Stepwise Internal Energy Change of Protonated Methanol Clusters By

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Stepwise Internal Energy Change of Protonated Methanol Clusters by Using the Inert Gas Tagging Takuto Shimamori, Jer-Lai Kuo, and Asuka Fujii J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b10140 • Publication Date (Web): 05 Nov 2016 Downloaded from http://pubs.acs.org on November 6, 2016

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The Journal of Physical Chemistry

Stepwise Internal Energy Change of Protonated Methanol Clusters by Using the Inert Gas Tagging Takuto Shimamori,a Jer-Lai Kuo,b and Asuka Fujiia* a) Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan b) Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, 10617, Taiwan * E-mail: [email protected]

Abstract Structural isomer population of a hydrogen-bonded cluster generally depends on temperature. Therefore, determination of an isomer population profile in a wide temperature range is important to understand the nature of hydrogen bond networks of the cluster.

To explore an isomer population profile, stepwise changes of internal

vibrational energy of a protonated hydrogen-bonded cluster are performed by inert gas tagging.

We observe infrared spectra of the protonated methanol pentamer with

various tag species. The bare protonated methanol pentamer practically has only two possible isomer types.

With the tagging, the relative population of the two isomer

types changes according to the biding energy with the tag species. The observed relative population follows its theoretically predicted temperature dependence.

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I.

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INTRODUCTION

Geometric structures of hydrogen-bonded clusters in the gas phase have attracted great interest in the last a few decades.1 So far, most of such studies have focused on determination of the most stable isomer structure of the cluster.

With elevation of

temperature of the cluster, however, higher energy isomers can be populated because of entropy. Therefore, to fully understand the hydrogen bond structure of the cluster, not only the most stable structure but also the temperature dependence of preferred structures is very important.2-16 Infrared (IR) spectroscopy combined with quantum chemical calculations has been one of the most powerful techniques to explore structures of hydrogen-bonded clusters. The recent development of cryogenic ion trap techniques enables us to control temperature of ionic (or, in many cases, protonated) hydrogen-bonded clusters and observe their temperature dependence of isomer distribution.4,

8, 14, 15, 17-28

IR

spectroscopy of such a temperature-controlled cluster, however, is not straightforward. IR spectroscopy of ionic clusters is usually based on dissociation spectroscopy. To keep the constant dissociation yield irrespective of the excitation vibrational frequency, attachment of a “tag (messenger)” to the cluster is frequently requested.

29

The tag is

generally an inert gas atom or molecule (typically rare gases, H2 (D2), and N2) which

2


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does not perturb the structure of the “bare” cluster. The weak intermolecular bond between the bare cluster and tag easily breaks as a result of vibrational excitation, and the mass change due to the dissociation of the tag can be detected as a sensitive probe of IR absorption. Though this “tagging” technique is generally utilized for IR spectroscopy of cryogenic ions to keep the constant dissociation yield,14,15,20-22,26,28,30 the weak intermolecular bond between the tag and bare cluster also effectively determines the upper limit of the internal energy of the tagged cluster because of thermal dissociation of the tag. Therefore, tagging itself has an effect to restrict internal energy of the cluster. Apart from cryogenic ion trap techniques, inert gas tagging of clusters has been widely utilized as a simple technique to observe low energy isomers and to change isomer populations.6, 7, 9, 11, 31-37 Protonated clusters formed in a supersonic jet expansion have large excess energy due to protonation. Jet expansion cooling is not efficient enough to cool down protonated clusters to standard vibrational temperature of neutral clusters in a jet (< ca.100 K), and produced protonated clusters are typically warm (ca. 200 K).4,11,13,37 Since inert gas tagging provides a cut off for the internal energy according to the binding energy with the tag, it would be possible to stepwise change the internal energy

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distribution of the protonated cluster by systematic changes of the tag species from strongly bound one to weakly bound one.

The internal energy distribution of such a

tagged cluster is not necessarily under thermal equilibrium. However, if suppression of the high energy population by tagging is good approximation of a fall in temperature, it enables us to explore a temperature dependence profile of isomer distribution of a protonated cluster without a cryogenic ion trap technique.

To test this idea, in the

present study, we apply IR spectroscopy to the protonated methanol pentamer cluster (H+(CH3OH)5, abbreviated to H+M5 in the following) with various tag species. We prepare H+M5 with a variety of tag species which have different binding energies, ranging from a π-hydrogen-bonded species to a rare gas atom. The observed IR spectra of the tagged clusters change with decreasing binding energy with the tag. The spectral changes can be reasonably interpreted by the temperature dependence profile of the isomer population evaluated by the harmonic oscillator approximation.

