Imaging Photoelectron Photoion Coincidence Spectroscopy Research Articles

Photoionization and Dissociative Photoionization of Acetaldehyde in the 10.0-13.7 eV Range by Synchrotron Photoelectron Photoion Coincidence Spectroscopy.

Photoionization and dissociative photoionization of acetaldehyde (CH3CHO) in the 10.0-13.7 eV energy range are studied by using synchrotron radiation double imaging photoelectron photoion coincidence spectroscopy (i2PEPICO). The X2A' and A2A" electronic states of CH3CHO+ as well as the Franck-Condon gap region between these two states have been populated with several vibrational sequences and assigned in the high-resolution slow photoelectron spectrum (SPES). The adiabatic ionization energies (AIEs) of the X2A' and A2A" states are measured at 10.228±0.006 and 12.52±0.05 eV, respectively. The present results show that the X2A' state is a stable state while the A2A" state is fully dissociative to produce CH3CO+, CHO+ and CH4 + fragment ions. The 0 K appearance energies (AE0K) of CH3CO+ and CHO+ fragment ions are determined through the modeling of the breakdown diagram, i. e., AE0K(CH3CO+)=10.89±0.01 eV (including a reverse barrier of ~0.19 eV) and AE0K(CHO+)=11.54±0.05 eV. In addition, the dissociation mechanisms of CH3CHO+ including statistical dissociation, direct bond breaking and isomerization are discussed with the support of the calculated dissociation limits and transition state energies.

Conformer-Specific Photoelectron Spectroscopy of Carbonic Acid: H2CO3.

Carbonic acid (H2CO3) is a fundamental species in biological, ecological, and astronomical systems. However, its spectroscopic characterization is incomplete because of its reactive nature. The photoionization (PI) and the photoion mass-selected threshold photoelectron (ms-TPE) spectra of H2CO3 were obtained by utilizing vacuum ultraviolet (VUV) synchrotron radiation and double imaging photoelectron photoion coincidence spectroscopy. Two carbonic acid conformers, namely, cis-cis and cis-trans, were identified. Experimental adiabatic ionization energies (AIEs) of cis-cis and cis-trans H2CO3 were determined to be 11.27 ± 0.02 and 11.18 ± 0.03 eV, and the cation enthalpies of formation could be derived as ΔfH°0K = 485 ± 2 and 482 ± 3 kJ mol-1, respectively. The cis-cis conformer shows intense peaks in the ms-TPES that are assigned to the C=O/C-OH stretching mode, while the cis-trans conformer exhibits a long progression to which two C=O/C-OH stretching modes contribute. The TPE spectra allow for the sensitive and conformer-selective detection of carbonic acid in terrestrial experiments to better understand astrochemical reactions.

Gas Phase Ionization Energy of Heptacene.

The ionization energy is a fundamental property that is relevant to charge transport in organic semiconductors. We report adiabatic ionization energies (AIEs) of heptacene at 6.21 and 7.20 eV for the X̃+B2g and Ã+Au states, respectively, as the next larger member of the acene series using mass- and isomer-selective double imaging photoelectron photoion coincidence spectroscopy. The X̃+ state energy decreases monotonically with an increase in size within the homologous series of acenes and approaches an asymptotic limit [AIE(polyacene) = 5.94 ± 0.06 eV] based on a fit with an exponential decay function. As byproducts of heptacene formation from cycloreversion of diheptacenes, 5,18-, 7,16-, and 6,17-dihydroheptacene can be detected, and their AIE is similar to that of their largest acene subunit (anthracene and tetracene, respectively), in very good agreement with computational treatments.

What a difference a chlorine makes: The remarkable unimolecular ion chemistry of phenyl formate and phenyl chloroformate.

