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Dissociative Photoionization of Sulfur Chlorides and Oxochlorides: Thermochemistry and Bond Energies Based on Accurate Appearance Energies Sampada Borkar,† Lauren Ooka,† Andras Bodi,‡ Thomas Gerber,‡ and Ba´lint Szta´ray*,† Department of Chemistry, UniVersity of the Pacific, Stockton, California 95211, USA, and Paul Scherrer Institut, Villigen 5232, Switzerland ReceiVed: June 4, 2010; ReVised Manuscript ReceiVed: July 8, 2010
The dissociative photoionization of four compounds, SCl2, S2Cl2, SOCl2, and SO2Cl2, were measured with threshold and imaging photoelectron photoion coincidence spectrometry (TPEPICO and iPEPICO). In all systems, the molecular ion loses a chlorine atom in a fast dissociation. The 0 K appearance energies of the first chlorine-loss fragment ions were determined to be 12.252 ( 0.012 eV, 11.205 ( 0.003 eV, 11.709 ( 0.003 eV, and 12.505 ( 0.003 eV, respectively. SCl2 was measured on the laboratory-based TPEPICO instrument, in which the second Cl-loss dissociation could not be observed within the available photon energy. For S2Cl2+ and SOCl2+, the appearance energy of the fragment ion after two chlorine-loss dissociations were determined to be 13.32 ( 0.02 eV and 14.88 ( 0.02 eV, respectively. On the basis of the analysis of the breakdown curves, it was concluded that assuming three-dimensional translational degrees of freedom yields a more reliable statistical model of the product energy distributions. The literature heat of formation of the neutral precursor molecule thionyl chloride, SOCl2 does not agree with our results based on the SO+ cation and is revised by more than 10 kJ mol-1 to -198.2 ( 2.4 kJ mol-1. A particularly broad Franck-Condon gap with vanishingly small threshold electron signal in the photon energy range for the second Cl-loss reaction in SO2Cl2+ is discussed with regard to the mechanism of threshold ionization. Introduction The chemistry involving sulfur species is of vital importance in the formation of aerosols in photochemical smog and in the Jungle layer of the lower stratosphere,1-4 as well as for acid rain.5 Indirect atmospheric sources of SO2 include many pesticides and fungicides. These mixtures use S2Cl2 and SCl2,6 and most pesticide runoffs ultimately hit a water source,7 which causes the release of SOx species to the atmosphere. Also, chlorine chemistry plays an important role in the chemistry of the stratosphere.8 Without reliable thermochemical information on sulfur-halide and sulfur-oxo-halide species, the sulfur and halogen cycles can only be considered independent of one another, even though chemical reactions between these cycles can occur. Sulfur species have also been detected in interstellar comets.9 SO2, SO, S2, along with small quantitites of chlorinated sulfur species such as SCl, S2Cl, or SO2Cl2 are present on Io, one of the moons of Jupiter.10-15 Furthermore, S2Cl2 is a powerful and important chlorinating agent for perchloration of aromatic compounds.16-18 SO2Cl2 is also an important sulfurating agent18 as well as chlorinating agent, known for chlorinating alkenes, alkanes, and aromatics. SOCl2 (thionyl chloride) is used in the industrial production of organochlorine compounds such as the conversion of carboxylic acids to acyl chlorides.19,20 However, the thermochemistry of these important sulfur species is not extensively explored, because of several issues that mar any attempt at accurate measurement. Impurities and side reactions can plague experimental work due largely to the instability of these gaseous molecules. In addition to experimental uncertainties, errors in large compendiums of thermo* Author to whom correspondence should be addressed. E-mail: bsztaray@ pacific.edu. † University of the Pacific. ‡ Paul Scherrer Institut.
chemical data, such as the JANAF tables,21 also add to the problems. Lodders recently discovered four errors due to incorrect data conversion for sulfur-containing molecules as reported in the third and fourth edition of the JANAF tables for the molecules HS(g), S2O(g), NS(g), and PS(g).22 Recently, computational chemistry has been used for predicting the thermochemistry of many sulfur-containing species.23-27 The known sensitivity of sulfur-containing species to basis set construction and basis set choice complicates these studies.28-30 Thermochemical accuracy in these calculations requires methods such as CCSD(T) in combination with large correlationconsistent basis sets, that is, such studies are costly in terms of computer time, memory, and disk space requirements.27 With the technique of photoelectron photoion coincidence (PEPICO) spectrometry, the dissociation energetics of energyselected ions can be determined with an accuracy far superior to traditional mass spectrometry.31 Recent advances in the TPEPICO and the iPEPICO techniques allow sub-kJ mol-1 accuracy for small molecule analysis.32-37 In this technique, the dissociation of energy selected photoions is measured with timeof-flight mass spectrometry, and very accurate 0 K appearance energies can be determined from the disappearance of the precursor ion signal:33
AB + hV f A+ + B + e-
(1)
If the heat of formation is well-known for any two of the three species, the experiment yields the third one. The PEPICO technique can also be used for parallel or consecutive dissociations, where statistical modeling of the reaction rates and the ion energy distributions help extracting accurate 0 K onsets. We have shown the applicability of the method to determine
10.1021/jp105151c 2010 American Chemical Society Published on Web 08/11/2010
