Dissociative Photoionization of Dimethyl Carbonate - American

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Dissociative Photoionization of Dimethyl Carbonate: The More It Is Cut, the Bigger the Fragment Ion Xiangkun Wu, Xiaoguo Zhou, Patrick Hemberger, and Andras Bodi J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b00544 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 22, 2017

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The Journal of Physical Chemistry

Dissociative Photoionization of Dimethyl Carbonate: The More It Is Cut, the Bigger the Fragment Ion Xiangkun Wu,1 Xiaoguo Zhou,1,* Patrick Hemberger,2 and Andras Bodi2,* 1

Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemical

Physics, National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, China 2

Laboratory for Femtochemistry and Synchrotron Radiation, Paul Scherrer Institute, 5232 Villigen,

Switzerland

Abstract The dissociation of internal energy selected dimethyl carbonate (DMC) cations was studied by imaging photoelectron photoion coincidence spectroscopy (iPEPICO) in the 10.3–12.5 eV photon energy range. Vibrational fine structure is observed in the ground state band of the threshold photoelectron spectrum up to the dissociation threshold, and Franck–Condon simulations identify the O–C–O bend mode, in particular in combination with the C=O stretch mode, to be active. The DMC ionization energy was determined to be 10.47 ± 0.01 eV. The first dissociative photoionization product, CH2OH+, dominates the breakdown diagram from 11.0 to 11.8 eV, and is also the lightest fragment ion at m/z = 31. Statistical modeling of the PEPICO data yields a 0 K appearance energy of E0 = 11.14± 0.01 eV. At higher photon energies, parallel dissociation channels are observed leading to the daughter ions C2H6O2+, probably OHCH2CH2OH+(m/z = 62, E0 = 11.16 eV), CH3CHOH+(m/z = 45, E0 = 11.46 eV), and CH3OCO+ (m/z = 59, E0 = 11.47 eV). The m/z = 62 ethylene glycol ion may lose a CHO fragment and yield CH3OH2+ (m/z = 33) in a sequential dissociation at E0 = 11.54 ± 0.03 eV. Reaction path calculations explain the observed dissociation processes, except for the formation of the ethylene glycol cation. Composite method calculations of isodesmic and isomerization reaction energies are used to obtain the enthalpy of formation of dimethyl carbonate at ∆fHo0K(DMC(g)) = –548.3 ± 1.5 kJ mol–1. This puts the heat of formation of the cation at ∆fHo0K(DMC+(g)) = 461.9 ± 1.8 kJ mol–1.

___________________________________________ †

Authors to whom correspondence should be addressed. Electronic mail:[email protected] (A. Bodi), [email protected](X. Zhou)

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1. Introduction Dimethyl carbonate (DMC) is the methyl ester of carbonic acid, and is known to be a weak, but environmentally friendly methylating agent.1It can be used instead of dimethyl sulphate and methyl halides in methylation reactions.2-3 DMC replaces phosgene as a methoxyl carbonylation agent in the formation of polycarbonates and carbamates. Thanks to its low toxicity and biodegradability, and recent advances in its synthesis in large quantities from methanol and carbon dioxide, it has been proposed not only as a green reagent, but also as a green solvent,4and as electrolyte in lithium ion batteries.5Since it is highly miscible with diesel fuels, it has potential as an oxygenated fuel blending agent promising cleaner combustion, thanks to its capability of reducing the concentration of soot precursor radicals.6-7It lowers the vapor pressure of the fuel, decreases carbon monoxide emissions, and does not increase NOx emissions.8-9 DMC has also attracted interest because of its multifaceted conformational behavior, which motivated infrared and microwave spectroscopy as well as computational studies.10-12 Lovas et al. reported the microwave spectrum of cis–cis dimethyl carbonate,13 and also calculated the relative energies of 13 isomers in the C3H6O3 family at the MP2/6-311++G(d,p) level of theory. They found that only three isomers, lactic acid, 3-hydroxypropanoic acid, and hydroxymethylacetate are more stable than DMC, by 55, 53, and 33 kJ mol–1, respectively. Photoionization mass spectrometry (PI-MS) is a powerful technique to study combustion and pyrolysis processes individually and in model systems.14-19PI-MS can be combined with the kinetic energy analysis of the coincident photoelectron in photoelectron photoion coincidence spectroscopy20 to identify reaction or pyrolysis decomposition products isomer selectively.21-22 However, when assigning an m/z peak in the mass spectrum, it is of constant concern whether the detected ion is a parent ion of a neutral reaction product or a fragment ion produced in a dissociative ionization process. Therefore, in pyrolysis or combustion studies using VUV photoionization, the dissociative ionization mechanism of the precursor has to be established in advance.23-24 In addition, the dissociative ionization of DMC may provide insights into reaction mechanisms, as well as quantitative energetics data, such as ionization and activation energies, and, should any of the dissociative ionization thresholds correspond to dissociation energies, even heats of formation.25 2


