Structure and dynamics of Zr 6 O 8 metal organic framework node surfaces probed with ethanol dehydration as a catalytic test reaction

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1 Supporting Information (SI) for the manuscript: Structure and dynamics of Zr 6 8 metal organic framework node surfaces probed with ethanol dehydration as a catalytic test reaction Dong Yang, 1 Manuel A. rtuño, 2 Varinia Bernales, 2 Christopher J. Cramer, 2* Laura Gagliardi, 2* Bruce C. Gates 1* 1 Department of Chemical Engineering, University of California, Davis, CA 95616, USA 2 Department of Chemistry, Chemical Theory Center, and Minnesota Supercomputing Institute, University of Minnesota, Minneapolis, MN 55455, USA S1

2 Table of Contents S1. Characterization of the ligands on the MF nodes S3S13 S2. Ethanol dehydration catalyzed by MF nodes S14S19 S3. Computational modeling S20S27 S2

3 Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) S1. Characterization of the ligands on the MF nodes Evidence of IR spectra characterizing formate ligands on the MF nodes Note that data are included here for nodes incorporating f in place of Zr and that, for brevity, this information is not included in any detail in the main text. The data characterizing the hafniumcontaining nodes support the data and the conclusions presented for the zirconiumcontaining nodes. 3 A B Wavenumber (cm 1 ) Wavenumber (cm 1 ) Figure S1. IR spectra in the (A) cm 1 region and (B) cm 1 region characterizing Ui66 (Cl, black), fui66 (Cl, red) and Ui66 (Cl) after treatment in liquid methanol at 298 K for 48 h (blue) and Ui66 after further treatment in water vapor/helium at 393 K for 30 min (pink) Wavenumber (cm 1 ) Wavenumber (cm 1 ) Figure S2. IR spectrum in the (A) cm 1 region and (B) the cm 1 region characterizing Ui67 (Cl). S3

4 Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) Wavenumber (cm 1 ) Wavenumber (cm 1 ) Figure S3. IR spectra in the (A) cm 1 region and (B) the cm 1 region characterizing NU1000 (black) and fnu1000 (red) Wavenumber (cm 1 ) Wavenumber () Figure S4. IR spectra in the (A) cm 1 region and (B) the cm 1 region characterizing a MF closely related in structure to those of this investigation, NU1000, after treatment in liquid methanol at 343 K for 48 h and followed by treatment in water vapor/helium at 393 K for 30 min (black), and the same sample after treatment in liquid DMF at 353 K for 24 h followed by washing with acetone 5 times (red). See refs. 3 and 4. S4

5 Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) Wavenumber (cm 1 ) Figure S5. IR spectra in the cm 1 region characterizing NU1000 (see preceding figure and refs 3 and 4) after treatment in liquid methanol at 343 K for 48 h and followed by treatment in water vapor/helium at 393 K for 30 min (black), and the same sample after treatment in liquid DMFD7 at 353 K for 24 h followed by washing with acetone 5 times (red) Wavenumber (cm 1 ) Wavenumber (cm 1 ) Figure S6. IR spectra in the region charactering Ui66 (acetic acid, black) and the same sample after treatment with formic acid vapor at 393 K for 30 min (red). S5