Imaging photoelectron photoion coincidence (iPEPICO) spectroscopy and tandem mass spectrometry were employed to explore the ionisation and dissociative ionisation of phenyl formate (PF) and phenyl chloroformate (PCF). The threshold photoelectron spectra of both compounds are featureless and lack a definitive origin transition, owing to the internal rotation of the formate functional group relative to the benzene ring, active upon ionisation. CBS-QB3 calculations yield ionisation energies of 8.88 and 9.03 eV for PF and PCF, respectively. Ionised PF dissociates by the loss of CO via a transition state composed of a phenoxy cation and HCO moieties. The dissociation of PCF ions involves the competing losses of CO (m/z 128/130), Cl (m/z 121) and CO2 (m/z 112/114), with Cl loss also shown to occur from the second excited state in a non-statistical process. The primary CO- and Cl-loss fragment ions undergo sequential reactions leading to fragment ions at m/z 98 and 77. The mass-analysed ion kinetic energy (MIKE) spectrum of PCF+ showed that the loss of CO2 occurs with a large reverse energy barrier, which is consistent with the computationally derived minimum energy reaction pathway.

Evidencing an elusive conical intersection in the dissociative photoionization of methyl iodide.

The valence-shell dissociative photoionization of methyl iodide (CH3I) is studied using double imaging photoelectron photoion coincidence (i2 PEPICO) spectroscopy in combination with highly-tunable synchrotron radiation from synchrotron SOLEIL. The experimental results are complemented by new high-level ab initio calculations of the potential energy curves of the relevant electronic states of the methyl iodide cation (CH3I+). An elusive conical intersection is found to mediate internal conversion from the initially populated first excited state, CH3I+(Ã2A1), into the ground cationic state, leading to the formation of methyl ions (CH3+). The reported threshold photoelectron spectrum for CH3+ reveals that the ν5 scissors vibrational mode promotes the access to this conical intersection and hence, the transfer of population. An intramolecular charge transfer takes place simultaneously, prior to dissociation. Upon photoionization into the second excited cationic state, CH3I+(B̃2E), a predissociative mechanism is shown to lead to the formation of atomic I+.

The Unimolecular Chemistry of Methyl Chloroformate Ions and Neutrals: A Story of Near-Threshold Decomposition.

The near-threshold dissociation of ionized and neutral methyl chloroformate (CH3COOCl, MCF) was explored with imaging photoelectron photoion coincidence spectroscopy. The threshold photoelectron spectrum (TPES) for MCF was acquired for the first time; the large geometry changes upon ionization of MCF result in a broad, poorly defined TPES. Franck-Condon simulations are consistent with an adiabatic ionization energy (IE) of 10.90 ± 0.05 eV. Ionized MCF dissociates by chlorine atom loss at a measured 0 K appearance energy (AE) of 11.30 ± 0.01 eV. Together with the above IE, this AE suggests a reaction barrier of 0.40 ± 0.05 eV, consistent with the SVECV-f12 computational result of 0.41 eV. At higher internal energies, the loss of CH3O• becomes competitive due to its lower entropy of activation. Pyrolysis of neutral MCF formed the anticipated major products CH3Cl + CO2 (R1) and the minor products HCl + CO + CH2O (R2). The thermal decomposition products were identified by their photoion mass-selected threshold photoelectron spectrum (ms-TPES). Possible reaction pathways were explored computationally to confirm the dominant ones: R1 proceeds by a concerted Cl atom migration via a four-membered transition state in agreement with the mechanism proposed in the literature. R2 is a two-step reaction first yielding 2-oxiranone by HCl loss, which then decomposes to CH2O and CO. Kinetic modeling of the neutral decomposition could simulate the observed reactions only if the vibrational temperature of the MCF was assumed not to cool in the expansion.

Pyrolysis of Trifluoroacetic Acid and Trifluoroacetic Anhydride Studied with Mass Spectrometry and Synchrotron Radiation: Decomposition and Free Radical Formation

AbstractThe remediation strategies for accumulated polyfluoroalkyl substances (PFAS) are a subject of great concern in environmental science. In this study, the thermal decomposition of trifluoroacetic acid (TFA) and trifluoroacetic anhydride (TFAN), taken as models of PFAS, were explored. Imaging photoelectron photoion coincidence (iPEPICO) spectroscopy was used to detect pyrolysis products from room temperature to 1000 °C under dilute conditions. The observed pyrolysis products of each molecule were CO, CO2, CF2, and CF2O (and CF3 for TFAN), with some species reflecting unimolecular rearrangement prior to dissociation. Electronic structure calculations using density functional theory and the SVECV‐F12 composite method were employed to evaluate the energy of the different decomposition channels. The results show the advantage of exploring the pyrolysis under dilute conditions to catch the first stages of unimolecular dissociation of these molecules for the first time.

