Understanding the Complex Dissociation Dynamics of Energy

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Understanding the Complex Dissociation Dynamics of Energy Selected Dichloroethylene Ions: Neutral Isomerization Energies and Heats of Formation by Imaging Photoelectron-Photoion Coincidence Andras Bodi,*,† William R. Stevens,‡,§ and Tomas Baer‡ † ‡

Molecular Dynamics Group, Paul Scherrer Institut, Villigen 5232, Switzerland Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290, United States ABSTRACT: The dissociative photoionization of 1,1-C2H2Cl2, (E)-1,2-C2H2Cl2, and (Z)-1,2-C2H2Cl2 has been investigated at high energy and mass resolution using the imaging photoelectron photoion coincidence instrument at the Swiss Light Source. The asymmetric Cl-atom loss ion time-of-flight distributions were fitted to obtain the dissociation rates in the 103 s-1 < k < 107 s-1 range as a function of the ion internal energy. The results, supported by ab initio calculations, show that all three ions dissociate to the same C2v symmetry ClCdCH2þ product ion. The 0 K onset energies thus establish the relative heats of formation of the neutral isomers, that is, the isomerization energies. The experimental rate constants, k(E), as well as ab initio calculations indicate an early isomerization transition state and no overall reverse barrier to dissociation. The major high energy channels are the parallel HCl loss and the sequential ClCdCH2þ f HCCHþ þ Cl process, the latter in competition with a ClCdCH2þ f ClCCHþ þ H reaction. A parallel C2H2Cl2þ f C2HCl2þ þ H channel also weakly asserts itself. The 0 K onset energy for the sequential Cl loss reaction suggests no barrier to the production of the most stable acetylene ion product; thus the sequential Cl-atom loss is preceded by a ClCdCH2þ f HC(Cl)CHþ reorganization step with a barrier lower than that of the second Cl-atom loss. The breakdown diagram corresponding to this sequential dissociation reveals the internal energy distribution of the first C2H2Clþ daughter ion, which is determined by the kinetic energy release in the first, Cl loss reaction at high excess energies. At low kinetic energy release, this distribution corresponds to the predicted two translational degrees of freedom, whereas at higher energies, the excess energy partitioning is characteristic of only one translational degree of freedom. New ΔfHo298K of 3.7, 2.5, and 0.2 ( 1.75 kJ mol-1 are proposed for 1,1-C2H2Cl2, (E)-1,2-C2H2Cl2, and (Z)-1,2-C2H2Cl2, respectively, and the proton affinity of ClCCH is found to be 708.6 ( 2.5 kJ mol-1.

’ INTRODUCTION The reactivities and energetics of isomeric species are of considerable interest because subtle structural differences often have large implications in terms of both. Isomerization barriers and energies play an important role: If these are high, we can expect specific reactivities, while if they are low so that the isomers can interconvert readily, their reactivities will not depend upon their structure. Tuckett, Mayhew, and co-workers investigated the reactivities of a number of unsaturated halogenated molecules, both their positive1 and their negative2 ions, and among other things, reported significant differences between the reaction products when the three dichloroethyelene (DCE) isomers, 1,1-C2H2Cl2, (E)-1,2C2H2Cl2 (trans), and (Z)-1,2-C2H2Cl2 (cis), are reacted with such ions as NH4þ, H3Oþ, CF3þ, etc. When the charge transfer reaction is energetically possible, it was found to dominate the product species. In the case of very stable ions, such as NH4þ, no reaction was observed. However, when the three DCE isomers were reacted with H3Oþ, the reactivities were quite different. The 1,1 isomer reacted only by proton transfer to produce H2O, while the two 1,2 isomers also generated the adduct ion as well as r 2011 American Chemical Society

generating HCl as a neutral product. The relative yields of these products differed, however, significantly. To shed light on these reactivities, Parkes et al.3 recently reported a threshold photoelectron photoion coincidence study of the dissociation dynamics of the three DCE ions. They reported dissociative photoionization onset energies for Cl loss as well as kinetic energy release values at higher excitation energies, which were based on fitting the broadened product ion time-of-flight distribution. They reported that the Cl loss onset energies for the 1,1-, E-1,2-, and Z-1,2 DCE isomers were 11.88, 11.84, and 11.88 ( 0.05 eV, respectively. An exhaustive review of the thermochemical literature of small chlorinated hydrocarbons by Manion4 has established that the heats of formation of the three isomers are in the following order: 1,1-C2H2Cl2 (2.4 ( 2 kJ mol-1), (E)-1,2C2H2Cl2 (-0.5 ( 2 kJ mol-1), and (Z)-1,2-C2H2Cl2 (-3.0 ( 2 kJ mol-1). This analysis has completely reversed the ordering reported by Pedley5 and Gurvich et al.6 Because these heats Received: November 12, 2010 Revised: December 14, 2010 Published: January 6, 2011 726

dx.doi.org/10.1021/jp110831q | J. Phys. Chem. A 2011, 115, 726–734

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of formation span a range of only 5.4 kJ mol-1 (56 meV), the 50 meV resolution in the Parkes et al. data does not allow for a firm conclusion about whether a common C2H2Clþ product ion was being produced, and if yes, whether the 0 K onset energy corresponds to the dissociative photoionization reaction energy. However, calculations by Rodriquez et al.7 suggested that there are two stable C2H2Clþ isomers, and the ClCdCH2þ ion has a substantially lower energy structure than the chlorine bridged acetylene isomer, which lies about 80 kJ mol-1 higher. Thus, it would appear that the Cl loss products from the three dichloroethene ions would have to be the same. However, the Rodriquez calculations of 1993 were done at the MP4/6-311G(2df,p) level, and it may be worthwhile to revisit these systems using more advanced model chemistries. The onset energies for Cl loss in the dissociative ionization of C2H2Cl2, ca. 12 eV, are considerably above the C2H2Cl2 ionization energies of ca. 9.9 eV. Consequently, the density of states in the parent ion will be high at the dissociation threshold, even for this relatively small molecule. This in turn suggests that the dissociation rate constant is low and the parent ions may show metastable behavior. For instance, the butadiene ion, consisting also of four heavy atoms and having a barrier to dissociation of 2.3 eV, is quite metastable, and its rate constant at the dissociation limit is sufficiently low that no product ions can be observed in the time scale of a PEPICO experiment.8,9 Such slow reactions show up as asymmetric TOF distributions in our experiment. By fitting these TOF distributions with a rate curve, k(E), obtained from the statistical theory of unimolecular reactions, it is possible to extrapolate the rate to the dissociation limit, where k(E0) = 0. The imaging PEPICO experiment10 at the VUV beamline of the Swiss Light Source (SLS) is ideally suited for preparing energy selected ions and measuring their dissociation rates in the 103-107 s-1 range. Thus, it can be hoped that accurate onset energies in combination with good heats of formation of the three C2H2Cl2 isomers could be used to establish whether the C2H2Clþ produced in the dissociation for the three isomers is the same. A difference of about 2 kJ mol-1 (i.e., 21 meV) should be readily measurable with our resolution of 2 meV. If identical product ions are generated from the three DCE isomers without any reverse barrier in the exit channel which is isomer specific, their onsets will provide additional information about the relative heats of formation of the three molecules, which could serve to reduce the uncertainty limits quoted by Manion.4 The C2H2Clþ ion also plays a role in the proton affinity of the chloroacetylene via the reaction:

from a bending magnet, dispersed by a grazing incidence monochromator and with higher orders suppressed by a compact gas filter,14 was used to ionize the sample in a 2 2 mm interaction region. The photon energy, with a resolution of about 2 meV, was calibrated using the well-known Ar and Ne autoionization lines. Upon ionization, an 80-120 V cm-1 electric field accelerated the electrons and ions in opposite directions. Velocity map imaging15,16 was used to focus the electrons onto a DLD40 Roentdek position sensitive delay-line detector with a kinetic energy resolution better than 1 meV at threshold. The ions were extracted from the same region by a two-stage Wiley-McLaren17 space focused time-of-flight (TOF) mass spectrometer with a 5.5 cm long first and a 1 cm long second acceleration region and a 55 cm drift region. The last 12 cm of the drift region can be maintained at a lower potential to separate the fragment ions generated in the drift region from metastable parent ions, from the undissociated parent ions.18 The ions were detected by a Jordan TOF C-726 nonimaging microchannel plate detector. The threshold electron signal is contaminated with energetic, hot electrons produced with no transversal velocity. This was corrected by determining the energetic electron background in a small ring around the threshold region of the electron image and subtracting this from the threshold signal.19 Using this setup, the energy resolution was ultimately limited by the photon energy resolution of about 2 meV. Electron hit times and positions and ion hit times were recorded in the triggerless mode of an HPTDC time to digital converter card, and electrons and ions were correlated on the fly to obtain time-of-flight distributions without dead time. This multiple-start/multiple-stop data acquisition scheme20 enables data acquisition at high ionization rates, which is beneficial at a high intensity light source, such as the synchrotron. The experimental data were analyzed and may be plotted in several ways: the threshold electron signal as a function of the photon energy yields a threshold photoelectron spectrum (TPES); the threshold electron signal detected in coincidence with an ion in a particular TOF range yields a mass-selected TPES; and the fractional ion abundances as a function of the photon energy yield the breakdown diagram, which has the significant benefit of being independent of volatile ambient parameters such as sample pressure, photon intensity, and Franck-Condon factors. The breakdown curves and the experimental rate constants based on the asymmetric peaks in the ion time-of-flight distributions were modeled on the basis of the room temperature internal energy distribution of the neutral, and the densities and numbers of states of the fragmenting ions and transition states, respectively. In our approach,21 the two variable parameters are the transitional frequencies and the 0 K appearance energy. The rest of the model parameters were calculated using density functional theory (DFT) with the Gaussian 09 computational chemistry suite.22 Isodesmic reaction energies were computed for accurate energetics using composite methods (e.g., G3B323 and W1(U)12,13). Transition states were also located and reaction paths obtained by constrained optimizations, in which either a bond angle (for H-atom transfer in the parent ion) or a bond length (for Cl-atom loss) was scanned.

C1-CtC-H þ Hþ f C2 H2 C1þ Although there is little experimental information about the heat of formation of chloroacetylene, it has been calculated at a high level by Parthiban et al.11 using W1 and W2 theory.12,13 In addition, one of the motivations of the 1993 calculations by Rodriquez et al.7 was the determination of the chloroacetylene proton affinity, which they reported to be 707 kJ mol-1. Our measured onset for Cl loss should finally provide some experimental information about these energies.

’ EXPERIMENTAL AND THEORETICAL APPROACHES The imaging photoelectron-photoion coincidence (iPEPICO) spectrometer10 located at the X04DB VUV beamline14 at the Swiss Light Source (SLS) of the Paul Scherrer Institut has been described in detail and is only briefly reviewed here. The pure sample is effusively introduced at room temperature into the ionization region through a Teflon tube. Synchrotron radiation

’ RESULTS AND DISCUSSION Dissociative Photoionization Mechanism up to 20 eV. The lowest energy dissociation channel for the C2H2Cl2þ ion is via the loss of a chlorine atom to produce the C2H2Clþ ion. However, within 1 eV of this onset, we also note the thermochemically favored loss of HCl to produce the HCtCClþ ion, a channel that is in 727

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Figure 2. Time-of-flight distributions for 1,1-C2H2Cl2 in the vicinity of the Cl-atom loss onset. The asymmetric shape of the fragment ion TOF is a result of the slow dissociation in the acceleration region. The peak at 17.8 μs is due to fragment ions born in the 43 cm long drift region. The experimental points are modeled (solid lines) taking into account the molecule’s thermal energy as well as the assumed dissociation rate constants as obtained by statistical theory. Dots correspond to measured ion counts, and the continuous lines correspond to modeled TOF distributions. The parent ion exhibits 9:6:1 isotope intensity ratios, whereas a 3:1 ratio is seen in the Cl loss daughter ion peak.