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SCHEME 1: Relationship between the Thermochemistry of the Four Systems Studied by TPEPICO and iPEPICO. Solid lines indicate ionization or dissociation observed in the experiments, and dotted lines show derived thermochemical relationships
second dissociation onsets for both parallel and consecutive dissociations.31 Two different instruments were used to carry out PEPICO experiments. The TPEPICO instrument based at University of the Pacific, Stockton, USA, was used to analyze the SCl2 molecule, and the iPEPICO instrument at the Swiss Light Source, PSI, Switzerland was used to analyze S2Cl2, SO2Cl2, and SOCl2. The details of the TPEPICO and the iPEPICO spectrometers have been described previously.31-33,35,38 A brief overview of some of the differences in these two apparatuses is as follows. In the TPEPICO experiment, a hydrogen discharge lamp is used as the photon source in an energy range of 5-14 eV. With 100 µm slits, the monochromator resolution is 1 Å, corresponding to about 8 meV at about 10 eV. The resolution in the electron analyzer is between 5-8 meV. The iPEPICO instrument was the first experimental station built on the new VUV (vacuum ultraviolet) beamline at the Swiss Light Source.35 At this beamline, the photon energy range is 5-30 eV, which is much wider than the range on the hydrogen discharge lamp, and the photon flux is approximately 1010-1011 s-1 with a ∆E/E ≈ 10-4. As its name suggests, the iPEPICO experiment uses 2D imaging detection for the photoelectrons, with a threshold electron energy resolution that is better than 1 meV. The iPEPICO endstation is equipped with both an effusive and a molecular beam inlet system, as well as heatable inlet for lowvolatility samples. Out of S2Cl2, SO2Cl2, SOCl2, and SCl2, the first three are stable molecules and are commercially available. These compounds have been measured on the iPEPICO instrument in Switzerland. However, SCl2 is neither stable nor commercially available. Due to the unavailability of a synthetic lab at the Swiss Light Source and the need for analyzing this compound immediately after its synthesis, SCl2 was measured with the laboratory-based TPEPICO in California. As shown in Scheme 1, all four of the studied compounds are interrelated through their ionic species. Therefore, by determining accurate heats of formation of the ionic fragments, SdO bond energies can also be obtained by relating the ionic fragments from the four different PEPICO experiments. The literature heat of formation of SCl2 at 298 K is listed in the NIST-JANAF thermochemical tables as -17.57 ( 3.3 kJ mol-1, and the 0 K value is -16.43 ( 3.3 kJ mol-1.21 The ionization energy of SCl2 has been measured by Solouki et al39 and is evaluated in the JANAF tables as 9.46 ( 0.02 eV, leading to a SCl2+ heat of formation of 896.4 ( 5.4 kJ mol-1.21 A recent value of the SCl neutral heat of formation calculated at the
Borkar et al. CCSD(T)/CBS level of theory including corrections for scalar relativistic, core-valence, and spin-orbit effects is 114 ( 2.1 kJ mol-1 at 298 K.26 This value is more than 40 kJ mol-1 lower than the experimental value of 156.46 ( 16.7 kJ mol-1 in the NIST-JANAF Table,21 which is concluded to be wrong by the authors of the theoretical paper. Therefore, in our thermochemical derivations, the CCSD(T) value is used. In the thermochemistry of S2Cl2, both the neutral S2Cl2, and S2 are fairly well-known, and can be used to establish the thermochemistry of the ions. Furthermore, the heat of formation of the S2Cl species is also listed in the JANAF tables, albeit with a large error bar. The JANAF 0 K heats of formation of S2Cl2, S2Cl, and S2 at 0 K are then -15.2 ( 4, 79.5 ( 8.4, and 128.3 ( 0.30 kJ mol-1, respectively. According to the recent theoretical paper published by Denis,26 the experimental heat of formation of S2 is most likely too high by 2-4 kJ mol-1, and he suggests a 298 K value of 125.9 ( 2.5 kJ mol-1, which, converted to a 0 K value of 125.6 ( 2.5, has been used in our thermochemical calculations. The adiabatic ionization energy of S2Cl2 was measured by Kaufel et al40 as 9.66 ( 0.03 eV, and a photoionization efficiency study on S2 done by Liao et al. yielded 9.356 ( 0.002 eV for the IE of S2.41 This latter value can be used to calculate the heat of formation of S2+, which then gives the literature-based 0 K appearance energy of the S2+ ion from S2Cl2 of 13.30 ( 0.05 eV. The SOCl2 0 K heat of formation is listed as -209.5 kJ mol-1 in the Wagman compilation.42 The ionization energy was determined to be 10.85 ( 0.05 eV by Mayer and Baer, who also determined the appearance energy of the SOCl+ ion to be 11.73 ( 0.01 eV, which leads to their SOCl2+ heat of formation of 802 ( 1 kJ mol-1. The heat of formation of sulfur monoxide is well-known; the JANAF tables21 list it as 5.03 ( 1.3 kJ mol-1 at 0 K. Norwood et al. has measured its ionization energy to be 10.294 ( 0.004 eV,43 which gives an SO+ heat of formation of 998.3 ( 1.4 kJ mol-1. This value, along with the Wagman heat of formation of SOCl2 then leads to an SO+ 0 K appearance energy from SOCl2 of 15.00 ( 0.01 eV. Similarly to SOCl2, the appearance energy of the SO2+ ion from SO2Cl2 can be calculated from literature values. The 0 K heat of formation for SO2Cl2 given by the NIST-JANAF table21 is -348.56 ( 2.1 kJ mol-1. The SO2 heat of formation is very accurately known, the JANAF 0 K value is -294.30 ( 0.21 kJ mol-1. With the adiabatic ionization energy, determined by Wang et al.44 to be 12.3494 ( 0.0002 eV, the heat of formation of SO2+ is 897.2 ( 0.2 kJ mol-1. This gives an SO2Cl2 f SO2+ + 2 Cl 0 K appearance energy of 15.39 ( 0.02 eV. Experimental Details TPEPICO Experiment on SCl2. SCl2 was synthesized by chlorination of sulfur using the procedure