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Highly oxygenated compounds often ionize dissociatively even at the ionization onset,26 and it is advantageous to establish a photon energy window in which the parent ion is stable. Because the parent ion peak is observed in the electron ionization mass spectrum of DMC,27 we are hopeful that such a window exists. Thus the decay mechanism of energetic DMC+ parent ions can be understood by using imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. At the same time, a statistical model of the dissociative photoionization will enable us to predict DPI processes as a function of sample temperature, as well, facilitating data analysis at elevated temperatures. The photoionization of DMC was studied in the 1970s by Meeks and McGlynn et al.,28-29and Wittelet al.30 by low resolution photoelectron spectroscopy. They assigned the photoelectron bands based on molecular orbital calculations and observed no vibrational fine structure. The overall DPI mechanism of DMC has not yet been studied. Appearance energies have been reported by electron ionization for m/z = 59 (11.5 or 12.2 eV)31-32and m/z = 60 (11.5 eV).The large difference in the former set of values does not justify the claimed 0.05–0.08 eV error bar, and the latter ion has not been observed in the experiments herein. Thus, it appears that some of these onsets have not stood the test of time. The dissociative photoionization of DMC, CH3OC(=O)OCH3, can be related to that of other small esters, as discussed more recently by Wang et al.33Generally, they found that dissociation channels often arise from complex isomerization processes in competition with direct bond breaking in the parent ion. This was also observed by Suh et al. for the methyl glycolate cation, HOCH2C(=O)OCH3+,34 in which HCO-loss takes place by double hydrogen transfer by the H-bonded intermediate H2C=O…H…O=COCH3+. Methyl glycolate cations may also rearrange to DMC+, which Suh et al. identified as the driving force for CH2OH+ (m/z = 31) formation.Methyl-β-hydroxypyruvate

[CH3OC(=O)C(=O)CH2OH]

and

dimethyl

oxalate [CH3OC(=O)C(=O)OCH3]

are also similar compounds to DMC. In

dissociative photoionization, both may lose CO to form a C3H6O3+ ion, isomer with DMC+.35-36 However, m/z = 60 and 61, i.e.,HOCOCH3+ and HOC(H)OCH3+ have been identified as the main collision induced dissociation and dissociative photoionization products of these C3H6O3+ isomers. Although the involvement of the DMC+ was invoked to explain isotope mixing in the fragmentation of the above mentioned ions, m/z = 60 and 61 ions are not observed as major products in the DPI 3



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of DMC (vide infra). This leads us to believe that we can rule out the isomerization of the DMC cation to the above mentioned H-bonded intermediate or methyl glyconate, and the reverse process, i.e. the formation of DMC+, is irreversible and quickly followed by dissociation. Thus, the fragmentation of DMC+ takes place in a comparatively separated subspace of the C3H6O3+ potential energy surface compared with the isomers studied before in detail. Trikoupis et al. studied another interesting dissociative ionization process of dimethyl oxalate, in which the molecular ion seemingly breaks in half, and does so 0.5 eV (50 kJ mol–1) below the calculated barrier to CH3OC=O + CH3OC=O+ formation.37Collision induced dissociation experiments unveiled the true mechanism, in which CO2 and CH3 are released without the intermediacy of the less stable CH3OCO radical. Since CO2 + CH3 are more stable separately than bound,31, 38 it is no surprise that 59 amu loss takes place in two steps. The intriguing aspect is that the intermediate, 15 or 44 amu loss cation has not been detected. In dimethyl oxalate, the proposed reaction energy curve involves a CO2-loss intermediate CH3OCOCH3+ potential energy well of at least 0.7 eV or almost 70 kJ mol–1.Them/z = 54 intermediate could be stabilized by kinetic energy release or rovibrational excitation of the leaving CO2 molecule, but has not been observed experimentally. A bifurcated CH3 + CO2 or CO2 + CH3 loss reaction pathway was identified in the DPI of acetic acid anhydride [CH3C(=O)OC(=O)CH3], in which the intermediate CO2-loss acetone cation also sits in a 0.8 eV deep potential energy well and is only formed with 0.16 eV excess energy at the transition state.25 However, the intermediate acetone cation as well as the minor methane-loss channel from the acetone cation39could be observed in very small amounts here. Generally, both in these dissociation mechanisms and in the water loss in adipic acid at the ionization onset,26 the underlying theme is that isomerization reactions leading to a significantly more stable constitutional isomer are associated with low barriers on the cation potential energy surface in these oxygenated organic compounds. This opens up fragmentation channels, sometimes even sequential ones, at energies much lower than any of the bond energies in the parent ion. The dissociative ionization mechanism of DMC, in particular the production of the first and lightest fragment at m/z = 31 will also be shown to be best explained by sequential methyl radical and carbon dioxide loss with an unseen and short-lived intermediate. We report imaging photoelectron photoionization coincidence (iPEPICO)40data 4


