A Computational Study of Lithium Cuprate Mixed Aggregates

Aug 24, 2012 - Lawrence M. Pratt*. Department of Physical, Environmental, and Computer Sciences, Medgar Evers College, The City University of New York...
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A Computational Study of Lithium Cuprate Mixed Aggregates Chau Nguyen Duy Khiem and Le Ngoc Thach* Department of Organic Chemistry, University of Science, Vietnam National University, 227 Nguyen Van Cuu, District 5, Ho Chi Minh City, Vietnam

Takanori Iwasaki and Nobuaki Kambe* Department of Applied Chemistry, Osaka University, Suita, Osaka 565-0871, Japan

Andrey A. Boguslavskiy UMASS Boston, 100 Morrissey Boulevard, Boston, Massachusetts 02125-3393, United States

Lawrence M. Pratt* Department of Physical, Environmental, and Computer Sciences, Medgar Evers College, The City University of New York, 1650 Bedford Avenue, Brooklyn, New York 11225, United States S Supporting Information *

ABSTRACT: Lithium dialkylcuprates may potentially form mixed aggregates with many species in solution. Those include excess alkyllithium used to prepare the cuprate, lithium halide, and lithium cyanide from cuprate preparation and from coupling reactions with alkyl halides, higher order cuprates, and species resulting from incomplete cuprate reactions. The M06 DFT method was used to elucidate the structures and energies of formation of potential mixed aggregates. A comparison was made to available experimental data.

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INTRODUCTION Lithuim dialkylcuprates undergo coupling reactions with alkyl halides and tosylates. They also undergo conjugate addition with enones as well as coupling reactions with acid halides. Although the mechanism can be quite complex, a common feature is oxidative addition to a Cu(I) species, forming a Cu(III) intermediate, which then undergoes reductive elimination of the product.1−3 Cross-coupling occurs most readily with alkyl tosylates and iodides, less readily with alkyl bromides, and poorly with alkyl chlorides. More than one mechanism is possible for the oxidative addition step, and both nonradical4−9 and radical10 mechanisms have been proposed. Solvent effects and additives can significantly change the reaction rate and possibly the mechanism. Lithium cuprate dimers and solventseparated ion pairs (SSIPs) have both been reported in the solid state.11 The presence of SSIPs has been deduced in solutions of cyanocuprates by 1H−6Li HOESY NMR.12 Lithium cuprates are prepared from a Cu(I) halide or cyanide, releasing one molar equivalent of lithium halide or LiCN in the process. The lithium halide or cyanide may form a mixed aggregate in equilibrium with the free lithium cuprate dimer and SSIP.13−15 Furthermore, coupling of the lithium © 2012 American Chemical Society

cuprate with an alkyl halide produces another molar equivalent of LiX, in addition to some RCuXLi. Under some conditions, a higher order cuprate of the structure R3CuLi2 is formed.16,17 The RCuXLi species may exist as a SSIP, a homodimer, or a mixed aggregate with the numerous lithium-containing species present in solution. If the coupling reaction is performed in the presence of excess alkyllithium, lithium cuprate−alkyllithium mixed aggregates may potentially form with any of the cuprate species in solution. This work is a comprehensive computational study of the mixed aggregates that may be formed during the preparation and coupling reactions of lithium cuprates. Understanding the various species that may be present will provide a better understanding of the sometimes complex reaction mechanisms of lithium cuprates.

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COMPUTATIONAL METHODS All geometry optimizations, frequency calculations, and singlepoint energy calculations were performed with the Gaussian 09 Received: April 10, 2012 Revised: August 20, 2012 Published: August 24, 2012 9027

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Figure 1. Optimized geometries [M06/6-31+G(d)] of THF solvated lithium dimethylcuprate mixed dimers. Upper left, LiCl; upper right, LiBr; lower left, LiI; and lower right, LiCN. Hydrogens were omitted for clarity. Gray, C; copper, Cu; bright red, O; green, Cl; dark red, Br; light violet, Li; dark violet, I; and blue, N.