given by Schlessinger.45 The chlorine gas was produced by reacting conc. HCl with excess KMnO4. The gas was bubbled through H2SO4 and water and then dried with a dehydrated CaCl2 plug. The first step included the formation of the crude orange-red S2Cl2 by passing dry chlorine over molten sulfur for approximately two hours. The usual procedure in the synthesis of pure S2Cl2, the addition of excess sulfur to the crude condensate to combine with the excess of dissolved chlorine, was not deemed suitable, as the product was going to be further chlorinated in any case. In the second step, the crude S2Cl2 was further chlorinated in the presence of steel wool, which acts as a halogen carrier. PCl3 was then added to the dark liquid SCl2 to prevent its immediate decomposition into chlorine and sulfur monochloride (S2Cl2), and it was allowed to stand for an hour. Since PCl3 would give
Photoionization of SCl2, S2Cl2, SOCl2, and SO2Cl2 time-of-flight peaks very close to and possibly overlapping with the peaks from SCl2, care was taken to separate it from our sample. Therefore, pure sulfur dichloride was collected by fractional distillation at 59-61 °C, and the sample was measured on the TPEPICO immediately. In the TPEPICO spectra, no contamination was found due to PCl3. The details of the TPEPICO spectrometer have been described previously.31-33 The sample container was kept immersed in an ice bath during the whole measurement to reduce the decomposition of SCl2 to chlorine gas and S2Cl2. The vapor of the sample was introduced into the ionization chamber at -40 °C through a temperature-controlled inlet system consisting of a large copper block in which chilled methanol was circulating, and the sample vapor traveled approximately 10 cm, thermalizing with the walls. Any chlorine formed during the measurement was repeatedly pumped out while freezing the sample. The ionization region is enclosed in copper plates attached to the cooled copper block to minimize the amount of background room-temperature molecules. The molecules were ionized with vacuum ultraviolet (VUV) light, while the photon energy scale was calibrated using the hydrogen Lyman-R and Lyman-β resonance lines. The electrons and ions were extracted in opposite directions in a constant electric field of 20 V cm-1. Threshold electrons were velocity map imaged in a 13 cm long drift tube set to 77 V through a 1.4 mm aperture onto a center Channeltron detector.32 Hot electrons are focused into concentric rings with diameters determined by their initial velocity perpendicular to the extraction axis. A fraction of these hot electrons are collected by a second channeltron located next to the central detector. The hot electron contamination of the threshold signalsi.e., the energetic electrons with a zero perpendicular velocity componentscan be eliminated by subtracting a fraction of the pure hot-electron signal from the central signal as described by Szta´ray and Baer.33 Ions were directed into a linear time-of-flight (LinTOF) mass spectrometer with a Wiley-McLaren space-focusing geometry. In the linear TOF, ions were first accelerated by a 20 V cm-1 field in the 5 cm first acceleration region, and then they were rapidly accelerated to 260 eV in a 5 mm long second acceleration region. After exiting the acceleration regions, they finally drifted across a 34 cm field-free region and were detected with tandem microchannel plates (MCPs). The two electron and the ion signals served as the start and stop signals, respectively, for two time-to-pulseheight converters, the output of which is fed into multichannel analyzer cards generating the two TOF spectra that correspond to the center electron detector and the hot-electron detector, respectively. As the sulfur dichloride ions dissociate rapidly on the time scale of ion extraction, their TOF peaks are symmetric and sharp, and the only information obtained from these data is the fractional abundance of the parent and fragment ions as a function of the photon energy, that is, the breakdown diagram. Because only the peak areas are of interest in the fast dissociation, in order to correct for hot electron contamination we simply multiply the integrated hot electron TOF peaks by a constant factor (usually 0.15 for this setup) and subtract it from the center TOF counts. This factor is obtained by taking the ratio of the precursor ion peaks for the center and off-center spectra at a photon energy in excess of the dissociation limit, where no parent ion is present. This factor remains constant for all wavelengths and, in general, from one molecule to the next if the collection efficiencies of the detectors do not change. iPEPICO Experiments. S2Cl2, SO2Cl2 and SOCl2 samples were purchased from Sigma-Aldrich and used without further purification. The details of the iPEPICO experiment have been
J. Phys. Chem. A, Vol. 114, No. 34, 2010 9117 described previously.35,38 Briefly, a room temperature gas-phase sample was introduced to the experimental chamber through a 30 cm long, 6 mm o.d. Teflon tube. The typical pressure outside the µ-metal shield was 2 × 10-6 to 6 × 10-6 mbar during measurement. The sample was ionized by the incident synchrotron radiation with a maximum spot size of 2 × 4 mm at the X04DB bending magnet beamline of the Swiss Light Source.46 The photon energy was calibrated against Ar and Ne autoionization lines, both in the first and second order.47 The photoelectrons are velocity map-imaged onto a DLD40 Roentdek position sensitive delay-line detector with a kinetic energy resolution better than 1 meV at threshold. The ions are extracted by the same 80 V cm-1 field in a 5 cm long acceleration region and then are further accelerated to -550 V to space focus them at a Jordan TOF C-726 microchannel plate detector. The hot electrons are accounted for by subtracting signal collected in a small ring around the central spot with typical i.d. and o.d. values of 0.8 and 1-1.5 mm, respectively. Electron hit times and positions and ion hits are recorded in the triggerless mode of an HPTDC time to digital converter card, and electrons and ions are correlated on-the-fly to obtain time-of-flight distributions without dead time. This multiple-start/multiplestop data acquisition scheme38 enables data acquisition at almost arbitrarily high ionization rates. In our experiments, both the first and second chlorine-loss dissociation onsets were measured for S2Cl2 and SOCl2. However, we could not measure the second dissociation onset for SO2Cl2 due to the presence of a most unusual Franck-Condon gap where there was no threshold electron production above hν ) 15.0 eV. Results and Discussion Quantum Chemical Calculations. Density functional calculations were performed on S2Cl2, SO2Cl2, SOCl2, and SCl2 using the hybrid B3LYP functional48 with the 6-311G(d,p) basis sets49-51 to obtain the vibrational frequencies and the rotational constants of the neutral precursors. These were then used to calculate the internal energy distribution of the neutrals. The statistical product energy distributions are based on the densities of states for the ionic and neutral fragments. Here, the neutral fragment was a chlorine atom in all cases, and B3LYP/6311G(d,p) calculations were performed on the S2Cl+ and the SOCl+ fragment ions, for both of which the triplet state was found to be more stable than the singlet, and was used in the calculation of the harmonic vibrational frequencies. A reaction network consisting of 10 reactions, as shown in Figure 1, has also been constructed. The 0 K reaction enthalpies were calculated using the G3,52 G3B3,53 CBS-QB3,54,55 and W1(U)56-58 composite methods and were compared to the reaction enthalpies based on the literature heats of formation. More than one G3B3 value seemed to be an outlier, and the G3B3 results were consequently excluded from the subsequent analysis. Similarly to the one used by Bodi et al.,59 an error function was defined to weigh different computational results according to their expected accuracy and also to obtain an estimate on the uncertainty of the results by considering the differences between different composite methods of calculation:
ε)
∑ i
fG3∆G3∆rHi + fCBS-QB3∆CBS-Q3∆rHi + fW1(U)∆W1(U)∆rHi σ(∆∆rH) (1a)
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Figure 1. Ab initio reaction network heat of formation offsets with respect to literature values. The uncertainties in the offset are based on the standard deviation of the different fit results (the literature error bars are also shown as barbell ranges). For more details about the reaction network fit, see text.
where f stands for weighting factors of the different computational methods, ∆∆rHi is the difference between the computed and literature-based 0 K reaction energy, and σ is the standard deviation. The error function was minimized by changing the heats of formation of SO2Cl2, Cl2O, SO2, SO, SCl2, SOCl2, S2, and S2Cl2 for the following f-sets: (1,0,0; 0,1,0; 1,2,0; 1,2,1; 1,2,10; 1,2,100; 1,2,1000, in the same order as in eq 1). The average offsets to the literature heat of formation of these species are shown in Figure 1 together with literature error bars and three times the standard deviation of the offset in the different f-set fits. The latter indicates how much the computational results depend on the method employed. For most species, the error bars overlap, indicating good agreement between calculation and literature values. The two exceptions are SOCl2 and S2Cl2. As will be discussed later, the experimental data confirm the discrepancy for SOCl2 but are largely consistent with the literature values for S2Cl2. Thus, the computational results for the SOCl2 heat of formation are in agreement with the experiment, but for S2Cl2, the discrepancy is most likely overestimated by theory. All calculations were performed using the Gaussian 03 Revision E.01 quantum chemical code.60 Experimental Data. SCl2. Time-of-flight spectra of energy selected SCl2 ions were collected in the photon energy range of 12.05-12.46 eV with the linear TOF (LinTOF) setup on the TPEPICO. The fractional ion abundances as a function of the photon energy, that is, the breakdown curves are shown in Figure 2. The experimental data are plotted as open circles, and the solid lines show the modeled breakdown curves and TOF distributions. The nominal-mass peak for the molecular ion, SCl2+ (m/z 102) and the isotope peaks (m/z 104 and 106) were detected in the time-of-flight range of 20.3-21.0 µs; the two peaks corrsponding to the fragment ion, SCl+ (m/z 67, and 69) were detected between 16.2 and 16.9 µs. The second chlorineloss dissociation was not observed due to the limited photon energy range offered by the hydrogen discharge lamp. S2Cl2. Time-of-flight spectra of energy selected S2Cl2 ions were collected in the photon energy range of 10.6-15.0 eV with the iPEPICO instrument. Below a photon energy of 11.2 eV, the nominal-mass peak for the molecular ion, S2Cl2+ (m/z 134) and the isotope peaks (m/z 136, and 138) were detected in the time-of-flight range of 25.1-26.5 µs, the two peaks corresponding to the fragment ion, S2Cl+ (m/z 99, and 101) were detected between 21.7 and 23.0 µs. In this experiment, above 13.3 eV, the fragment ion due to the second chlorine loss (S2+) was detected at a time-of-flight of 17.8 µs. The experimental breakdown curves constructed from the integrated peak areas
Borkar et al.
