Article pubs.acs.org/JPCA
IR Photodissociation Spectroscopy of H7+, H9+, and Their Deuterated Analogues J. W. Young, T. C. Cheng, B. Bandyopadhyay, and M. A. Duncan* Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States S Supporting Information *
ABSTRACT: Cluster ions of H7+/D7+ and H9+/D9+ produced in a supersonic molecular beam with a pulsed discharge source are mass selected and studied with infrared laser photodissociation spectroscopy. Photodissociation occurs by the loss of H2 (D2) from each cluster, producing resonances in the 2000−4500 cm−1 region. Vibrational patterns indicate that these ions consist of an H3+ (D3+) core ion solvated by H2 (D2) molecules. There is no evidence for the shared proton structure seen previously for H5+. The H3+ ion core vibrational bands are weakened and broadened significantly, presumably by enhanced rates of intramolecular vibrational relaxation. Computational studies at the DFT/B3LYP or MP2 levels of theory (including scaling) are adequate to reproduce qualitative details of the vibrational spectra, but neither provides quantitative agreement with vibrational frequencies.
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INTRODUCTION Since J. J. Thomson discovered H3+ in 1911,1 small hydrogen ions and their clusters have been seen in many mass spectrometry and plasma environments. These fascinating ions are also proposed to be present in interstellar gas clouds, playing a key role in the ion−molecule chemistry at low temperature.2−6 The ion−molecule reactions of small hydrogen cluster ions have been well-studied in mass spectrometry,7−21 and the structures of these ions are also highly interesting. The laboratory infrared spectrum of H3+ was reported by Oka and co-workers, proving the symmetric D3h structure for this ion, and subsequently it has been investigated extensively by several laboratories.22−25 It was identified in numerous interstellar sources by Oka and co-workers and is now a key indicator species for interstellar chemistry.26−28 H5+ has not been detected in space, but it is the intermediate in the symmetric proton transfer from H3+ to H2, and its deuterated analogues have been proposed as intermediates in deuterium fractionation in the interstellar medium.2−6,19−21 It is the smallest protonbound dimer, and it has been the subject of recent spectroscopic and theoretical investigations.29−48 In the present report, we extend these new measurements to H7+ and H9+ and their deuterated analogues. After much computational study and recent spectroscopy, H5+ is now recognized as a symmetric (D2d) proton-bound dimer.29−48 Its potential has a double minimum with limiting H3+−H2 structures, but the maximum of the zero-point density lies at the D2d saddle point, giving rise to the symmetric structure with an equally shared proton. Okumura, Yeh, and Lee (OYL) used mass-selection of ions followed by infrared laser photodissociation measurements, to detect the first infrared spectroscopy of H5+, H7+, and H9+, as well as larger Hn+ species.44 More recently, our group used more efficient ion cooling and broader IR laser tuning (2000−4500 cm−1) to obtain improved data for H5+ and new spectra for D5+.45 These © 2013 American Chemical Society
spectra were assigned in coordination with a full anharmonic potential and Diffusion Monte Carlo (DMC) spectral analysis in a collaboration with Bowman and co-workers.45 In a second study, we extended the spectral coverage for H5+ and D5+ to the mid- and far-IR regions below the one-photon dissociation threshold using a free electron laser and the method of multiple photon photodissociation.46 These new infrared data have refocused attention on the H5+ ion and its unusual vibrational dynamics, and these data continue to be investigated with new theoretical methods.47,48 Unfortunately, the new IR spectroscopy methods applied to H5+ have not yet been extended to larger hydrogen cluster ions. According to theory, these ions should have structures very different from that of H5+, with a central H3+ ion core solvated by H 2 molecules, giving rise to simpler vibrational patterns.31−34 To investigate this scenario and to elaborate on its details, we here extend the spectroscopy on the larger hydrogen cluster ions H7+, D7+, H9+, and D9+, examining their photodissociation behavior in the 2000−4500 cm−1 region. The H7+ measurements are also stimulated by a recent computational study and potential analysis on this system by Barragán et al.49