Figure 1. Breakdown diagram of 1,1-C2H2Cl2 from hν = 11.5-20 eV. The overall appearance of the (Z)- and (E)-1,2-C2H2Cl2 breakdown curves agrees with that of the 1,1 isomer (see Figure 4).

competition with the Cl loss channel. Because the HCl loss process has a reverse barrier in the exit channel and a tighter transition state than the Cl loss, HCl loss is only a minor channel. At still higher energies, sequential Cl-atom and H-atom loss reactions as well as the parallel H-atom loss reaction generate more ions. An overview of the dissociation mechanism can be obtained from the overall breakdown diagram between 11.5 and 20 eV (Figure 1). The breakdown diagram is a plot of the fractional abundance of the parent and fragment ions detected in coincidence with threshold electrons as a function of the photon energy. The appearance of the breakdown diagram is very similar for the three isomers, which indicates their quick interconversion at higher internal energies. Low extraction fields lead to high residence times, which are advantageous when low dissociation rates are to be measured and also to determine the kinetic energy release (KER) based on the peak shapes.24 However, at or below a field strength of 80 V cm-1, the thermal broadening and the kinetic energy release make the 35Cl and 37Cl Cl loss peaks partially overlap with the 35Cl and 37Cl HCl loss peaks. Thus, a higher field strength is needed to resolve these dissociation channels accurately. Therefore, the breakdown diagram shown in Figure 1 was obtained on the basis of 120 V cm-1 time-of-flight mass spectra. The shape of the breakdown curves also provides clues about the reaction mechanism. For instance, the slow rise of the HCl channel relative to the Cl loss channel indicates a parallel channel, the slope of which is determined by the relative activation entropies of the two channels. At 16 eV, the rapid fall of the C2H2Clþ ion signal relative to the C2H2þ signal is characteristic of a sequential reaction in which the intermediate ion loses another Cl atom. The slope for this transition is determined by the internal energy distribution in the fragment C2H2Clþ ion after the loss of one Cl atom, a process that can be modeled quantitatively. The C2HClþ signal rises again slowly above 17 eV, indicating a parallel process in which an H-atom is lost from the C2H2Clþ ion instead of the second Cl-atom. The overall mechanism up to 20 eV can be summarized by the following scheme:

also appears and disappears in the energy range of a TPES peak, suggesting that nonstatistical ion decay processes may be at play. Experimental data as well as calculations show that the three DCE isomer cations have a shared phase space volume even at the threshold of the first Cl loss (vide infra). Therefore, the above reaction scheme applies equally to all isomers. Chlorine Atom Loss: Dissociation Rates and Shared Phase Space Volume. Time-of-flight distributions for 1,1-C2H2Cl2 obtained at an extraction field of 80 V cm-1, showing the parent ion and the Cl-atom loss fragment in the vicinity of hν = 12 eV, are plotted in Figure 2. As expected on the basis of the barrier to dissociation in the ion, the loss of a Cl atom is a slow dissociation and thus results in asymmetric product ion TOF distributions. In addition, a peak to the right of the parent ion at 17.8 μs appears. This is a result of fragment ions generated in the first 43 cm of the drift region. By slowing the ions down for the last 12 cm, we can separate these fragments from the parent ion peak.18,21 The points are the experimental data, while the solid lines through them are the fits using the PEPICO modeling program.21 The modeled fits take into account the thermal energy distribution of the neutral sample at 298 K based on harmonic frequencies and rotational constants calculated at the B3LYP/cc-pVTZþd level of theory. Densities and numbers of states of the parent ion and transition states, respectively, are calculated using harmonic B3LYP/6-31G(d) frequencies as employed in the composite G3B3 method. The two variable parameters in our RRKM model are the 0 K appearance energies and a factor to scale the two transitional frequencies in the transition state, converging to the energy invariant relative rotations in the products, to model the experimental TOF asymmetries. Figure 3 shows two breakdown diagrams for 1,1-C2H2Cl2. The two diagrams differ by how the drift peak at 17.8 μs is treated. If we include it in the fragment ion peak, we count all fragment ions generated within 0 < τ < 13 μs as fragment ions. On the other hand, if we include the drift peak with the parent ions, we count only fragment ions generated in the first acceleration region (0 < τ < 3.3 μs). The shift in the two breakdown diagrams is thus sensitive to the dissociation rate. The fact that both of these diagrams as well as

in which the dashed arrow showing the loss of the H atom signifies a very minor channel (less than 5%). This H loss channel 728



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Figure 4. Comparison of the breakdown diagrams of the three C2H2Cl2 isomers in the vicinity of the Cl-atom loss onset energy. Because the kinetics of the dissociative photoionization are the same for all isomers, the energy shifts in the breakdown curves correspond exactly to the relative 0 K neutral heats of formation, that is, the 0 K isomerization energies.

Figure 3. Experimental and modeled breakdown curves for 1,1C2H2Cl2 in the vicinity of the Cl-atom loss appearance energy. The red curve (shifted to lower energy) includes the “drift” peak at 17.8 μs with the fragment ion peak, while the blue (higher energy) curve includes the “drift” peak with the parent ion signal. The shift is directly related to the kinetic shift. The arrow at 12.060 eV indicates the 0 K onset energy for the Cl-atom loss derived from the simultaneous fitting of the TOF distributions and the breakdown diagrams.