Figure 2. Breakdown diagram of SCl2+. Circles are experimentally measured ion abundances, and lines are the best-fit modeling of the data. The downward arrow indicates the 0 K dissociation onset.
Figure 3. Breakdown diagram of the first chlorine-loss dissociation of the S2Cl2+ ion. Circles are experimentally measured ion abundances, and lines are the best-fit modeling of the data. The downward arrow indicates the 0 K dissociation onset.
Figure 4. Breakdown diagrams of the chlorine-loss dissociation of the S2Cl+ ion, leading to the second fragment ion, S2+. Circles are experimentally measured ion abundances, and lines are the best-fit modeling of the data assuming (a) 2 translational degrees of freedom: double line; (b) 3 translational degrees of freedom: single line. The downward arrows indicate the 0 K dissociation onsets.
are shown in Figure 3 and Figure 4 (open circles) together with the results of the modeling (solid lines). SOCl2. Time-of-flight spectra of energy selected SOCl2 ions were collected in the photon energy range of 11.0-16.0 eV with the iPEPICO instrument. Below a photon energy of 11.2
Photoionization of SCl2, S2Cl2, SOCl2, and SO2Cl2
J. Phys. Chem. A, Vol. 114, No. 34, 2010 9119 Modeling and Thermochemistry. To obtain the 0 K dissociation energies, the experimental data is modeled in terms of the thermal energy distribution of the precursor ion.31 The fractional ion abundances corresponding to the threshold ion signal 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 alignment. For most molecules that dissociate fast on the time scale of the PEPICO experiment, it is a good approximation that the neutral internal energy distribution is transposed onto the ion manifold, and those species that have more internal energy than the dissociation energy will dissociate.31 Thus, the breakdown curve corresponds to the cumulative distribution function of the ion internal energy and, thus, of the neutral internal energy at the experimental temperature:
Figure 5. Breakdown diagram of the first chlorine-loss dissociation of the SOCl2+ ion. Circles are experimentally measured ion abundances, and lines are the best-fit modeling of the data. The downward arrow indicates the 0 K dissociation onset.
Figure 6. Breakdown diagrams of the chlorine-loss dissociation of the SOCl+ ion, leading to the second fragment ion, SO+. Circles are experimentally measured ion abundances, and lines are the best-fit modeling of the data assuming (a) 2 translational degrees of freedom: double line; (b) 3 translational degrees of freedom: single line. The downward arrows indicate the 0 K dissociation onsets.
eV, the nominal mass peak for the molecular ion, SOCl2+ (m/z 118) and the isotope peaks (m/z 120, and 122) were detected in the time-of-flight range of 23.55-23.85 µs. The two peaks corrsponding to the fragment ion, SOCl+ (m/z 83, and 85) were detected between 19.55 and 20.8 µs. Similar to the S2Cl2 experiment, above 14.8 eV, the fragment ion due to the second chlorine loss (SO+) was detected at a time-of-flight of 15.4 µs. The experimental breakdown curves constructed from the integrated peak areas are shown in Figures 5 and 6 (open circles) together with the results of the modeling (solid lines). SO2Cl2. Time-of-flight spectra of energy selected SO2Cl2 ions were collected in the photon energy range of 12.1-15.5 eV with the iPEPICO instrument. The nominal mass peak for the molecular ion, SO2Cl2+ (m/z 134), and the isotope peaks (m/z 136, and 138) were detected in the time-of-flight range of 25.5-26.5 µs, the two peaks corrsponding to the fragment ion, SO2Cl+ (m/z 99, and 101), were detected between 21.5 and 23.0 µs. The breakdown curves corresponding to the first dissociation of the SO2Cl2+ ion is shown in Figure 7, similarly to the previous compounds. Although, according to the literature, the second chlorine-loss dissociation was expected to be seen at around 15.39 eV, virtually no threshold electron signal was observed in this Franck-Condon gap.
