Excess Electrons in LiAlH4

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J. Phys. Chem. C 2009, 113, 1104–1108

Excess Electrons in LiAlH4 Clusters: Implication for Hydrogen Storage Shihai Yan and Jin Yong Lee* Department of Chemistry, Institute of Basic Science, Sungkyunkwan UniVersity, Suwon 440-746, Korea ReceiVed: October 6, 2008; ReVised Manuscript ReceiVed: NoVember 23, 2008

The structures, frontier molecular orbitals, electron populations, stabilization energies, vertical detachment energies, and hydrogen interactions with nearby atoms of the anionic LiAlH4 (LAH) clusters and their hydrogenated species are investigated with density functional theory. For the hydrogenated anionic LAH dimers and trimers, there are two states, the dipole-bound state and the surface state. The importance of excess electrons (EE) for hydrogen storage is addressed because of the calculated results. The hydrogen storage capacity of the LAH clusters (diads and triads) is improved with the existence of EE as evidenced by the enhanced weight percent. The head-to-head trimers shrink with the addition of hydrogen atoms because of the strong coupling between the central hydrides and Li cations. Furthermore, the anionic LAH cluster size and the coupling mode between Li+ and the AlH4- fragment are important factors influencing the hydrogen storage capacity. Introduction Hydrogen has been targeted as the utopian fuel for transportation systems because hydrogen is potentially an ideal energy carrier and pollution free. One of the key challenges that must be overcome is hydrogen storage.1 A solid-state hydrogen storage system has the unique advantage of offering high volumetric densities and low parasitic losses compared with those of a compressed and cryogenic hydrogen storage system. Solid-state hydrogen storage is now pursued as a safe and effective way for a routine treatment of hydrogen. Many metals and alloys are capable of reversibly absorbing and desorbing large amounts of hydrogen that exist as atoms, not molecules, on the interstitial sites of the host metal lattice. Upon absorption, molecular hydrogen is dissociated at the surface; two H atoms recombine to H2 during the desorption process.2 An ideal solid hydrogen storage material for practical applications should satisfy the following qualifications: high storage capacity, low desorption temperature, reversibility, fast kinetics, low toxicity, possibly inert to oxygen and water, and low cost needed for automotive applications. However, no known material matches all of these requirements.3 The non-transition metal hydrides could be more efficient for hydrogen storage than the intermetallic hydrides because of the involvement of lightweight elements such as lithium, sodium, and aluminum.4 Ternary aluminum hydrides (MxAlyHz, M is an alkali or alkaline earth) as potential hydrogen storage materials with enhanced storage capacity (e.g., the theoretical hydrogen content of LiAlH4 and NaAlH4 is 10.6 and 7.5 wt %, respectively) have recently received considerable attention. Generally, these hydrides are comprised of M+ cations and AlH4- anions, where the negative charge is allocated to the tetrahedrally coordinated H atoms. These studies presented new insights into the metal-hydrogen bond mechanism in ternary metal hydrides and offered a possibility of synthesizing improved hydrogen storage systems for practical use.5 Lithium aluminum hydride (LAH) is technologically very attractive because it is a viable candidate for practical use as an on-board * Corresponding author. E-mail: [email protected]. Phone: +82-31-2994560. Fax: +82-31-290-7075.

hydrogen storage material with enhanced storage capacity.6 Usually LAH is deposited and sold as a solution in THF, and it exists in cluster forms. The measurements of clusters can provide detailed information that is difficult to obtain in bulk.7 The ionic nature of LAH offers the possibility for LAH clusters to host an excess electron because of Li+ terminals. The excess electron (EE) may bind on the cluster surface or reside in its interior as a solvated electron (SE). The SE may trap electron deficient H atoms and furnish a higher weight percent of hydrogen storage capacity (the targets of the U.S. Department of Energy are 6.5 wt % and 62 kg H2 m-3). Thus, the EE system of LAH clusters may be a novel candidate for hydrogen storage materials. Such anions can be produced by injecting electrons into high-density cluster vapor8 as well as by attachment of low-energy electrons to pre-existing clusters9 depending on the cluster size as extensively investigated for water clusters.10 Excess electrons are widely occurring and are commonly proposed for reactions that occur in water.11 The EE in a water hexamer is shared by all the participants with electropositive terminals. A similar characteristic may be observed in anionic LAH clusters. A novel type of dihydrogen bond has been observed recently with experimental techniques12 and theoretical approaches13 in various cluster systems for the existence of EE. A wide range of LixAly- clusters has been observed and studied with the anion photoelectron spectroscopy techniques,14 which indicates the ability of the LixAly cluster to combine with the EE and the stability of anionic LixAly- clusters. The EE also plays a crucial role in the understanding of phenomena associated with electron transfers, radial chemical reactions, fission reactors, and polarons. No investigation of the relationship between EE and hydrogen storage material design has occurred until now. Therefore, our investigation will shed light on the design of hydrogen storage materials and improve our understanding of EE. It is significant to explore characteristics such as structure, electron population, bonding, stability, and the storage capacity of the anionic LAH clusters as a prototype model.