II.

EXPERIMENTAL SECTION

The tagged H+M5-X cluster was produced by discharge to the supersonic jet expansion of the methanol/tag species (X)/carrier gas (He or Ar) mixture. The total stagnation pressure of the jet expansion was 3 - 9 atm. The cluster of interest was mass-selected by

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the first stage of a tandem-type quadrupole mass spectrometer,11 and was introduced to an octopole ion guide. The cluster was irradiated by IR light in the ion guide, and the predissociation fragment by the loss of the tag was detected at the second stage of the mass spectrometer. By monitoring the fragment intensity while scanning the IR frequency, an IR spectrum of the tagged cluster was measured. To estimate the temperature dependence profile of the isomer population based on

classical

statistical

approximation.11,

13, 37

mechanics,

we

employed

the

harmonic

oscillator

Details of the calculation method are summarized in the

Supporting Information. Briefly, each isomer was regarded as an ensemble of harmonic oscillators. Canonical probability of an isomer as a function of temperature is calculated from the relative weight of its vibrational partition function in those of all the possible isomers. As noted in Introduction, vibrational temperature of the tagged cluster is not necessarily well-defined under the present experimental condition. Therefore, as approximation, we consider effective temperature of the tagged cluster. If we fit vibrational energy distribution of the tagged cluster with thermal distribution, resulting temperature is effective temperature in this study. Effective temperature would not exceed temperature Tmax at which vibrational energy under the thermal equilibrium is

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equal to the dissociation energy between the tag and bare H+M5. Under the harmonic oscillator approximation, Tmax was evaluated by using the vibrational partition function and the binding energy between the tag and bare cluster. All the calculations were performed by the Gaussian 09 program at the ωB97X-D/6-311++G(3df,3pd) level.38

III.

RESULTS AND DISCUSSION

Observed IR spectra of H+M5-X clusters (X = (b) bare, (c) C6H6, (d) C2H2, (e) CS2, (f) CO2, (g) CO, and (h) Ar) in the OH and CH stretching vibrational region are shown in Figure 1. The tagged species are ordered with the estimated binding energy between the tag and bare cluster.

On the top and bottom of the observed spectra, simulated spectra

of (a) linear and (i) cyclic type isomers of H+M5 are also displayed, respectively.

All

of the observed clusters were prepared by pulsed discharge of -400 V to a supersonic free jet expansion of methanol/tag species/carrier gaseous mixture.

The bands higher

than ~3600 cm-1 are assigned to “free” OH stretch (some of them are π-hydrogen bonded to or weakly interacting with the tag).

Those lower than ~3600 cm-1 are

hydrogen-bonded OH stretch, and the relatively sharp bands around 3000 cm-1 are CH stretch.

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Possible isomer structures of H+M5 have been thoroughly studied by both of IR spectroscopy and theoretical analyses.37, 39, possible for H+M5.

40

Only two isomer types are practically

One is the linear type and the other is the cyclic type (both types

have some different stable conformers which have the same hydrogen bond network topology and quite similar energies and IR spectra).

The cyclic type includes the most

stable isomer and its population is predominant at low temperature. On the other hand, the linear type is more flexible, and becomes predominant in higher temperature because of larger entropy (higher vibrational state density).

The statistical weight

evaluation under the harmonic approximation at the B3LYP/6-31+G(d) level has predicted that the switching of the major isomer type occurs at ~125 K.

37

In theory,

the third isomer type, “cyclic with a tail” (cyclic structures with a side chain growing from the ring moiety) is also stable.

Its free energy, however, is much higher than

those of the linear and cyclic types in all the temperature range, and its population is negligible. 37 The IR spectra of bare and Ar-tagged clusters of H+M5 have been measured in our previous study.

37

The observed spectra of the bare and Ar-tagged clusters (Figs.

1(b) and 1(h), respectively) are totally different from each other, and they are uniquely attributed to the liner and cyclic isomers, respectively, as demonstrated by the

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comparison with the simulated spectra of Figs. 1(a) and 1(i). The bare cluster is warmer because of the large excess energy in the protonation and limit of the standard jet expansion cooling efficiency, and its effective temperature is roughly estimated to be ~200 K.