flask and adding 6 mL of THF, which had been dried over sodium and freshly distilled under nitrogen. While maintaining a positive nitrogen pressure, 2.0 mL of 2 M butyllithium (4.0 mmol) was added via syringe at −78 °C, and the mixture was stirred for 1 h. The alkyl halide was then added by syringe (1-choloro- or 1-bromooctane, 0.76 mmol), and 0.1 mL aliquots were removed and quenched in 1.0 M HCl after 5, 15, 30, 60, and 120 min. The coupling products were extracted from the quenched solutions into ether and analyzed by gas chromatography. The percent conversions of the cuprate were calculated from the GC peak areas relative to a mesitylene internal standard. GC analysis showed no further coupling reaction occurred after 15 min. Description of NMR Experiments and Parameters. Deuterium-free 1H NMR spectra were recorded with a JEOL JNM-Alice 400 (400 MHz) spectrometer. All 1H NMR chemical shifts were reported in ppm relative to TMS at δ 0.00. Butyllithium was purchased from Kanto Chemical Company. The complex formed from CuCl + 2BuLi is stable even at 50° for a short time, but some degradation is detected by NMR in about 1 h. To a 5 mm NMR tube was added CuCl (5.0 mg, 0.05 mmol). The tube was capped with a septum and purged with N2, and then, THF (0.5 mL, 0.1 M, containing TMS as an internal standard) was added. To a suspension was added nBuLi (1.6 M in hexane, 63 μL, 0.1 mmol) at 0 °C. The mixture was sonicated to dissolve CuCl. The resulting dark purple solution was analyzed by deuterium-free NMR.

program.18 Geometry optimizations were performed at the M06/6-31+G(d)19 level of theory, followed by frequency calculations at the same level. Free energy corrections were calculated at 298.15 K from the frequency calculations and added to the electronic energies at each level of theory, to obtain approximate free energies of each species. Solvent effects were modeled by placing explicit THF ligands on the lithium atoms. Two ligands were used for each higher order cuprate, and four ligands were used for the cuprate dimers (two ligands per lithium atom). Special care is taken to ensure consistent handling of standard states.20,21 Specifically, a correction term RT ln(c°RT/P°) must be added per mole of each species in the reaction under consideration, which represents the change in free energy involved in compressing the system from standard pressure P° (or a concentration of P°/RT) used in gas phase calculations to the standard concentration of c° = 1 mol/L commonly used for solutions. This term is numerically equal to +1.8900 kcal/mol at 298.15 K. While it cancels from both sides when the net change in the number of moles due to reaction Δn = 0, it is a non-negligible correction in cases where Δn ≠ 0. Yet another correction is required for cases where a THF ligand dissociates, as in RLi·nTHF ⇄ RLi·mTHF + (n − m)THF

for which ΔG° = −RT ln

[RLi·mTHF] [THF] − (n − m)RT ln [RLi·nTHF] c°

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(1)

Because the concentration of pure THF is different from the standard concentration c°, it was evaluated from its molar volume at 1 atm and 298.15 K using the empirical expression provided by Govender et al.22 and incorporated into the second term of eq 1. Numerically, this correction to ΔG° amounts to −1.4883 kcal/mol per THF at 298.15 K. This approach to modeling solvation effects on organolithium compounds has been used in other studies23−28 and has been found to give results in agreement with available experimental results.

RESULTS AND DISCUSSION

Aggregates Formed during Lithium Cuprate Preparation and at Low Percent Conversion. Mixed aggregates may potentially form from the lithium halide or cyanide produced from the Cu(I) salts or from lithium halides produced during the coupling reaction with alkyl halides. The free energies of mixed dimer formation in the gas phase and in THF solution were calculated according to eqs 2 and 3, respectively, and the optimized geometries of the THF solvated lithium dimethylcuprate mixed dimers are shown in Figure 1. Lithium bromide has been shown to be dimeric in THF.29 The calculated gas phase energies range from slightly endergonic to slightly exergonic, as shown in Table 1. The mixed dimer formation

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EXPERIMENTAL METHODS Cuprate Coupling Procedure. The Gilman reagent was prepared by placing 0.38 g (2.0 mmol) of CuI in an oven-dried 9028

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was formed. A separate experiment using 1-halononanes showed that octane is formed in small amounts, and thus, the octane formation is not a result of 1-halooctane reduction. 1-Bromooctane was more reactive, and the reactivity was enhanced by the addition of butyllithium and lithium bromide. Those additives also changed the product distribution. Coupling of 1-iodooctane went with quantitative conversion with and without excess butyllithium and lithium iodide, and the additive effects on the product distribution were minimal. The data are shown in Table 2. These results suggest the possibility that lithium cuprate−butyllithium and lithium cuprate−lithium halide mixed aggregates may be formed or that the presence of excess butyllithium may perturb the equilibrium of other aggregates, thus affecting the reactivity of the lithium cuprate. The free energies of lithium cuprate mixed aggregate formation with alkyllithium were calculated in the gas phase and in THF solution according to eqs 4 and 5, respectively. The results are shown in Table 3. Alkyllithiums exist in

