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Charge-Polarized Coordination Space for H2 Adsorption Jun-ya Hasegawa,*,† Masakazu Higuchi,†,‡ Yuh Hijikata,† and Susumu Kitagawa†,‡ Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto UniVersity, Kyoto-Daigaku-Katsura, Nishikyo-ku, Kyoto 615-8510, Japan, and RIKEN, SPring-8 center, 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan ReceiVed September 23, 2008. ReVised Manuscript ReceiVed March 17, 2009
We investigated the concept “charge-polarized coordination spaces” in porous coordination polymers to increase the adsorption of H2. Ab initio calculations were performed to investigate the H2-adsorption capabilities of organic ligands. The adsorption energy of a pyrazine-carboxylate ligand (2.4 kcal/mol), relevant to a coordination polymer with a pillared layer structure, was found to be larger than that obtained by van der Waals interactions alone. An ab initio energy decomposition analysis showed that the stabilization arose mainly from the electronic polarization of H2. We therefore propose a concept of “charge-polarized spaces” to promote polarization. Under the dipolar electric field created by ideal chargepolarized spaces, the adsorption energy markedly increased to 5-8 kcal/mol, which is in the ideal energy range for H2 adsorption in storage materials. Charge-neutral ligands can also provide electrostatically polarized spaces. Although the binding energy decreases to around 2 kcal/mol, the H2 polarization effect still increases the adsorption energy. The charge-polarized and electrostatically polarized spaces are therefore an important direction for molecular design of H2 storage materials.
Introduction Establishing H2 storage materials is one of the key steps for realizing a H2-based energy system.1 To design a material capable of reversible H2 adsorption/desorption at reasonable pressures and temperatures, the interaction between H2 and the binding site of the materials should be appropriately tuned to provide around 5-10 kcal/mol of stabilization.2,3 van der Waals interactions are the driving force for H2 adsorption in carbon-based materials.4 However, these adsorption energies have been calculated to be around a few kJ/mol,4,5 which is too weak for stable adsorption at ambient temperatures. On the other hand, chemical-bond formations result in dissociative adsorption, as seen in hydride complexes and metal clusters.2 Another class of materials that has emerged in the past decade is porous coordination polymers (PCPs),6-8 which have gas-adsorption capabilities due to their microporous structures (coordination spaces).6 Some PCPs have already * Corresponding author. E-mail:
[email protected] (J. H.). † Kyoto University. ‡ RIKEN/SPring-8.
(1) Schlapbach, L.; Zuettel, A. Nature (London) 2001, 414, 353–359. (2) Fichtner, M. AdV. Eng. Mater. 2005, 7, 443–455. (3) Lochan, R. C.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2006, 8, 1357–1370. (4) Patchkovskii, S.; Tse, J. S.; Yurchenko, S. N.; Zhechkov, L.; Heine, T.; Seifert, G. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10439–10444. (5) Hu¨lbner, O.; Glo¨ss, A.; Fichtner, M.; Klopper, W. J. Phys. Chem. A 2004, 108, 3019–3023. (6) Kondo, M.; Okubo, T.; Asami, A.; Noro, S.; Yoshitomi, T.; Kitagawa, S.; Ishii, T.; Matsuzaka, H.; Seki, K. Angew. Chem., Int. Ed. 1999, 38, 140–143. (7) Eddaoudi, M.; Moler, D. B.; Li, H.; Chen, B.; Reineke, T. M.; O’Keeffe, M.; Yahgi, O. M. Acc. Chem. Res. 2001, 34, 319–330. (8) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem., Int. Ed. 2004, 43, 2334–2375.