In the Ar-tagged cluster, on the other hand, the high energy component

cannot survive because of thermal evaporation of the Ar tag. Therefore, the vibrational energy of the survived cluster should be lower than the binding energy between the bare cluster and Ar.

Tmax of the Ar-tagged cluster has been estimated to be 85 K by the

ωB97X-D/6-311++G(3df, 3pd) level calculation.

37

If we assume that effective

temperature of the Ar-tagged cluster is lower than this Tmax value, the predominant population of the cyclic type is consistent with the temperature dependence of the isomer population based on the harmonic approximation. The observed spectra of the CO, CO2, and CS2-tagged clusters show essentially the same spectral features as those of the Ar-tagged cluster. Therefore, these clusters are attributed to the cyclic type structures.

The small red-shift of the highest frequency

band (“free” OH stretch) in the Ar to CS2 tags indicates the weak interaction between the bare cluster and the tag through the free OH moiety.

On the other hand, the

spectrum of the C6H6-tagged cluster is similar to that of the bare cluster and this means that the C6H6-tagged cluster has the linear type structure.

The proton affinity (PA) of

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C6H6 (750.4 kJ/mol) is slightly lower than that of methanol (754.3 kJ/mol). 41 Moreover, even in the H2O-H+-C6H6 cluster, it has been proved that water (PA= 691kJ/mol) holds the excess proton because of the large solvation energy by C6H6. 42 Therefore, it is quite reasonable that the methanol moiety holds the structure of the bare protonated cluster and C6H6 is π-hydrogen-bonded to the methanol moiety. The spectrum of the C2H2-tagged cluster seems unique.

At glance, this spectrum is different from both of

the bare and Ar-tagged clusters.

However, as shown in the comparison with the

simulated spectra of the linear and cyclic isomers in Figure 2, the observed spectrum is well interpreted as a combination of the cyclic and linear type isomers with an additional band due to the free acetylenic CH stretch at 3270 cm-1. Therefore, the cyclic and linear type isomers coexist under the C2H2-tagging condition. Comparison between the observed and simulated spectra of all the tagged clusters is seen in Figure S1 in the Supporting Information. The observed spectra of the H+M5-X tagged clusters show that the major isomer type changes from the cyclic type to the linear type with increasing binding energy with the tag species.

From the Ar to CS2 tags, the cyclic type is dominant. The

two isomer types coexist at the C2H2 tag, and the dominant isomer type is switched to the linear type at the C6H6 tag.

To confirm this spectral behavior, we calculated the

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temperature dependence of the relative isomer populations of bare H+M5.

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In this time,

all the calculations were performed at the ωB97X-D/6-311++G(3df, 3pd) level to include the influence of dispersion. The results are shown in Figure 3. The profile of the relative isomer population of the bare cluster is essentially same as that calculated at B3LYP/6-31+G(d) in our previous work. 37 In low temperature, the cyclic type, which is more stable in energy than the linear type, is predominant, but with elevation of temperature, the more flexible liner type becomes predominant because of the entropy factor.

The population of the “cyclic with a tail” type is negligible, and this type is

ignored in the following. The population switching temperature of the major isomer types is ~140 K in the ωB97X-D/6-311++G(3df, 3pd) level calculation, and this value is ~15 K higher than that estimated at the B3LYP/6-31+G(d) level. This shift may be caused by difference of calculated frequencies and that of sampling of possible isomer structures. At the present stage, we have no experimental reference data to absolutely judge the precision of the calculated isomer population profiles, and this approach cannot be free from some ambiguity in temperature though its magnitude is reasonably acceptable. We calculated Tmax of the tagged isomers at the ωB97X-D/6-311++G(3df, 3pd) level and plotted them on the temperature dependence of the relative statistical

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populations of the bare isomers in Figure 3. The calculated values are also summarized in Table 1. The estimated Tmax of each tagged cluster is successfully consistent with the calculated relative isomer population and the observed dominant isomer type.

For the

CO, CO2, and CS2 tags, Tmax is lower than 130 K, and the cyclic type is dominant. Tmax of the C2H2 tag is 141 and 164 K for the linear and cyclic types, respectively. These values are very close to the calculated population switching temperature of the major isomer, and this explains well the observation of the coexistence of the two isomer types (Note that Tmax is slightly different in the linear and cyclic isomer types. Therefore, the sum of the populations of the two isomer types at the marked points cannot be unity). With the C6H6 tag, Tmax goes over the switching temperature, and the linear type becomes dominant.

