Dissociative Photoionization of Diethyl Ether - The Journal of Physical

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Dissociative Photoionization of Diethyl Ether Krisztina Voronova, Chrissa M. Mozaffari Easter, Kyle J. Covert, Andras Bodi, Patrick Hemberger, and Balint Sztaray J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b08091 • Publication Date (Web): 07 Oct 2015 Downloaded from http://pubs.acs.org on October 7, 2015

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The Journal of Physical Chemistry

Dissociative Photoionization of Diethyl Ether

Krisztina Voronova,† Chrissa M. Mozaffari Easter,† Kyle J. Covert,† Andras Bodi,‡ Patrick Hemberger,‡ Bálint Sztáray†,*

†

Department of Chemistry, University of the Pacific, Stockton, CA 95211, USA.

‡

Molecular Dynamics Group, Paul Scherrer Institut, Villigen 5232, Switzerland.

*

Phone: (209) 946-2654; fax: (209) 946-2607; e-mail: [email protected].

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Dissociative Photoionization of Diethyl Ether ABSTRACT The dissociative photoionization of internal energy selected diethyl ether ions was investigated by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. In a large, 5 eV energy range Et2O+ cations decay by two parallel and three sequential dissociative photoionization channels, which can be modeled well using statistical theory. The 0 K appearance energies of the CH3CHOCH2CH3+ (H-loss, m/z = 73) and CH3CH2O=CH2+ (methyl-loss, m/z = 59) fragment ions were determined to be 10.419 ± 0.015 and 10.484 ± 0.008 eV, respectively. The reemergence of the hydrogen-loss ion above 11 eV is put down to transition state switching, in which the second, outer TS is rate determining at high internal energies. At 11.81 ± 0.05 eV, a secondary fragment of the CH3CHOCH2CH3+ (m/z = 73) ion, protonated acetaldehyde, CH3CH=OH+ (m/z = 45) appears. Based on the known thermochemical onset of this fragment, a reverse barrier of 325 meV was found. Two more sequential dissociation reactions were examined, namely ethylene- and formaldehyde-loss from the methyl-loss daughter ion. The 0 K appearance energies of 11.85 ± 0.07 and 12.20 ± 0.08 eV, respectively, indicate no reverse barrier in these processes. The statistical model of the dissociative photoionization can also be used to predict the fractional ion abundances in threshold photoionization at large temperatures, which could be of use in, e.g., combustion diagnostics.


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INTRODUCTION Diethyl ether, Et2O, is widely used as solvent, engine starting fluid, and as an octane and oxygen enhancer in internal combustion engines.1-6 It is also known to become unstable and form peroxides spontaneously upon exposure to air and UV radiation, a process to which numerous laboratory accidents can be ascribed.7-8 The Et2O oxidation mechanism is therefore of interest in combustion-relevant environments,9-10 as well as in the atmosphere.11 Thermochemical input parameters have been shown to be the major source of uncertainty in combustion models,12 which spurred us to revisit the dissociative photoionization of Et2O with a view to confirm or revise enthalpies of formation. Photoelectron photoion coincidence techniques with vacuum ultraviolet (VUV) synchrotron radiation have recently been embraced as a highly isomer specific detection tool13 in flame diagnostics.14-16 Electron kinetic energy analysis, usually with velocity map imaging, makes it possible to record the threshold photoionization signal corresponding to a given m/z of the coincident photoion. As a function of the photon energy, this yields the mass selected threshold photoelectron spectrum (ms-TPES). It is important to emphasize, however, that the ms-TPES corresponds to the mass of the photoion, which may result from dissociative photoionization of the precursor neutral, as well. The fractional ion abundances in threshold photoionization can also be plotted in the breakdown diagram as a function of the photon energy. Rather complex unimolecular reaction mechanisms can often be unveiled with only a cursory analysis of the breakdown diagram of pure precursors.17-18 Information in the breakdown curves can also be used in the ms-TPES analysis of reaction mixtures, to apportion the ms-TPES signal to dissociative photoionization of the intact precursor and to photoionization of the reactive intermediate.19 This is trivial if the dissociative photoionization of the parent molecule does not interfere with the ms-TPES signal of


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interest,20 but an a priori knowledge of the precursor breakdown diagram may help signal apportioning in mixtures. Our secondary motivation is to construct a statistical model to describe the room temperature breakdown diagram of Et2O,21 which will also make it possible to predict it at higher temperatures, relevant in combustion experiments. The third motivation is that energy selected diethyl ether cations dissociate into small oxygenated ionic species of considerable interest, by several but tractable parallel and sequential dissociative photoionization channels in a wide energy range. On the one hand, breakdown diagrams of comparable range have been reported and analyzed within a statistical framework for organometallic compounds22-23 and Arduengo-type carbenes.17 On the other, nonstatistical aspects and a de-coupling between the electronic and nuclear degrees of freedom were observed in methanol24 as well as in halogenated organic25-26 and organometallic compounds.27 As yet another organic molecule, diethyl ether can thus offer further insights into the applicability of the statistical approach to the decay of highly energetic intermediates. In the absence of a well-resolved peak for the 0–0 transition in the photoelectron spectrum of Et2O, its ionization energy is poorly known. NIST reports an evaluated 9.51 ± 0.03 eV value, 0.10 eV below the band maximum at the vertical IE at 9.61 eV.28 Botter et al. argued that the maximum in the ground state photoelectron band at 9.61 eV in fact corresponds to the adiabatic IE, and also reported the 0 K appearance energies of the two main fragment ions, C4H9O+ (m/z = 73, by H-loss from Et2O, 10.37 ± 0.02 eV) and C3H7O+ (m/z = 59, by CH3-loss from Et2O, 10.48 ± 0.02 eV).29 They noted that the H-loss signal plateaus at a photon energy of 10.5 eV first, but starts to rise again just below 11 eV. They also reported on the formation of fragment ions C3H5+, CH2OH+, CH3CHOH+, and C2H5+, at appearance energies of 11.6, 11.7, 11.8, and 12.5 eV, respectively. Later, Koizumi et al.30 and ACS Paragon Plus Environment