6 Table S1. IR bands of hydroxyl and formate groups observed for Ui66, Ui67, and NU 1000, with frequencies stated in cm 1. MFs region C (C D) vibration C (C D) vibration in the region of cm 1 formate ligand formate ligand ZrUi , 2875, 2897, 2924 fui , 2881, 2903, 2931 ZrUi , 2872, 2892, 2915 ZrNU (2039) 2858, 2875, 2900, 2936, 2955, 2992 (2170) fnu , 2882, 2907, 2944, 2961, 3001 We recorded IR spectra of fui66 (the synthesis was Cl modulated; the structure of the MF is reported elsewhere 1 ) and ZrUi66 (Cl modulated, the structure is reported elsewhere 2 ), as shown in Figure S1. In the region, the sharp band at 3674 cm 1, which has been assigned to µ 3 vibrations in ZrUi66, was observed at 3679 cm 1 when the MF incorporated f instead of Zr. And the band observed at 2746 cm 1, previously assigned to hydrogenbonded / 2 groupsand herein reassigned to a C vibration of the formate ligand on the nodes, was observed at 2754 cm 1 in the fcontaining MF. Similarly, the sharp bands observed for the Zrcontaining MF between 3000 and 2800 cm 1 (2861, 2875, 2897, and 2924 cm 1 ) also appeared at higher wavenumbers (2866, 2881, 2903, and 2931 cm 1 ) when the MF contained f instead of Zr (Figure 1). All these results point to the similarity of the two MFs and to subtle differences in their electrondonor properties. Similar results were observed for Ui67 (Figure S2) and NU1000 (Figure S3). We note that we were unable to synthesize the fcontaining Ui67 by using Cl as a modulator, possibly because of poor stability of this MF. All the results are summarized in Table 1. Moreover, the groups represented by these bands between cm 1 in the spectra of Zr Ui66, fui66, ZrNU1000, and fnu1000 were all missing when the topology was converted (the ligands were changed) from formate to terminal groups (previously referred to 35 as site 2). We also found that NU1000 with terminal groups does not react with 2, but does react with DMF to convert back to the formate topology with bands between 2800 and 3000 cm 1 and a band at 2746 cm 1 (Figure S4). Using this chemistry, we allowed NU1000 (with terminal node groups) to react with DMFD7, and observed C D vibration of DC at 2170 and 2039 cm 1. All the evidence indicates that the IR bands between 2800 and 3000 cm 1 characterizing Ui66, Ui67, and NU1000 are similar in having the band at 2746 cm 1 on the same MF nodes, which are assigned to formate groups on the node. Table 1 is a summary of the formate bands on the nodes of Ui66, Ui67, and NU1000. Because these formate groups are directly bonded to the node defect sites, they are expected to be sensitive to the electrondonor properties of the nodes. There is more evidence in support of these assignments. We observed increases in intensity of all the bands (at 2746, 2861, 2875, 2897, and 2924 cm 1 ) when we treated the Ui66 (acetic acid) S6

7 sample with formic acid vapor in helium at 393 K, indicating the replacement of acetate ligands with formate ligands (Figure S6). S7

8 Intensity (a.u.) IR evidence of DMF ligands on the nodes Wavenumber (cm 1 ) Figure S7. IR spectra in the ν C region characterizing Ui66 (Cl, black) and the same sample after methanol (red) and ethanol (blue) treatments at 298 K for 24 h and activation under high vacuum at 423 K for 12 h Figure S7 shows a band at 1656 cm 1 characterizing Ui66 (Cl), which disappeared after methanol or ethanol treatment of the sample at 298 K. This band is assigned to C vibrations of chemisorbed DMF molecules, and the bands are slightly shifted to lower frequencies relative to physisorbed DMF (1667 cm 1 ). We emphasize that we did not observe any IR band characterizing N vibrations of dimethyl amine in the region of cm 1, indicating a lack of such species on the MF nodes. S8

9 Normalized Intensity (a.u.) Dissolution of MFs and characterization of the resultant solutions by NMR spectroscopy D 3 C C 3 N C Chemical shift (ppm) Figure S8. 1 NMR spectra characterizing Ui66 Cl (black) and the same sample after liquid methanol treatment at 298 K for 24 h (red). Both samples were dissolved in 1M Na/D 2. The 1 NMR spectrum of the dissolved product formed from Ui66 modulated by Cl and washed with acetone is shown in Figure 1. Besides peaks characterizing the bdc linker, peaks characterizing the ligands formate and dimethyl amine were also observed. The data are consistent with the IR spectra, showing that formate and DMF ligands were bonded to the nodes. These data characterizing the three species were normalized, and the results are summarized in Table 1. The amount of formate was found to be 1.44 per 12 node binding sites (referred to for brevity as 1.44/12), and DMF was found to be 0.18/12. The total number of vacancies calculated from this NMR spectrum (the sum the number of formate and dimethyl amine) is 1.62/12, which is close to the number, 1.4/12, determined by TGA. The results indicate that majority of the vacancies were covered with formate ligands. The Ui66 (Cl) sample was treated with methanol at 298 K for 24 h, and the NMR spectrum of the resultant sample is shown in Figure S8 (red). The formate and DMF ligands had been removed. Simultaneously, peaks for methoxy ligands appeared, consistent with the IR data. 3 S9