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EXPERIMENTAL SECTION Hydrogen cluster ions are produced in a pulsed discharge source using needle electrodes with an expansion of pure hydrogen.50 Ions are mass-analyzed and size-selected for photodissociation studies in a specially designed reflectron time-of-flight mass spectrometer.51 Infrared excitation in the 2000−4500 cm−1 region is accomplished with a Nd:YAG Special Issue: Joel M. Bowman Festschrift Received: December 21, 2012 Revised: January 28, 2013 Published: January 29, 2013 6984
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energy surface fit by Barragán et al. found De (H7+) = 1728 cm−1.49 The photodissociation spectra of H7+ and H9+ are shown in Figure 2, where they are compared to the spectrum of H5+
(Spectra Physics Pro-230) pumped OPO/OPA laser system (LaserVision). Photodissociation proceeds by elimination of H2 molecules, and the fragment ion intensity is recorded versus the wavelength to generate a spectrum. Computational studies were done at the MP2/6-311+G** and DFT/B3LYP/level using the Gaussian 03 program package.52 Vibrational frequencies predicted by DFT calculations were scaled by the standard factor of 0.9688 for comparison to the spectra.53
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RESULTS AND DISCUSSION As shown in Figure 1, the mass spectrum of the ions produced in this experiment contains primarily protonated hydrogen
Figure 2. IR photodissociation spectra of H5+ reported previously compared to the new spectra for H7+ and H9+.
taken from ref 45. The band positions from these spectra are presented in Table 1. These spectra required extensive signal Figure 1. Mass spectrum measured for hydrogen cluster cations produced by the pulsed-discharge source.
Table 1. Vibrations of H7+ and H9+ and Their Deuterated Analogues Compared to Those of Other Hydrogen Molecules and to the Predictions of Theory (DFT/B3LYP), All in cm−1
clusters of the form Hn+, where n is odd. The intensities of these peaks drop noticeably after H9+. This is not surprising because previously computed structures show that three hydrogen molecules bind to H3+, with one on each corner forming the first solvation shell.31−34 Binding energies for subsequent hydrogen molecules drop substantially after this H3+(H2)3 shell closing,16,17 consistent with the computed structures. In addition to the main sequence of peaks, there is a very small amount of some even-numbered hydrogen clusters. Of these, the signal is greatest for H6+, which has only been studied before computationally.54 Because the density of mass-selected ions is too low for direct absorption spectroscopy, photodissociation methods were employed in all previous studies of hydrogen ion infrared spectroscopy.44−46 These measurements are most effective at energies above the one-photon dissociation threshold. Experimental dissociation energies of H5+ (to eliminate H2) have been measured between 5.0 and 10.0 kcal/mol (1750−3500 cm−1),14−17 with the most recent value from ion equilibria at 6.9 kcal/mol (2415 cm−1).16 DMC calculations find D0 = 6.37 and 6.87 kcal/mol for H5+ and D5+ ions, respectively (2227 and 2402 cm−1).36 Our single-photon photodissociation experiments found bands beginning just above these two thresholds, but none below,45 consistent with the DMC results. Multiple photon methods and much higher laser powers were employed to investigate lower frequency spectra.46 The dissociation energies measured for H7+ and H9+ with ion equilibria mass spectrometry are D0 = 3.3 and 3.2 kcal/mol, respectively (1155 and 1120 cm−1),16 and we expect single-photon photodissociation to be possible in the region above these energies. For comparison to the H7+ value, the full-dimensional potential
ion H2 D2 H3+ D3+ H7+ D7+ H9+ D9+ a
H3+ asym. str. exp.
2521.4a 1834.7a 2200 ± 100 2163
H3+ asym. str. scaled DFT
2605 1842 2298 1626 2167 1534
H−H/D−D str. exp.