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Cool et al.31 re-measured the low-energy part of the photoionization efficiency curves with a view to obtain absolute photoionization cross sections to aid combustion diagnostics. The dissociation mechanism of the Et2O cation and the possible fragment ions have been discussed at length in the literature. Concerning the first H-loss, Botter et al. surmise a CH3CHOCH2CH3+ structure, which appears to be the most stable isomer accessible by simple bond rupture. The C2H5O+ fragment ion at m/z = 45 may be a secondary fragment of the Hloss ion. McLafferty32 and Harrison et al.33 suggested and Philips et al. later confirmed by deuteration34 that its structure is CH3CH=OH+, protonated acetaldehyde. Thus, the loss of a methylene hydrogen is followed by H-transfer to the oxygen from the opposing methyl group so that ethylene can be eliminated. Because of the early onset of the methyl-loss channel, its product ion can easily be identified as CH3CH2O=CH2+. From this fragment ion, at higher energies and after a 1,3hydrogen shift, an ethylene molecule can be eliminated in a consecutive dissociation reaction to form hydroxymethyl ions, CH2OH+ (m/z = 31).35-37 The mechanism of the small but measurable parallel H2O-loss from CH3CH2O=CH2+ is far from obvious; a series of ion–neutral complexes are involved. First, a 1,3-hydrogen migration leads to a protonated formaldehyde–ethylene complex. The CH2OH+ moiety rotates, and protonated allyl alcohol, CH2=CHCH2OH2+ is formed by multiple hydrogen shifts. This intermediate easily loses water, and yields the allyl cation (CH2CHCH2+, m/z = 41).37

EXPERIMENTAL AND THEORETICAL APPROACH Et2O was purchased from Sigma–Aldrich and introduced into the ionization chamber of the imaging Photoelectron Photoion Coincidence (iPEPICO) endstation38 at the VUV beamline39 of the Swiss Light Source. Room temperature Et2O was seeded through a 6 mm Teflon tube ACS Paragon Plus Environment

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from the headspace of a glass vial directly into the ionization region of the experimental chamber. The pressure in the experimental chamber was set to 1–3 × 10–6 mbar using a needle valve. VUV synchrotron radiation was collimated, dispersed in grazing incidence by a 600 grooves/mm laminar grating, focused at the exit slit in a differentially pumped gas filter at a resolution of 3–5 meV and used to ionize the sample in a 2 mm × 2 mm interaction region. Higher harmonic radiation was suppressed by a 10 cm long chamber in the gas filter filled with 10 mbar of Ne or a Ne–Ar–Kr mixture, depending on the photon energy. The photon energy was calibrated using Ar 11sʹ–14sʹ autoionization lines in the first and second order of the grating. A constant, 40 or 120 V cm–1 electric field was used to extract photoions and photoelectrons from the interaction volume in opposite directions. The electrons, which provided the start signal for the ion time-of-flight analysis, were kinetic energy analyzed using velocity map imaging on a Roentdek DLD40 position sensitive delay-line detector with sub-meV kinetic energy resolution at threshold. Zero kinetic energy, threshold electrons are detected in the center of the image, while non-zero kinetic energy electrons with an off-axis momentum component are detected in a ring around the center image. The contamination of the threshold electron signal by “hot”, kinetic energy electrons without an off-axis momentum component was approximated by the average count rate in a ring area around the center spot, and the corresponding coincidence spectrum was subtracted from the center signal to keep only coincidences with true threshold electrons.40 The ions were mass analyzed by a twostage Wiley–McLaren time-of-flight mass spectrometer41 with a 5.5 cm long extraction, a 1 cm long acceleration region, and a 55 cm drift region and detected by a Jordan TOF C-726 microchannel plate detector. The long extraction region, with 40 or 120 V cm–1 electric field results in ion residence times on the order of several µs. Metastable parent ions that dissociate in the extraction region are, thus, detected at a TOF between the daughter ion and parent ion times of flight, and yield quasi-exponential TOF distributions characteristic of the first-order ACS Paragon Plus Environment