The modeled fits of the 1,1 isomer are shown in Figure 3. The breakdown diagrams for all three DCE isomers are compared in Figure 4, where the drift peak is included in the fragment ion signal. From the combined TOF distributions and breakdown diagrams for the three isomers, it is possible to extract accurate 0 K dissociative photoionization onsets for Cl loss of 12.060 ( 0.005, 12.086 ( 0.007, and 12.102 ( 0.005 eV for the 1,1, E, and Z C2H2Cl2 isomers, respectively. Even though the dissociation is slow at threshold, the fits only show a kinetic shift of about 15 meV when the drift peak is included in the fragment signal. The larger error bar on the E isomer onset is due to a fine structure apparent in the breakdown curve, associated with the vibrational structure in the TPES, and similar to the one observed for iodomethane.30 The internal energy distribution of threshold ions produced in direct ionization, (i.e., at photon energies corresponding to TPES peaks) differs from that of ions produced by autoionization (i.e., in between TPES peaks), thereby leading to peaks in the breakdown diagram. The derived 0 K onsets can be compared to those reported by Parkes et al.3 by converting their 298 K onsets to 0 K onsets. Adding the average ro-vibrational energy of 80 meV to their 298 K onsets yields the following 0 K onsets: 12.09 ( 0.05, 12.05 ( 0.05, and 12.09 ( 0.05 eV, respectively, for the 1,1, E, and Z isomers. As the differences (30, -40, and -10 meV, respectively) are all within their error bars of 50 meV, their results are in agreement with ours. However, our difference between the 0 K onset energies for the lowest energy Z and the highest energy 1,1 isomers of 42 meV matches very closely (well within the mutual error limits) with the heats of formation difference between these two isomers of 56 meV as reported by Manion.4 This indicates that the three ions generate the same C2H2Clþ product ion via a shared reaction coordinate and that the differences in the dissociation onset energies reflect differences in the neutral isomer energies. Our onset energies have an error of only 5-7 meV (0.5-0.7 kJ mol-1); therefore, we should be able to improve on the latest heats of formation by Manion.4 The concept of a shared reaction coordinate may be surprising at first. In the 1,1 isomer, the Cl-atom loss reaction is a simple bond rupture, whereas the 1,2 isomers require a rearrangement to produce the CH2CClþ product. Fitting of the experimental rate data provides more detailed information about the potential energy surface. If the three isomers reacted along separate potential

the daughter ion peak shapes are fitted with a single RRKM model provides ancillary support to the experimental rate curve. The HCl loss channel does not open up below 12.3 eV, so only parent and Cl loss daughter ions are produced in the energy range of the Cl loss breakdown diagram. The TOF distributions and breakdown diagrams can, thus, be accurately modeled using the rate constants, k(E), obtained from the RRKM statistical theory eq 1.25 The RRKM model contains two adjustable parameters, the appearance energy (E0) and the factor to adjust the transitional mode frequencies, that is, the number of states of the transition state, Nq(E-E0). Additionally, the parent ion density of states, F(E), is also contained in this rate equation: kðEÞ ¼

σN q ðE - E0 Þ hFðEÞ

ð1Þ

where σ is the reaction degeneracy, and h is Planck’s constant. Both E and E0 in eq 1 are referenced to the ion ground-state energy. The density and sums of states are calculated using harmonic vibrational frequencies with the direct count of states algorithm.26 The ionization energy (IE) of the E isomer has been measured very accurately with laser based techniques by two groups: Woo et al.27 using pulsed field ionization spectroscopy, and by Bae et al.28 using mass analyzed threshold ionization (MATI). To within the nearest meV, both groups reported an IE of 9.631 eV. The Z isomer IE was reported by Lau and Ng29 to be 9.658 eV. There is no accurate experimental IE value for the 1,1C2H2Cl2 isomer available. Our calculated G3B3 IE values for the E and Z isomers are 9.608 and 9.639 eV, respectively. We note that these are both too low by 23 and 19 meV, respectively. Thus, we have determined the IE of the 1,1 isomer by adding 21 meV to its calculated value, which yields an IE of 9.833 eV. However, as has been mentioned previously and will be shown later, the Cl loss reaction coordinate in the parent ion is shared between the three DCE isomers. H-atom transfer is also possible in the Z and E isomer ions. As the phase space is shared at the dissociation threshold, the total density of states will be the sum of the densities of states of the three parent isomers and the two H-transfer isomer ions. 729



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energy curves and generated the Cl loss product via different transition states, the k(E) curves needed to fit the data would differ, and so would the activation entropies. In fact, the best fit activation entropies are 81.5, 81.4, and 81.5 J K-1 mol-1 (calculated at 600 K) for the 1,1, E, and Z isomers, respectively, which show remarkable agreement. In addition to the span of the fitted appearance energies, this is further experimental evidence that the reaction coordinate is shared, and in the energy range of the dissociation, the three DCE isomer cations interconvert readily. This is also supported by ab initio calculations for the H-atom transfer transition states. As is shown in Figure 5, H atom transfer transition states for isomerization of the 1,2 isomer ions to the higher energy 1,1 isomer lie about 0.23 eV below the dissociation energy. The reaction surface includes the structure that finally leads to the CH2CClþ product by a simple bond rupture. From the similarity of the three breakdown diagrams up to 19 eV, we conclude that this isomerization to a common parent ion structure is very rapid. Thus, it is established that the relative shifts in the breakdown curve correspond to the isomerization energies of the neutrals, that is, the relative 0 K heats of formation of the neutral isomers, because the dissociating species does not “know” from which neutral isomer it has been made. The derived 0 K dissociation energies are then determined by taking into account the slow rates in the vicinity of the Cl loss onset. For rate extrapolation to zero to be useful in accurate thermochemistry, that is, in determining the C2H2Cl2 f CH2CClþ þ Cl þ e- reaction energy, there should be no overall reverse barrier along the reaction coordinate. In fact, a more exacting criterion in modeling the kinetic shift and equating the k(E0) = 0 limit to the dissociative photoionization reaction energy is that no local energy maximum should be rate determining in the dissociation. Ab initio calculations at the B3LYP/6-311þþG(d,p) level can be used to shed light on this issue: first, when the C-Cl bond is elongated in a series of constrained optimizations in the H-transfer Z and E isomer parent ions, the ion structure gravitates toward the 1,1 isomer ion. In other words, the 1,1 isomer ion acts as a gateway to dissociation. Second, when the C-Cl bond length is scanned in the 1,1 isomer ion, the resulting potential energy curve (Figure 6) is not entirely attractive as is the case for most ionic dissociations taking place without a rearrangement, but shows several minima. However, these minima are all below the dissociation threshold. Furthermore, the vibrational frequencies of the highest lying transition state at R(C-Cl) = 2.8 Å yield an activation entropy of 60 J K-1 mol-1, that is, correspond to a tighter transition state than was found in the modeling. When we employ these transition state frequencies, the corresponding rates indicate a 60 meV larger kinetic shift and a bad fit to the experimental data. Therefore, the modeled appearance energy clearly corresponds to the dissociation energy and not a local energy maximum along the reaction coordinate. Because of the shared reaction coordinate, the 0 K isomerization energies correspond directly to the energy shifts in the breakdown diagrams. These can be determined more accurately than the absolute 0 K onset energies, as uncertainties in the TOF distributions and, therefore, in the kinetic shift do not perturb the isomerization energies. The best overlap with the (Z)-C2H2Cl2þ breakdown curve is achieved if the breakdown curves for the 1,1-C2H2Cl2þ and the (E)-C2H2Cl2þ are shifted 37 ( 5 meV (3.6 kJ mol-1) and 22 ( 7 meV (2.1 kJ mol-1) to higher energies (cf., 42 ( 7 and 26 ( 9 meV, respectively, based on the E0