Probing the pyrolysis of ethyl formate in the dilute gas phase by synchrotron radiation and theory.

The thermal decomposition of the atmospheric constituent ethyl formate was studied by coupling flash pyrolysis with imaging photoelectron photoion coincidence (iPEPICO) spectroscopy using synchrotron vacuum ultraviolet (VUV) radiation at the Swiss Light Source (SLS). iPEPICO allows photoion mass-selected threshold photoelectron spectra (ms-TPES) to be obtained for pyrolysis products. By threshold photoionization and ion imaging, parent ions of neutral pyrolysis products and dissociative photoionization products could be distinguished, and multiple spectral carriers could be identified in several ms-TPES. The TPES and mass-selected TPES for ethyl formate are reported for the first time and appear to correspond to ionization of the lowest energy conformer having a cis (eclipsed) configuration of the O=C(H)-O-C(H2 )-CH3 and trans (staggered) configuration of the O=C(H)-O-C(H2 )-CH3 dihedral angles. We observed the following ethyl formate pyrolysis products: CH3 CH2 OH, CH3 CHO, C2 H6 , C2 H4 , HC(O)OH, CH2 O, CO2 , and CO, with HC(O)OH and C2 H4 pyrolyzing further, forming CO + H2 O and C2 H2 + H2 . The reaction paths and energetics leading to these products, together with the products of two homolytic bond cleavage reactions, CH3 CH2 O + CHO and CH3 CH2 + HC(O)O, were studied computationally at the M06-2X-GD3/aug-cc-pVTZ and SVECV-f12 levels of theory, complemented by further theoretical methods for comparison. The calculated reaction pathways were used to derive Arrhenius parameters for each reaction. The reaction rate constants and branching ratios are discussed in terms of the residence time and newly suggest carbon monoxide as a competitive primary fragmentation product at high temperatures.

Threshold photoelectron spectroscopy and dissociative photoionization of benzonitrile.

The threshold photoionization and dissociative ionization of benzonitrile (C6H5CN) were studied using double imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy at the Vacuum Ultraviolet (VUV) beamline of the Swiss Light Source (SLS). The threshold photoelectron spectrum was recorded from 9.6 to 12.7 eV and Franck-Condon simulations of ionization into the ionic ground state, X̃+, as well as the B̃+ and C̃+ states were performed to assign the observed vibronic structures. The adiabatic ionization energies of the X̃+, B̃+ and C̃+ states are determined to be (9.72 ± 0.02), (11.85 ± 0.03) and, tentatively, (12.07 ± 0.04) eV, respectively. Threshold ionization mass spectra were recorded from 13.75 to 19.75 eV and the breakdown diagram was constructed by plotting the fractional abundances of the parent ion and ionic dissociation products as a function of photon energy. The seven lowest energy dissociative photoionization channels of benzonitrile were found to yield CN˙ + c-C6H5+, HCN + C6H4˙+, C2H4 + HC5N˙+, HC3N + C4H4˙+, H2C3N˙ + C4H3+, CH2CHCN + C4H2˙+ and H2C4N˙ + c-C3H3+. HCN loss from the benzonitrile cation is the dominant dissociation channel from the dissociation onset of up to 18.1 eV and CH2CHCN loss becomes dominant from 18.1 eV and up. We present extensive potential energy surface calculations on the C6H5CN˙+ surface to rationalize the detected products. The breakdown diagram and time-of-flight mass spectra are fitted using a Rice-Ramsperger-Kassel-Marcus statistical model. Anchoring the fit to the CBS-QB3 result (3.42 eV) for the barrier to HCN loss, we obtained experimental dissociation barriers for the products of 4.30 eV (CN loss), 5.53 eV (C2H4 loss), 4.33 eV (HC3N loss), 5.15 eV (H2C3N loss), 4.93 eV (CH2CHCN loss) and 4.41 eV (H2C4N loss). We compare our work to studies of the electron-induced dissociative ionization of benzonitrile and isoelectronic phenylacetylene (C8H6), as well as the VUV-induced dissociation of protonated benzonitrile (C6H5CNH+). Also, we discuss the potential role of barrierless association reactions found for some of the identified fragments as a source of benzonitrile(˙+) in interstellar chemistry and in Titan's atmosphere.