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dissociation rate constant.21 Breakdown diagrams are constructed by plotting the fractional ion abundances in the coincidence spectra as a function of photon energy. The Gaussian 09 suite of programs was used to carry out quantum chemical calculations.42 Vibrational frequencies and rotational constants at the B3LYP/6-311++G(d,p) level of theory were used to calculate the thermal energy distribution of the neutral precursor molecules, as well as densities and numbers of states in the rate equation (see below) and to obtain the internal energy distribution of intermediate fragments based on statistical distribution of product internal energies.43 Reaction paths and approximate transition state (TS) structures were located by constrained optimizations in which either a bond length or a bond angle was scanned, or by using the Synchronous Transit-Guided Quasi-Newton (STQN) method.44-45 The minimum energy structures and saddle points that likely play a role in the dissociative photoionization process were also evaluated using the G4 composite method.46 Even though the enthalpy of formation of Et2O appears to be well established (see Table 1),47 isodesmic reaction energy calculations were carried out using the G4, CBS-APNO, and W1U methods for confirmation.48-49 Concerning the ionization energy of Et2O, Botter et al. suggested that the 100 meV difference between the onset of the Et2O ion signal and the band maximum was due to hot bands and that the adiabatic and vertical ionization energies coincided in Et2O. This has not been considered by NIST, and composite method calculations have been carried out on the ionization energy, too, in order to provide quantum-chemical support for resolving this issue. Details of the RRKM and energy distribution calculations within the statistical framework, used to model the experimental data have been described in detail earlier.21 Briefly, we employ statistical thermodynamics and rate theories to calculate the thermal energy distribution, dissociation rate constants, branching ratios, and product energy distributions. Model parameters, such as appearance energies and transitional frequencies are ACS Paragon Plus Environment

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subsequently varied to fit the model to the experimental results. The unimolecular rate constant of each dissociation pathway, k(E), can be calculated as 𝑘 𝐸 =

σ𝑁 ‡ 𝐸 − 𝐸! , ℎ𝜌 𝐸

where σ is the reaction symmetry, N‡(E – E0) is the sum of states of the transition state from 0 to E – E0, h is Planck’s constant, and ρ(E) is the density of states of the parent ion at energy E.50 The densities and sums of states are calculated using harmonic vibrational frequencies by the Beyer–Swinehart direct count algorithm.51

RESULTS AND DISCUSSION

Dissociative Photoionization Processes Time-of-flight mass spectra of internal energy selected Et2O+ ions [1] were collected in the 10.0–15.0 eV photon energy range using 120 V cm–1 constant extraction field to map out the valence photoionization fragmentation processes which are shown in Scheme 1.

[14] [7]

[4]

[1]

[8]

O

O

O

O

+ H + CH 3

+H

H

+ CH 3

H

+ CH2O + CH 3

[13] H

O

+ C2H 4 + CH 3

Scheme 1. General dissociative photoionization scheme for the energy selected Et2O+ cations [1] in the 10.0–15.0 eV photon energy range (for structures of the intermediates and transition states see Fig. 6).


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The relatively low field combined with a long extraction region is essential to obtain unimolecular rate information manifested in asymmetric TOF distributions. As a side effect, the instrumental peak widths are too large for baseline separation of the parent and the hydrogen-loss daughter ions [4]. Therefore, a deconvolution procedure was applied to extract the fractional abundances.52-53 The center of gravity of the parent and daughter ion band is determined according to µ=

∫ 𝑡 ∙ TOF 𝑡 d𝑡 ∫ TOF 𝑡 d𝑡

,

where µ is center of mass of the combined peaks for which the time-of-flight spectrum, TOF(t) is integrated. Then, the fractional abundance (a and [1 – a]) of the two peaks can be determined by solving the following equation: µ = at1 + (1 – a)t2 where a, 0 ≤ a ≤ 1, is the contribution of the parent ion; t1 and t2 are the arrival times of the C4H10O+ (11.23 µs) [1], and C4H9O+ (11.16 µs) [4] ions, respectively. To plot the breakdown diagram with two parallel dissociation channels where the second daughter ion, the methylloss product [8] here, is well-separated from the parent ion peak, the equation had to be modified: µ = [at1 + (1 – b – a)t2] / (1 – b) where b represents the fractional abundance of the well-separated daughter ion. Because the parent and H-loss fragment ions are not baseline separated, the kinetic information, which could otherwise be obtained by center of gravity analysis of the fragment ion,21 cannot be extracted from the H-loss peak. However, as the first-order dissociation rate constant, which governs the asymmetric shape of the daughter ions, belongs to the dissociation of the parent ion, it could also be extracted from another parallel channel. In the case of diethyl ether, this is the methyl loss channel, which rises at almost the same energy as the hydrogen atom loss ACS Plus Environment from the α-carbon. Consequently, theParagon total rate constant of dissociative photoionization can be

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determined based on the methyl-loss daughter peak, and the breakdown diagram yields branching ratios, thus the absolute rate constants for both processes. As there is very little asymmetry in the methyl-loss peak shape, we have also carried out measurements using lower, 40 V cm–1 extraction field, in the 10.00–10.75 eV region. The fractional abundances of the Et2O+ and, methyl- [8] and hydrogen-loss [4] fragment ions obtained with different extraction fields are compared in Fig. 1.

Fig. 1. Comparison of the breakdown diagrams obtained with 120 V cm–1 (solid polygons) and 40 V cm–1 (crosses) extraction fields for Et2O in the 10.00–10.75 eV photon energy range.