Table 1. Free Energies of Lithium Dialkylcuprate Mixed Aggregate Formation with Lithium Halides and LiCN (kcal/mol) ΔGf° R2CuLi/LiX

gas phase

THF solution

Me2CuLi/LiCl Me2CuLi/LiBr Me2CuLi/LiI Me2CuLi/LiCN Bu2CuLi/LiCl Bu2CuLi/LiBr Bu2CuLi/LiI Bu2CuLi/LiCN

−0.858 −1.26 −0.585 0.500 0.149 −0.572 0.291 1.29

2.62 −3.23 −1.94 −2.53 −0.728 −3.89 −0.160 −2.41

energies in THF were calculated from the lithium halide tetrasolvated dimers. Because those salts are likely to coexist as a monomer and dimer and in more than one solvation state, there is some uncertainty introduced in the calculated energies of mixed dimer formation. The calculated energies range from +2.6 to −3.9 kcal/mol, suggesting that the cuprate homodimer, mixed dimer, and lithium halide all exist in equilibrium with the lithium cuprate SSIP. As lithium halide is added to the solution, more cuprate homodimer will react to form the mixed dimer, and some of the SSIP will be removed from solution as the equilibrium adjusts to replenish the homodimer. Depending on the relative reactivity of the three species, addition of lithium halide may therefore affect the reactivity of the lithium cuprate.

Table 3. Free Energies of Lithium Dialkylcuprate Mixed Aggregate Formation with Alkyllithiums (kcal/mol) ΔGf° gas phase

THF solution

Me2CuLi/MeLi Bu2CuLi/BuLi

5.67 6.94

2.16 4.14

different aggregation states in solution, depending on the alkyllithium and the solvent.30−32 Tetramers are common in THF, sometimes in equilibrium with other aggregates. Higher aggregates are common in hydrocarbon solvents. The free energies of cuprate mixed dimer formation were calculated from the cuprate homodimer and the alkyllithium tetramer and were found to be endergonic in all cases. The computational results are consistent with the proton NMR spectrum of Bu2CuLi (prepared from LiI and butyllithium) with a second molar equivalent of butyllithium, shown in Figure 2. We therefore conclude that butyllithium does not form a

1/2(R 2CuLi)2 (gas) + 1/2(LiX)2 (gas) → R 2CuLi·LiX(gas) (2)

1/2(R 2CuLi)2 ·4THF + 1/2(LiX)2 ·4THF → R 2CuLi· LiX· 4THF

R2CuLi/RLi

(3)

Lithium dibutylcuprate was coupled with 1-halooctanes in THF solution. 1-Chlorooctane was unreactive toward coupling, with conversions of the starting material under 10% by comparison to an internal standard. A small amount of octane

Table 2. Percent Conversion of Alkyl Halide by Lithium Dibutylcuprate Alone and with Additives, Relative to Internal Standard (Octane:Dodecane:Hexadecane Ratio) R−X

Bu2CuLi

Bu2CuLi + BuLi

Bu2CuLi + LiX

1-chlorooctane 1-bromooctane 1-iodooctane

2.7 (9.5:0:0) 15.3 (8.0:19.6:1.1) 100 (19.6:24.2:8.7)

8.2 (10.4:0:0) 44.8 (15.2:24.0:0) 100 (22.1:18.5:8.5)

9.8 (10.5:0:0) 50.6 (9.9:29.8:1.8) 100 (17.5:21.7:9.4)

Figure 2. Proton NMR spectrum of lithium dibutylcuprate with excess butyllithium. Triplet (centered at 0.40 ppm) is the (CH3CH2CH2CH2)2CuLi peak at the left. Free butyllithium (right) appears as a complex peak between −1.3 and −1.0 ppm. 9029

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mixed aggregate with lithium dibutylcuprate to a significant extent.

Table 4. Free Energies of Lithium Dialkylcuprate Lithium Alkyl-Halocuprate Mixed Aggregate Dissociation to Homodimers (kcal/mol)

1/2(R 2CuLi)2 (gas) + 1/4(RLi)4 (gas) → R 2CuLi· RLi(gas)

ΔGrxn°

(4)

1/2(R 2CuLi)2 · 4THF + 1/4(RLi)4 · 4THF + THF → R 2CuLi· RLi· 4THF

(5)

Aggregates Formed at Higher Percent Conversions of the Lithium Cuprate: R2CuLi and RCuXLi Both Are Present. As one alkyl group of R2CuLi is consumed during the coupling reaction, a unit of RCuXLi is formed, where X is the halogen from the alkyl halide undergoing coupling. If the coupling reaction takes place via the lithium dialkylcuprate dimer, the initial product will be a mixed dimer of R2CuLi·RCuXLi. If the SSIP is the reactive species, a unit of RCuXLi will be formed, presumably also as a SSIP. In any case, the SSIPs may coexist in equilibrium with the mixed dimers, whose optimized geometries are shown in Figure 3. The mixed dimers may also equilibrate with the R2CuLi and RCuXLi homodimers, as indicated in eqs 6 and 7, for the gas phase and THF solvated species, respectively. The data in Table 4 show that Keq for those reactions will be close to unity in both the gas phase and in THF solution. Thus, all three species are likely to be present, in addition to any SSIP species.

