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.

The dissociative photoionization reactions of two small, oxygenated organics, namely 1,3-dioxolane and methyl vinyl ketone, were studied by photoelectron photoion coincidence (PEPICO) spectroscopy. Experiments involving 1,3-dioxolane were carried out in the photon energy range of 9.5‒13.5 eV. The statistical thermodynamics model shows that a total of six dissociation channels are involved in the formation of three fragment ions, namely C3H5O2+ (m/z 73), C2H5O+ (m/z 45) and C2H4O+ (m/z 44), with two channels contributing to the formation of each. By comparing the results of ab initio quantum chemical calculations to the experimentally derived appearance energies of the fragment ions, the most likely mechanisms for these unimolecular dissociation reactions are proposed, including a description of the relevant parts of the potential energy surface.In the case of methyl vinyl ketone, an important atmospheric intermediate in the oxidation of isoprene, between 9.5‒13.8 eV four main fragment ions were detected at m/z 55, 43, 42, and 27 aside from the parent ion at m/z 70. The m/z 55 fragment ion (C2H3CO+) is formed from ionized MVK by direct methyl loss, while breaking the C–C bond on the other side of the carbonyl group results in the acetyl cation (CH3CO+, m/z 43) and the vinyl radical. The m/z 42 fragment ion is formed via a CO loss from the molecular ion after a methyl shift. The lightest fragment ion, the vinyl cation (C2H3+ at m/z 27), is produced in two different reactions: acetyl radical loss from the molecular ion and CO-loss from C2H3CO+. Their contributions to the m/z 27 signal are quantified based on the acetyl and vinyl fragment thermochemical anchors and quantum-chemical calculations. Based on the experimentally derived appearance energy of the m/z 43 fragment ion, a new, experimentally derived heat of formation is proposed for gaseous methyl vinyl ketone (ΔfH0K = −94.2 ± 4.8 kJ mol−1; ΔfH298K = −110.4 ± 4.8 kJ mol−1), together with cationic heats of formation and bond dissociation energies.

Photoelectron Photoion Coincidence (PEPICO) Spectroscopy studies on two unsaturated hydrofluorocarbons (HFCs), also known as hydrofluoroolefines (HFOs), are presented here. Previously, the Sztáray group has studied the dissociation of trans-1,3,3,3-tetrafluoroprop-1-ene (ElixClean), which is a fourth-generation refrigerant and propellant and has lower global warming potential than its precursors. My study is an extension of the ElixClean study as it aims to explore how the different number of fluorine atoms impact the dissociation reactions of these molecules. Both 3,3,3-trifluoropropene (TFP) and cis-1,2,3,3,3- pentafluoropropene (PFP) are also utilized as propellants and refrigerants.Measurements were carried out with remote access to the CRF-PEPICO (combustion reactions followed by photoelectron photoion coincidence spectroscopy) endstation of the vacuum-ultraviolet (VUV) beamline at the Swiss Light Source (SLS). Gas phase samples were intersected with tunable vacuum ultraviolet synchrotron light to create photoions and photoelectrons which are then detected in coincidence. Breakdown diagrams were then obtained by integrating the photoelectron-photoion coincidence signal for times of flight corresponding the precursor and fragment ions and plotting their fractional abundances at each photon energy. Analysis of these dissociation mechanisms was aided by ab initio calculations. Dissociative photoionization of TFP was studied over the photon energy range of 11.8–16.0 eV. Besides the molecular ion (m/z 96), there were four main fragment ions detected: m/z 95 (H-loss), 77 (F-loss), 46 (CF2-loss), 27 (CF3-loss). The experimental data for the dissociative photoionization of PFP was taken over the photon energy range of 12.0–16.5 eV and indicated the formation of m/z 113(F-loss), m/z 82 (CF2-loss), m/z 69 (C2HF2-loss), and m/z 51 (C2F3-loss), but an H-loss is not detected. Quantum-chemical calculations at the B3LYP level were used to explore the potential energy surface and identify the most likely structures that play a role in the dissociative photoionization processes of these fluorinated propenes. The energies of the most relevant stationary points were refined utilizing G4 composite method. The dissociation/isomerization pathways of energy-selected TFP and PFP ions will be explored, and I will discuss the most likely mechanisms that lead to the lowest-energy products.

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.

Photoelectron Photoion Coincidence (PEPICO) Spectroscopy studies on two small oxygenated species are presented here. Diethyl Ether (Et2O) and Acetic Anhydride (AcOAc) were chosen because of their and their fragments' relevance to combustion chemistry. The Imaging PEPICO (iPEPICO) experiment at the VUV beamline of the Swiss Light Source (SLS) was utilized to provide dissociative ionization data of the two molecules of interest. In this experiment, the unimolecular fragmentation pathways of energy selected ions can be studied with high energy resolution. The iPEPICO experimental setup also allows the measurement of the dissociation rates, which is indispensable to derive accurate thermochemical information on large ions. The experimental data on the fragmentation of ions of interest are then examined through modeling the experimental ion fractional abundances (breakdown curves, BDCs) and reaction rates, in a modeling framework based on the RRKM statistical theory. In our first project, diethyl ether was studied to provide the appearance energies of its daughter ions along with the dissociation pathways of the molecular ion, leading to thermochemical data (such as heats of formation) pertinent to combustion chemistry. A revised ionization energy (IE) differing from the reviewed National Institute of Standards and Technology (NIST) was also proposed. In the second project presented, AcOAc was also measured on the iPEPICO apparatus to understand its dissociative photoionization processes. The appearance of trace amounts of acetone in the ionization spectra, discrepancies in the statistical models of the branching ratios, and the quantum chemical calculations all point to the existence of a post-transition-state bifurcation, when a single TS separates multiple products, namely a methyl-loss fragment and acetone, as well. The acetyl cation, as well as the methyl cation at higher energies, appear to be formed by both parallel and sequential dissociation processes.