10.1021/jp808841g CCC: $40.75  2009 American Chemical Society Published on Web 12/30/2008

Hydrogen Storage in Anionic LiAlH4 Clusters

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Computational Details The computationally inexpensive density functional theory (DFT) has been successfully employed on large free radicals and intermolecular complexes.15 Especially, the hybrid B3LYP16 method has proved its merits in dealing with valence-type molecular anions.17 All the geometry optimizations were carried out using the B3LYP exchange-correlation functional with the 6-311++G(d, p) basis set implemented in a suite of Gaussian 03 programs.18 The diffuse basis functions were employed with special emphases to describe the expected very diffuse state of the EE and allow the electron to spread from the system if the energy can be lowered. The absence of an imaginary frequency ensured that all the structures were in local minima on the potential energy surfaces. The zero-point vibrational energy (ZPE) was corrected for the relative thermodynamic stabilization energy (E0), which was obtained by subtracting the energy of monomers (neutral or negatively charged) from that of the anionic clusters. The hydrogen binding energy (EHB) refers to the energy difference before and after hydrogenation for anionic clusters. The ZPE is also included as a correction for EHB. Here, two successive steps are considered during the hydrogenation process. In each step, one hydrogen atom is stored in the anionic cluster. The EHBs are obtained according to eq 1. EHB ) E(LAH) - E(LAH) - mEH n+Hm n

(1)

The vertical detachment energy (VDE), a value that corresponds to the most intense Franck-Condon character in the electron detachment spectrum, is the energy required to detach an electron from the negatively charged cluster without structural distortion, reflecting the capacity of the cluster to integrate the electron. The VDEs are calculated on the basis of eq 2 VDE ) E(LAH)n+1 - E(LAH) n+1

(2)

where, E(LAH)n+1 denotes the energy of the neutral system bearing the optimized structure of the anionic cluster. A positive VDE indicates that the reaction is endothermic, going from an anionic cluster to a neutral system. A negative VDE means that the anionic system is metastable, and the reaction from an anionic cluster to a neutral system is exothermic. At first, the VDE of Al2H6- is calculated at the B3LYP/6-311++G(d, p) level to verify the reliability of the selected method. The result is 57.2 kcal/mol, which is in excellent agreement with the experimental value of 55.36 ( 3.46 kcal/mol.19 To reveal the electron population and the bonding feature of the anionic LAH clusters, the molecular orbital and the natural bond orbital (NBO)20 have been analyzed. The orbital interactions between the donor (Lewis type) orbitals and the acceptor (non-Lewis type) orbitals are estimated according to the secondorder perturbation theory. For each donor NBO orbital (i) and acceptor NBO orbital (j), the interaction energy E(2) associated with delocalization i f j is given by the formula

E(2) ) ∆Eij ) qi

F(i, j)2 εj - εi

(3)

where qi is the donor orbital occupancy and F(i, j) is the offdiagonal NBO Fock matrix element. εi and εj are diagonal elements (orbital energies). Results and Discussion Geometries. Before analysis of the clusters, it is necessary to recognize the features of the monomer. During the optimiza-

Figure 1. Optimized structures for neutral monomers, anionic dimers, and the (di)hydrogenated species.