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on DMC taken at the VUV beamline of the Swiss Light Source, use quantum chemical calculations to establish the dissociation mechanism of the DMC+ cation, and set up a statistical model to determine experimental 0 K appearance energies for the DPI channels. Plotting and analyzing the fractional threshold ionization parent and daughter ion abundances in the breakdown diagram has been shown to reveal directly complex dissociative photoionization mechanisms involving multiple isomerization steps.41-43We have determined accurate 0 K appearance energies for the main DPI channels leading to the fragments of m/z = 31, 62, 33, 45, and 59. The small abundance of m/z = 61 and 60 confirms that DMC+ is unlikely to form the hydrogen bonded intermediate H2C=O…H…O=COCH3+, known to be an important crossroads in the dissociation of other C3H6O3+ isomers. Furthermore, the threshold photoelectron spectrum shows vibrational fine structure at the ionization onset, which is modeled to confirm the adiabatic ionization energy of DMC. Composite method calculations are used to derive the heat of formation of DMC, which anchors the derived appearance energies to the thermochemical heat of formation scale.

2.Methods 2.1 Experimental approach The photoionization experiments were performed at the VUV beamline of the Swiss Light Source, Paul Scherrer Institute, using the Imaging Photoelectron Photoion Coincidence (iPEPICO) endstation. A detailed description of the beamline and the iPEPICO experiment can be found elsewhere,40, 44and only a brief overview is given here. Bending magnet synchrotron radiation is collimated, dispersed by a grazing incidence monochromator using a 600 lines/mm laminar grating, and focused into a differentially pumped gas filter to suppress higher-order radiation. The gas filter is filled with 10 mbar of a neon, argon and krypton mixture, effectively extinguishing radiation above 14 eV in a 10 cm long absorption chamber. The high-harmonic free, monochromatic VUV radiation enters the ionization chamber of the iPEPICO endstation and ionizes the sample in a 4×2mm2cross section interaction region. The photon energy is calibrated using argon autoionization lines, recorded in first and second order of the grating, and the photon energy resolution is 3–4 meV. Dimethyl carbonate was purchased from Sigma–Aldrich and used without further purification. It was kept at room temperature in a sample holder and seeded 5



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into the ionization chamber through a needle valve and a 6 mm outer diameter Teflon tube. The typical pressure in the experimental chamber was3×10–6 mbar during measurement. In the interaction region, the VUV light crosses the effusive sample beam and the resulting photoelectrons and photoions are extracted in opposite directions by a constant, 120Vcm–1, electric field. The electrons are then velocity map imaged onto a position sensitive Roentdek DLD40 delay-line detector with a kinetic energy resolution better than 1 meV at threshold. As their time of flight (TOF)is negligible, electron hit times can be used as start signal for the ion TOF analysis. The ions are extracted in a 5 cm long first extraction region, then undergo further acceleration to ensure space focusing conditions at a Jordan TOF C-726 microchannel plate detector at the end of a 55 cm long field-free drift region. The arrival times of the photoelectrons are correlated with that of the photoions to obtain the ion TOF distributions. The ion hits as stop signals and arrival times are recorded using a triggerless multiple-start/multiple-stop data acquisition scheme.45 Threshold electrons are imaged onto a central spot on the detector together with kinetic energy electrons with negligible off-axis momentum component. The latter make up the hot electron contamination, which can be approximated based on a small ring around the center spot and subtracted from the center signal as proposed by Sztáray and Baer.46 The threshold photoelectron signal is plotted as a function of the photon energy in the threshold photoelectron spectrum (TPES). The coincident threshold photoionization mass spectra are two dimensional, i.e., they correspond to the ion signal as a function of TOF and photon energy. The fractional parent and fragment ion abundances are plotted in the breakdown diagram as a function of photon energy, which shows the open fragmentation channels and their branching ratio as a function of photon energy, and is independent of the photon flux or the sample pressure, and only depends on the temperature and, in the case of a slow dissociation, the residence time of the parent ion in the extraction region. The use of a relatively low extraction field leads to ion residence times in the acceleration region of several µs, and yields a quasi-exponential daughter ion peak shape in metastable dissociations. It is indicative of the dissociation rate constant in the kinetic time window, in our case 103 s–1

Dissociative Photoionization of Dimethyl Carbonate - American

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