H−H/D−D str. scaled DFT
4158.5a 2991.9a
4281 3029
3982 2871 4033 2894
4089 2892 4136 2926
Reference 24.
averaging because the detected photodissociation signals were quite small (see discussion below). Therefore, we did not pursue spectra at frequencies lower than 2000 cm−1 because of the lower laser powers available in that region. The spectrum of H5+ has been discussed extensively in our previous reports on this ion.45,46 Its shared-proton structure gives rise to complex anharmonic couplings with activity in the proton stretch vibration, the H2 stretch, and a complex pattern of combination and overtone bands involving torsional modes.45,46 The higher frequency region shown here has three bands, which shift upon deuteration by roughly a factor of (2)−1/2. The vibrational assignments made with DMC methods included terminal H2 stretching vibrations mixed with proton stretch overtones and torsional motions.45 However, these assignments have recently been reinvestigated by other groups.47,48 In contrast to this, the spectra of H7+ and H9+ each contain a single high frequency band in the region where an H−H stretch might be expected. 6985
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This band occurs at 3982 and 4033 cm−1 for H7+ and H9+. These positions agree well with those reported originally by Okumura, Yeh, and Lee (OYL) at 3980 and 4020 cm−1,44 although the bands seen here are sharper than those seen before. The 3982 cm−1 band here has a line width of about 60 cm−1 (fwhm) compared to the 3980 cm−1 band of OYL with a line width of about 80 cm−1. The 4033 cm−1 band measured here has a line width of only about 20 cm−1 compared to the 4020 cm−1 band of OYL at 60 cm−1. These simple spectra already indicate that the structures and vibrational patterns of H7+ and H9+ are quite different from that of H5+. In bands not reported previously, there is also evidence here for a very weak broad feature near 2200 cm−1 for H7+, which becomes more clearly resolved for H9+. This band occurs at slightly lower frequency than the position of the e′ asymmetric ν2 stretch of isolated H3+, which has been well-documented previously at 2521.4 cm−1.22−25 To investigate these spectra more fully, we have undertaken computational studies at both the DFT/B3LYP/6-311+G** and MP2/6-311+G** levels. The structures of these ions have been investigated with theory in several previous studies.31−34 In the case of H7+, high level CCSD(T) and MP2 computations and a full potential surface analysis were recently reported by Barragán et al.49 However, these studies presented only IR frequencies (no intensities) and did not include the deuterated species. Our computational studies were conducted to provide the frequency and intensity information needed to generate predicted spectra for both ions and their isotopomers. Additionally, we were curious to see how different the DFT and MP2 results would be compared to higher level calculations. DFT is known to perform poorly for the energetics of weakly bound systems,55 but we have found it to be more reliable for vibrational frequencies of these systems when standard scaling factors are applied to harmonic spectra produced by both methods.56 The structures derived from our computational studies for both H7+ and H9+ have H3+ core ions solvated by H2, consistent with previous theoretical work. The qualitative structures are shown as insets in Figures 3 and 4, while the full details of these structures are presented in the Supporting Information (SI). Table 1 presents the experimental bands measured for H7+ and H9+ and their perdeuterated analogues, compared to the band
Figure 4. IR photodissociation spectra of H9+ and D9+ compared to the spectra predicted by DFT theory (harmonic calculations with scaled frequencies).
