energy range of 10.0–13.1eV. There are interesting dissociation dynamics involving breaking the C–¬C bond: the lowest energy product (CH₃ loss) is quickly outcompeted by a kinetically favored CH₄ loss. At low internal energies of < 0.3 eV, the loss of CH₄ dominates through a roaming pathway, when the leaving CH₃ abstracts a hydrogen atom from the other methyl group. At higher energy, the direct loss of CH₃• quickly takes over as its transition state is much less tight and, thus, it is kinetically favored. The statistical model fitted to the experimental data yielded the appearance energy corresponding to the thermochemical limit for the CH₃-loss dissociation and the 0 K heats of formation of the CH₃CHOH⁺ ion was found to be in good agreement with ATcT values and with our previous study on ethanol.

Pages

116

Recommended Citation

Covert, Kyle John. (2019). Roaming Transition States and Highly Accurate Thermochemistry: A PEPICO Study of Two Small Combustion Systems. University of the Pacific, Dissertation. https://scholarlycommons.pacific.edu/uop_etds/3624