BD(hν) )
∫0E -IE Pi(E, hν)dE = ∫0E -hν Pn(E)dE 0
0
(2) where Pi is the ion’s internal energy distribution, as a function of the photon energy. Pn is the neutral molecule’s internal energy distribution function that can be calculated by the simple Boltzmann-formula: Pn(E) ) Fn(E) e-E/kT. The Fn(E) is the neutral molecule’s density of states function calculated using the molecule’s vibrational frequencies and rotational constants determined by density functional theory. It is evident that at hν ) E0 the above integral becomes 0, therefore the disappearance of the parent ion signal directly gives the 0 K dissociation energy. This quantity, however, can be obtained to a better precision by modeling the whole of breakdown curve and varying the assumed E0 for the best fit of the calculated and measured breakdown curves. This method also takes the slight broadening due to photon and electron energy resolution, into account. The modeled breakdown curve corresponding to the Cl-loss dissociation of SCl2+ is shown with solid lines in Figure 2. The good agreement with the experimental ion ratios indicates that the thermal assumption of the experimental temperature of -40 °C is valid even for this very small molecule. Obtaining the best fit of the modeled curves to the experimental data gave a 0 K appearance energy of SCl+ of 12.252 ( 0.012 eV. Using this value together with the known heat of formation of SCl2 and Cl from the NIST-JANAF tables,21 -16.43 ( 3.3 kJ mol-1 and 119.621 ( 0.006 kJ mol-1, respectively, gave the heat of formation of SCl+ as 1046.1 ( 3.5 kJ mol-1. As the heat of formation of the neutral SCl is also known from the high-level calculations by Denis,26 one can obtain the adiabatic IE of SCl as 9.67 ( 0.04 eV. This is in good agreement with the W1U calculated ionization energy of 9.62 eV. Thus, it further confirms that the 0 K heat of formation of SCl of 155.65 ( 16.7 kJ mol-1 given in the NIST-JANAF tables is too high, by more than 40 kJ mol-1. Similarly, modeling the first chlorine-loss dissociation of S2Cl2+ yields a 0 K appearance energy of the S2Cl+ fragment ion as 11.205 ( 0.003 eV. To model the breakdown curve corresponding to the second dissociation, one has to calculate the internal energy distribution of the first fragment ion. The excess energy after the first dissociation is distributed between the ionic and the neutral fragments. To calculate the internal energy distribution of the first fragment ion, one can use the canonical formalism, assume a virtual temperature and use the Klots’ equation to calculate how much energy is taken away by the leaving atom. For n
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translational degrees of freedom, (n/2) · kBT is assigned to the translation. Another approach is to use the microcanonical formalism and calculate how the excess energy is distributed between the fragment ion and the neutral at each point in the energy distribution function using the densities of states of the fragment ion and the translational degree of freedom:
p(Ei, E - E0) )
FA+(Ei)Ftr(E - E0 - Ei) E-E0 FA+(x)Ftr(E - E0 - x)dx 0
∫
(3) where p(Ei, E - E0) is the probability of the fragment ion to have Ei energy from the E - E0 total excess energy, FA+ and Ftr are the densities of states of the fragment ion, and that of the translational degrees of freedom, respectively. To complicate matters further, in the case of a chlorine-loss, the existence of a spin-orbit state lying at 882.36 cm-1 had also be taken into account in the calculation of the energy taken away by the neutral fragment. In our previous PEPICO studies, we have encountered cases where either one- or two-dimensional translation provided a best fit to the experimental data. In this work, we have used both 2D and 3D descriptions, and the results are shown in Figure 4. It is clear that neither model fits the data perfectly. The 3D model offers a better fit close to the dissociation onset, whereas at high energies, a 2D model reproduces the experimental data better. This latter can be due to deviations from a purely statistical behavior: for a very fast first dissociation, there may not be enough time for a complete statistical distribution of the excess energy among all degrees of freedom. For the determination of the second 0 K appearance energy, it is more important to have a good fit close to the onset of the second fragment ion, therefore the 3D model is likely to prove more reliable in extracting the true 0 K onset. Furthermore, as the thermochemistry of both the second fragment ion, S2+, and the neutral S2Cl2 is fairly well established, the literature appearance energy can be compared to our results. From the 2D model, the 0 K appearance energy of S2+ is 13.51 eV, whereas the 3D model gives 13.32 ( 0.003 eV; and the difference between them is much larger than the uncertainty in the literature heats of formation. If one calculates the expected E0 for S2+, based on the Denis heat of formation of 125.6 ( 2.5 kJ mol-1,26 our 3D value of 13.32 ( 0.02 eV agrees well with the literature value of 13.30 ( 0.04 eV. This is a strong argument for the validity of this model. On the basis of the Denis S2 heat of formation, the revised heat of formation for the neutral S2Cl2 is -17.6 ( 3.2 kJ mol-1. The sign of this correction of -2.4 kJ mol-1 agrees with the reaction network calculations, but it is a fair bit smaller, and is still within the 4 kJ mol-1 error bar of the literature S2Cl2 heat of formation value. Using this revised heat of formation of S2Cl2, and the iPEPICO first E0, one can calculate the heat of formation of S2Cl+ to be 943.9 ( 3.2 kJ mol-1. As the heat of formation of the neutral S2Cl is also known, its IE is obtained as 8.96 ( 0.09 eV. Modeling the first chlorine-loss dissociation of the SOCl2+ molecular ion results in a 0 K appearance energy of 11.709 ( 0.003 eV for the SOCl+ ion. This number is in reasonable agreement with an earlier TPEPICO measurement by Mayer and Baer61 with their reconstructed E0 of 11.73 ( 0.01 eV. Their confidence interval may have been slightly optimistic as their analysis was hindered by hot-electron contamination, which makes an accurate determination of the E0 much less straightforward. Similarly to S2Cl2, the second chlorine-loss was also
Figure 7. Breakdown diagram of SO2Cl2+. Circles are experimentally measured ion abundances, and lines are the best-fit modeling of the data. The downward arrow indicates the 0 K dissociation onset.