VUV photoprocessing of oxygen-containing polycyclic aromatic hydrocarbons: iPEPICO study of the unimolecular dissociation of ionized benzofuran

Oxygen-containing polycyclic aromatic hydrocarbons (OPAHs) are potential contributors to the 11.3 µm band in interstellar observations. To further explore their role in the interstellar medium, we investigated their fate after photoprocessing by VUV radiation; in particular, we studied the dissociative photoionization of the simplest OPAH, benzofuran, with imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. Ionized benzofuran dissociates by loss of CO, followed by a sequential H-atom loss. The parallel HCO-loss channel, leading to the same bicyclic C7H5+ fragment ion, is not competitive at low excess energies above the ionization threshold. However, the collision-induced dissociation tandem mass spectrometry results suggest that CO and HCO may be formed in parallel at higher energies. A Rice–Ramsperger–Kassel–Marcus fragmentation model reproduced the iPEPICO data well, assuming the initial 1,2-H shift transition state to be rate determining to CO loss. The breakdown diagram and the measured dissociation rates agreed well at the CBS-QB3-calculated activation energy of 2.99 eV, which could be relaxed to 3.25 eV, and only a slight adjustment of the ab initio activation entropy. The model barrier to sequential H loss is larger than the computed H-loss threshold and the breakdown diagram rises less steeply than predicted, which indicates suprastatistical kinetic energy release after the tight H-transfer transition state of the first step. HCO cleavage is possible after a ring-opening transition state, which is looser than and isoenergetic with the CO-loss transition state. However, a subsequent ring formation transition state at 3.85 eV is moderately tight, which suppresses HCO loss at low excess energies.

Probing the pyrolysis of methyl formate in the dilute gas phase by synchrotron radiation and theory

AbstractThe thermal dissociation of the atmospheric constituent methyl formate was probed by coupling pyrolysis with imaging photoelectron photoion coincidence spectroscopy (iPEPICO) using synchrotron VUV radiation at the Swiss Light Source (SLS). iPEPICO allows threshold photoelectron spectra to be obtained for pyrolysis products, distinguishing isomers and separating ionic and neutral dissociation pathways. In this work, the pyrolysis products of dilute methyl formate, CH3OC(O)H, were elucidated to be CH3OH + CO, 2 CH2O and CH4 + CO2 as in part distinct from the dissociation of the radical cation (CH3OH+• + CO and CH2OH+ + HCO). Density functional theory, CCSD(T), and CBS‐QB3 calculations were used to describe the experimentally observed reaction mechanisms, and the thermal decomposition kinetics and the competition between the reaction channels are addressed in a statistical model. One result of the theoretical model is that CH2O formation was predicted to come directly from methyl formate at temperatures below 1200 K, while above 1800 K, it is formed primarily from the thermal decomposition of methanol.

Threshold Photoelectron Spectroscopy of Quinoxaline, Quinazoline, and Cinnoline

The threshold photoelectron spectra of cinnoline, quinazoline, and quinoxaline, three small naphthalene-analogue polycyclic nitrogen-containing hydrocarbons of C8H6N2 composition, were recorded. The spectra are assigned to understand their electronic structure and the role of isomerism. Furthermore, this work provides reference data for the selective identification of such species as gas-phase reaction products at low number densities. Imaging photoelectron photoion coincidence spectroscopy was used at the VUV beamline of the Swiss Light Source to record the spectra from the ionization onset to 12 eV. To assign and interpret the spectral features, we relied on (time-dependent) density functional theory and EOM-IP-CCSD calculations and computed vertical and adiabatic ionization energies as well as Franck-Condon factors to simulate ground- and excited-state spectra. Vibrational progressions belonging to four electronic states could be simulated in each of the samples, and we report a total of 12 adiabatic ionization energies, including the ones to the ground and excited cation states. Such a wealth of spectral information, as well as the reliable ab initio modeling, is promising with regards to analytical applications. While cinnoline can be easily distinguished by its lowest adiabatic ionization energy, quinazoline and quinoxaline show different vibrational fingerprints, which can be used to distinguish the three isomers even in complex reaction mixtures. Finally, we also relate the cation electronic states to the neutral molecular orbitals and note that Koopmans' approximation fails in these N2-containing species very much like it does in N2.

