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C1 coupling reactions over zeolite catalysts are central to sustainable chemical production strategies. However, questions persist regarding the involvement of CO in ketene formation, and the impact of this elusive oxygenate intermediate on reactivity patterns. Using operando photoelectron photoion coincidence spectroscopy (PEPICO), we investigate the role of CO in methyl chloride conversion to hydrocarbons (MCTH), a prospective process for methane valorization with a reaction network akin to methanol to hydrocarbons (MTH) but without oxygenate intermediates. Our findings reveal the transformative role of CO in MCTH at the low pressures, inducing ketene formation in MCTH and boosting olefin production, confirming the Koch carbonylation step in the initial stages of C1 coupling. We uncover pressure‐dependent product distributions driven by CO‐induced ketene formation, and its subsequent desorption from the zeolite surface, which is enhanced at low pressure. Inspired by the above results, extension of the co‐feeding approach to CH3OH as another simple oxygenate showcases the additional potential for improved catalyst stability in MCTH at ambient pressure. [ABSTRACT FROM AUTHOR]

Pyrolysis and low-temperature oxidation of dimethoxymethane (methylal, MeOCH 2 OMe) 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 MeOCH 2 OMe isomerization is sometimes disregarded. In this work, we investigate the pyrolysis of MeOCH 2 OMe 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 MeOCH 2 OMe, pyrolysis products and radical intermediates were observed at m/z 15 (CH 3), 28 (CO), 29 (HCO), 30 (H 2 CO), 31 (CH 2 OH), 32 (CH 3 OH), 45 (CH 3 OCH 2), and 75 (H-loss from methylal). Only the m/z 45 and 75 ions are found to be dissociative photoionization products of MeOCH 2 OMe, the other mass spectral peaks are attributed to ionization of the neutral MeOCH 2 OMe 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 MeOCH 2 OMe is proposed. Here, H-migration precedes the production of methoxymethylene (CH 3 OCH) and methanol, while dimethyl ether and formaldehyde are probably formed in multi-step processes, too. The sequential dissociation of CH 3 OCH 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 MeOCH 2 OMe isomerization and to help improve predictive combustion models. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Dissociation of Cr(CO) 6 + ions was studied by Photoelectron Photoion Coincidence. • Cr(CO) 6 + ions dissociate by losing all six CO ligands, yielding the Cr+ ion. • By modeling the breakdown diagram and fitting rate curves to measured dissociation rates, bond energies were obtained. • Repulsive doublet states lead to non-statistical dissociation at intermediate energies. • Bond energies disagree with literature, leading to revised Cr(CO) 6 thermochemistry. Abstract The fragmentation processes of internal energy selected chromium hexacarbonyl cations, Cr(CO) 6 +, were investigated by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy at the VUV beamline of the Swiss Light Source. In the 9.3–21.5 eV photon energy range, Cr(CO) 6 + dissociates by six sequential carbonyl ligand losses. The fragment ion fractional abundances, plotted in the breakdown diagram, along with the time-of-flight mass spectra for the first three metastable CO-loss channels were modeled using a statistical approach. Between 12 and 16 eV, the statistical model overestimates the degree of fragmentation, which is explained by enhanced kinetic energy release in impulsive CO loss on repulsive electronic states of the parent ion Cr(CO) 6 +, as confirmed by TD-DFT calculations. This is the first reported example for an embedded non-statistical unimolecular dissociation regime, bracketed by statistical regimes at low and at high energies. The statistical model was employed to derive 0 K appearance energies for Cr(CO) n + (n = 0–5). The Cr(CO) 5 + appearance energy and the literature CO Cr(CO) 5 + bond dissociation energy yield an adiabatic Cr(CO) 6 + ionization energy of 8.195 ± 0.120 eV, independent of photoelectron spectroscopy measurements. The 0 K appearance energy of Cr+ is 61 kJ mol–1 higher than predicted based on literature enthalpies of formation, which we suggest is most likely due to an error in the enthalpy of formation of Cr(CO) 6 , and propose a revised Δ f H [Cr(CO) 6 , g] = –972.1 ± 4.1 and –968.9 ± 4.1 kJ mol–1 at 0 and 298 K, respectively, and Δ f H 298K [Cr(CO) 6 , c] = –1040.6 ± 4.2 kJ mol–1 for the crystalline state, based on the known enthalpy of sublimation. The measured Cr CO bond dissociation energies in [(CO) n Cr CO]+ (n = 0–5), and the enthalpies of formation of the chromium carbonyl ion series are also reported. [ABSTRACT FROM AUTHOR]