Figure 8. The low-energy region of the threshold photoelectron spectrum of SOCl2. Dots are the experimental data points, and the solid line shows an extrapolation to zero intensity to determine the adiabatic ionization energy.
observed in our iPEPICO experiments, and the data was modeled assuming both two and three translational degrees of freedom. The results are shown in Figure 6, and is clear that close to the dissociation onset, the 3D model gives a better fit.
Figure 9. The low-energy region of the threshold photoelectron spectrum of SO2Cl2. Dots are the experimental data points, and the solid line shows an extrapolation to zero intensity to determine the adiabatic ionization energy. The arrow indicates the anticipated position of the second chlorine-loss dissociation, not observed because of negligible threshold-electron intensity.
Photoionization of SCl2, S2Cl2, SOCl2, and SO2Cl2
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TABLE 1: Auxiliary and Derived Thermochemical Data species SCl2 SCl2+ SCl SCl+ S2Cl2 S2Cl S2Cl+ S2 S2+ SOCl2 SOCl2+ SOCl+ SO SO+
∆fH°0K (kJ mol-1) -16.4 ( 3.3 896.4 ( 5.4b 113.0 ( 2.1d 155.6 ( 16.7b 1046.1 ( 3.5a 1055.7 ( 2.9b -15.2 ( 4b -17.6 ( 3.2a 79.5 ( 8.4b 943.9 ( 3.2a 125.6 ( 2.5e,g 128.30 ( 0.30b b
1028.4 ( 2.5e,h 1031.02 ( 0.4b,h -209.5i -198.2 ( 2.4a 845.8 ( 3.0a 811.9 ( 2.4a 5.03 ( 1.3b
SO2Cl2
998.3 ( 1.4b,k -348.56 ( 2.1b
SO2Cl2+ SO2Cl+ SO2 SO2+
811.19 ( 2.9a 738.4 ( 2.1a -294.30 ( 0.21b 897.24 ( 0.21b,m
∆fH°298K (kJ mol-1) -17.6 ( 3.3 895.3b 114 ( 2.1e 156.5 ( 16.7b 1046.3 ( 3.5a 1055.8b -16.7 ( 4b -19.1 ( 3.2a 78.6 ( 8.4b
E0 (eV)
IE (eV) 9.46 ( 0.02c
b
9.67 ( 0.04a 12.252 ( 0.012a 9.66 ( 0.03f 11.205 ( 0.003a
125.9 ( 2.5e 128.60 ( 0.30b
13.32 ( 0.02a 13.30 ( 0.05e,h 13.32 ( 0.04b,h
-212.5i -201.3 ( 2.4a
11.709 ( 0.003a 11.73 ( 0.01j
5.01 ( 1.3b
14.88 ( 0.02a 15.00 ( 0.01b,i,k
-354.80 ( 2.1b
-296.84 ( 0.21b
12.505 ( 0.003a 15.391 ( 0.02b,m
8.96 ( 0.09a 9.356 ( 0.002h
10.85 ( 0.05j 10.82 ( 0.02a
10.294 ( 0.004k 12.02 ( 0.02a 12.05l 12.3494 ( 0.0002m
a This work. b Chase et al. (ref 21). c Solouki et al. (ref 39). d Converted from the corresponding 298 K value using H°298K - H°0K ) 9.819 kJ mol-1. e Denis (ref 26). f Kaufel et al. (ref 40). g Converted from the corresponding 298 K value using H°298K - H°0K ) 9.124 kJ mol-1. h Liao et al. (ref 41). i Wagman et al. (ref 42). j Mayer et al. (ref 61). k Norwood et al. (ref 43). l Chadwick et al. (ref 62). m Wang et al. (ref 44).
The appearance energy from the 2D model is 15.22 eV while the 3D model results in an E0 of 14.88 ( 0.02 eV. This latter should be compared to the literature E0 of 15.00 ( 0.01 eV calculated from the Wagman heat of formation of SOCl2 and the well-established SO+ heat of formation.43 The two numbers clearly do not agree within the claimed experimental uncertainty and, since the SO+ is well established, in line with the highlevel ab initio studies, the SOCl2 heat of formation is most likely too low. Using the SO+ 0 K heat of formation of 998.3 ( 1.4 kJ mol-1 with our iPEPICO second E0 gives an SOCl2 neutral heat of formation of -198.2 ( 2.4 kJ mol-1. This value was then used to calculate the 0 K heat of formation of the SOCl+ ion of 811.9 ( 2.4 kJ mol-1. If the Mayer and Baer61 value of 802 ( 1 kJ mol-1 for the heat of formation of SOCl+ is updated based on the new heat of formation of SOCl2 to 814.0 ( 2.6 kJ mol-1, then the agreement with our value is restored. The threshold photoelectron spectrum of SOCl2 was also measured in our experiments, and the low-energy part is shown in Figure 8. By extrapolating to zero intensity, an adiabatic IE of 10.82 ( 0.02 eV was determined, in agreement with the Mayer and Baer value of 10.85 ( 0.05 eV, which can be used to obtain the 0 K heat of formation of SOCl2+ as 845.8 ( 3.0 kJ mol-1. Modeling the first chlorine-loss dissociation of the SO2Cl2+ molecular ion results in a 0 K appearance energy of 12.505 ( 0.003 eV. This value can be used together with the known heat of formation of SO2Cl2 and the Cl atom from the NIST-JANAF tables,21 -348.56 ( 2.1 kJ mol-1 and 119.621 ( 0.006 kJ mol-1, respectively, to calculate the heat of formation SO2Cl+ as 738.4 ( 2.1 kJ mol-1. Similarly to SOCl2, the low-energy part of the TPES was also recorded as shown in Figure 9, and the extrapolated adiabatic IE of SO2Cl2 is determined to be 12.02