These results show that effective temperature of the

tagged clusters is practically approximated by Tmax. Moreover, a temperature dependence profile of isomer distribution can be probed by stepwise internal energy changes with variation of tag species.

IV.

CONCLUDING REMARKS

The present results demonstrate the potential of inert gas tagging to explore a temperature dependence profile of isomer distribution. Finally, we summarize some

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remarks on this method. (1) Though the present method is simple, interpretation of experimental results requests several assumptions and approximation, such as effective temperature equal to Tmax and harmonic oscillator approximation.

Quantitativeness of them has not necessarily been

verified well. Comparison with cryogenic ion trap experiments will be helpful to examine the quantitativeness of the present method.

However, we should note that in

the tagged cluster prepared by a supersonic jet expansion, effective temperature of rotation and vibration can be very different, while all the degrees of freedom should be at the same temperature in collisional cooling type ion trap experiments.

It has been

found for H+M5 that inclusion of the rotational degrees of freedom in the statistical weight rather results in worse agreement with the observed isomer population behavior of the tagged clusters. 37, 40 (2) To apply the present method, the intermolecular bond between the tag and bare cluster should be much weaker than those in the bare cluster. Otherwise, the tag cannot be a spectator to the structure of the bare cluster. In this meaning, this method is suitable for (ionic or protonated) hydrogen-bonded clusters but not for van der Waals type clusters. (3) Temperature of the cluster cannot be determined only with experiments. Therefore,

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accuracy of determined temperature directly depends on that of the level of theory. Nevertheless, the present results suggest that magnitude of the error would be acceptable with a reasonable choice of the level of theory. (4) It has been demonstrated that inert gas tagging sometimes results in anomalous isomer population which clearly deviates from effective temperature estimated by the binding energy.35,

36, 43, 44

This anomaly might be caused by kinetic trap to local

minima in the potential landscape. We should be careful of this anomaly. This anomaly, however, tends to occur with very weakly binding tags (corresponding to very low effective temperature range), and would not be a serious problem to observe an isomer population profile in wide temperature range.

SUPPORTING INFORMATION Comparison between the observed and simulated spectra of all the tagged clusters. Details of the calculation methods of temperature dependent isomer population and maximum effective temperature. The complete author lists of Refs. 14, 15, and 38.

ACKNOWLEDGEMENTS We would like to acknowledge to Dr. Toshihiko Maeyama and Dr. Yoshiyuki Matsuda

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for their helpful discussion. This study was supported by the Grant-in-Aid for Scientific Research (Project No. 26288002) from JSPS.

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30. Kelleher, P. J.; Johnson, C. J.; Fournier, J. A.; Johnson, M. A.; McCoy, A. B. Persistence

of

Dual

Free

Internal

Rotation

in

the

Helium

“Tagged”

NH4+(H2O)·Hen=0-3 Ion-Molecule Complexes: Expanding the Case for Quantum Delocalization in He Tagging. J. Phys. Chem. A 2015, 119, 4170-4176. 31. Pivonka,N. L.; Kaposta, C.; Brümmer, M.; von Helden G.;, Meijer, G.; Wöste, L.; Neumark, D. M.; Asmis, K. R. Probing a Strong Hydrogen Bond with Infrared Spectroscopy: Vibrational Predissociation of BrHBr¯•Ar. J. Chem. Phys. 2003, 118, 5275-5278. 32. Headrick, J. M.; Diken, E. G.; Walters, R. S.; Hammer, N. I.; Christie, R. A.; Cui, J.; Myshakin, E. M.; Duncan, M. A.; Johnson, M. A., Jordan, K. D. Spectral Signatures of Hydrated Proton Vibrations in water Clusters. Science, 2005, 308, 1765-1769. 33. Solca, N.; Dopfer, O. Hydrogen-Bonded Networks in Ethanol Proton Wires: IR Spectra of (EtOH)qH+-Ln Clusters (L = Ar/N2, q ≤4, n ≤ 5). J. Phys. Chem. A 2005, 109, 6174-6186. 34. Ricks, A. M.; Reed, Z. E.; Duncan, M. A. Infrared Spectroscopy of Mass-Selected Metal Carbonyl Cations. J. Mol. Spectrosc. 2011, 266, 63-74. 35. Mizuse, K.; Fujii, A. Infrared Photodissociation Spectroscopy of H+(H2O)6·Mm (M = Ne, Ar, Kr, Xe, H2, N2, and CH4): Messenger-Dependent Balance between H3O+ and