Photoionization of Two Potential Biofuel Additives: γ-Valerolactone and Methyl Butyrate.

The photoionization of two potential biofuel additives, γ-valerolactone (GVL, C5H8O2) and methyl butyrate (MB, C5H10O2) has been studied by imaging photoelectron photoion coincidence spectroscopy (iPEPICO) at the VUV beamline of the Swiss Light Source (SLS). The vibrational fine structure in the photoelectron spectrum is compared with a Franck-Condon simulation for the electronic ground-state band of the GVL cation. In the lowest energy dissociative photoionization channel of GVL, CO2 is lost, resulting in a 1-butene fragment ion with a 0 K appearance energy of E0 = 10.35 ± 0.01 eV. A newly calculated 1-butene ionization energy of 9.595 ± 0.015 eV establishes the reverse barrier height to CO2 loss as 66.6 ± 4.3 kJ mol-1. Methyl butyrate cations undergo McLafferty rearrangement, which explains the missing ion signal at the computed adiabatic ionization energy of 9.25 eV. After H transfer, ethylene is lost in the lowest energy dissociation channel to yield the methyl acetate enol ion at E0 = 10.24 ± 0.04 eV. This value connects the energetics of methyl butyrate with that of methyl acetate enol ion, which is established at ΔfHo0K[CH2C(OH)OCH3+] = 502 ± 6 kJ mol-1. Parallel to ethylene loss, methyl loss is also observed from the enol tautomer of the parent ion. Both samples exhibit low-energy nonstatistical dissociative ionization channels. In GVL, the methyl-loss abundance rises quickly but levels off suddenly in the energy range of the first electronically excited states, indicating nonstatistical competition between CH3 and CO2 loss. In MB, the major parallel dissociation channel is the loss of a methoxy radical. Calculations indicate that McLafferty rearrangement is inhibited on the excited-state surface. Indeed, breakdown curve modeling of this and a sequential CO-loss channel confirms a second statistical regime in dissociative photoionization, decoupled from ethylene loss.

Iodide-Coordinated Single-Site Pd Catalysts for Alkyne Dialkoxycarbonylation

Heterogeneous single-metal-site catalysts (HSMSCs) have attracted considerable interest, but most studies have focused on the metal atoms in the active site while ignoring the key role of ligands. The unique coordination environment of a single-site catalyst is crucial for realizing its potential. Constructing this kind of catalyst via a feasible and practical fabrication method is challenging. Herein, a single-site Pd catalyst with iodide ligands supported on activated carbon (Pd1/AC) was successfully fabricated by atomic dispersion of large Pd nanoparticles (NPs). Intermediate I• radicals were detected during the atomic dispersion process of Pd NPs by in situ imaging photoelectron photoion coincidence spectroscopy (in situ iPEPICO) with vacuum ultraviolet synchrotron radiation. The molecular structure of single-site Pd was established as [Pd(CO)I4(O═AC)]2– through combined characterization. Alkyne dialkoxycarbonylation with high selectivity toward 1,4-dicarboxylic acid esters (>94%) and high acetylene conversion (>99%) was achieved. A sulfonic promoter on the Pd1/AC catalyst for alkyne dialkoxycarbonylation was avoided because of the iodide ligand. Good durability and a broad substrate scope were successfully achieved.

Dissociation of High-Lying Electronic States of NO2+ in the 15.5-20 eV Region.

We present a dissociative photoionization study of NO2 in the 15.5-20 eV energy range using synchrotron radiation-based double imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy. The high-lying electronic states of the NO2+ cation, c 3B1, C 1B1, d 3A1, e 3B2, and D 1B2, are prepared in well-resolved vibronic states in order to study their individual dissociation mechanisms. Up to eight dissociation limits of NO2+ are reached, and mass-selected threshold photoelectron spectra (TPES) show that the c 3B1, C 1B1, and d 3A1 states predominantly dissociate into the NO+ + O products, while the e 3B2 and D 1B2 states can undergo fragmentation into both the NO+ + O and the O+ + NO channels, as well as the O2+ + N channel with a small yield. Overall, these product yields are found to be quite sensitive to autoionization processes. Mass-selected high-resolution electron and ion kinetic energy correlation diagrams reveal dissociative mechanisms that possess strong state-specific character.