tion, two stable isomers, M1 and M2 (Figure 1), were found for lithium aluminum hydride (LAH). In the neutral monomer M1/ M2, Li coordinates with three/two interstitial hydrides. The distance between Li and Al, RLi-Al, is significantly different in M1 and M2 (2.229 Å in M1, 2.463 Å in M2). These two structures satisfy the well-known Li+[AlH4-] feature. Upon electron attachment, the RLi-Al lengthens to 2.395 and 2.594 Å for M1 and M2, respectively. This is attributed to the fact that Li+ moves easily around the nearly spherical AlH4-.21 During the formation of D1 (Figure 1), the configuration reorganizes and the AlH4- groups change their coupling feature with the Li+ ions. The detailed process is represented in the Supporting Information. The Li+ ion still coordinates with three interstitial hydrides in D1. D2 is formed when two M2 monomers interact through the bridging EE with the terminal lithium ions. The distance between two lithium ions, RLi-Li, is 3.188 Å, which carries some features of the Li2- anion, where the bond length is 3.027 Å. Four interstitial hydrides are located in the plane perpendicular to that formed by the four terminal hydrides. The symmetry of D1 is destroyed upon trapping one hydrogen atom (D1+H), as is evidenced by its large dipole moment (µ ) 8.6 D). This isomer can hold more than one electron.22 In D2+H, the hydrogenated D2, one hydrogen atom is located in the center of the two Li+ ions, where the EE is located, and the hydrogen atom can have maximum attractions. The addition of this bridging hydrogen increases the RAl-Al from 8.342 to 8.547 Å (Table 1). The hydrogen storage capacity is improved from 10.6 wt % in D1/D2 to 11.8 wt % in D1+H/D2+H. Additional hydrogen storage is investigated for these two dimers. For D1+2H, the system expands slightly when the hydrogen atoms are trapped, as is evidenced by the increased RLi-Al (from 2.549 to 2.611 Å). D2 can store two hydrogen atoms like B2H6 and Al2H6 structures,19 considering the six hydrogen atoms around the two lithium atoms as seen in Figure 1. The two bridging hydrogen atoms between the two lithium ions are located in the same plane as the terminal hydrogen atoms. The distance between two bridging hydrogen atoms is 1.011 Å, which is larger than the H2 bond length of 0.744 Å. At this distance, the interaction energy is -88.1 kcal/mol at the CCSD(T)/6311++G** level. This phenomenon is in agreement with the assessment that H2 is dissociated on the surface, while two H atoms recombine in the desorption process.2 The Li · · · Li distance in dihydrogenated D2+2H increases further (3.454 Å) as compared to that in D2+H. The enhancement of the hydrogen storage capacity is observed for D1+2H/D2+2H (12.9 wt %) as compared with that of D1+H/D2+H (11.8 wt %). Three trimers, T1, T2, and T3, are optimized (Figure 2). T1 is a head-to-tail-linked cyclic trimer, while T2 and T3 are head-

1106 J. Phys. Chem. C, Vol. 113, No. 3, 2009

Yan and Lee

Figure 2. Optimized structures of anionic trimers and the corresponding (di)hydrogenated species.

to-head-linked ones. In these trimers, the monomers are arranged to be an equilateral (planar) triangular shape with the terminal Li atoms inside and the AlH4- groups outside. This arrangement is impossible for the neutral trimers. The distance from the Li+ ion to the center of the equilateral triangular shape (center of the EE), RLi-e, is 1.801, 1.885, and 1.877 Å in T1, T2, and T3, respectively (Table 1). The RLi-Al is shorter in T2 (2.322 Å) than in the other two. T1 and T2 give the same hydrogenated structure, T12+H, when one hydrogen atom is stored in their centers. The volume of T1 slightly increases during the hydrogen storage process. Upon hydrogenation, T2 is remarkably compressed, as noted by the reduced Al · · · Al distance (RAl-Al) from 7.268 to 5.552 Å for T2 f T12+H. For T12+2H, the interaction of the two stored hydrogen atoms should be very weak with their long distance (1.414 Å), although they are all stored in the center of the system. The system is further compressed for T3+H f T3+2H, as noted by the contraction of the Al · · · Al distances from 7.463 Å in T3+H to 6.281 Å in T3+2H. The distance of the two additional hydrogen atoms is 1.054 Å, which is much shorter than that of T12+2H. During the hydrogen storage process, the hydrogen storage capacity increases from 10.6 wt % in T2/T3 to 11.4 wt % in T12+H/T3+H and to 12.2 wt % in T12+2H/T3+2H. Here, the hydrogen multicenter bonds,23 proposed by Janotti and Van de Walle as a concept parallel with the purely and polar covalent hydrogen bond, intermolecular hydrogen bond, and three-center bond, are observed in (di)hydrogenated trimers. These bonds, ranging from 1.74 to 1.89 Å, must play an important role in the hydrogen storage process. NBO Analysis. The [AlH4-]Li+ feature is demonstrated from the NBO charges (Supporting Information) of M1 and M2. Upon electron attachment, the NBO charges located on Li change from 0.925 and 0.920 to 0.018 and -0.002 for M1 and M2, respectively, while the NBO charges populated on the interstitial hydrogen atoms become less negative from -0.423 and -0.489 to -0.399 and -0.454 for M1 and M2, respectively. Therefore, the EE is located predominantly on Li, and the coupling mode for anionic monomers is [AlH4-]Li. The interactions between the three Al-H (interstitial hydrides) bonds and the empty 2s orbital of Li, i.e., σAl-H f n*Li, in M1 (-7.25 kcal/mol) are stronger than that of the M1 anion (-3.70 kcal/mol). Similarly, the σAl-H f n*Li interactions are weakened from -12.55 kcal/ mol in M2 to -5.50 kcal/mol in the M2 anion, which may contribute to the RLi-Al bond lengthening (from 2.463 to 2.594 Å) upon electron attachment. The frontier molecular orbital analysis reveals that the excess electron is located on the empty 2s orbital of Li+.