( 0.02 eV, in reasonable agreement with the 12.05 eV measured by Chadwick et al.62 Using this ionization energy, we can also determine the heat of formation of the SO2Cl2+ molecular ion as 811.19 ( 2.9 kJ mol-1. We have attempted to measure the appearance of the second chlorine-loss fragment ion, SO2+. This ion, according to the reasonably well-established literature thermochemistry, should appear at approximately 15.39 ( 0.02 eV.44 However, at this photon energy, there was no threshold electron signal. In order to explain this phenomenon, the production of threshold electrons in Franck-Condon gaps has to be considered. The mechanism Guyon et al.63 and Chupka et al.64 proposed, and Bodi et al. employed to explain a vibrational fine structure in the breakdown curve of iodomethane,65 relies upon intermediate Rydberg states. Normally, there is a quasi continuum of these converging to various ion states, and, thus, it is Franck-Condon allowed for the neutral molecule to absorb a photon and access these states in regions where the probability of direct ionization is low, that is, in a Franck-Condon gap. The neutral molecule can cross over to a neutral dissociative surface from these states. As the molecule dissociates, it may recross to a lower energy ion state, generating a threshold electron. In SO2Cl2, however, the next ionic state is too far up in energy, the Franck-Condon gap is broad and the quasi continuum of these intermediate Rydberg states is, apparently, missing. The auxiliary and derived thermochemical values are listed in Table 1. In the calculation of the 298 K heats of formation, the H°298K - H°0K values of the various neutral and ionic species were taken and, for the elements, the following values were used: S: 4.412 kJ mol-1, Cl2: 9.181 kJ mol-1, O2: 8.683 kJ mol-1.
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TABLE 2: Summary of the SdO and S-Cl Bond Energies in the Ions SdO bond energies (kJ mol-1) SOCl2+ SO2Cl2+ SOCl+ SO2Cl+
297.5 ( 6.2 281.4 ( 4.2 481.0 ( 4.2 320.3 ( 3.2
S-Cl bond energies (kJ mol-1) SOCl+ SCl2+ S2Cl+ S2Cl2+ SOCl2+ SO2Cl2+
305.5 ( 3.4 269.4 ( 2.2 203.6 ( 4.5 149.2 ( 2.9 85.8 ( 2.0 46.8 ( 2.0
Bond Energies. Even though there is no SdO bond breakage in the low-energy PEPICO experiments, as shown in Scheme 1, the SdO bond energies can be determined from the heats of formations of the various ions, as determined above. These SdO bond energies have been summarized in Table 2. It can be concluded that (a) the second SdO bond is somewhat weaker in the molecular ion, that is, the SdO bond is stronger in SOCl2+ than in SO2Cl2+; (b) this difference is even more pronounced after the loss of a chlorine; and (c) the SdO bond get significantly stronger after the removal of a chlorine, for both molecular ions. The S-Cl bond energies are also listed in Table 2 and are weakened in the order of SOCl+, SCl2+, S2Cl+, S2Cl2+, SOCl2+, and SO2Cl2+. Conclusions The dissociative photoionization of sulfur chlorides and oxochloridessS2Cl2, SO2Cl2, SOCl2 and SCl2swas studied by producing energy selected parent ions using monochromatic VUV radiation and recording their dissociation reactions as a function of photon energy. The TPEPICO experiment on SCl2 and the iPEPICO experiments on S2Cl2, SO2Cl2, and SOCl2 have yielded breakdown diagrams that have allowed us to determine accurate heats of formation for the fragment ions and to establish trends in the SdO and S-Cl bond energies. By way of the ion cycle bypass, a discrepancy several times the supposed error bar in the heat of formation of SOCl2 (thionyl chloride) has been found, and its revised heat of formation, -198.2 ( 2.4 kJ mol-1, is also supported by ab initio reaction network calculations. The same calculations also suggest a lower S2Cl2 heat of formation, which only agrees in sign but not in magnitude with the experimental data. An unusually broad Franck-Condon gap in the TPES of SO2Cl2 meant that the second Cl-loss reaction was impossible to record. The vanishingly small threshold electron signal in this energy region can be explained by the absence of Rydberg-states to which transitions are FranckCondon allowed. Acknowledgment. The TPEPICO measurements were carried out on the instrument built in the laboratory of Prof. Tomas Baer at the University of North Carolina at Chapel Hill, supported by the grants from the U.S. National Science Foundation, the US Department of Energy, and the Hungarian Science Fund (OTKA No. 71644). This experiment was donated to the University of the Pacific in 2009, for which we are very grateful. B. S., S. N. B. and L. O. gratefully acknowledge the support of the ACS Petroleum Research Fund, the Pacific Fund, and the Paul Scherrer Institute for a generous allocation of synchrotron beamtime. Supporting Information Available: Detailed results of the ab initio reaction network calculations are given as electronic supporting material. This information is available free of charge via the Internet at http://pubs.acs.org.
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