positions predicted by scaled DFT/B3LYP theory. The experimental spectra for these ions are compared to the predictions of DFT theory in Figures 3 and 4. The corresponding frequencies resulting from MP2 computations (SI) are all higher still and do not agree as well with the experiment as the DFT frequencies. Figures S9 and S10 in the Supporting Information show the comparison of DFT/B3LYP and MP2 spectral predictions for H7+ and H9+. In an understandable way, the structures and vibrational frequencies for these ions vary slightly with method and basis sets, but the qualitative picture of the structures derived from our work is completely consistent with that from previous studies. According to all computational studies done, H7+ and H9+ are best described as solvated H3+ ions. Figure 3 shows the spectrum for H7+ in the region of 2000− 4500 cm−1 compared to that of D7+ and the spectra predicted with DFT computations for both of these ions. The spectra predicted by theory (red traces) extend to lower frequencies than the experimental data to provide a more complete picture of the vibrational structure. As shown, these spectra are characteristic of weakly interacting H3+ and H2 units. A band is predicted for H7+ at 4089 cm−1 for an out-of-phase stretch of the two H2 units, while the stronger feature predicted at 2298 cm−1 is the asymmetric stretch of the H3+ core ion. Because of the asymmetric environment, the symmetric stretch of the H3+ is predicted to become weakly IR-active at 3058 cm−1, but no band is detected here at our signal levels. A lower frequency band predicted at 1765 cm−1 is the bend of the H3+ core. The single band in the experimental spectrum at higher frequency for H7+ at 3982 cm−1 becomes a similar single band for D7+ at 2871 cm−1, which has not been reported previously. The shift to lower frequency is very close to the (2)−1/2 ratio and agrees well with the shift predicted by theory. Like the H7+ spectrum, the single band seen at high frequency for D7+ is very different from the multiplet band structure seen for D5+. These bands are assigned to the H−H and D−D vibrations of the external H2 and D2 ligands solvating the core H3+ ion. Their frequencies can be compared to those of the isolated H2 and D2 molecules, at 4158.5 and 2991.9 cm−1.24 Binding to the H3+ core ion shifts the vibrations of both H2 and D2 to slightly lower frequencies. The H−H stretch for H7+ can also be compared to the
Figure 3. IR photodissociation spectra of H7+ and D7+ compared to the spectra predicted by DFT theory (harmonic calculations with scaled frequencies). 6986
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harmonic frequency for this vibration from the fit to the full dimensional potential energy surface recently done by Barragán et al.49 The PES value of 4296 cm−1 is much higher than the value determined here experimentally at 3982 cm−1. Another new feature here is the very broad (200 cm−1 fwhm) band occurring for H7+ at ∼2200 cm−1. Even though this feature is predicted to be quite strong, it is not at all obvious in Figure 2. This region has been scanned with extensive averaging and as shown in an expanded view in Figure 5, a band is just barely
seen more clearly here than it was for H7+. It appears as a weak somewhat broad band centered at 2163 cm−1, compared to its predicted position of 2167 cm−1. Again, the predicted intensity of this lower frequency band is much greater than that observed. This band is shown in an expanded view in Figure 5 where it is compared to the corresponding feature for H7+. From the vibrational band patterns seen here and their reasonable agreement with the frequencies predicted by theory, it is apparent that H7+/D7+ and H9+/D9+ all represent solvated H3+/D3+ structures. The spectra all have the expected single H−H/D−D stretch at high frequency consistent with external H2/D2 molecules whose frequencies are slightly shifted from those of the free molecules. At lower frequency, both H7+ and H9+ have a vibration characteristic of the core H3+ moiety. For D7+ and D9+ this band is predicted but lies outside the frequency range studied. Although this core band for both ions has much weaker intensity than predicted by theory, this is understandable (see below) and its position agrees well with that predicted by theory. It is interesting to note that there is no evidence for the more complex multiplet structure seen before for H5+ in this region of the spectrum. H7+ or H9+ could conceivably have H5+ as their core ion, which would then be solvated by H2 to form H5+(H2) or H5+(H2)2 structures, but the vibrations known for H5+ are not seen. The same is true for the deuterated analogues of these ions. In these hypothetical structures, the H2 molecules would act essentially as weakly bound ″tags″ like those that are used throughout ion spectroscopy to enhance photodissociation yields.57−62 The use of H2 molecules for tagging was first described by Lee and co-workers,57 and hydrogen tagging is now common in cryogenically cooled ion traps.63,64 It has recently been pointed out that hydrogen tagging is more inert than argon tagging in the study of protonated water cluster ions.65 However, in the delicate situation here, where hydrogen interactions are the basis for the bonding, hydrogen ″tagging″ is apparently too strong a perturbation on the system. The H3+ ion is apparently solvated more effectively by external hydrogen, making H3+based structures more favorable than H5+-based structures. Although all of the vibrations in these complexes resemble those of the component species H3+ and H2, all of the bands measured lie at lower frequencies than those of these isolated molecules. The external H−H stretches for H7+ and H9+ at 3982 and 4033 cm−1 are both well below the isolated vibration of H2 at 4158.5 cm−1.24 Likewise, the D−D stretches for D7+ and D9+ at 2871 and 2894 cm−1 are also well below the D2 value at 2991.9 cm−1.24 This kind of red shift in these vibrations has been seen before for other cation−hydrogen complexes, where the charge-quadrupole electrostatic interaction leads to the cation interacting side-on with the hydrogen. The partial charge transfer removes bonding electron density from the σ orbital, weakening the bond slightly and lowering its vibrational frequency. This effect has been well documented, for example, in the case of metal cation−H2 complexes.66 In the present system, this effect is greater for H7+/D7+ than it is for H9+/D9+. This is consistent with a stronger polarization effect when there are fewer ligands, and a weaker one when there are more ligands and the interaction is distributed more, consistent with trends seen before for other cation−ligand systems.61 According to the measurements available and the theory for both ions, the asymmetric stretch vibration for the core H3+/ D3+ ion is also red-shifted with respect to this vibration in the isolated ions. Additionally, the band is also much broader in the case of the H7+, and somewhat broader for H9+, compared to
Figure 5. Expanded view of the IR photodissociation spectra of H7+ and H9+ in the region of the core H3+ ion’s asymmetric stretch vibration.