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H5O2+ Core Isomers. Phys. Chem. Chem. Phys. 2011, 13, 7129-7135. 36. Rodriguez, Jr., O.; Lisy, J. M. Revisiting Li+(H2O)3-4Ar1 Clusters: Evidence of High-Energy Conformers from Infrared Spectra. J. Phys. Chem. Lett., 2011, 2, 1444-1448. 37. Li, Y. –C.; Hamashima, T.; Yamazaki, R.; Kobayashi, T.; Suzuki, Y.; Mizuse, K.; Fujii, A.; Kuo, J. –L. Hydrogen-Bonded Ring Closing and Opening of Protonated Methanol Clusters H+(CH3OH)n (n = 4 – 8) with the Inert Gas Tagging. Phys. Chem. Chem. Phys. 2015, 17, 22042-22053. 38. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al.,

Gaussian 09, Revision B.01, Gaussian, Inc., Wallingford CT, 2009. 39. Bing, D.; Hamashima, T.; Nguen, Q. C.; Fujii, A.; Kuo, J. –L. Comprehensive Analysis on the Structure and Proton Switch in H+(CH3OH)m(H2O)n (m + n = 5 and 6). J. Phys. Chem. A 2010, 114, 3096-3102. 40. Fifen, J. J.; Nsangou, M.; Dhaouadi, Z.; Motapon, O.; Jaidane, N. –E. Structures of Protonated Methanol Clusters and Temperature Effects. J. Chem. Phys. 2013, 138, 184301. 41. Hunter, E. P.; Lias, S. G. Evaluated Gas Phase Basicities and Proton Affinities of

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Molecules: An Update. J. Phys. Chem. Ref. data 1998, 27, 413-457. 42. Cheng, T. C.; Bandyopadhyay, B.; Mosley, J. D.; Duncan, M. A. IR Spectroscopy of Protonation in Benzene−Water Nanoclusters: Hydronium, Zundel, and Eigen at a Hydrophobic Interface. J. Chem. Soc. Am. 2012, 134, 13046-13055. 43. Brites, V.; Lisy, J. M.; Gaigeot, M. -P. Infrared Predissociation Vibrational Spectroscopy of Li+(H2O)3-4Ar0,1 Reanalyzed Using Density Functional Theory Molecular Dynamics. J. Phys. Chem. A 2015, 119, 2468-2474. 44. Asmis, K. R.; Wende, T.; Brümmer, M.; Gause, O.; Santambrogio, G.; Cristina Stanca-Kaposta, E.; Döbler, J.; Niedziela, A.; Sauer. J. Structural Variability in Transition Metal Oxide Clusters: Gas Phase Vibrational Spectroscopy of V3O6–8+. Phys. Chem. Chem. Phys. 2012, 14, 9377-9388.

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Table 1. Binding energy (BE / kJmol-1) between the tag and bare cluster and maximum effective temperature (Tmax / K) estimated by the binding energy. All the calculations were performed at the ωB97X-D/6-311++G(3df, 3pd) level under the harmonic oscillator approximation.

C6H6

C2H2

CS2

CO2

CO

BE

33.9

21.0

12.7

12.3

11.7

Tmax

192

141

100

95

97

BE

41.3

24.5

16.8

16.0

15.8

Tmax

226

164

125

120

78

Isomer\tag

Linear

Cyclic

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Figure 1

Observed IR spectra of H+(methanol(M))5-X clusters (X = (b) bare, (c)

C6H6, (d) C2H2, (e) CS2, (f) CO2, (g) CO, and (h) Ar) in the OH and CH stretching vibrational region. Simulated spectra of (a) linear and (i) cyclic type isomers of bare H+M5 are also displayed, respectively. The simulations were performed at the ωB97X-D/6-311++G(3df, 3pd) level.

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Figure 2

Comparison between the observed and simulated spectra of H+M5-C2H2.

The simulations were performed at the ωB97X-D/6-311++G(3df, 3pd) level.

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Figure 3

Plots of the relative population of the isomer types (liner (L), cyclic (C),

and cyclic with a tail (Ct)) of bare H+M5 and maximum effective vibrational temperature Tmax of the tagged clusters. All the calculations were performed at the ωB97X-D/6-311++G(3df, 3pd) level under the harmonic oscillator approximation.

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TOC Graphic

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Figure 1 230x721mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2 100x113mm (300 x 300 DPI)


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Figure 3 124x90mm (300 x 300 DPI)


Stepwise Internal Energy Change of Protonated Methanol Clusters By

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