To roam or not to roam, that is the question for the methyl group in isopropanol cations

Isopropanol (2-propanol) dissociatively photoionizes primarily into the CH2CHOH+, CH3CHOH+, CH3CHCH3+ fragment ions, and, as a minor product, into (CH3)2COH+ in the 10.0–13.1 eV photon energy range as shown by Imaging Photoelectron Photoion Coincidence (iPEPICO) spectroscopy. At internal energies of below 0.3 eV, the loss of CH4 dominates via a roaming pathway, in which the leaving CH3 abstracts a hydrogen atom from the other methyl group. At higher energies, the kinetically favored direct loss of a methyl radical quickly takes over as its transition state is looser. We use the measured CH3-loss appearance energy of 10.44 ± 0.01 eV to confirm the heat of formation of protonated acetaldehyde, CH3CHOH+, as ΔfH° = 608 ± 1 kJ mol−1 at 0 K. The highest-energy dissociation observed leads to CH3CHCH3+ + OH, which corresponds to C–O bond scission. This process is the premier example of a non-statistical dissociation which can be modeled using a statistical model, albeit with a physically meaningless appearance energy. This channel is shown to be non-statistical due to preferential OH loss from the first electronically excited ion state.

Threshold photoelectron spectroscopy of the HO2 radical.

We report a synchrotron radiation vacuum ultraviolet photoionization study of the hydroperoxyl radical (HO2), a key reaction intermediate in combustion and atmospheric chemistry as well as astrochemistry, using double imaging photoelectron photoion coincidence spectroscopy. The HO2 radical is formed in a microwave discharge flow tube reactor through a set of reactions initiated by F atoms in a CH4/O2/He gas mixture. The high-resolution threshold photoelectron spectrum of HO2 in the 11 eV-12 eV energy range is acquired without interferences from other species and assigned with the aid of theoretically calculated adiabatic ionization energies (AIEs) and Franck-Condon factors. The three vibrational modes of the radical cation HO2 +, the H-O stretch, the H-O-O bend, and the O-O stretch, have been identified, and their individual frequencies are measured. In addition, the AIEs of the X3A″ ground state and the a1A' first excited electronic state of HO2 + are experimentally determined at 11.359 ± 0.003 eV and 11.639 ± 0.005 eV, respectively, in agreement with high-level theoretically computed results. Furthermore, the former AIE value provides validation of thermochemical networks used to extract the enthalpy of formation of the HO2 radical.

Hydrogen migration as a potential driving force in the thermal decomposition of dimethoxymethane: New insights from pyrolysis imaging photoelectron photoion coincidence spectroscopy and computations

Pyrolysis and low-temperature oxidation of dimethoxymethane (methylal, MeOCH2OMe) play an important role in the ignition of blended diesel fuels, but the underlying mechanisms are still debated. In these kinetic models, bimolecular hydrogen abstraction or unimolecular C–O bond fission are considered as the primary initial steps, while MeOCH2OMe isomerization is sometimes disregarded. In this work, we investigate the pyrolysis of MeOCH2OMe combining imaging photoelectron photoion coincidence spectroscopy with vacuum ultraviolet (VUV) synchrotron radiation and CBS-QB3 theoretical calculations to unveil reaction paths and energetics. In the mass spectrum of MeOCH2OMe, pyrolysis products and radical intermediates were observed at m/z 15 (CH3), 28 (CO), 29 (HCO), 30 (H2CO), 31 (CH2OH), 32 (CH3OH), 45 (CH3OCH2), and 75 (H-loss from methylal). Only the m/z 45 and 75 ions are found to be dissociative photoionization products of MeOCH2OMe, the other mass spectral peaks are attributed to ionization of the neutral MeOCH2OMe pyrolysis products. The m/z 31 peak was assigned to the methoxy radical in the previous studies. However, our photoion mass-selected threshold photoelectron spectrum (ms-TPES) confirms that it originates from dissociative photoionization of the primary pyrolysis fragment methanol. Based on the experimental and computational results, a thermal decomposition mechanism of MeOCH2OMe is proposed. Here, H-migration precedes the production of methoxymethylene (CH3OCH) and methanol, while dimethyl ether and formaldehyde are probably formed in multi-step processes, too. The sequential dissociation of CH3OCH and of dimethyl ether yields enhanced m/z 15, 28 and 29 signals at high temperature. Rate constants have been calculated to confirm the dominant role of MeOCH2OMe isomerization and to help improve predictive combustion models.

