J. Phys. Chem. C 2008, 112, 11779–11785
11779
Water Clusters Confined in Nonpolar Cavities by Ab Initio Calculations Lu Wang,† Jijun Zhao,*,† and Haiping Fang‡ State Key Laboratory of Materials Modification by Laser, Electron, and Ion Beams, School of Physics and Optoelectronic Technology and College of AdVanced Science and Technology, Dalian UniVersity of Technology, Dalian 116024, China and Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China ReceiVed: May 31, 2008
Encapsulation of (H2O)n clusters (n ) 1-22) in fullerene cages of different diameters (0.73-1.41 nm) has been investigated using gradient-corrected density functional theory. A linear relationship between cavity volume and maximum number of the encapsulated water molecules has been obtained. The interaction between water molecules and the fullerene wall was identified as physisorption with an adsorption energy of about 1.1 kcal/mol per molecule. The equilibrium configurations of small confined water clusters (n < 12) roughly resemble those of gas-phase clusters, whereas larger water clusters tend to adopt cage-like configurations when they are encapsulated in fullerene cages of sufficiently large diameter (i.e., >1.4 nm). The dipole moments of water clusters in the confined phase are smaller than those in the gas phase due to the screening effect of the outer fullerene cage. These results might shed some light on the behavior of water clusters confined in the nonpolar cavities of biological interests. 1. Introduction Water has been recognized as the matrix of life and plays a crucial role in many biological and chemical systems. In many of these systems, water is found to be confined in clefts of nanometer scale, for example, hydrophobic cavities in proteins such as crambin,1 Scarpharca dimeric hemoglobin,2 and human interleukin 1β.3,4 Inside these cavities, water molecules may form small clusters stabilized by intermolecular hydrogen bonding. In additional to the interior of those biological macromolecules, small water clusters have been observed in a variety of crystalline hosts5–10 such as organic molecular crystals and metal organic framework structures; and their spatial coordinates have been crystallographically determined. When water is confined in the nanoscale, it usually exhibits behavior different from either bulk water or water molecules/ clusters in gas phase.11–25 One of the motivations to study the behavior of water confined in nanoscale is the water transport across the channels in the cellular membrane that conduct water in and out of the cell (aquaporins).11–13 Because of the complexity of the biological system, simple pores modeled by carbon nanotubes are usually used to exploit the primary behavior, and molecular dynamics (MD) simulations have achieved great success in those studies.14–21 For example, the phase transition due to confinement, concerted hydrogen-bond orientation, and flipping and gating of water permeation across the narrow nanoscale channels have been observed. Compared to the comprehensive studies on water confined in polar cavities such as nanotubes, much less is known about the behavior of water inside nonpolar cavities. Vaitheeswaran et al.22 simulated the structure and thermodynamics of water clusters in idealized nonpolar cavities (modeled by carbon fullerenes) using the Monte Carlo (MC) method with an empirical force field. They found that the structures and thermodynamic properties of these encapsulated water clusters * Corresponding author. E-mail:
[email protected]. † Dalian University of Technology. ‡ Chinese Academy of Sciences.