Figure 3. HOMOs of (LAH)2-, (LAH)3-, and their mono- and dihydrogenated species.

The highest-occupied molecular orbitals (HOMOs) of the dimers, trimers, and their hydrogenated species are represented in Figure 3. The HOMO of D1 demonstrates that the dipolebound electron is distributed on the σLi-Li orbital symmetrically. A similar feature was also observed when the more diffuse basis functions of 6-311++G(3df, 3pd) were used in the calculations. The HOMO electrons of D1+H are populated on the σLi-H orbital, where the added hydrogen atom can be stored. The charges populated on Al and Li increase upon hydrogenation, especially for the dihydrogenated D1+2H. The charges of Al and Li are 0.632 and 0.495 in D1 and 0.704 and 0.781 in D1+2H, respectively. The most significant contribution to the stability of D1+2H is the interaction between the lone pair orbitals of the two trapped hydrogen atoms and the empty 2s orbitals of the two Li ions, i.e., nH f n*Li, which is -24.22 kcal/mol. As shown in the HOMO of D2, the EE is solvated by the two LAHs with their Li+ terminals. This orbital is composed of s-type σ orbitals of Li. The interaction energy between these two σLi orbitals is relatively weak, -16.13 kcal/mol. Thus, the EE must be helpful for the storage of the electron deficient hydrogen atom. The charges located on the Li ion increase continuously upon hydrogen storage from 0.458 in D2 to 0.826 in D2+H and then to 0.850 in D2+2H. In D2+H, the hydrogen stored in the middle of the two LAH moieties carries -0.798 electron and interacts with the two Li ions, and the electron deficient three-center two-electron (3c-2e) chemical bond24 is formed. Correspondingly, the four-center three-electron (4c-3e) bond is formed when two hydrogen atoms are stored in the middle of the two LAH moieties of D2. The interactions between the lone pair orbital of one stored hydrogen and the Rydberg antibonding orbital of the other stored hydrogen, nH f σ*H, give the largest contributions (-21.21 kcal/mol) to the stabilization energy. The EE plays a vital role in the stability of the trimer. Similar to the HOMO electron population of D2, D2+H, and D2+2H, the HOMO electrons of T1/T2, T12+H, and T12+2H also show the distribution of the excess electrons among the LAH fragments (Figure 3). The positive charges populated on Li cations increase from 0.672 and 0.617 in T1 and T2 to 0.837 in T12+H and then to 0.856 in T12+2H. For T12+H, the significant negative NBO charge (-0.786) of the additional hydrogen atom leads to the strong interaction with three positive Li cations. The stabilization energy mainly comes from the orbital interac-

Hydrogen Storage in Anionic LiAlH4 Clusters

J. Phys. Chem. C, Vol. 113, No. 3, 2009 1107

TABLE 1: Structural Parameters, Hydrogen Storage Capacity (wt %), Stabilization Energy (E0), Hydrogen Atom Binding Energy (EHB), and Vertical Detachment Energy (VDE) of Anionic LAH Clustersa dimer RLi-Al RAl-Al RLi-eb wt % E0 EHB VDE a

D1

D1+H

D1+2H

2.549 4.241

2.511 4.740

2.611 4.219

10.6 -31.2

11.8 -98.9 -67.7 93.6

12.9 -100.9 -66.6 140.2

8.5

trimer D2

D2+H

D2+2H

T1

T2

T12+H

T12 +2H

2.577 8.342 1.594 10.6 -29.2

2.590 8.547 1.684 11.8 -110.5 -81.2 191.0

2.547 8.544 1.727 12.9 -123.4 -94.1 70.4

2.563 5.483 1.801 10.6 -62.6

2.322 7.268 1.885 10.6 -42.0

44.1

88.4

2.591 5.552 1.739 11.4 -149.6 -113.7 130.8

2.619 5.644 1.753 12.2 -154.5 -118.6 90.0

63.5

T3

T3+H

T3+2H

2.525 7.631 1.877 10.6 -46.0

2.531 7.463 1.775 11.4 -135.2 -89.2 142.5

3.141 6.281 1.801 12.2 -159.9 -113.9 91.5

100.6

b

Distances are in Å, and energies (E0, EHB, VDE) are in kcal/mol. RLi-e denotes the distance from the center of the excess electron to Li.