10 Normalized Intensity (a.u.) D Chemical shift (ppm) Figure S9. 1 NMR spectra characterizing Ui67 Cl (black). Samples were dissolved in 1M Na/D 2. Ui67 modulated by Cl had only formate ligands, no detectable dimethyl amine. The loading of formate on the nodes is 1.53/12, by the abovestated convention. S10

11 Normalized Intensity (a.u.) D 3 C C 3 N C Chemical shift (ppm) Figure S10. 1 NMR spectra characterizing a MF related to the ones of this investigation, NU 1000 (black) and the same sample after liquid methanol treatment at 343 K for 48 h (red) and NU1000 after further treatment with water vapor/helium at 393 K for 30 min. All the samples were dissolved in 1M Na/D 2. See refs. 3 and 4. NU1000 as synthesized (black), after methanol treatment at 343 K (red), after water vapor treatment at 393 K (blue). Figure S10 shows NMR data characterizing NU1000. Formate and dimethyl amine are both identified in this sample. owever, because the linker is not soluble in the Na/D 2 solution, we cannot normalize the data for all the ligands. owever, we can quantify the formate ligands by the desorption data shown in Figure S12. The results show the amounts of these two ligands are 3.99/12 and 0.01/12, indicating that a large majority of the ligands are formate. We treated NU1000 with liquid methanol at 343 K, as reported previously 3. Formate and dimethyl amine ligands were both removed as a result of the treatment, and methoxy ligands S11

12 Normalized Intensity (a.u.) formed. Further treatment of this sample with water vapor at 393 K led to removal of almost all the methoxy ligands, consistent with reported IR data. 3 D 3 C C 3 N C 3 C 3 C Chemical shift (ppm) Figure S11. 1 NMR spectra characterizing Ui66 (acetic acid, black) and the same sample after ethanol/helium treatment at 473 K for 20 h (red). Each sample was dissolved in 1M Na/D 2. Ui66 modulated by acetic acid has three ligands on the node vacancies, including formate, acetate, and DMF. Their amounts are 0.19/12, 0.86/12, and 0.02/12, respectively. Again, we treated the sample with ethanol vapor in helium at 473 K for 20 h. Formate, acetate, and DMF were all removed, and ethoxy ligands appeared. In summary, the NMR data provide further evidence of the ligands on the MF node vacancies. Formate, acetate, and DMF ligands can be readily removed by the reaction of each with methanol or ethanol. S12

13 Weight (% Zr 2 ) Weight (% Zr 2 ) Integrated area Desorption of formate ligands during ethanol treatment Time on stream (min) Figure S12. Desorption of formate by formation of ethyl formate characterizing Ui66 (Cl, black) and NU1000 (red) during ethanol vapor/helium treatment at 423 K. Ethyl formate was monitored by gas chromatography. TGA measurements 250 A Zr 6 6 (BDC) 6 [=2.22 6Zr 2 ] 300 Zr 6 6 (BPDC) 6 [=2.84 6Zr 2 ] B 200 Ui66 (acetic acid) 250 Ui67 (Cl) Ui66 (Cl) Zr Zr Temperature ( o C) Temperature ( o C) Figure S13. TGA trace of (A) activated Ui66 (Cl) and Ui66 (acetic acid), and (B) Ui67 (Cl). S13

14 S2. Ethanol dehydration catalyzed by MF nodes SEM images Ui66 Cl (200nm) Ui66 Acetic acid (200nm) Ui67 Cl (500nm1µm) NU1000 (1.5 5 µm) Figure S14. SEM images of MF samples S14

15 Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) Reaction rate (/10 6, mol gcatal 1 s 1 ) Catalytic activity for ethanol dehydration to diethyl ether 3 2 Ui66 Cl Ui66 Methoxy Ui66 terminal Ui67 Cl NU1000 Al 2 3 Zr Time on stream (min) Figure S15. Catalytic activities of various samples for ethanol dehydration at 523 K. IR Evidence of ethoxy as ethanol dehydration intermediate Wavenumber (cm 1 ) Wavenumber (cm 1 ) Wavenumber (cm 1 ) Figure S16. IR spectra characterizing Ui66 modulated by Cl (black), and the respective samples after ethanol dehydration catalysis at 523 K for 10 min, followed by treatment of the sample in flowing helium at 1.0 bar and 523 K for 10 min (red). The IR spectra indicate the quickly removal of formate ligands (2746 cm 1 ) during ethanol dehydration, and simultaneous formation of ethoxy ligands (C vibration at 2971, 2931 and 2869 cm 1, and C vibration at 1151 cm 1 ). S15