The dissociation mechanisms can be predicted based on the overall appearance of the breakdown diagram (Fig. 2, for structures see Scheme 1 and Fig. 6). Below 11 eV, the parent ion CH3CH2OCH2CH3+ (m/z = 74) [1] and its first two fragments, C4H9O+ [4] by H loss and C3H7O+ [8] by methyl loss (m/z = 73 and 59, respectively) were detected. Based on the TOF ACS Paragon Plus Environment

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distribution and the fact that the methyl-loss channel overtakes H-loss rapidly, the latter probably corresponds to a tight transition state. Methyl loss, on the other hand, yields wellresolved symmetric fragment ion peaks, which indicates it to be a fast dissociation leading to CH3−CH2−O=CH2+ [8]. Above 11 eV, a puzzling feature can be observed, namely the reemergence of the C4H9O+ ion [4] signal. This was also noted by Botter et al.,29 and we propose it is caused by transition state switching in the H-loss channel: as the internal energy increases, the effective transition state for this reaction becomes the loose one.54 In the absence of low-lying electronically excited states, this phenomenon cannot be the result of a non-statistical dissociation taking place on an excited electronic surface. Furthermore, the gentle slope of the C3H7O+ curve [8] in the 11.0–11.7 eV photon energy range also suggests a competing parallel dissociation. Above 11.7 eV, the sharp rise in the CH3O+ [13] signal and the coincident drop in the C3H7O+ [8] abundance indicates a consecutive channel from the methyl-loss product leading to CH2=OH+ [13] by C2H4-loss at a phenomenological appearance energy of about 11.8 eV. At approximately the same energy, C3H5+ (m/z = 41) also appears as a minor channel with a maximum intensity of less than 2%. This minor product ion can be formed by a complex H2O-loss step with a consecutive dissociation from the CH3−CH2−O=CH2+ (m/z = 59) ion [8], and has not been included in the statistical model. The CH3CH=OH+ [7] (m/z = 45) fragment ion, which appears at around 11.8 eV, is the product of a sequential dissociation channel from C4H9O+ [4] by ethylene loss. The C2H5+ [14] (m/z = 29) fragment ion appears above 12 eV, formed by a consecutive C–O bond cleavage from C3H7O+ [8].


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Fig. 2. Breakdown diagram for Et2O in the 10 to 15 eV photon energy range. Open polygons are experimentally measured ion abundances, and lines are the best-fit modeling of the data.

Statistical Modeling of the Dissociation Processes The adiabatic ionization energy of Et2O was reported to be 9.52 ± 0.07, 9.50 ± 0.01 and 9.61 by Bowen and MacColl,55 Cocksey et al.,56 and Botter et al.,29 respectively. In order to examine these values, we calculated the ionization energy of Et2O using the G3, G4, CBSQB3, CBS-APNO and W1U composite methods to be 9.60, 9.55, 9.62, 9.58, and 9.60 eV, respectively. Different geometry optimization, electron correlation, and basis set extrapolation approaches all point to the same conclusion with remarkable confidence, namely that Botter et al. were right to claim that the vertical and adiabatic ionization energies coincide and that the photoionization signal at lower energy is in fact due to hot bands. Therefore, we use 9.60 eV as the ionization energy in the parent ion density of states calculation in the statistical model. A similarly large discrepancy but with an opposite sign has been observed between the ACS Paragon Plus Environment

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previously published and the true adiabatic ionization energy of ethanol.57 In the latter case, negligible Franck–Condon factors for the origin transition make adiabatic ionization inaccessible by one photon transition. Both cases lend weight to the observation that ionization energies derived from photoelectron spectra may carry systematic errors several times larger than the reported error bars unless the vibrational fine structure is observed and, whenever practicable, modeled.58 In the first step of the modeling procedure, the two hydrogen-loss channels [4], one with a tight and one with a loose transition state, were fitted together with methyl-loss fragmentation channel [8], up to 11 eV. Asymmetric daughter ion peak shapes are characteristic for slow dissociations. In this case, however, we can firmly establish that the well-separated methyl-loss peak is symmetric, which indicates no kinetic shift and allows the determination of 0 K thresholds with a small error. The relaxed model temperature, 320 K in the best fit, agreed reasonably with the experimental temperature of 300 K. As both the 120 V cm–1 and 40 V cm–1 field measurements confirmed that the kinetic shift is very small, the experimental data contains accurate information only on the relative tightness of the hydrogen- [4] and methyl-loss [8] channels. This was confirmed by varying the tightness of the transition states corresponding to both of these channels and finding that irrespective of the absolute activation entropies, best fit was obtained with the same activation entropy difference. Thus, in the final fitting to the breakdown curves and the TOF peak shapes in the 120 V cm–1 data, the transition state vibrational frequencies of the hydrogen-loss channel were set at the B3LYP/6-311++G(d,p) calculated values and the tightness of the methyl-loss transition state was optimized. The relative activation entropy between the formation of the methyl-loss CH3−CH2−O=CH2+ [8] and the H-loss C4H9O+ fragment ions [4] was found to be ∆∆‡S600K = 65 ± 1 J (K mol)–1 with 0 K appearance energies (E0) of 10.484 ± 0.008 and 10.419 ± 0.015 eV, respectively. ACS Paragon Plus Environment

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These appearance energies (Fig. 2) and reaction rates (Fig. 3) were also confirmed by fitting the center of gravity of the Et2O+ and H-loss product ion combined peaks in the timeof-flight spectra obtained at 40 V cm-1 extraction field (Fig. 4), based on which the appearance energies of methyl- [8] and hydrogen-loss [4] fragment ions were determined to be 10.485 ± 0.008 eV, and 10.417 ± 0.015, respectively.