shown H2 adsorption.9-11 Since PCPs are composed of metal-organic complexes, their interaction with H2 is controllable by choosing appropriate organic ligands. Recent theoretical studies have shown that an alkali-metaldoped organic linker enhanced the H2 adsorption energy in PCPs.12,13 Since side-on binding structures were reported in the previous reports,13 it is believed that H2 acts as an σ-donor to the vacant s orbital of the alkali metal. A comprehensive study was also carried out for the intermolecular interaction between H2 and metals, ligands, and complexes.3 However, the effects of electronic polarization of H2 were not considered in the adsorption process. We analyzed the potential energy for the adsorption of H2 in a coordination polymer with a pillared layer structure (CPL-1)6 and focused on the polarization contribution to the H2 adsorption energy. Recently, classical Monte Carlo simulations that included molecular polarizability in the force field were performed.14 In this paper, we performed ab initio theoretical calculations, including electron correlations, to analyze molecular interactions between H2 and the organic linkers used in CPL1. The molecular adsorption energy was decomposed into several physical terms using an ab initio energy decomposition analysis, which clearly indicated the significance of polarization. On the basis of these results, we propose (9) Kubota, Y.; Takata, M.; Matsuda, R.; Kitaura, R.; Kitagawa, S.; Kata, K.; Sakata, M.; Kobayashi, T. C. Angew. Chem., Int. Ed. 2005, 44, 920–923. (10) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe, M.; Yaghi, O. M. Science 2003, 300, 1127–1129. (11) Takamizawa, S.; Nakata, E. CrystEngComm 2005, 7, 476–479. (12) Han, S. S.; Goddard, W. A., III J. Am. Chem. Soc. 2007, 129, 8422– 8423. (13) Klontzas, E.; Mavrandonakis, A.; Tylianakis, E.; Froudakis, G. E. Nano Lett. 2008, 8, 1572–1576. (14) Belof, J. L.; Stern, A. C.; Eddaoudi, M.; Space, B. J. Am. Chem. Soc. 2007, 129, 15202–15210.
10.1021/cm802566z CCC: $40.75 2009 American Chemical Society Published on Web 04/17/2009
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Figure 1. Potential energy curves for H2 binding in (a) pzdm and (b) pzmc.
designing charge-polarized and electrostatically polarized coordination spaces in PCPs suitable for H2 storage. Computational Details To understand the H2-ligand interactions in CPL-1, we applied ab initio MP2/aug-cc-pVDZ level calculations with basis set superposition error (BSSE) corrections15 for obtaining adsorption structures and H2 binding energies. This level of computational detail is the requisite minimum for calculating reliable potential energy surfaces for H2 adsorption. To establish a stable computational procedure, some preliminary calculations were performed to evaluate the potential energy curves for H2 binding to dimethylpyrazine (pzdm) and pyrazine-monocarboxylate (pzmc). These resulting potential energies are shown in Figure 1 for various bond lengths, R. The rest of the coordinates were fixed to those in the X-ray structures.9 For newly introduced structures (for instance, when a hydrogen atom replaced a carboxylate group in pyrazine-dicarboxylate) the coordinates were optimized at the B3LYP/6-31G* level. To describe the dispersion contribution of the van der Waals interaction, the effect of electron correlation must be taken into account. We employed MP216 theory since this method offers moderate computational effort among the standard electroncorrelation approaches. Density functional theory17 with the B3LYP18,19 functional is not able to describe the dispersion forces,20 as will be shown later. (15) Boys, S.; Bernardi, F. Mol. Phys. 1970, 19, 553. (16) Møller, C.; Plesset, M. S. Phys. ReV. 1934, 46, 618. (17) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and Molecules; Oxford University Press: Oxford, 1989. (18) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (19) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (20) Krystyan, S.; Pulay, P. Chem. Phys. Lett. 1994, 229, 175.