tions between the lone pair of this hydride and the empty 2s orbitals of the lithium ions, i.e., nH f n*Li, which is -21.10 kcal/mol. Here, the four-center two-electron (4c-2e) hydrogen multicenter bond is generated by the central hydride and three peripheral Li cations. Two hydrogen atoms stored in T12+2H interact with each other through their s orbitals to form a novel σH-H bond, the energy of which is -119.0 kcal/mol. The σH-H f n*Li coupling produces an electron deficient five-center threeelectron (5c-3e) hydrogen multicenter bond. A similar tendency is also observed for T3. The charges on Li cations increase as hydrogen atoms are added, 0.608 for T3, 0.837 for T3+H, and 0.856 for T3+2H. The solvated electrons of T3+H are populated on the HOMO-9 orbital instead of the HOMO orbital like those in T12+H. The central hydride interacts with the Li cations, which provides the largest contribution to the stabilization energy by the nH f n*Li couplings (-20.3 kcal/mol). Two stored hydrogen atoms in T3+2H couple with each other through their s orbitals. Considering the distance (1.054 Å) between two hydrogen atoms, the binding energy was calculated to be -197.8 kcal/mol when all the other parts were removed except H2. However, because of the environmental effect of the anionic LAH cluster, the dihydrogen interaction may be much reduced. On the other hand, the stored dihydrogen has an interaction energy of -113.9 kcal/mol with the anionic LAH cluster. For the dihydrogen interactions in T3+2H, the largest contribution (-15.28 kcal/mol) is from the orbital interactions through their s orbitals. Here, the 5c-3e hydrogen multicenter bond is observed. Energies. The stabilization energies (E0s) are all represented in Table 1. For the anionic LAH dimers D1 and D2, the stability is significantly enhanced upon hydrogen storage. The most significant contribution comes from the interactions between the stored hydrogen and the Li cations as discussed above. The E0 of D2 is less than that of D1, while after hydrogenation, the E0s of D2+H and D2+2H are higher than those of D1+H and D1+2H. This indicates that the hydrogen storage capacity for the dipole-bound state (D2) is higher than that for the surface state (D1). The E0s of trimers (T1, T2, and T3) also increase significantly when the hydrogen atoms are stored. The headto-tail-linked cyclic isomer (T1) is more stable compared to the head-to-head coupling mode isomers (T2 and T3). Dihydrogenated species, T12+2H and T3+2H, possess higher E0s than the corresponding monohydrogenated isomers, T12+H and T3+H. These results suggest that the EE has a high capability for hydrogen storage. As a criterion of hydrogen storage capacity, the hydrogen binding energy (EHB) is calculated with the ZPE correction. From the EHBs collected in Table 1, some useful information can be drawn. First, the EHBs of D2+H and D2+2H are larger than those of D1+H and D1+2H, which implies that D2 has a higher hydrogen storage capacity than D1. Second, the larger EHB value