Fig. 3. RRKM rate constants plotted as a function of parent ion internal energy. *Formation of C4H9O+ fragment ion [4] through a loose, outer (i.e., centrifugal) TS near the bimolecular products.


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Fig. 4. The center of gravity of the Et2O+ and H-loss product ion peaks obtained with 40 V cm-1 extraction field. Open squares are experimentally measured data, and the line is the bestfit model.

For technical reasons, the sequential dissociation product abundances from methyl loss, i.e. m/z = 31 [13], and 29 [14], were added to their parent ion, C3H7O+ [8], and the breakdown diagram was modeled up to a photon energy of 15 eV to establish the appearance energy of the consecutive ethylene loss channel from the second H-loss product C4H9O+, yielding CH3CH=OH+ [7] at m/z = 45. The appearance energies and transition states fitted in the previous step were kept constant, only optimizing the model parameters for the second H loss and sequential ethylene loss channels, yielding appearance energies of 10.67 ± 0.03 and 11.81 ± 0.05 eV, respectively. As can be seen in Fig. 3 the latter is in good agreement with 11.8 eV as determined by Botter et al.29 However, based on the heats of formation of Et2O and CH3CH=OH+,59 the 0 K thermochemical onset lies significantly lower, at 11.484 ± 0.008


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eV (Table 1). This indicates a 0.325 eV, i.e. 31 kJ mol–1 reverse barrier in the formation the CH3CH=OH+ ion [7]. In the final modeling step, the sequential dissociations from the methyl-loss first fragment were modeled. Kinetic energy release and the fragment ion internal energy distribution have been calculated assuming two translational degrees of freedom and using the vibrational frequencies of the C3H7O+ ion [8] and of the CH3 neutral fragment.21 Between 12.5 eV and 13.5 eV, the fit is inaccurate as the modeled C3H7O+ ion [8] abundance drops to zero faster than the experimental data points. Consequently, the abundance of CH2OH+ [13] is overestimated. However, fitting the rising edge of sequential and parallel dissociation products is most important in obtaining accurate appearance energies, which remains unaffected by this discrepancy. Thus, the 0 K onset of these dissociation channels could be determined as 11.85 ± 0.07 eV and 12.20 ± 0.08 eV for CH2OH+ [13] and C2H5+ [14], respectively. Two explanations can be put forward for the deviations from the statistical dissociation model. First, the predicted dissociation rates at this energy, corresponding to 3.4 eV internal energy in the parent ion, are in the 1014 s–1 range (Fig. 3). The resulting ps–fs parent ion lifetime may not allow for a complete statistical redistribution of the internal energy, leading to suprastatistical kinetic energy release. This will lead to less excess energy deposited in the methyl-loss intermediate, which leads to its increased stability and, hence, abundance. The opposite effect was observed in the sequential dissociation of sulfur oxides and oxochlorides and in the statistical dissociative photoionization regime of carbon tetrachloride.60-61 In these samples, kinetic energy release was apparently suppressed at high excess energies. Alternatively, it is also possible that a thermodynamically more stable methyl-loss C3H7O+ ion [10] is formed above 12 eV.37 While we could not locate a low-lying transition ACS Paragon Plus Environment

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state explaining this channel, we cannot rule out its existence, and the system could be trapped in the isomer structure because of the need to pass the high-energy transition state [9]‡ prior to further dissociation, thereby increasing the observed abundance of the methyl-loss intermediate. With a statistical model of the unimolecular decomposition pathways of diethyl ether, one can also change the model temperature and predict the product abundances in high temperature settings, e.g. in a combustion environment.62 This can be useful in combustion diagnostics, in order to assign the ion signal to its neutral source as well as to measure the temperature easily by threshold photoionization. In molecular beam experiments, the temperature of the ionization volume was reported in the range of 100–500 K depending on the measurement procedure.15,63-64 A simulated spectrum at 500 K is shown in Fig. 5 together with the room temperature model. Two effects are evident: a ca. 0.2 eV or 20 kJ mol–1 redshift in the breakdown curves, due to the thermal energy of the neutral contributing to the internal energy of the parent ion, as well as a broadening of the crossover regions in the breakdown diagram. Since Franck–Condon factors are expected to be negligible for hot bands well below the ionization energy, little or no photoionization signal is expected in the lowenergy part of the breakdown diagram, resulting in weak parent ion signal in the mass spectrum at high temperatures.


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Fig. 5. Diethyl ether breakdown diagram modeled at 320 K and predicted at 500 K. Dashed lines are the best-fit modeling of the data at room temperature, and solid lines are the predicted breakdown diagram at 500 K.