Hasegawa et al. To choose a basis set, Bas I, II, and III sets were numerically tested. In set I, an aug-cc-pVTZ basis set was used for the H2, methyl group in pzdm, and carboxylate in pzmc. The rest of the atoms were treated with a cc-pVDZ basis set. In set II, the augcc-pVTZ basis set of Bas I was replaced with an aug-cc-pVDZ set. In set III, this was further changed to a daug-cc-pVDZ set. To correct for the BSSE on the adsorption energies, counterpoise corrections (CP)15 were performed. The results of the test calculations are shown in Figure 1. It was found that the B3LYP calculations failed to describe the very shallow potential minimum in pzdm-H2. Furthermore, without BSSE corrections by the CP method, the H2 binding energies were significantly overestimated. Similar potential energy curves were obtained by the MP2/Bas I/CP, MP2/Bas II/CP, and MP2/Bas III/CP methods. Since the MP2/Bas II/CP level of theory is the requisite minimum for calculating H2 binding energies, the Bas II basis set was chosen for the electronic structure calculations. BSSE corrections by the CP method were performed at the structuraloptimization level. To calculate the H2 adsorption energy, geometry optimization calculations were performed for the ligand-H2 and the ligand-H2-countercation complexes. The atomic coordinates of the ligands and countercation were fixed during the optimization. The structures of the ligands were optimized as isolated systems at the B3LYP/6-31G* level. Countercations were placed 6 Å away from the carboxylate O and sulfonate O atoms in the ligands. The reason for the distance (6 Å) is as follows. In the ligand-H2 systems without countercations, the O-H2 and H-H distances were found to be 2.3-2.6 Å and 0.8 Å, respectively. In addition, the Na-H distance in the Na+-H2, end-on complex was calculated to be 2.6 Å at the MP2/bas II/CP level. Assuming an end-on binding configuration in the ligand-H2-countercation complexes, the sum of the O-H, H-H, and H-Na distances was found to be 5.7-6.0 Å. Another reason is in the carboxylate groups in the coordination space inside CPL-1. These carboxylate groups are in the face-toface configuration, and the O-O distance is around 8 Å. In a pzmc-Na+ complex, the O-Na+ distance was calculated to be around 2 Å. If Na+ was introduced into CPL-1, the H2 would bind in the 6 Å space between the O atom and the Na+ ion. We also evaluated the toluene-H2-toluene system (ph-ph) for comparison.4 The structure of an isolated toluene molecule was optimized at the B3LYP/6-31G* level. The two toluenes were placed such that their molecular planes were parallel. The distance between planes was 6 Å since this separation approximately maximizes the van der Waals interaction, according to the previous study.4 The H2 molecule was added and its coordinates were optimized, while the toluenes were frozen. Frequency analysis was not performed. Since the binding structures are simple, the potential energy curves are similar in shape to those shown in Figure 1. We confirmed that the energy decreased during the optimization processes. The physical forces underlying H2 binding are of fundamental importance, not only for understanding the nature of the interaction but also for molecular design of the binding site. Energydecomposition analysis21 is a very useful tool for analyzing the molecular interaction energies. We used a modified variant21 of the reduced variational space (RVS) method.22 The dispersion-energy contribution was defined as the difference between the MP2 and HF binding energies. The RVS analysis was used to decompose the HF interaction energy into electrostatic, polarization, exchange-repulsion, charge-transfer, and higher -order effects.22 We briefly outline this analysis for a demonstrative case (21) Morokuma, K. J. Chem. Phys. 1971, 35, 1236–1244. (22) Stevens, W. J.; Fink, W. H. Chem. Phys. Lett. 1987, 139, 15–22.
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Figure 3. H2 binding-energy decomposition analysis. The dispersion-energy contribution was defined as the difference between the MP2 and HF energies. The HF adsorption energy was decomposed using the RVS analysis. 1 ) pzmc, 2 ) pzmc-Na+, 3 ) pzmc-Li+, 4 ) bs, 5 ) bs-Na+, 6 ) ph, and 7 ) ph-ph.
Figure 2. Structures and H2 binding energies of (a) pz with four different H2 orientations, (b) dmpz, (c) pzmc with and without counter cations, (d) benzenesulfonate (bs) with and without countercation, (e) phenyl group in toluene (ph), and (f) ph-ph sandwich system (ph-ph).
of two interacting molecules. First, the electrostatic and exchangerepulsion effects are calculated using the wave functions of the two noninteracting molecules. Second, the molecular orbitals are allowed to relax for the system of both molecules; however, intermolecular orbital mixing is not allowed. This treatment gives the polarization term. Third, the orbitals of the two molecules are allowed to mix, which indicates intermolecular electron transfer. This stabilization is termed as the charge-transfer effect. Higher order effects are included in the “Others” term in Figure 3. The BSSE correction and structural relaxation effects are minor terms that are included in the “Others” term. The Gaussian 0323 and GAMESS24 packages were used for the MP2 and RVS calculations, respectively.