for the trimer demonstrates its superiority for hydrogen storage compared with that of the dimer. This result illustrates that the size effect is important for hydrogen storage capacity. Third, it is also indicated that the systems prefer to store two hydrogen atoms rather than trap only one. Furthermore, we also compared the hydrogen storage capacity of the anionic dimer and the neutral dimer. For the neutral dimer, we studied only the case corresponding to D1 and found that the related EHBs are -20.5 kcal/mol for one hydrogen atom and -67.3 kcal/mol for two hydrogen atoms. Although the EHB is a little larger than that of the anionic D1+2H, the additional hydrogen atoms combine with the original hydrogen atoms and depart from the framework (Supporting Information). This fragile feature indicates that it is difficult for the neutral dimer to store hydrogen steadily. Therefore, it is clear that the hydrogen storage capacity is enhanced upon electron attachment. Lastly, the large EHB values of T12+H and T12+2H in comparison with those of T3+H and T3+2H demonstrate the merits of T1/T2 in hydrogen storage. Table 1 also reports the vertical electron detachment energies (VDEs). The VDE of D1 indicates that the interaction of the LAH dimer with the surface state electron is weak, and the excess electron can be easily detached, while the binding energy of EE with the LAH dimer (D2) is stronger (63.5 kcal/mol). Therefore, the lifetime of the EE in the dipole-bound state is longer than that of the surface state. This table also demonstrates that the anionic LAH trimers exist more steadily than the dimer noted from the increased VDE value. Upon hydrogen storage, all the VDE values are greatly enhanced for both dimers and trimers. This indicates that the stability of both dimers and trimers is enhanced upon hydrogenation. Furthermore, the VDE of D1+2H is higher than that of D1+H, which implies a higher stability and stronger interactions of the additional hydrogen with the dipole-bound electron of D1+2H. The higher VDE value of D2+H demonstrates a higher stability of its 3c-2e bond compared to that of the 4c-3e bond in D2+2H, whereas the VDE values of the singly hydrogenated trimers are larger than those of the dihydrogenated ones, which indicates a higher stability of the electron deficient 4c-2e bond compared to that of the 5c-3e bond. Conclusions Anionic lithium aluminum hydride (LAH) clusters, dimer and trimer, are explored with a special emphasis on hydrogen storage capacity. The Li+ · · · e interaction is significant for anionic clusters with excess electrons (EE). Compared with anionic clusters bearing the surface state electron, anionic clusters bearing the dipole-bound state are more efficient in improving the hydrogen storage capacity of LAH clusters. When the hydrogen is stored, the system shrinks because of the large coupling between the central hydride and the Li cations. The