detectable above our noise level. This band is in the region of the asymmetric stretch of H3+ noted above, but is also shifted to lower frequency compared to the vibration in the isolated H3+ ion. The predicted H−H and D−D stretches for both the H7+ and D7+ ions are higher than the experimental values. In the case of the H7+, the core vibration of H3+ is also predicted higher than observed, although the experimental band is very broad and noisy. Figure 4 shows the spectrum for H9+ in the region of 2000− 4500 cm−1 compared to that of D9+ and the predicted spectra resulting from DFT computations for both of these ions. Again, the spectra predicted by theory extend to lower frequencies than the experimental data to provide a more complete picture of the vibrational structure. The predicted spectra are much like those for H7+/D7+, with a high frequency H−H/D−D stretch and a low frequency band from the asymmetric stretch of the H3+/D3+ core ion. The H3+ core bending mode seen as a separate vibration for H7+ is now degenerate with the asymmetric stretch. The high frequency (degenerate) H−H/ D−D stretch of the ligands is predicted to be a single band because of the high symmetry of the system. In both ions, this vibration is observed as a single relatively sharp band in about the same position as predicted (4033 cm−1 for H9+; 2894 cm−1 for D9+), but again theory predicts this frequency higher than where it occurs. Both of these H−H and D−D stretches are at higher frequencies for H9+/D9+ than they are for H7+/D7+. The core ion vibration for D9+ is predicted to be at lower frequency than the scan range shown here, but this vibration for H9+ is 6987
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the predissociation process, which is also common for ion spectra measured with photodissociation.57−62 Power broadening is conceivable, but the widths of these lines did not change with the laser power. However, the core H3+ vibrations, which were measured under the same ion beam conditions, are much broader than the higher frequency bands, consistent with some additional line broadening mechanism. The hydrogen bonding vibrations in many small clusters containing water are well-known to exhibit significantly greater linewidths,56,67 which has been attributed to different lifetime broadening for these specific vibrations.67 This has been suggested to result from faster rates of intramolecular vibrational relaxation (IVR) for core ions that are more strongly coupled to the surrounding network than molecules at its periphery.67 The faster IVR results in shorter lifetimes for the states initially excited and consequently broader linewidths. Photodissociation rates can be slowed further in these strongly coupled vibrations because the energy spreads more efficiently and completely throughout the molecule, which can depress the photodissociation yield. It therefore seems plausible that the combined effects of faster IVR, consequent lifetime broadening, and depressed photodissociation yields explain the broad widths and weak intensities seen here for the core H3+ vibrations.