Results and Discussion The CPL-1 with a pillared layer structure, ([Cu2(pzdc)2(pz)]n),6 contains pyrazine (pz) and pyrazine-dicarboxylate (pzdc), whose H2 storage has been structurally studied.9 Our first step was to investigate the H2 affinity of the pz,
dimethylpyrazine (dmpz), and pyrazine-monocarboxylate (pzmc) ligands. For the pz case, several possible H2 orientations were examined, as shown in Figure 2a. The H2 adsorption energies ranged from 0.19 to 0.66 kcal/mol, which indicates that the H2-pz interactions originate from the dispersion force. A similar energy of 0.27 kcal/mol was obtained for dmpz. These results are in accordance with those obtained at the MP2/TZVPP, CCSD(T)/TZVPP, and MP2-R12/aug-cc-pVQZ’ levels.5 The binding energy for H2 with the pzmc ligand was found to be 2.44 kcal/mol (Figure 2c), which is larger than that for the pz ligand. This result indicates that the H2-pzdc interaction would be the driving force for H2 storage in CPL1. The calculated H2-O distance of 2.30 Å is reasonably close to the 2.37 Å distance observed in the crystal structure.9 RVS calculations were carried out to analyze the physical origin of the adsorption. In Figure 3(1), the adsorption energy of the pzmc-H2 system was decomposed into dispersion, electrostatic, exchange-repulsion, polarization, chargetransfer, and other higher order contributions.21,22 The electrostatic and the polarization terms are attractive, while the exchange term is repulsive. The results from pzmc were compared with those from toluene (ph) in Figure 3(6). H2 binding to ph is dominated by the dispersion interaction. The electrostatic and exchange-repulsion effects cancel each other in both pzmc and ph. However, polarization of H2 yields an attractive contribution of ∼2 kcal/mol in pzmc, but not in ph. The H2 electron-density difference upon (23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R. J. A.; Montgomery, J.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J.; Ayala, P. Y.; Morokuma, K.; Voth, G.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03 (ReVision C.02); Gaussian, Inc.: Pittsburgh, PA, 2003. (24) Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem. 1993, 14, 1347–1363.
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Figure 5. H2 binding structures of (a) pzmc(0) (neutral), (b) pzdc(-1) (anionic), and (c) pzdc(0) (neutral) ligands with a Na+ counterion.
Figure 4. Electron density difference upon H2 binding to (a) pzmc and (b) pzmc with Na+ counterion. The calculations were performed at the MP2/aug-cc-pVDZ level.