1108 J. Phys. Chem. C, Vol. 113, No. 3, 2009 higher stabilization energy, stronger hydrogen bonding energy, and larger vertical detachment energy demonstrate that the systems are stabilized upon hydrogen storage. The size influence (i.e., the LAH number in the cluster) on hydrogen storage capacity is significant, and the coupling mode of Li+ with the AlH4- group also has an influence on the hydrogen storage capacity. Although our discussions are based on the theoretical results for the anionic LAH clusters as a prototype model, a similar phenomenon should also be observed for other ternary metal hydrides by extrapolation. This study will be helpful in designing novel efficient storage materials with higher hydrogen capacity. Acknowledgment. This work was supported by Korea Science and Engineering Foundation (KOSEF) Grants R012008-000-10653-0 and R01-2007-012-03002-0 (2008) funded by the Korean Ministry of Education, Science, and Technology (MEST). Supporting Information Available: Details for the coordinates and NBO charge populations, the reorganization process of D1, and the optimized structures of the neutral dimer and its hydrogenated species. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Bogdanovic´, B.; Schwickardi, M. J. Alloys Compd. 1997, 253, 1. (b) Grochala, W.; Edwards, P. P. Chem. ReV. 2004, 104, 1283. (c) Lubitz, W.; Tumas, B. Chem. ReV. 2007, 107, 3900. (d) Orimo, S.; Nakamori, Y.; Eliseo, J. R.; Zu¨ttel, A.; Jensen, C. M. Chem. ReV. 2007, 107, 4111. (e) Struzhkin, V. V.; Militzer, B.; Mao, W. L.; Mao, H.; Hemley, R. J. Chem. ReV. 2007, 107, 4133. (f) Xiong, Z. T.; Yong, C. K.; Wu, G. T.; Chen, P.; Shaw, W.; Karkamkar, A.; Autrey, T.; Jones, M. O.; Johnson, S. R.; Edwards, P. P.; David, W. I. F. Nat. Mater. 2008, 7, 138. (g) Tarakeshwar, P.; Kumar, T. J. D.; Balakrishnan, N. J. Phys. Chem. A 2008, 112, 2846. (h) Kumar, T. J. D.; Tarakeshwar, P.; Balakrishnan, N. J. Chem. Phys. 2008, 128, 194714. (2) Schlapbach, L.; Zu¨tte, A. Nature 2001, 414, 353. (3) Wolverton, C.; Siegel, D. J.; Akbarzadeh, A. R.; Ozolinsˇ, V. J. Phys.: Condens. Matter 2008, 20, 064228. (4) (a) Sun, Q.; Jena, P.; Wang, Q.; Marquez, M. J. Am. Chem. Soc. 2006, 128, 9741. (b) Wu, H. J. Am. Chem. Soc. 2008, 130, 6615. (c) Balde´, C. P.; Hereijgers, P. C.; Bitter, J. H.; de Jong, K. P. J. Am. Chem. Soc. 2008, 130, 6761. (5) Olofsson-Ma˚rtensson, M.; Kritikos, M.; Dore´us, D. J. Am. Chem. Soc. 1999, 121, 10908. (6) (a) Ashby, E. C.; Dobbs, F. R.; Hopkins, H. P., Jr. J. Am. Chem. Soc. 1973, 95, 2823. (b) Chen, J.; Kuriyama, N.; Xu, Q.; Takeshita, H. T.; Sakai, T. J. Phys. Chem. B 2001, 105, 11214. (c) Balema, V. P.; Wiench, J. W.; Dennis, K. W.; Pruski, M.; Pecharsky, V. K. J. Alloys Compd. 2001, 329, 108. (d) Hauback, B. C.; Brinks, H. W.; Fjellva˚g, H. J. Alloys Compd. 2002, 346, 184. (e) Vajeeston, P.; Ravindran, P.; Vidya, R.; Fjellva˚g, H.; Kjekshus, A. Phys. ReV. B 2003, 68, 212101. (f) Kang, J. K.; Lee, J. Y.; Muller, R. P.; Goddard, W. A., III. J. Chem. Phys. 2004, 121, 10623. (g) Løvvik, O. M.; Opalka, S. M.; Brinks, H. W.; Hauback, B. C. Phys. ReV. B 2004, 69, 134117. (h) Resan, M.; Hampton, M. D.; Lomness, J. K.; Slattery, D. K. Int. J. Hydrogen Energy 2005, 30, 1413. (i) Labazan, I.; Krstulovic´, N.; Milosˇevic´, S. Chem. Phys. Lett. 2006, 428, 13. (j) Jun, W.; Armin, D.; Ebner, J.; Ritter, A. J. Am. Chem. Soc. 2006, 128, 5949. (k) Zheng, X.; Li, P.; Islam, S. H.; An, F.; Wang, G.; Qu, X. Int. J. Hydrogen Energy 2007, 32, 4957. (l) Senoh, H.; Kiyobayashi, T.; Kuriyama, N. Int. J. Hydrogen Energy 2008, 33, 3178. (7) Jordan, K. D. Science 2004, 306, 618. (8) Haberland, H.; Langosch, H.; Schindler, H. G.; Worsnop, D. R. J. Phys. Chem. 1984, 88, 3903.