How does successive hydrogen addition to PAH ions impact their unimolecular chemistry?

Hydrogenated polycyclic aromatic hydrocarbons (PAHs) have been proposed to contribute to the formation of interstellar H2 by acting as a surface catalyst for the adsorption of hydrogen atoms and desorption of H2 molecules. In the present study, imaging photoelectron photoion coincidence (iPEPICO) spectroscopy and tandem mass spectrometry were employed to examine the unimolecular chemistry of four hydrogenated polycyclic aromatic hydrocarbon ions: 9,10-dihydroanthracene (DHA+•) and 1,2,3,4,5,6,7,8-octahydroanthracene (OHA+•), having opposite patterns of hydrogenation, and 1,2,3,4-tetrahydrophenanthrene (THP+•) and 1,2,3,4,9,10-hexahydrophenanthrene (HHP+•). DHA+• exhibits the same reactions previously observed for 1,2-dihydronaphthalene and 9,10-dihydrophenanthrene, namely competing loss of H• and CH3•. However, the energy required for H•-loss, as predicted by RRKM modeling of the iPEPICO results, was lower than the latter ions, presumably due to charge delocalization across the central ring upon dehydrogenation. OHA+• behaves similarly to ionized tetralin, displaying losses of H•, CH3•, C2H4 and C3H5• in its collision induced dissociation (CID) mass spectra, but under iPEPICO conditions CH3•-loss is not observed. THP+• and HHP+• have aspects of both DHA+• and OHA+• chemistries, displaying losses of H•, CH3•, C2H4 and C3H5•. RRKM modeling produced minimum energies for all observed reaction channels, which were also computationally explored at the B3LYP/6–31+G(d,p) level of theory. The results indicate that small PAH ions may not be effective surfaces for the catalytic formation of H2 in the ISM, but rather sources of small hydrocarbons.

What a little branching can do – Dissociative photoionization of two butanol isomers

The fragmentation processes of two internal energy-selected C4H10O+ isomers, 1-butanol and isobutanol cations, were investigated by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. The first dissociation channel leads to the formation of C4H8+ ions (m/z 56) by water loss in both isomers. Using statistical energy distribution and rate models including the isomerization of the parent ion, the 0 K appearance energies (E0) were determined to be 10.347 ± 0.015 eV and 10.57 ± 0.05 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 10.61 ± 0.02 eV. It appears only as a minor channel in 1-butanol with an E0 of 10.74 ± 0.09 eV. The methanol-loss channel, forming propylene ion, opens up at E0 = 10.94 ± 0.04 eV and 10.72 ± 0.02 eV in 1-butanol and isobutanol, respectively. The next two fragmentation pathways correspond to complementary pair formation of C3H7(+/o) and CH2OH(o/+). The former is assigned as the isopropyl (ion) in both butanol isomers. The channel corresponds to simple bond cleavage in isobutanol with the E0 difference corresponding to the ionization energy difference of the fragments at E0 = 10.97 ± 0.05 eV (C3H7+) and at E0 = 11.11 ± 0.20 eV (CH2OH+). However, there is an internal hydrogen shift necessary in 1-butanol and, therefore, the complementary ions appear at the same E0 = 11.10 ± 0.03 eV, which corresponds to a shared rate limiting transition state. The sequential dissociation product from m/z 56, C3H5+, appears above 11.6 eV as a minor channel in both isomers. Finally, we determined −279.1 ± 1.6 kJ mol−1 as the 298 K heat of formation of gaseous isobutanol. We also propose revised ionization energies for the two butanols isomers based on composite method calculations.