Figure 5. Isomerization and dissociation channels in dichloroethylene cations. The energies are relative to the neutral 1,1-C2H2Cl2 isomer and have been computed using the G3B3 method. Low-energy H-atom transfer transition structures indicate that H-migration is possible below the Cl-atom loss onset. Cl-C bond length scans from the H-atom transfer structures gravitate toward the 1,1 isomer cation, indicating that the latter acts as a gateway structure to dissociation. A low energy transition state for Cl-atom transfer has been found in the Cl loss fragment that enables a second Cl-atom loss to form the acetylene cation above 16.24 eV (G3B3 value, 16.155 eV literature-based value).

differences). Thus, our results suggest that the 0 K heats of formation of the 1,1 and (E)-1,2 isomers lie 3.6 ( 0.5 and 2.1 ( 0.7 kJ mol-1 above that of the lowest energy (Z)-1,2 isomer. HCl Loss Reaction. The HCl loss, evident in the breakdown diagram in Figure 1, was not reported by Parkes et al.,3 because their mass resolution was insufficient to resolve HCl from the dominant Cl-atom loss. However, their peak width-based KER analysis may have been affected by HCl loss. Even though this is the energetically most favored process, it is curious that HCl loss never becomes dominant. This suggests that the reaction involves a highly energetic and/or tight transition state, due to a reverse barrier in the exit channel. We can estimate the dissociation energy on the basis of literature values as well as calculate it directly. As shown in Table 1, the heat of formation of chloroacetylene has been calculated by two groups. Its IE was also measured by several groups to be 10.6 eV.31 Combining the most recent heat of formation with this ionization energy yields an HCCClþ heat of formation of 1252 kJ mol-1, from which we can infer that the thermochemical dissociative photoionization onset for HCl loss from C2H2Cl2 is about 11.9 eV. Our G3B3 calculation of this onset yields a value of 11.88 eV. This is well below the measured Cl-atom loss onset energies, thus indicating a reverse barrier in the HCl loss exit channel. This conclusion is confirmed by the Cl loss potential energy curve calculations. As can be seen in Figure 6, the last minimum along the reaction coordinate is a Cl 3 3 3 HC(H)CClþ 730



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Table 1. Thermochemical Data (kJ mol-1) ΔfHo298K 1,1-C2H2Cl2

3.7 ( 1.75a 2.4 ( 2

(E)-1,2-C2H2Cl2

0.2 ( 1.75

14.5a

7.5

a

13.8a

5.6

b

13.7b

CH4 C2H4

-74.53 ( 0.06 52.6 ( 0.2c

C2H3Cl

a

14.3b a

228.3 ( 0.2

CH3Cl

13.8a

b

C2H2

CH2Cl2

9.1a

13.8b

2.5 ( 1.75

-3.0 ( 2

H298K - H0K

b a

-0.5 ( 2 (Z)-1,2-C2H2Cl2

ΔfHo0K

228.9 ( 0.2

c

c

d

-66.56 ( 0.06 61.1 ( 0.2c

-95.1 ( 2.5b

-88.3

e

-95.5 ( 1.3

-88.7

-93.8 ( 2.0 f

-87.0

-81.9 ( 1.5b

-74.0

22.9 ( 1.5a

10.0c d

30.5 ( 1.5a

10.02d 10.54c 11.9a

10.4a 11.8a

22.0 ( 3.0b C2HCl

229.4 ( 2g 226.4 ( 10b

228.8 ( 2g

11.6a

C2H3þ

1115.6 ( 1.0a

1119.9 ( 1a

11.3a

C3H7þ

805.9 ( 0.50h

CH3þ Cl

823.1 ( 0.50h

15.57h

1099.4 ( 0.1i 121.3 ( 0.01d

119.60 ( 0.01d

6.28d 9.18e

Cl2

8.47e

H2 a

b

4 c

This work, W1(U) thermal enthalpies. Manion (2002). Ruscic (private communication 2010).32 d Ruscic (2010).33 e Chase (1998).45 f Feller et al. (2003).41 g Parthiban et al. (2002)11 h Stevens et al. (2010).44 i Taken from Bodi and Baer (2009).30

Figure 6. Cl-atom loss and HCl loss reaction energy curves in the 1,1dichloroethylene cation. The energies are relative to the cation energy minimum. The B3LYP/6-311þþG(d,p) bond length scans in the 1,1DCE ion show several minima (a), one of which is a Cl 3 3 3 H-C structure. As the inset (not to scale) and (b) show, this can lead to a Cl-H 3 3 3 C structure with an H-atom transfer. After H-atom migration, HCl may be lost from the parent ion.

Cl-atom, the energy distribution is given by:21,25 þ ðEi ÞF ðE - Ei Þ F tr PC2 H2 Clþ ðEi , EÞ ¼ R E C2 H2 Cl þ ðyÞF ðE - yÞ dy F tr 0 C2 H2 Cl

complex. A pseudo-orthogonal mode to the Cl loss from this structure is the H-transfer to the Cl, yielding a new, ClH 3 3 3 C(H)CClþ complex that can dissociate along a purely attractive potential energy curve to lead to the HCl loss product (Figure 6b). The calculated barrier, 2.6 eV above the 1,1-DCE ground-state ion, that is, 12.4 eV above the neutral, is in good agreement with the observed HCl loss onset energy, which makes the proposed dissociation pathway quite likely. As far as the transition state is concerned, B3LYP/6-311þþG(d,p) frequencies at the transition state along the proposed HCl loss coordinate yield an activation entropy of 61.3 J mol-1 K-1 (600 K). The HCl loss partial ion abundances from 1,1-DCE are well reproduced with a 12.4 eV total energy transition state and a fitted activation entropy of 68.9 J mol-1 K-1 for the HCl loss, indicating a rather loose transition state for a reaction with a reverse barrier. Sequential Loss Reactions. At higher energies, the dominant reaction is the sequential loss of Cl from the C2H2Clþ intermediate. The analysis of this step involves a calculation of the internal energy distribution of the intermediate ion C2H2Clþ. The statistical theory provides a rigorous route for determining this energy distribution by making the assumption that the energy is statistically distributed between the translational degrees of freedom of the C2H2Clþ and Cl pair, and the rovibrational internal energy of the C2H2Clþ ion. Neglecting the spin-orbit and higher excited electronic states in the leaving