Yan and Lee (9) Knapp, M.; Echt, O.; Kreisle, D.; Recknagel, E. J. Phys. Chem. 1987, 91, 2601. (10) (a) Wallqvist, A.; Martyna, G.; Berne, B. J. J. Phys. Chem. 1988, 92, 1721. (b) Barnett, R. N.; Landman, U.; Cleveland, C. L.; Jortner, J. J. Chem. Phys. 1988, 88, 4429. (c) Coe, J. V.; Lee, G. H.; Eaton, J. G.; Arnold, S. T.; Sarkas, H. W.; Bowen, K. H.; Ludewigt, C.; Haberland, H.; Worsnop, D. R. J. Chem. Phys. 1990, 92, 3980. (d) Makov, G.; Nitzan, A. J. Phys. Chem. 1994, 98, 3459. (e) Ayotte, P.; Johnson, M. A. J. Chem. Phys. 1997, 106, 811. (f) Silva, C.; Walhout, P. K.; Yokoyama, K.; Barbara, P. F. Phys. ReV. Lett. 1998, 80, 1086. (g) Bragg, A. E.; Verlet, J. R. R.; Kammrath, A.; Cheshnovsky, O.; Neumark, D. M. Science 2004, 306, 669. (h) Paik, D. H.; Lee, I.-R.; Yang, D.-S.; Baskin, J. S.; Zewail, A. H. Science 2004, 306, 672. (i) Hammer, N. I.; Shin, J.-W.; Headrick, J. M.; Diken, E. G.; Roscioli, J. R.; Weddle, G. H.; Johnson, M. A. Science 2004, 306, 675. (j) Verlet, J. R. R.; Bragg, A. E.; Kammrath, A.; Cheshnovsky, O.; Neumark, D. M. Science 2005, 307, 93. (k) Turi, L.; Wheu, W.-S.; Rossky, P. J. Science 2005, 309, 914. (l) Shkrob, I. A.; Glover, W. J.; Larsen, R. E.; Schwartz, B. J. J. Phys. Chem. A 2007, 111, 5232. (m) Zappa, F.; Denifl, S.; Ma¨hr, I.; Bacher, A.; Echt, O.; Ma¨rk, T. D.; Scheier, P. J. Am. Chem. Soc. 2008, 130, 5573. (11) (a) Migus, A.; Gauduel, Y.; Martin, J. L.; Antonetti, A. Phys. ReV. Lett. 1987, 58, 1559. (b) Rossky, P. J.; Schnitker, J. J. Phys. Chem. 1988, 92, 4277. (c) Long, F. H.; Lu, H.; Eisenthal, K. B. Phys. ReV. Lett. 1990, 64, 1469. (d) Schwartz, B. J.; Rossky, P. J. J. Chem. Phys. 1994, 101, 6902. (e) Lee, S.; Lee, S. J.; Lee, J. Y.; Kim, J.; Kim, K. S.; Park, I.; Cho, K.; Joannopoulos, J. D. Chem. Phys. Lett. 1996, 254, 128. (f) Kim, J.; Park, J. M.; Oh, K. S.; Lee, J. Y.; Lee, S.; Kim, K. S. J. Chem. Phys. 1997, 106, 10207. (g) Turi, L.; Borgis, D. J. Chem. Phys. 2002, 117, 6186. (h) Tauber, M. J.; Mathier, R. A. J. Am. Chem. Soc. 2003, 125, 1394. (i) Boero, M. J. Phys. Chem. A 2007, 111, 12248. (12) (a) Kevan, L. Acc. Chem. Res. 1981, 14, 138. (b) Gutowski, M.; Hall, C. S.; Adamowicz, L.; Hendricks, J. H.; de Clercq, H. L.; Lyapustina, S. A.; Nilles, J. M.; Xu, S.-J.; Bowen, K. H., Jr. Phys. ReV. Lett. 2002, 88, 143001. (13) (a) Kim, K. S.; Park, I.; Lee, S.; Cho, K.; Lee, J. Y.; Kim, J.; Joannopoulos, J. D. Phys. ReV. Lett. 1996, 76, 956. (b) Kim, K. S.; Lee, S.; Kim, J.; Lee, J. Y. J. Am. Chem. Soc. 1997, 119, 9329. (c) Anusiewicz, I.; Skurski, P.; Simons, J. J. Phys. Chem. A 2002, 106, 10636. (d) Hao, X.-Y.; Li, Z.-R.; Wu, D.; Wang, Y.; Li, Z.-S.; Sun, C.-C. J. Chem. Phys. 2003, 118, 83. (e) Yan, S.; Bu, Y.; Cukier, R. I. J. Chem. Phys. 2006, 124, 124314. (14) Kuznetsov, A. E.; Birch, K. A.; Boldyrev, A. I.; Zhai, H.-J.; Wang, L.-S. Science 2003, 300, 622. (15) (a) Adamo, C.; Barone, V. In Recent DeVelopment in Density Functional Methods; Chong, D. P., Ed.; Part II; World Scientific: Singapore, 1997. (b) O‘Malley, P. J. J. Am. Chem. Soc. 1998, 120, 11732. (c) Yan, S.; Bu, Y. J. Phys. Chem. B 2004, 108, 13874. (d) Yan, S.; Bu, Y. J. Chem. Phys. 2005, 122, 184324. (e) Yan, S.; Zhang, L.; Cukier, R. I.; Bu, Y. ChemPhysChem 2007, 8, 944. (16) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (b) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (c) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200. (d) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (17) (a) Gutsev, G. L.; Bartlett, R. J. J. Chem. Phys. 1996, 105, 8785. (b) O’Malley, P. J. J. Am. Chem. Soc. 1999, 121, 3185. (c) RienstraKiracofe, J. C.; Tschumper, G. S.; Schaefer, H. F., III; Nandi, S.; Ellison, G. B. Chem. ReV. 2002, 102, 231. (18) Frisch, M. J.; et al. Gaussian 03; Gaussian, Inc.: Wallingford, CT, 2004. (19) (a) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211. (b) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735. (c) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1998, 88, 899. (20) Rao, B. K.; Jena, P.; Burkart, S.; Gantefo¨r, G.; Seifert, G. Phys. ReV. Lett. 2001, 86, 692. (21) Gutsev, G. L.; Boldyrev, A. I. Chem. Phys. 1981, 56, 277. (22) Gutsev, G. L.; Jena, P.; Bartlett, R. J. Phys. ReV. A 1998, 58, 4972. (23) Janotti, A.; Van de Walle, C. G. Nat. Mater. 2007, 6, 44. (24) (a) Belderrain, T. R.; Paneque, M.; Carmona, E.; Gutie´rrez-Puebla, E.; Monge, M. A.; Ruiz-Valero, C. Inorg. Chem. 2002, 41, 425. (b) Tian, S. X. J. Phys. Chem. A 2005, 109, 4428. (c) Okulik, N. B.; Peruchena, N. M.; Jubert, A. H. J. Phys. Chem. A 2006, 110, 9974.

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