Dimethyl ether (DME) is a widely recognized alternative fuel which can be sustainably produced from different feedstock. Its oxidation mechanism belongs to the most deeply understood within oxygenated fuels. The oxidation of DME in the presence of NO has gained renewed attention in recent experimental and modeling efforts because of a rather complex behavior in accelerating or inhibiting DME consumption, depending on the respective temperature and mixture conditions, similarly to what has been already observed for n- and iso- alkanes. The present investigation focuses on the interaction chemistry of DME–O 2 –NO mixtures in the low- to intermediate-temperature regime. Previously reported flow reactor data from mass spectrometric analysis with and without NO addition are extended by isomer-resolved detection of some key intermediates, including species of formula {HNO 2 } and {CH 3 NO 2 } by using double-imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy. Specifically trans- HONO is highlighted as the most abundant isomer. Starting from the recently published CRECK mechanism for DME oxidation and other models available in the literature, the relevant kinetics of DME/NO x interactions are included and analyzed. The model thus obtained is compared with new experimental data from this study and others from the literature and is used to interpret observed discrepancies. A systematic polynomial chaos expansion (PCE) analysis is also performed to assess the joint uncertainty of key influential reactions under the present conditions. Nevertheless, remaining differences of model and experiment can only be addressed jointly, demonstrating the value of a concurrent experimental–modeling approach. [ABSTRACT FROM AUTHOR]

Photoelectron photoion coincidence measurements have been performed for the thiazole (C3H3NS) molecule in gas phase, using time-of-flight mass spectrometry in the electron-ion coincidence mode and vacuum ultraviolet synchrotron radiation. photoelectron photoion coincidence spectra have been recorded as a function of the photon energy covering the valence range from 10 to 21 eV. The resulting photoionization products as well as the dissociation pathways leading to the ionic species were proposed and discussed. We have also performed density functional theory and ab initio calculations for the neutral molecule, its cation and the ion fragments produced in order to determine their electronic and structural parameters. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Molecular LiCl and Li2Cl2 have been studied in the vapor phase with valence photoelectron and photoelectron–photoion coincidence spectroscopies. These two techniques determine the binding energies in fundamentally different ways. Binding energies obtained from photoelectron spectra are usually taken as the vertical ionization energies of the corresponding electronic states. In cases with several overlapping bands, corresponding to different electronic states, the coincidence measurement can separate the bands if the respective final states fragment differently. This applies well to the monomer case. To facilitate the determination of state-specific ionization energies in the dimeric molecule, a theoretical Franck–Condon analysis has been carried out. Moreover, ab initio coupled-cluster and density-functional-theory calculations have been used to analyze the fragmentation pattern based on asymptotic dissociation energies. The fragmentation pattern is largely common to all the accessible valence-ionized states of the dimer, consistent with rapid conversion to the ionic ground state before fragmentation. However, the highest-lying state of Li2Cl2 +, 2 A g , shows enhanced propensity for Li+ as dissociation product. [Copyright &y& Elsevier]

Abstract: Dissociation of acetonitrile molecules following resonant core excitations of carbon and nitrogen core electrons to the LUMO orbital was investigated using the electron-energy-resolved photoelectron–photoion coincidence (PEPICO) technique. The fragment ion mass spectra were recorded in coincidence with the resonant Auger electrons, emitted in the decay process of the core-excited states. Deuterated and 13 C-substituted samples were used for fragment identification. The results showed the initial core hole localization to be of minor importance in determining the dissociation pattern of the molecular cation. The participator and spectator Auger transitions produce entirely different fragmentation patterns and the latter indicates that complex nuclear rearrangements take place. Using also the findings from the 13 C labelling, it is suggested that the bending motion, induced in the core-excited state by the Renner–Teller effect, plays an important role in determining the most intense dissociation pathways of the spectator Auger final electronic states. [Copyright &y& Elsevier]