ð2Þ

where FC2H2C1þ(E) is a density of states as a function of internal energy, Ei is the ion energy referenced to the ground state of the ion, and E is the total available energy referenced from the dissociation products. The denominator is the normalization integral. The ro-vibrational density of states of the ion is readily calculated knowing the ion vibrational frequencies and moments of inertia. The translational density of states is given by the classical expression, Ftr(E) = CEz, with z = (D - 2)/2, where D is the dimensionality of the system, and C is a constant. In phase space theory, a two-dimensional translational energy distribution is assumed; consequently, Ftr(E) is a constant, independent of the energy. The only adjustable parameter in eq 2 is, thus, the dimensionality of the translational degrees of freedom. For a second, fast, consecutive Cl loss reaction, the breakdown curve of the first daughter ion as a function of the photon energy hν will be determined by the ratio of the first daughter ion internal energy distribution above the second dissociation onset energy, E00: Z E 0 0 - E0 þ BDC2 H2 Cl ðhνÞ ¼ PC2 H2 Clþ ðEi , hν - E0 Þ dEi ð3Þ 0

If the excess energy needed, E00 - E0, is large relative to the width of the energy distribution, P(Ei,E), the breakdown curve of eq 3 will approximately correspond to the cumulative distribution function of the internal energy distribution of the first 731



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The dichloromethane ΔfH298K is known from the analysis of Manion4 to be -95.1 ( 2.5 kJ mol-1, as well as from a high level calculation by Feller et al.,41 who reported its value to be -93.8 ( 2.0 kJ mol-1. The ΔfH(C2H3Cl) is based on a calculated value for the ΔfH0K(C2H3þ) = 1119.6 ( 3 kJ mol-1,42 and the measured dissociative photoionization onset by Shuman et al.,43 who reported a 0 K onset of 12.530 ( 0.010 eV for the vinyl ion from vinyl chloride: C2H3Cl þ hν f C2H3þ þ Cl þ e-. The following two reactions were used to confirm the Lago and Baer42 ΔfH(C2H3þ): C2 H3 þ þ CH4 f CH3 þ þ C2 H4 þ

C2 H3 þ CH4 f C3 H7

The ΔrE0K for eq 6 was calculated to be 106.9 kJ mol (W1 theory). Using the heats of formation of the methyl ion,30 ethylene,32 and methane33 listed in Table 1 yields a ΔfH0K(C2H3þ) = 1120.2 kJ mol-1. An experimental C3H7þ ion 0 K heat of formation was recently reported by Stevens et al.44 to be 823.1 ( 0.50 kJ mol-1. Using ΔfH0K(CH4) = -66.56 ( 0.06 kJ mol-1,33 the ΔrE0K = -230.0 kJ mol-1 (W1 reaction energy) for 7 yields a ΔfH0K(C2H3þ) = 1119.7 kJ mol-1. The results from these two reactions agree to within 0.5 kJ mol-1, so that the vinyl ion heat of formation can be assumed to be ΔfH0K(C2H3þ) = 1119.9 ( 1 kJ mol-1, a value having smaller error limits than the previous calculation.42 Thus, the vinyl chloride heat of formation is ΔfH0K(C2H3Cl) = 30.5 ( 1.5 kJ mol-1. The dissociative photoionization onsets from C2H2Cl2 cannot be used to determine accurate neutral heats of formation, because no accurate experimental heat of formation is available for the fragment ion C2H2Clþ. However, the isomerization energies can be accurately determined, so there are two approaches to the DCE heats of formation: (a) to calculate the W1 isodesmic reaction energies for eqs 4 and 5 for all three isomers individually, or (b) to calculate an average heat of formation based on the isodesmic reactions and use the experimental isomerization energies to obtain the three individual heats of formation. The first approach yields 10.4, 8.8, and 6.9 kJ mol-1 for the 1,1, E, and Z isomer heats of formation at 0 K and 4.5, 3.6, and 1.0 kJ mol-1 at 298 K, respectively. The second approach, in which the experimental isomerization energies are taken into account, yields 4.7, 3.2, and 1.1 kJ mol-1 at 298 K for the 1,1, E, and Z isomers. The close agreement simply demonstrates that the calculated isomerization energies are very close to our measured energy differences among the three isomers. These heats of formation can be compared to the values of Manion at 298 K of 2.4, -0.5, and -3 kJ mol-1, respectively. The difference between the average literature and average ab initio-isodesmic heats of formation is 3.4 kJ mol-1, which is somewhat larger than the 2.3 kJ mol-1 internal difference between the eqs 4 and 5 W1 reaction energy results. The latter discrepancy is, however, reduced to 1 kJ mol-1 if the Feller et al.41 calculated heat of formation for dichloromethane is used instead of the experimental one reported by Manion. Using this heat of formation also increases the discrepancy between our calculated heats of formation and those recommended by Manion to 4.1 kJ mol-1. If we use the presumably less accurate G3B3, CBS-QB3, and G3 calculations with reactions 4and 5, the calculated heats of formation are reduced from the W1 values by 2.3, 1.7, and 2.3 kJ mol-1, thereby moving closer to the Manion values, but still higher by about 2 kJ mol-1. This indicates that the literature heats of formation are 2.5 ( 1.5 kJ mol-1 too low. The isomerization heats also