binding to pzmc is shown in Figure 4a. Density slightly decreases in the H atom closer to the anionic O atom of the pzmc ligand and slightly increases in the other H atom. A Mulliken population analysis of the MP2 electron density yielded -0.076 and +0.054 charges for the H atoms farther and closer to the ligand, respectively. This polarization introduces an attractive, Coulombic interaction with the pzmc ligand. It is concluded that this polarization effect is the origin of the large adsorption energy in the pzmc ligand. The polarization of H2 should be enhanced if a counterion is added. As a very simple model system, a Na+ ion was placed 6 Å away from the pzmc O atom, and H2 was placed between them (see Figure 2c). The calculated adsorption energy was found to be 7.58 kcal/mol, which is about 3 times larger than the 2.44 kcal/mol that was found without the Na+ ion. This binding energy is much larger than that previously obtained for a graphene sandwich system, which was 3.11 kcal/mol.4 We tested a ph-ph sandwich system with 6.0 Å separation between the two rings and calculated an adsorption energy of 1.9 kcal/mol. The sandwich structure doubles the adsorption energy compared to the ph monomer case (1.0 kcal/mol), which is due to the additivity of the dispersion term as shown in Figure 3(7). We note that the effect of the charge-polarized space is more than additive since the electrostatic field enhances the H2 polarization, as seen in the pzmc-Na+ case. The H2 adsorption energy of Na+ in the end-on configuration was 0.43 kcal/mol at the same level of theory. According to the RVS analysis for pzmc-Na+ shown in Figure 3(2), the attractive components increased much more than the repulsive ones relative to pzmc. In particular, the polarization contribution significantly increases. An electrondensity analysis (Figure 4b) shows that the polarization of H2 is enhanced by Na+. Additionally, electron density at the
pzmc O atom slightly decreases and the density slightly increases around H2, which explains the increasing chargetransfer stabilization shown in Figure 3(2). The Mulliken population analysis also confirmed the charge transfer by yielding -0.921, -0.076, and +0.997 charges for the pzmc, H2, and Na+, respectively. It is clear that the charge-polarized space causes H2 to polarize and provides an efficient H2 binding site. Since these physical effects are general, the “chargepolarized coordination space (CPCS)” concept is applicable to other, similar systems. As an example, a substitution of the Na+ ion to Li+ resulted in a calculated adsorption energy of 7.89 kcal/mol, as shown in Figure 2c. The RVS analysis for the pzmc-Li+ system, shown in Figure 3(3), showed that the polarization term significantly increases, as the addition of an ion did in the pzmc-Na+ case. The H2 binding affinity of another organic acid, benzenesulfonate (bs), was examined. When the Na+ counterion was introduced, the adsorption energy increased by 3.78 to 5.24 kcal/mol, as shown in Figure 2d. The RVS analysis for the bs and bs-Na+ systems (Figure 3(4 and 5), respectively) clearly showed that the polarization term remarkably increases, as it did in the pzmc systems. In the above examples, CPCSs were considered for organic ligands in the anionic form. To investigate how the charge state of the ligands affects the enhancement in the adsorption energy, we examined the neutral form of pzmc (pzmc(0)), and pyrazine dicarboxylate, pzdc, in anionic and neutral forms, pzdc(-1) and pzdc(0), respectively, as shown in Figure 5. Atomic coordinates for the C, O, and N atoms in the pzmc and pzdc ligands were taken from the X-ray crystal structure.9 Positions of the H atoms in the ligands were optimized by B3LYP/6-31G* calculations. Na+ ions were placed 6 Å from the O atom. The H2 binding structures were obtained by the energy minimizations at the MP2/Bas II/CP level, during which the coordinates of the ligands and Na+ were fixed. As shown in Table 1, although the H2 binding energy decreased to 1.7-2.5 kcal/mol, the presence of Na+ still increases the binding energy by 1.3-1.9 kcal/mol in the neutral cases. This result indicates that the polarized coordination space (with counterion) has a higher H2 binding capability than monopolar coordination spaces (without counterion).
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Table 1. H2 Adsorption Energies (in kcal/mol) for pzmc, pzdc, and a Polarized Coordination Space (PCS) Model Eads (kcal/mol) ligands
protonation state
with Na+ without Na+ ∆a (kcal/mol)
pzmc(-1)
anion
7.58
2.44
5.14
pzmc(0)
neutral (O site)b neutral (OH site)b
2.45 1.74
0.56 0.39
1.89 1.35
pzdc(-1)
anion
5.68
2.28
3.40
pzdc(0)
neutral (O site)b neutral (OH site)b
2.18 1.66
0.49 0.37
1.69 1.29
0.86d
1.16
Na+
0.48
PCS model neutral
2.02c
a
Counterion effect on the adsorption energy. bs-Na+. d Without bs-Na+.
b
See Figure 5.