are similar to those in the gas phase and that water filling is highly sensitive to the cavity size as well as the strength of water-wall interaction. Using the MD technique, the same Hummer-Rasaiah group investigated metastable water clusters in the nonpolar cavities of the thermostable protein tetrabrachion and suggested that the large hydrophobic cavities may act as binding sites for two proteases.23 Despite the great success of empirical MD/MC simulations, ab initio calculations are essential for exploring their behaviors directly related to quantum properties of the systems, such as the molecular orbitals and the dipole moments. For polar cavities, Li et al.24 performed ab initio calculations to study the behavior of a water chain confined in an armchair single-walled (6, 6) tube and found that the electrostatic field of water dipoles polarizes the nanotube and induces a charge distribution on the nanotube surface, resulting in a reduction of dipole moments of water clusters. So far, ab initio investigations on water clusters confined in nanoscale nonpolar cavities were only limited in the small endohedral complexes of (H2O)n@C60 (n ) 1-4).25 There are still many unclear issues to be addressed. For instance, how many water molecules can be exothermically encapsulated inside a nonpolar cavity of given volume, and how are the structure and properties of the water cluster influenced by the guest-host interaction? In this paper, we present the results of ab initio calculations on water clusters encapsulated in a series of fullerene cages of different diameters. Most importantly, we found the modification of lowest-energy configurations and the reduction of dipole moments of water clusters due to confinement, which might have some implications for the behavior of water clusters in the nonpolar cavities of biological macromolecules. 2. Methods and Benchmark Calculations All computations are performed using all-electron density functional theory (DFT) with a double numerical basis set plus polarization functions (DNP), as implemented in the DMol program.26 The generalized gradient approximation (GGA) with
10.1021/jp8048185 CCC: $40.75 2008 American Chemical Society Published on Web 07/10/2008
11780 J. Phys. Chem. C, Vol. 112, No. 31, 2008 PW91 functional27 was employed to describe the exchangecorrelation interaction. Self-consistent field calculations were done with a convergence criterion of 10-6 a.u. on the total energy. The real-space global orbital cutoff radius was chosen to be 5.0 Å. For the gas-phase molecular clusters, full geometry optimizations were performed without symmetry constraint. For the water clusters in the confined phase, the water molecules were put in a preoptimized carbon fullerene cage. During the geometry optimization, all atoms in water clusters are allowed to relax, whereas the fullerene cages were set to be rigid, similar to a previous MC simulation.22 The accuracy of the present computational scheme has been assessed by benchmark calculations on water monomer and dimer. From our calculations, the H2O molecule has a H-O bond length of 0.96 Å and a H-O-H angel of 103.4°, very close to the experimental values of 0.96 Å and 103.9°,28 respectively. The dipole moment of a water monomer is 1.99 debye at the present PW91/DNP level, compared to the experimental data of 1.86 debye.29 The computed vertical ionization energy of a H2O molecule is 12.81 eV, whereas the experimental value is 12.65 eV.30 From our calculations, the vibrational frequencies for a H2O molecule in equilibrium geometry are 1618, 3724, and 3840 cm-1, respectively, which agree well with the measured values of 1595, 3657, and 3756 cm-1,30 respectively. For the water dimer, the computed O-O bond length of 2.887 Å at the PW91/DNP level is comparable to the previous experimental value of 2.98 Å.31 The strength of the intermolecular hydrogen bond is 5.64 kcal/mol from our calculations, which falls within the range of experimental data of 5.0 ( 0.7 kcal/mol32 and is close to a previous result of 5.37 kcal/mol from high-level MP2/6-311++G(2d,2p) calculations.33 The dipole moment of water dimer from our calculation is 2.82 debye, which is close to the experimental value of 2.64 debye.29 The vertical ionization potential of a water dimer is computed to be 10.74 eV, compared to the experimental value of 11.21 eV.30 From the above results on the water monomer and dimer, we can conclude that our computational scheme with PW91 functional and all-electron DNP basis set is reasonably reliable for describing both intramolecular and intermolecular bonding in the water clusters. 3. Results and Discussions 3.1. Structures of (H2O)n (n ) 1-16) Clusters Encapsulated in a C180 Cage. Previously, the gas-phase water clusters (H2O)n up to n ) 20 have been intensively studied.33–37 Here, the initial structures of water clusters in the gas phase and the confined phase were taken from the most stable configurations and some important metastable structures reported in the literature.33–37 In addition, for each cluster size, we generated an initial configuration of confined cluster via randomly filling H2O molecules inside the fullerene cage. Upon relaxation, it is interesting to find that the randomly filled H2O molecules automatically form a hydrogen-bonded water cluster with welldefined structure, similar to the result in a recent MD simulation.23 We first concentrate on a fullerene cage with 180 carbon atoms, corresponding to a diameter of ∼1.23 nm. Up to 16 water molecules were filled inside the C180 cage, and the lowest-energy configurations for the confined phase of water clusters were selected from a number of candidate structures (typically three to five isomers for each size), as shown in Figure 1. The Cartesian coordinates of all of the lowest-energy configurations (H2O)n@C180 (n ) 1-16) can be found in the Supporting