−0.155 0.508 −0.0439 0.679 0.210 1.18

ΔGrxn°

(6)

(7)

The R2CuLi·RCuXLi mixed dimer may also eliminate a molar equivalent of lithium halide to form the higher order cuprates that were reported by Nakamura, Bertz, and co-workers,16,17 as indicated in the gas phase by eq 8 and in THF solution by eq 9. The optimized geometries of these R3Cu2Li species are shown in Figure 4. The data in Table 5 show the reaction to be energetically unfavorable in the gas phase and marginally favorable in THF solution, except for the R2CuLi·RCuBrLi mixed dimer.

R2CuLi·RCuXLi

gas phase

THF solution

Me2CuLi·MeCuClLi Me2CuLi·MeCuBrLi Me2CuLi·MeCuILi Bu2CuLi·BuCuClLi Bu2CuLi·BuCuBrLi Bu2CuLi·BuCuILi

4.28 7.79 6.86 4.20 7.06 7.11

−1.06 0.967 −0.594 −2.72 1.24 −1.37

RCuXLi. It is possible for some R2CuLi to be regenerated as a mixed dimer with CuX2Li according to eqs 10 and 11 in the gas phase and in THF, respectively. The optimized geometries of those mixed dimers are shown in Figure 5. From the data in Table 6, it can be seen that the homodimers (R2CuXLi)2 do not disproportionate to (R2CuLi)2 and CuX2Li forming mixed dimer R2CuLi·CuX2Li to a significant extent in the gas phase. That reaction is even less favorable in THF solution. Thus, as R2CuLi units are depleted during coupling reactions, they will not be significantly replenished from RCuXLi structural units.

R 2CuLi· RCuXLi(gas) → R3Cu 2Li(gas) + 1/2(LiX)2 (gas) (8)

R 2CuLi· RCuXLi· 4THF → R3Cu 2Li·2THF + 1/2(LiX)2 ·4THF

THF solution

−0.0542 −0.220 0.233 −0.0163 −1.62 0.380

Table 5. Free Energies of Lithium Dilkylcuprate Lithium Alkyl-Halocuprate Mixed Aggregate Dissociation to Higher Order Cuprates and Lithium Halide (kcal/mol)

R 2CuLi· RCuXLi· 4THF → 1/2(R 2CuLi)2 ·4THF + 1/2(RCuXLi)2 ·4THF

gas phase

Figure 4. Optimized geometries [M06/6-31+G(d)] of THF solvated R3Cu2Li higher order cuprates. Left, Me3Cu2Li; and right, Bu3Cu2Li.

R 2CuLi· RCuXLi(gas) → 1/2(R 2CuLi)2 (gas) + 1/2(RCuXLi)2 (gas)

R2CuLi·RCuXLi Me2CuLi·MeCuClLi Me2CuLi·MeCuBrLi Me2CuLi·MeCuILi Bu2CuLi·BuCuClLi Bu2CuLi·BuCuBrLi Bu2CuLi·BuCuILi

(9)

At even higher percent conversion of the lithium dialkylcuprate, most of the R2CuLi species have been consumed and converted to

RCuXLi(gas) → R 2CuLi·CuX 2Li(gas)

(10)

RCuXLi· 4THF → R 2CuLi·CuX 2Li· 4THF

(11)

Figure 3. Optimized geometries [M06/6-31+G(d)] of THF solvated lithium dimethylcuprate−lithium halomethylcuprate mixed dimers. Left, MeCuClLi; center, MeCuBrLi; and right, MeCuILi. 9030

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Figure 5. Optimized geometries [M06/6-31+G(d)] of THF solvated lithium dimethylcuprate−CuX2Li mixed dimers. Left, Me2CuLi·CuCl2Li; center, Me2CuLi·CuBr2Li; and right: Me2CuLi·CuI2Li.