the higher frequency bands in each case (Figure 5). Both the red shift and the broadening are known for other ions involved in hydrogen bonding. Although this system does not have the standard hydrogen bonding configuration, the mechanism of the frequency red shift appears to be the same. A high frequency vibration (O−H stretch in the case of water; asymmetric H3+ stretch here) has an attractive interaction from complex formation at its extended position, which induces a drag on the motion and hence a lower frequency. This kind of interaction can also explain the red-shifted core vibration in these complexes. Perhaps the most noticeable difference between the experiment here and the spectra predicted by theory is in the intensity of the core H3+ vibration in H7+ and H9+. In both cases this band is predicted by theory to have much stronger IR intensity here (>1000 km/mol; see SI) than it does in the isolated H3+ ion (192 km/mol) because of the charge transfer nature of the vibration in the clusters. However, for H7+ this band is barely detected, while for H9+ it is much weaker relative to the H−H stretching band than predicted. Part of this can be attributed to the lower laser power (3−5×) in the low frequency end of the spectrum compared to that at high frequency. These spectra could be normalized to the laser power to correct for this, but we do not do this because the laser beam spot size and shape (and hence the overlap with the ion beam) change as the laser scans. Another significant consideration is the dynamics of IR laser absorption, energy flow, and eventual fragmentation. The spectra predicted by theory are those for the linear absorption intensity, but we measure the photodissociation yield. The lower frequency vibrations may simply be depressed in relative intensity because dissociation yield is lower with lower energy photons. A final issue is the relationship between band intensity and line width. If lines are broader, their peak intensities are lower, which is another contributing factor here. The computed intensity should therefore be compared to the integrated intensity over the broad band to achieve an accurate comparison. Considering the issues of lower laser power, broader line width, and the possibility of a depressed dissociation yield, the weak intensity of the core vibrational bands is at least somewhat understandable. Another interesting issue is the widths of the bands in the spectrum, especially the greater widths observed for the core vibrational bands. As noted earlier, the high frequency band for H7+ and H9+ have widths of about 60 and 20 cm−1 respectively. As shown in Figure 5, the line width of the core H3+ vibrational band for H7+ is about 120−150 cm−1, while that for H9+ is 60− 80 cm−1. Both the high frequency band and the H3+ core band are sharper for H9+ than they are for H7+, contributing in part to the better signal levels seen in the H9+ spectrum. All of these bands are narrower than those detected by Lee and co-workers in the earlier work.44 This fact, and the difference in the linewidths for the higher frequency bands for H7+ versus H9+, suggests that the widths may arise at least partly from the rotational contours of the bands. Rotational simulations of these contours (using PGopher; oblate top; perpendicular-type band; 1−2 cm−1 laser line width) suggest that the 20 cm−1 line width of the H9+ complex band corresponds to a temperature of about 20 K. This temperature is consistent with that for other ions studied in our lab. The corresponding band for H7+ is broader, which could be expected for the larger rotational constants or a somewhat higher temperature. Both of these bands could also be affected by some lifetime broadening from
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CONCLUSIONS Infrared photodissociation spectroscopy has been measured for the mass-selected H7+, D7+, H9+, and D9+ ions produced in a pulsed discharge/supersonic beam source. Vibrational resonances are measured for each of these ions corresponding to H−H and D−D stretches of external hydrogen molecules. Core H3+ asymmetric stretches are also detected for H7+ and H9+. The comparison of these vibrations to the spectra predicted by theory confirm that these complexes are best described as solvated-H3+ rather than solvated-H5+ systems. Unusual vibrational dynamics that depress the photodissociation yield are suggested to explain the broad bands and very weak experimental intensities for the H3+ core asymmetric stretch vibrations. Computational studies at the B3LYP and MP2 levels of theory find structures consistent with those proposed earlier and are able to predict the qualitative aspects of the vibrational structure. However, harmonic calculations and standard vibrational scaling factors produce vibrations that have higher frequencies than those observed for both methods. These hydrogen cluster ions provide fascinating examples of vibrational spectroscopy and dynamics that can now be investigated with higher levels of theory.
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ASSOCIATED CONTENT
S Supporting Information *
Full citation for ref 52 along with full details of the DFT and MP2 computations done in support of the spectroscopy presented here. Structures, energetics, and vibrational frequencies for each of the complexes considered are included. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 6988
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ACKNOWLEDGMENTS Support from the National Science Foundation is gratefully acknowledged for this work, through grant no. CHE-0956025.
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