Thermochemistry Ancillary thermochemical data and the results of this work are summarized in Table 1. In order to confirm the Et2O heat of formation, last revised to –252.1 ± 0.7 kJ mol–1 in 1986,47 reaction energy calculations for the isodesmic 2 C2H5OH + (CH3)2O → 2 CH3OH + (C2H5)2O reaction and also for 2 C2H5OH → (C2H5)2O + H2O were carried out. In these, diethyl ether is anchored to compounds in version 1.112 of the Active Thermochemical Tables.59,65-66 While the two reactions may seem to be redundant as both of them contain DEE and ethanol on the product and reactant sides, respectively, the reason is somewhat different for the two. The 2 C2H5OH → (C2H5)2O + H2O connects our title molecule directly to EtOH through the wellknown heat of formation of water, while the other reaction, in which ethyl and methyl alcohol are linked to their respective symmetrical ethers is strictly isodesmic, preserving the nature of ACS Paragon Plus Environment every bond broken in the hypothetical reaction.

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We used the known elemental thermal enthalpies67 with W1U harmonic frequencies to convert the experimental 298 K enthalpy of formation of Et2O to 0 K, as shown in Table 1. Harmonic thermal enthalpies are expected to be accurate to 0.1 kJ mol–1, while the effect of internal hindered rotations are larger in diethyl ether, and the three approximations implemented in Gaussian 09 yield 0.15 ± 0.25 kJ mol–1 and will partially cancel out in reaction energy calculations.68-69 Thus, thermal effects are not expected to introduce an error larger than 0.2 kJ mol–1 and are negligible, compared to the expected accuracy of the electronic energy calculations. Ab initio reaction energies are compared with the literature values in Table 2. Based on this, one can conclude that the previously reported diethyl ether enthalpy of formation appears to be correct within the error bars of these calculations.

Table 1. Auxiliary and derived thermochemical data molecule

∆fHo0 K

∆fHo298 K

Ho298K – Ho0K

/ (kJ mol–1) CH3CH2OCH2CH3

–221.7a,b

CH3CHOCH2CH3+

CH3CH2OCH2+

CH3CHOH+

/eV

± 0.7

20.49c

567.6d

± 1.6

17.61c

562.8e

±4

544.8 f

–252.1b

507.1f

E0

10.419 ± 0.015d 10.37 ± 0.02f

±4

640.0d

± 1.0

639.6e

±4

633.9f

601.7f

±4

609.24g

594.82g

± 0.40

13.88c

10.484 ± 0.008d 10. 48 ± 0.02f

13.0h

11.81 ± 0.05d 11.484 ± 0.008i

CH2OH+

C2H5+

711d

±7

718.00g

710.06g

± 0.24

717.7j

709.8j

± 0.7

911d

±8

915.5k

± 1.3

915.1g

902.9g

10.147i

± 0.38

11.85 ± 0.07d 11.902 ± 0.008i 12.20 ± 0.08d 12.224 ± 0.008i


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a

H

216.034g

217.998g

± 0.000

CH3

149.88g

146.49g

± 0.08

C2H4

61.07g

52.56g

± 0.15

CH2O

–105.33g

–109.16g

± 0.11

CH3OH

–189.82g

–200.70g

± 0.18

CH3CH2OH

–216.85g

–234.57g

± 0.22

CH3OCH3

–166.51g

–184.02g

± 0.44

H2 O

–238.919g

–241.822g

± 0.027

c-C4H8O

–154.76l,a

–184.20l

± 0.71

12.98c

c-C4H8OH+

557.0m,a

523.8m

± 3.1

13.48c

H+

1528.084g

1530.047g

± 0.000

Page 20 of 36

Converted to T = 0 K using W1U thermal enthalpy. bPedley.47 cBased on W1U calculation of

vibrational frequencies. dThis work. eRecalculated based 0 K appearance energies reported by Botter.29 fBotter.29 gActive Thermochemical Tables (ATcT).59 hBodi et al.57 iCalculated based on literature heats of formations. jBorkar et al.24 kBorkar and Sztáray.70 lPell and Pilcher.71 m

Calculated based on proton affinity72 and heat of formation71 of THF and supported by

isodesmic reaction energy calculations.

Table 2. Calculated and literature reaction enthalpies and their deviation from the literature based values confirm the heat of formation of Et2O. ∆rHo0 K / (kJ mol–1) 2 C2H5OH + (CH3)2O →

2 C2H5OH → (C2H5)2O + H2O

2 CH3OH + (C2H5)2O literature

–1.14 ± 0.95

–26.93 ± 0.83

G4

–2.72 (Δ = –1.58)

–26.96 (Δ = –0.03)

CBS-APNO

–3.31 (Δ = –2.17)

–28.80 (Δ = –1.87)

W1U

–2.14 (Δ = –1.00)

–26.33 (Δ = 0.60)

diff.