16 Intensity (a.u.) XRD and BET measurements NU Ui67 (Cl) 0 Ui66 (Cl) Theta ( o ) Figure S17. XRD patterns characterizing samples of Ui66 (Cl) and Ui67 (Cl) and of these samples after ethanol dehydration catalysis at 523 K in a flow reactor for 4 h (red). Table S2. BET surface area changes during ethanol dehydration catalysis in a flow reactor MF (modulator) Treatment condition BET surface area (m 2 /g) Ui66 (Cl) None 1566 Ethanol dehydration catalysis at 523 K for 4 h 48 Ethanol dehydration catalysis at 473 K for 20 h 1045 Ui66 (acetic acid) None 1391 Ethanol dehydration catalysis at 473 K for 20 h 1548 Ui67 (Cl) None 2231 Ethanol dehydration catalysis at 523 K for 4 h 277 NU1000 None 2194 Ethanol dehydration catalysis at 523 K for 4 h 1196 S16

17 Intensity (a.u.) Evidence for the reaction between MF linkers and ethanol Wavenumber (cm 1 ) Figure S18. IR spectra in the ν C region characterizing Ui66 (Cl, black) and the same sample after ethanol treatment at 473 K for 1 h (red) and activation under high vacuum at 473 K for 12 h. The 1733 cm 1 band is assigned to ester species formed in the reactions of linkers with ethanol. The 1656 cm 1 band represents chemisorbed DMF molecules, which were removed after the treatment. S17

18 Integrated area of Ethyl Formate (a.u.) Integrated Area of Ethyl acetate (a.u.) Desorption of acetate and formate during ethanol dehydration catalysis A 5000 B Time on stream (min) Time on stream (min) Figure S19. Desorption of (A) ethyl formate and (B) ethyl acetate of Ui66 (Cl, black) Ui 66 (acetic acid, red) during ethanol dehydration catalysis at 473 K. To improve the stability of Ui66 as a catalyst, we synthesized it with acetic acid as a modulator and tested it for the ethanol dehydration reaction at a lower temperature, 473 K (Figure 2A). Ui66 (Cl) underwent only relatively slow deactivation at this temperature; the activity decreased from to mol (g of catalyst) 1 (s) 1 over 20 h of operation in the flow reactor. The XRD data shown in Figure 2B demonstrate that Ui66 (Cl) lost its crystallinity, but still had a high BET surface area, 1045 m 2 /g (Table S2). The Ui66 (acetic acid) sample performed differently from Ui66 (Cl). The activity first increased from to (g of catalyst) 1 (s) 1 in 4 h, and then decreased slowly to (g of catalyst) 1 (s) 1 in the following 16 h. The XRD data of Figure 2B show that Ui66 (acetic acid) retained its crystallinity, and was slightly higher in surface area, 1548 m 2 /g (compared with 1391 m 2 /g for the assynthesized sample), indicating the high stability of this MF sample, Table S2). When acetic acid was used to modulate the Ui66 synthesis, some acetate ligands remained bonded to the nodes (Table 1). Evidently, these ligands blocked active sites for catalysis, but only temporarily; they slowly reacted with ethanol to form ethyl acetate, which was observed as desorption product by gas chromatography (Figure S19). S18