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Table 6. Free Energies of R2CuLi·CuX2Li Mixed Dimer Formation (kcal/mol)

(1) Bertz, S. H.; Cope, S.; Murphy, M.; Ogle, C. A.; Taylor, B. J. J. Am. Chem. Soc. 2007, 129, 7208−7209. (2) Hu, H.; Snyder, J. P. J. Am. Chem. Soc. 2007, 129, 7210−7211. (3) Bertz, S. H.; Cope, S.; Dorton, D.; Murphy, M.; Ogle, C. A. Angew. Chem., Int. Ed. 2007, 46, 7082−7085. (4) Bartholomew, E. R.; Bertz, S. H.; Cope, S.; Murphy, M.; Ogle, C. A. J. Am. Chem. Soc. 2008, 130, 11244−11245. (5) Yoshikai, N.; Nakamura, E. J. Am. Chem. Soc. 2004, 126, 12264− 12265. (6) Pratt, L. M.; Voit, S.; Mai, B. K.; Nguyen, B. H. J. Phys. Chem. A 2010, 114, 5005−5015. (7) Yoshikai, N.; Iida, R.; Nakamura, E. Adv. Synth. Catal. 2008, 350, 1063−1072. (8) Nakamura, E.; Mori, S. J. Am. Chem. Soc. 1998, 120, 8273−8274. (9) Johnson, C. R.; Dutra, G. A. J. Am. Chem. Soc. 1973, 95, 7783− 7788. (10) Ashby, E. C.; Coleman, D. J. Org. Chem. 1987, 52, 4554−4565. (11) Jost, S.; Kuhnen, M.; Gunther, H. Magn. Reson. Chem. 2006, 44, 909−916. (12) Gschwind, R. M.; Rajamohanan, P. R.; John, M.; Boche, G. Organometallics 2000, 19, 2868−2873. (13) Nakamura, E.; Mori, S. Angew. Chem., Int. Ed. 2000, 39, 3750− 3771. (14) Burns, D. H.; Miller, J. D.; Chan, H.-K.; Delaney, M. O. J. Am. Chem. Soc. 1997, 119, 2125−2133. (15) Nakamura, E.; Yamanaka, M. J. Am. Chem. Soc. 1999, 121, 8941−8942. (16) Nakamura, E.; Yoshikai, N. Bull. Chem. Soc. Jpn. 2004, 77, 1−12. (17) Bertz, S. H.; Dabbagh, G. J. Am. Chem. Soc. 1988, 110, 3668− 3670. (18) 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.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision A.1; Gaussian, Inc.: Wallingford, CT, 2009. (19) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215−241. (20) Thompson, J. D.; Cramer, C. J.; Truhlar, D. G. J. Chem. Phys. 2003, 119, 1661−1666. (21) Pratt, L. M.; Nguyen, N. V.; Ramachandran, B. J. Org. Chem. 2005, 70, 4279−4283. (22) Govender, U. P.; Letcher, T. M.; Garg, S. K.; Ahluwalia, J. C. J. Chem. Eng. Data 1996, 41, 147−150. (23) Pratt, L. M.; Streitwieser, A. J. Org. Chem. 2003, 68, 2830−2838.

ΔGrxn° R2CuLi·CuX2Li

gas phase

THF solution

Me2CuLi·CuCl2Li Me2CuLi·CuBr2Li Me2CuLi·CuI2Li Bu2CuLi·CuCl2Li Bu2CuLi·CuBr2Li Bu2CuLi·CuI2Li

4.46 4.52 1.77 4.34 5.28 1.62

7.70 7.11 4.05 4.92 9.84 4.08

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CONCLUSIONS Lithium dialkylcuprates exist as several different aggregates in equilibrium with the SSIP in THF solution. Mixed dimers of the form R2CuLi·LiX are formed as the cuprate is prepared from Cu(I) halides or cyanide. They are also formed as the cuprate is consumed, and additional lithium halide is formed during coupling reactions with alkyl halides. In contrast, mixed dimers of the lithium cuprate with excess alkyllithium are not formed to a significant extent. As the reaction proceeds, RCuXLi is formed, which can exist as a homodimer or a mixed dimer with remaining R2CuLi. It can also eliminate lithium halide to form the higher cuprate R3Cu2Li, which is favored by polar solvents. As the reaction proceeds to even higher percent conversions, the remaining lithium dialkylcuprate is consumed, and the major species present will be RCuXLi or the higher order cuprates.

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ASSOCIATED CONTENT

S Supporting Information *

Tables of M06 optimized geometries and energies of all structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.N.T.), [email protected]. osaka-u.ac.jp (N.K.), and [email protected] (L.M.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSF Grant #INT-0744375. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under contract #DE-AC03-76SF00098. Thanks to Allan Pinhas and Phung Chau, University of Cincinnati, for helpful discussions. 9031

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