fragment ion. In other words, the energy range in which the second fragment ion first appears relates to small kinetic energy release and higher energy points to large kinetic energy release. In Figure 7, we plot the breakdown diagram associated only with the sequential loss of two Cl-atoms and compare the experimental breakdown diagram to modeled sequential loss breakdown curves with the assumption of D = 1, 2, and 3 translational degrees of freedom. In obtaining an onset energy for C2H2þ þ 2 Cl, E00, by fitting the second dissociation step, the energy range in which C2H2þ first appears is of pivotal importance, and the error function has been weighted by a factor 10 in the 15.9-16.4 eV energy range. Using 0 K heats of formation of 228.9 kJ mol-1 for C2H2,32 119.6 kJ mol-1 for Cl,33 9.4 kJ mol-1 for 1,1-DCE (vide infra), and IE = 11.401 eV34 for C2H2 yields E00 = 16.155 eV. The D = 1, 2, and 3 fits are optimized with E00 = 16.316, 16.202, and 16.072 eV, respectively. While the D = 2 fit reproduces the lowenergy shape of the breakdown curve and yields an acceptable fit close to the onset energy, its error is still more than 4 kJ mol-1. It is evident that only the D = 1 model can fit the data at higher energies, that is, high kinetic energy release, at which fewer Cl atoms are ejected in the first dissociation step than predicted by phase space theory (D = 2).35,36 This is probably associated with the complex series of potential wells that create a nonisotropic potential energy surface, which prevents the departing Cl atom from exploring the full 2-D translational phase space. It also means that for the reverse association reaction, the Cl atom should approach the C2H2Clþ ion head on with a low impact parameter. Such reduced translational phase space volume at large excess energies has also been observed by Borkar et al.,37 who found that the second sequential dissociation in S2Cl2þ and SOCl2þ is best described by D = 3 at the onset energy and by D = 2 at higher energies. Thermochemistry: Experiment and Calculations. The neutral heats of formation were calculated with the help of isodesmic reactions and the G3,38,39 CBS-QB3,40 G3B3,23 and W1(U)12,13 composite methods. The most reliable ones are based on 0 K W1 reaction energies (ΔrE0K) of the following reactions: ð4Þ

C2 H2 Cl2 þ C2 H4 f 2C2 H3 Cl

ð5Þ

ð7Þ -1

Figure 7. Experimental and modeled breakdown curves for the second sequential Cl-atom loss from the 1,1-DCE isomer. The shape of the breakdown curve is mainly determined by the kinetic energy release. Fits are shown with one, two (as predicted by phase space theory), and three translational degrees of freedom. The 2D fit yields the most accurate onset energy, but does not reproduce the experimental abundances at high photon energies.

C2 H2 Cl2 þ CH4 f CH2 Cl2 þ C2 H4

ð6Þ

þ

732


The Journal of Physical Chemistry A show differences that, although within the error bars, appear to be significant. The breakdown diagrams suggest that, relative to the most stable Z isomer, the E isomer is 2.1 kJ mol-1 less stable (cf., 2.6 kJ mol-1 W1 result and 2.5 kJ mol-1 literature value), whereas the 1,1 isomer is 3.6 kJ mol-1 less stable (cf., 3.5 kJ mol-1 W1 result and 5.4 kJ mol-1 literature value). All in all, these small differences warrant a small revision of the previously published data: the average heat of formation is increased by 2.5 kJ mol-1, whereas the isomerization heats are updated as the average of the experimental and the W1 calculated values. Thus, the revised 298 K heats of formation for the 1,1, E, and Z isomers are 3.7, 2.5, and 0.2 kJ mol-1, respectively, with the uncertainty in the revised heats of formation reduced to 1.75 kJ mol-1. The dissociative photoionization onset energies for C2H2Cl2 þ hν f C2H2Clþ þ Cl þ e- and the revised neutral heats of formation yield a new ΔfH for the protonated chloroacetylene, C2H2Clþ. Taking the average 0 K onset energy of 12.083 eV (1165.8 kJ mol-1), average ΔfH0K(C2H2Cl2) = 2.1 kJ mol-1, together with the 0 K heat of formation of Cl yields ΔfH0K(C2H2Clþ) = 1048.3 kJ mol-1. Together with the 0 K heat of formation of HCCCl of 228.8 ( 2 kJ mol-1,11 this yields a 0 K proton affinity for chloroacetylene of 708.6 ( 2.5 kJ mol-1, in excellent agreement with the results of Rodriquez et al.7 of 707 kJ mol-1.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Addresses §

U.S. Environmental Protection Agency, 109 T.W. Alexander Dr., Research Triangle Park, NC 27711.