c
With
The calculated H2 adsorption energy for the model coordination space, shown in Figure 6, was 2.02 kcal/mol at the MP2/bas II/CP level, as summarized in Table 1. It was found that charge compensation reduced the adsorption energy relative to the charged complexes. However, an increase in the adsorption energy due to the countercation was noted here as well. Without the bs-Na+ complex, the stabilization energy decreases to 0.86 kcal/mol. The positive electrostatic potential of the bs-Na+ complex increased the adsorption energy by around 1.2 kcal/mol. Since the Na+ itself only contributes 0.4 kcal/mol (see Table 1), the effect of the bs-Na+ complex is more than additive. This is confirmed by the electron difference density upon H2 binding, plotted in Figure 6b, which shows polarization of the H2 electrons in response to the polarized electrostatic potential of the coordination space. These computational results suggest that molecular design of intensely polarized coordination spaces should be an important direction for realizing useful H2 storage materials. The results for these charge-polarized spaces indicate ideal examples. Conclusions
Figure 6. (a) Two-dimensional electrostatic potential (ESP) contour map (in Hartree units) of the electrostatically polarized coordination space model system. The plane is defined by the positions of the two O nuclei of the carboxylate group and the Na+. (b) Difference electron density distribution, defined as ∆F ) F(with H2) - F(without H2). Increases and decreases in the density are shown in violet and green, respectively.
In the above examples, the electrostatic potential in the CPCS was provided by bare, charged species. However, neutral complexes can also induce polarization of the electrostatic potential (ESP). Figure 6a shows a model system composed of the Cu(II) complex in CPL-1,6 [Cu(II)(pzmc)(ac)(aa)(pz)] (ac ) acetate, aa ) acetic acid), and a bs-Na+ complex in a Cd(II) complex.25 Both the Cu(II) model complex and bs-Na+ complex were charge neutral. The atomic coordinates of these two complexes were taken from X-ray crystallographic data.6,25 The two complexes were placed such that the Na+ of bs-Na+ was on the same plane as the carboxylate group and 6 Å away from the carboxylate O atom. The ESP was calculated from the electron density obtained by a HF/6-31G* calculation. The two-dimensional contour plot clearly shows that the ESP is polarized, and the Na+ extends the positive ESP region to the outer area. In contrast, a negative ESP region is available due to the O atoms in the Cu(II) model complex. (25) Horike, S.; Matsuda, R.; Tanaka, D.; Mizuno, M.; Endo, K.; Kitagawa, S. J. Am. Chem. Soc. 2006, 128, 4222–4223.
To improve the H2 storage capability of a binding site, polarization of H2 should be used to tune the adsorption energy, in addition to the dispersion interaction. This strategy can be realized by introducing charge-polarized and electrostatically polarized spaces at the porous H2 binding sites. Since the physical picture is very intuitive, the polarized space concept is generally applicable to designing H2 adsorption sites. In this study, ab initio calculations were performed to investigate the H2 adsorption capabilities of some organic ligands. Starting with the organic linker used in CPL-1, we found a large adsorption energy for the pyrazine-carboxylate ligand of 2.4 kcal/mol, which indicated stabilization due to larger forces than just van der Waals forces. The ab initio energy decomposition analysis showed that the stabilization arose mainly from electronic polarization of H2. Under the dipolar electric field created by the idealized, chargepolarized spaces, the adsorption energy markedly increased to 5-8 kcal/mol, which is the ideal energy range for molecular adsorption in H2 storage materials. Charge-neutral ligands can also provide an electrostatically polarized space. Although the binding energy decreases to around 2 kcal/mol, the polarization effect still increases the adsorption energy by around 1.2 kcal/mol. We therefore propose charge-polarized spaces, or electrostatically polarized spaces in the general sense, to promote the polarization of H2 and thus to enhance the adsorption capability of PCPs. Acknowledgment. This study was supported by a Grant-inAid for Scientific Research on the Priority Area “Chemistry of Coordination Space” from the MEXT, Japan. Support for Young Scientists was provided by the ACCMS and IIMC, Kyoto University. Some of the computations were performed at the RCCS in the IMS, Okazaki, Japan. CM802566Z