19 Number of ligands on the node vacancies (/node) Formate Acetate Ethoxy Total Vacancies Time on stream (h) Figure S20. Evolution of ligands on the node vacancies of Ui66 (acetic acid) characterized by 1 NMR during ethanol dehydration catalysis at 473 K. Each sample was treated under high vacuum at 423 K for 12 h before dissolving in 1 M Na/D 2 for 1 NMR testing. Note: We observed terminal groups in the IR spectrum (with band at 3781 cm 1 ) of the 3h reaction sample, but not in other samples. We infer terminal groups are the product of esterification after the removal of acetate or formate ligands on the nodes by ethanol, which had disappeared after all acetate and formate ligands were removed. Because 1 NMR spectroscopic cannot quantify ligands, the total vacancy calculated by organic ligands of the 3h reaction sample, which is not shown, was lower than it was for the other three samples. We monitored the evolution of ligands on the node vacancies of Ui66 (acetic acid) by 1 NMR spectroscopy at various times on stream of ethanol dehydration catalysis in the flow reactor at 473 K. The results shown in Figure S20 confirm that formate and acetate ligands block the active sites, and they also confirm that ethoxy ligands were the dominant intermediate species in ethanol dehydration. Further, the results demonstrate that ethoxy ligands can also be used as markers to determine the number of node vacancies. From the data, we observe that the total number of node vacancies gradually increased with time on stream in the flow reactor as the ethanol dehydration reaction was taking place, which indicates that ethanol replaced linkers on the nodes and generated more defects. We emphasize that this process was accompanied by the degradation of the MF structure, indicated by the gradual decrease in its crystallinity in the 20 h of catalytic reaction, as shown in Figure 2B. We infer that the slow deactivation of Ui66 (acetic acid) after complete removal of acetate and formate ligands was caused by the MF degradation. S19

20 S3. Computational modeling Zrcontaining MF nodes were modeled as finite clusters extracted from DFToptimized periodic calculations to allow a detailed investigation of the modified complexes. All calculations for Ui66 were obtained from ajek et al., 6 whereby selected linkers were truncated as benzoate and formate groups. All calculations for NU1000 were performed using the mixs topology from Planas et al. 7, whereby eight linkers were truncated as benzoate and the / 2 ligands were replaced by formate. In all cases, the pcarbon atoms of benzoate groups were kept fixed to mimic the rigidity of the framework. To evaluate the choice of capping formate against capping acetate groups (Figure S21), we computed the coordination of one ethanol molecule in Ui66 models. The results, Table S3, show negligible differences between these models; thus, we choose the formate capping for computational efficiency. As noted in the main text Computational details section, for computational efficiency, splitvalence basis sets were used on nonmetal atoms in geometry optimizations, while for increased accuracy, singlepoint energetics were subsequently computed with valence triplezeta basis sets on these atoms. To assess sensitivity to choice of functional, we also computed the energy differences between structures BS N 23 and B1 at the B3LYPD3 and B3PW91D3 levels; the variation in electronic energy difference over these 3 functionals is 2 kcal/mol, which is ~5% of the computed activation energy (Table S8). This same due diligence comparison was made when computing possible formatefunctionalized node geometries and IR spectra, in all cases using valence triplezeta basis sets to achieve higher accuracy. Figure S21. Ethoxydecorated Ui66/67 model capped with formate groups (left) and acetate groups (right). Color code: Carbon atoms are gray, hydrogen is white, zirconium is cyan, and oxygen is red. a S20

21 Table S3. Ethanol binding energies. Compound/model E / hartree / hartree G / hartree Ethanol Ethoxyacetatemodel Ethoxyformatemodel Ethanolethoxyacetatemodel Ethanolethoxyformatemodel Ethanol binding energy in kcal mol 1 Ethoxyacetatemodel Ethoxyformatemodel a Energies computed at the M06L/BS1 level of theory S21

22 DFT characterization of formatefunctionalized Zrcontaining MFs Structures Figure S22. Plausible mechanism for formatefunctionalized Zrcontaining MFs. All linkers are omitted for clarity. Table S4. Formatefunctionalized Ui66 nodes (Figure S22). Energies relative to reactants a 4b 5 Water Formic acid Energy / hartree E K G K K G K Energy / kcal mol 1 ΔE Δ K ΔG K Δ K ΔG K a Energies computed at the M06L/BS1 level of theory Frequencies We have performed gasphase calculations of the IR frequencies of various C containing formyllike molecules with three different density functionals and MP2 in order to benchmark our M06L results, comparing them also with available experimental results (Table S5). We observe that there is a redshift in the computed frequency peaks going from a local functional toward hybrids and MP2. No significant difference was observed between B3LYP and B3PW91 functionals. A red shift is also observed between experimental and computed frequency, which is consistent within the computational methodology employed. Thus, in order to overcome the disagreement between theory and experiment, a specific scaling factor has been derived from these sets of molecules to arise from errors associated with the computational method. S22