–1.0 ± 1.1 ACS Paragon Plus Environment

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Only a few measurements were made to determine thermochemical onsets for C4H9O+ ions, and the CH3CHOCH2CH3+ ion [4] heat of formation estimated to be 544.8 ± 4 kJ mol–1 at 0 K by Botter et al.29 However, this reported value is 23 kJ mol–1 off, compared to our 567.6 ± 1.6 kJ mol–1. In fact, the major source of error is likely an incorrect conversion of the 298 K Et2O heat of formation to 0 K by Botter et al. Based on their reported appearance energy of the CH3CHOCH2CH3+ ion [4], one can re-calculate the 0 K heat of formation of CH3CHOCH2CH3+ to be 562.8 ± 4 kJ mol–1, which agrees within the error bars with our newly revised and more accurate result. However, there is still inconclusive evidence regarding a potential reverse barrier along the dissociation coordinate leading to the hydrogen loss, which would mean that the dissociative photoionization onset does not correspond to the thermochemical limit. In order to address this issue, reaction energy calculations were carried out for the isomerization of CH3CHOCH2CH3+ [4] to protonated tetrahydrofuran (THFH+). The THFH+ heat of formation was calculated to be 557.0 ± 3.1 kJ mol–1 based on the proton affinity, PA298K(THF) = 822.1 kJ mol–1,72 and the THF heat of formation,71 and was also confirmed by G4, CBS-APNO and W1U reaction energy calculations, which give an average of PAcalc,298K(THF) = 821.2 ± 3 kJ mol–1. Based on the ab initio isomerization reaction energies, the heat of formation of the hydrogen-loss fragment ion was calculated to be 563.4 ± 5.0 kJ mol–1, which is lower than the experimental value of 567.6 ± 1.6 kJ mol–1. Based on this, even though we cannot firmly exclude the possibility that the rate determining step in the H-loss dissociation channel is a higher-lying transition state, it is somewhat unlikely. Using the Botter et al. 0 K appearance energy, the reported heat of formation of the CH3CH2OCH2+ ion [8], has been we recalculated, as well. The corrected 639.6 ± 4 kJ mol–1 is in agreement with the 640.0 ± 1.0 kJ mol−1 determined in this work. However, the heat of formation of CH3CH2OCH2+ as reported by Lossing,73 which was also based on the ACS Paragon Plus Environment

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dissociative ionization onset from Et2O and converts to 617 kJ mol−1 at 0 K is certainly too low. Another experimental result ibid., derived from the dissociative ionization of CH3CH2OCH2CH2OH, 638 kJ mol−1, is in agreement with the 0 K value determined in this work. To support the revised heat of formation, the thermochemical onset of CH3CH2OCH2+ ion [8] was calculated using G4, CBS-APNO and W1U composite methods to be 10.48, 10.53, and 10.51 eV, respectively. The calculated 0 K methyl-loss ion heat of formation, 642.1 ± 3.9 kJ mol−1 is in good agreement with the 640.0 ± 1.0 kJ mol−1 determined experimentally herein. For the ions formed in sequential dissociations from the methyl-loss intermediate [8], namely CH2OH+ [13] and C2H5+ [14], the 0 K heats of formation can be derived as 711 ± 7 and 911 ± 8 kJ mol−1, respectively, which is found to be in agreement with the reported enthalpies of formation by Sztáray and Borkar,24,70 and in Active Thermochemical Tables.59,65-66 While the accuracy of the appearance energies for these high-lying channels is certainly inferior to that of the already available thermochemical data, they prove that the dissociation can take place at its thermochemical onset and the rate determining step is not a higher-lying transition state.

Potential Energy Surface


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Fig. 6. Computed reaction coordinate for the pathways for dissociation of the Et2O+ cation [1] with the G4 energies at 0 K for the stationary points. Normal mode analysis was carried out to confirm the minimum and transition state geometries, the latter being marked by ‘‡’. Primary dissociation channels are shown with red platform, while secondary with black platform. All energies are reported with respect to the neutral diethyl ether and are given in eV.

To explore the dissociative ionization channels of Et2O+ cation [1] in the theoretical realm, density functional theory was employed with the B3LYP/6-311++G(d,p) functional, and the transition states were located with the help of internal coordinate scans and synchronous transit-guided quasi-Newton (STQN) calculations. The energies of the stationary points on the potential energy surface were then refined with the G4 composite method. Fig. 6 shows an overview of the obtained results. The first dissociation channel, as reported in the literature and discussed here previously, is the loss of the H-atom from an α-carbon. This is supported by the G4 energy + ACS Paragon Plus Environment calculation, since the CH3CHOCH 2CH3 structure [4] appears to be the more stable isomer

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accessible by simple bond rupture. H-atom loss often involves reverse barriers due to the low polarizability of the H-atom. The C–H bond length was scanned and a tight transition state [2]‡ was located at 10.31 eV followed by a shallow minimum [3] at 10.13 eV, where the C–H bond distance is 3 Å. The apparently slow dissociation and relatively tight transition state are caused by this reverse barrier. The second dissociation channel is a simple methyl-loss, which appears less than 0.1 eV higher than the CH3CHOCH2CH3+ ion. With a calculated thermochemical onset of 10.48 eV and due to the loose transition state along this channel, formation of the CH3CH2OCH2+ ion [8] quickly dominates over the first, but slow H-loss channel. To reasonably resolve the remarkable reemergence of the H-loss fragment ion, the isomerization of Et2O+ [1] and possible C4H9O+ ion structures were also explored. The only possible m/z = 73 isomer with thermochemical onset below 10.9 eV is the protonated THF (not pictured). However, its formation involves a hydrogen-shift from the terminal carbon to the oxygen atom and a ring formation together with H-loss. The onset of this process lies more than 1 eV above the appearance of the second C4H9O+ ion. Thus the formation of a second m/z = 73 isomer can be excluded in this energy range. Rather, the reappearance of the C4H9O+ ion [4] can be explained by the transition state switching model.54 Two transition states were required to fit the experimental data adequately by the statistical model: a tight (i.e., configurational) TS occurring near the unimolecular reactant and a second looser one, outer (i.e., centrifugal) TS occurring near the bimolecular products, or possibly involving the terminal methyl-group hydrogens, leading to a higher phase space volume for the TS. This is also supported by the mild slope of the CH3-loss curve [8] in the 11.0–11.7 eV photon energy range which suggests a competing parallel dissociation. At higher energies, the H-loss daughter ion further fragments into CH3CHOH+ by loss of C2H4 [7]. This process takes place via a 1,3-hydrogen shift TS [5]‡, which lies 150 meV ACS Paragon Plus Environment