’ ACKNOWLEDGMENT The experimental work reported here was carried out at the VUV Beamline of the Swiss Light Source at Paul Scherrer Institut. ’ REFERENCES (1) Mikhailov, V. A.; Parkes, M. A.; Tuckett, R. P.; Mayhew, C. A. J. Phys. Chem. A 2006, 110, 5760. (2) Kennedy, R. A.; Mayhew, C. A.; Peverall, R.; Watts, P. Phys. Chem. Chem. Phys. 2000, 2, 3145. (3) Parkes, M. A.; Ali, S.; Howle, C. R.; Tuckett, R. P.; Malins, A. E. R. Mol. Phys. 2007, 105, 907. (4) Manion, J. A. J. Phys. Chem. Ref. Data 2002, 31, 124. (5) Pedley, J. B. Thermochemical Data and Structures of Organic Compounds; Thermodynamics Research Center: College Station, TX, 1994. (6) Gurvich, L. V.; Veyts, I. V.; Alcock, C. B. Thermodynamic Properties of Individual Substances, 4th ed.; Hemisphere Publishing Co.: Bristol, PA, 1991. (7) Rodriquez, C. F.; Bohme, D. K.; Hopkinson, A. C. J. Org. Chem. 1993, 58, 3344. (8) Keister, J. W.; Baer, T.; Evans, M.; Ng, C. Y.; Hsu, C. W. J. Phys. Chem. 1997, 101, 1866. (9) Werner, A. S.; Baer, T. J. Chem. Phys. 1975, 62, 2900. (10) Bodi, A.; Johnson, M.; Gerber, T.; Gengeliczki, Z.; Sztaray, B.; Baer, T. Rev. Sci. Instrum. 2009, 80, 034101. (11) Parthiban, S.; Martin, J. M. L.; Liebman, J. F. Mol. Phys. 2002, 100, 453. (12) Martin, J. M. L.; de Oliveira, G. J. Chem. Phys. 1999, 111, 1843. (13) Parthiban, S.; Martin, J. M. L. J. Chem. Phys. 2001, 114, 6014. (14) Johnson, M.; Bodi, A.; Schulz, L.; Gerber, T. Nucl. Instrum. Methods Phys. Res., Sect. A 2009, 610, 597. (15) Baer, T.; Li, Y. Int. J. Mass Spectrom. 2002, 219, 381. (16) Chandler, D. W.; Parker, D. H. Adv. Photochem. 1999, 25, 59. (17) Wiley, W. C.; McLaren, I. H. Rev. Sci. Instrum. 1955, 26, 1150. (18) Stevens, W.; Sztaray, B.; Shuman, N.; Baer, T.; Troe, J. J. Phys. Chem. A 2009, 113, 573. (19) Sztaray, B.; Baer, T. Rev. Sci. Instrum. 2003, 74, 3763. (20) Bodi, A.; Sztaray, B.; Baer, T.; Johnson, M.; Gerber, T. Rev. Sci. Instrum. 2007, 78, 084102. (21) Sztaray, B.; Bodi, A.; Baer, T. J. Mass Spectrom. 2010, 45, 1233. (22) Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (23) Baboul, A. G.; Curtiss, L. A.; Redfern, P. C.; Raghavachari, K. J. Chem. Phys. 1999, 110, 7650. (24) Stockbauer, R. Int. J. Mass Spectrom. Ion Phys. 1977, 25, 89. (25) Baer, T.; Hase, W. L. Unimolecular Reaction Dynamics: Theory and Experiments; Oxford University Press: New York, 1996. (26) Beyer, T.; Swinehart, D. R. Commun. ACM 1973, 16, 379. (27) Woo, H. K.; Wang, P.; Lau, K. C.; Xing, X.; Ng, C. Y. J. Phys. Chem. A 2004, 108, 9637. (28) Bae, Y. J.; Lee, M.; Kim, M. S. J. Phys. Chem. A 2006, 110, 8535. (29) Lau, K.-C.; Ng, C. Y. Acc. Chem. Res. 2006, 39, 823. (30) Bodi, A.; Shuman, N. S.; Baer, T. Phys. Chem. Chem. Phys. 2009, 11, 11013. (31) Maier, J. P.; Thommen, F. Relaxation dynamics of open-shell cations studied by photoelectron-photon coincidence spectroscopy.

’ CONCLUSIONS The lowest energy kinetically allowed channel in the dissociative photoionization of dichloroethylene isomers is shown to be the Cl-atom loss, which, along a shared reaction coordinate among the three (1,1; 1,2-Z, and 1,2-E) isomers, leads to the same ClCCH2þ fragment ion. As not only the product energetics, but also the fragmentation kinetics are the same, relative shifts in the breakdown curves of the parent ion correspond to the isomerization energies of the neutrals. In fortunate circumstances, such dissociative photoionization measurements can enable direct isomerization energy measurements even when the neutral isomerization barriers are high, and neutral equilibrium measurements do not yield these values. Relative to the most stable cis isomer (Z)-C2H2Cl2þ, the 1,1-C2H2Cl2þ and the (E)-C2H2Cl2þ isomers are found to be more energetic by 3.6 ( 0.5 and 2.1 ( 0.7 kJ mol-1 at 0 K. Combined with ab initio isodesmic reaction energies, the 298 K heats of formation for the 1,1, E, and Z and isomers have been slightly revised to 3.7, 2.5, and 0.2 ( 1.75 kJ mol-1, respectively. Also, in a combined theoretical and experimental approach, the proton affinity of chloroacetylene has been confirmed to be 708.6 ( 2.5 kJ mol-1. The breakdown curve corresponding to the second sequential dissociation reaction provides information about the internal energy distribution of the fragments in the first dissociation. In dichloroethenes, the fragment ion has been found to have a sharper overall internal energy distribution than predicted by phase space theory. The experimental distribution could best be modeled assuming one translational degree of freedom at high photon energies (high kinetic energy release) and two translational degrees of freedom, as predicted by phase space theory, at low photon energies (low kinetic energy release). This is the second time such a reduction in the translational phase space volume has been observed at large excess energies. 733



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In Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academic Press: Orlando, FL, 1984; pp 357-391. (32) Ruscic, B. Private communication of interim ATcT results based on the Core (Argonne) Thermochemical Network ver. 1.078. Unpublished work, 2010. (33) Ruscic, B. Active Thermochemical Tables; http://atct.anl.gov/ index.html. (34) Pratt, S. T.; Dehmer, P. M.; Dehmer, J. L. J. Chem. Phys. 1993, 99, 6233. (35) Klots, C. E. J. Chem. Phys. 1976, 64, 4269. (36) Klots, C. E. Z. Naturforsch. 1972, 27a, 553. (37) Borkar, S.; Ooka, L.; Bodi, A.; Gerber, T.; Sztaray, B. J. Phys. Chem. A 2010, 114, 9115. (38) Curtiss, L. A.; Raghavachari, K.; Redfern, P. C.; Rassolov, V.; Pople, J. A. J. Chem. Phys. 1998, 109, 7764. (39) Curtiss, L. A.; Redfern, P. C.; Raghavachari, K.; Pople, J. A. J. Chem. Phys. 2001, 114, 108. (40) Montgomery, J. A., Jr.; Frisch, M. J.; Ochterski, J. W.; Petersson, G. A. J. Chem. Phys. 1999, 110, 2822. (41) Feller, D.; Peterson, K. A.; de Jong, W. A.; Dixon, D. A. J. Chem. Phys. 2003, 118, 3510. (42) Lago, A. F.; Baer, T. J. Phys. Chem. A 2006, 110, 3036. (43) Shuman, N. S.; Ochieng, M. A.; Sztaray, B.; Baer, T. J. Phys. Chem. A 2008, 112, 5647. (44) Stevens, W. R.; Bodi, A.; Baer, T. J. Phys. Chem. A 2010, 114, 11285. (45) Chase, M. W., Jr. NIST-JANAF Thermochemical Tables, 4th ed.; 1998; Vol. Monograph 9.

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Understanding the Complex Dissociation Dynamics of Energy

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