23 Table S5. Experimental and calculated frequency benchmark for formyltype of C stretches in gasphase (in cm 1 ). Intensities are reported in parentheses. Computed frequencies Estimated scaling factor Molecule Phase Type of stretching Experimental a M06L B3LYP B3PW91 MP2 M06L B3LYP B3PW91 MP2 C gas C 2943 (formic acid) (59.13) (44.27) (45.59) (35.13) C 2 C gas C 2943 (methyl formate) (63.00) (59.00) (59.69) (32.32) CCl gas C 2929 (formyl chloride) (24.10) (21.47) (21.97) (18.57) C gas C 2800 (2propenal) (107.82) (92.74) (92.96) (78.64) Average scaling factor a Experimental results taken from M. E. Jacox, "Vibrational and Electronic Energy Levels of Polyatomic Transient Molecules" in NIST Chemistry WebBook, NIST Standard Reference Database Number 69, Eds. P.J. Linstrom and W.G. Mallard, National Institute of Standards and Technology, Gaithersburg MD, 20899, doi: /t4d303, (retrieved September 4, 2017). Frequencies were computed by using the BS1 set of basis functions. Table S6. Vibrational frequencies for structures 3, 4b, and 5 for Ui66/67 and 4b for NU1000 stable structures after posttreatment with formic acid (Figure S22). Frequencies in cm 1 and scaling factor obtained as stated in Table S5 for the M06L functional. Intensities are reported in parentheses. Structure In cm 1 3 4b 4bNU υ[c ] intensity / υ[c ] intensity , δ[c ] intensity υ[c ] symm +δ[c ] (scaling factor ) ( ) ( ) ( ) ( ) Note: Frequencies were computed by using the M06L/BS1 level of theory. S23

24 Catalytic ethylene formation from diethyl ether via model B We also considered further dehydration of the diethyl ether bound to the node in B3 (Figure S23). The corresponding E2type TS BE22 was found at 39.6 kcal mol 1, which is slightly less stable than BS N 23 at 38.7 kcal mol 1. owever, the diethyl ether ligand in B3 is relatively bulky and lacks hydrogen donors to stabilize the neighboring ethoxy group. As a result, the displacement of diethyl ether by either water (B4 at 8.3 kcal mol 1 ) or ethanol (B5 at 6.0 kcal mol 1 ) is quite favorable and prevents further dehydration to ethylene. Figure S23. E2type and S N 2type transition states for model B. ΔG 523 in kcal mol 1. All linkers are omitted for clarity. S24

25 Ethanol dehydration catalyzed by dehydroxylated Ui66 nodes It has reported that at high temperatures the nodes of Ui66 can be hydroxylated from Zr 6 () 4 () 4 to Zr From a periodic DFT structure, 8 we designed a cluster node model with one defect, four benzoate (with all pcarbon atoms kept fixed) and seven formate groups, namely, model C. Following the calculations for models A and B stated in the main text, we explored E2type and S N 2type transition states, as shown in Figure S24. With a similarity to model B, S N 2type TSs (CS N 22 and CS N 23) are favored over the E2type TS (CE21) by ca. 8 kcal mol 1. If this particular node were present under catalytic reaction conditions, it could also contribute to the exclusive formation of diethyl ether. Figure S24. E2type and S N 2type transition states for model C. ΔG 523 in kcal mol 1. All linkers are omitted for clarity. S25

26 Energies of all DFToptimized species (ethanol dehydration mechanism) Table S7. Electronic and free energies at the M06L level for all optimized species. Species E a / hartree G a,b / hartree E c / hartree Imag. freq. / cm NA Et NA Et NA A NA AE i AS N i AS N i AS N i B NA B NA BE i BS N i BS N i BS N i B NA BE i B NA B NA C NA C NA CE i CS N i CS N i CS N i CS N i a Basis set BS1 b Lowfrequency corrected free energy at 523 K c Basis set BS2 Table S8. Electronic energies (hartrees) for selected M06Loptimized species. Density Functional B1 BS N 23 B3PW91D B3LYPD Geometries of all DFToptimized species All Cartesian coordinates can be found in the attached file coordinates.xyz. S26

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