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above the thermochemical onset of the products. This can be compared with the 325 meV reverse barrier measured based on the known thermochemical onset of protonated acetaldehyde.59 A deep minimum between the transition state and products also exists at 10.87 eV [6]. This intermediate could be best described as a complex between CH3CHOH+ and ethylene, with the hydroxyl hydrogen atom bridging the ethylene unit. The CH3CH2OCH2+ ion [8] decomposes by either ethylene or formaldehyde elimination, as well as through a minor water-loss channel. Similarly to CH3CHOH+ formation, CH2OH+ [13] is the result of a 1,3-hydrogen shift TS [11]‡ at 11.61 eV, which leads to [12] at 11.15 eV. The intermediate structure is characterized with long C–H distances and with the hydroxyl hydrogen atom of the CH2OH+ fragment bridging the ethylene unit. At higher energies, this channel is outcompeted by the formation of C2H5+ [14], which takes place along a loose transition state without a reverse barrier. The deviation between the statistical model and the experimental breakdown diagram around 13 eV could be explained by the production of the more stable m/z = 59 CH3CHOCH3+ methyl-loss isomer [10], 490 meV below [8]. Despite numerous attempts, we have not found a low-energy direct path to [10] from [1] bypassing [8], only a 1,3-hydrogen shift TS [9]‡ with a reverse barrier of 1.62 eV relative to [8]. However, if such a path were to exist, it could lead to a methyl-loss product, which is trapped and stabilized by the same transition state [9]‡.

CONCLUSIONS The unimolecular dissociation of internal energy selected diethyl ether cations was investigated by imaging Photoelectron Photoion Coincidence Spectroscopy using VUV synchrotron radiation. Et2O+ ions dissociate by numerous parallel and sequential dissociation channels. Modeling fractional ion abundances, plotted in the breakdown diagram, yielded ACS Paragon Plus Environment

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Page 26 of 36

accurate dissociation onsets to hydrogen- and methyl-loss reactions in the molecular ion. High-lying consecutive dissociation channels were examined in terms of thermochemical onsets and reverse barriers. Quantum chemical calculations were used to map out the potential energy surfaces, helping to discern the dissociation mechanisms. The first dissociation channel is hydrogen atom loss, which is quickly overtaken by the fast methyl-loss channel. The 0 K appearance energy of CH3CHOCH2CH3+ [4] and CH3CH2OCH2+ [8] was determined to be 10.419 ± 0.015 and 10.484 ± 0.008 eV, respectively. Using a combined experimental and theoretical thermochemical network, the 0 K heats of formation values of 567.6 ± 1.6 and 640.0 ± 1.0 kJ mol–1 were determined and confirmed for the CH3CHOCH2CH3+ [4] and CH3CH2OCH2+ [8] ions respectively. The reappearance of the H-loss C4H9O+ ion [4] can be explained by a transition state switching model, where the second TS lies 250 meV (E0 = 10.67 ± 0.03 eV) higher than the first, tight TS. Three

high-lying

dissociation

reactions

were

analyzed

in

terms

of

the

thermochemically available channels and the reverse barriers. The reverse barrier to ethyleneloss [7] from CH3CHOCH2CH3+ [4] is found to be 325 meV, based on the thermochemical onset and the 0 K appearance energy of 11.81 ± 0.05 eV. The CH3CH2OCH2+ [8] ion can decompose by ethylene and formaldehyde-loss channel. The 0 K appearance energies of 11.85 ± 0.07 and 12.20 ± 0.08 eV, respectively, along with the corresponding 711 ± 7 and 911 ± 8 kJ mol−1 fragment ion heats of formation suggest that the dissociations take place at their thermochemical onsets. The deviation between in the statistical model around 13 eV could be explained by the excess energy deposited in the CH3-loss intermediate at high, 1013–1014 s–1 dissociation rates. Consequently, the abundance of CH3CH2OCH2+ [8] ion will be increased, which is the only experimental feature not taken into account in the simple dissociation model throughout the 5 eV photon energy range. ACS Paragon Plus Environment

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The constructed dissociation model or dissociative photoionization appearance energies and transition state models can also be used to predict the breakdown diagram of diethyl ether at arbitrarily higher temperatures; such an approach can be useful in interpreting high temperature mass spectra as well as in signal apportioning and temperature measurements in combustion experiments.

ACKNOWLEDGEMENTS This work has been funded by the National Science Foundation (CHE-1266407). The iPEPICO experiments were performed at the VUV beamline of the Swiss Light Source (Paul Scherrer Institut, Villigen, Switzerland). A.B. and P.H. gratefully acknowledge funding Swiss Federal Office for Energy (BFE Contract Number 101969/152433). The help of Krisztián G. Torma is also gratefully acknowledged.

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Dissociative Photoionization of Diethyl Ether - The Journal of Physical

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