Wang et al. Information. For comparison, gas-phase (H2O)n clusters have been studied using the same computational scheme, and their lowest-energy configurations were determined. Encapsulation of water monomer and dimer in a C180 cage does not result in noticeable changes of their geometrical parameters. Inside the C180 cage, water monomer and dimer locate off-center, with the oxygen atom 2.78 and 2.27 Å away from the center of the cage, respectively. This is consistent with previous results on H2O@C60 by Sathyamurthy et al.25 For H2O@C180, the minimum C-H and C-O distance is about 3.2 Å and 3.5 Å, respectively; both are typical van der Waals (vdW) distances. Inside a C180 cage, (H2O)n clusters with n ) 3-5 adopt cyclic structures stabilized by the intermolecular hydrogen bonds. Each water molecule forms two hydrogen bonds with two neighboring water molecules, thus forming hydrogen-bonded rings. The present results of cyclic configurations for confined (H2O)n clusters (n ) 3-5) are consistent with previous observation from MC simulations.22 In previous experiments, it was found that the water tetramer inside crystals of a Fe3 cluster-containing compounds also adopts a quasi-planar cyclic structure7 with an average O-O distance of 2.768 Å, compared to the average O-O distance of 2.704 Å for (H2O)4@C180 from the present simulations. Meanwhile, pentagonal rings of water molecules with average O-O distance of 2.8 Å were observed in crystals of protein crambin.1 Inside the C180 cage, the (H2O)6 cluster signals a crossover from cyclic to cage-like configurations. Similar two-dimensional (2D) to three-dimensional (3D) transition was observed by Vaitheeswaran et al.22 Among the gas-phase clusters, the water hexamer has attracted substantial attention because the cyclic hexamer is the building block of many ice forms and because there are several nearly isoenergetic isomers, for example, ring (or cyclic), prism, and cage (or boat).36 From our PW91/DNP calculations, the 3D prism configuration is the most energetically preferred one for the gas-phase (H2O)6 cluster, whereas the 2D cyclic isomer lies only 0.06 kcal/mol above, and another 3D configuration of cage is 2.69 kcal/mol higher in energy. After confinement inside the C180 fullerene, the sequence for the relative energies of these isomers remains the same, but the energy difference changes to 0.94 kcal/mol (between prism and ring) and 1.17 kcal/mol (between prism and cage), respectively, implying a tendency of forming 3D structures. In the experimental observations of metal organic framework structures encapsulated with water hexamer, both ring8 and cage9 configurations have been identified. Within the size range of n ) 7-11, the most stable structures of (H2O)n clusters confined in the C180 cage are an edge-capped trigonal prism (n ) 7), a cube (n ) 8), an edge-capped cube (n ) 9), a pentagonal prism (n ) 10), and an edge-capped pentagonal prism (n ) 11), respectively. Previously, a water octamer, (H2O)8 with slightly distorted cube configuration and average O-O distance of 2.845 Å, was observed in the crystalline phase of [A2B](H2O)8 complex.10 Our present results on the confined-phase cubic (H2O)8 cluster with the average O-O distance of 2.754 Å agree with this experiment. For the (H2O)n (n ) 2-11) clusters discussed above, we found that the lowest-energy structures of the clusters in confined phase roughly resemble those of the gas phase,33–37 with small distortions due to water-wall interaction. For example, in the most stable structure of gas-phase (H2O)11 cluster, the angle between the plane formed by the two hydrogen bonds associated with the edge-capped water molecule and the upper pentagonal ring of the prism is about
Water Clusters Confined in Nonpolar Cavities
J. Phys. Chem. C, Vol. 112, No. 31, 2008 11781
Figure 1. Equilibrium structures of (H2O)n (n ) 1-16) clusters confined in a fullerene cage of C180.
110°, whereas it becomes ∼90° in the (H2O)11 cluster confined in a C180 cage. Comparing the average O-O distances in the gas-phase and confined (H2O)n clusters shows that the O-O distances of the confided (H2O)n are slightly elongated by 0.01-0.015 Å due to vdW attraction from the C180 cage for n ) 3-7, whereas the O-O distances are shortened by 0.01-0.04 Å for larger water clusters with n ) 8-11 because of the repulsion by the fullerene wall. Overall, the confinement effect due to guest-host interaction causes little changes on the structures of those hydrogenbonded water clusters, whereas the details of the structural change depend on the size and equilibrium configuration of the water clusters. Starting from a (H2O)12 cluster, the most stable configurations in the confined phase deviate from those in the gas phase. As shown in Figure 1, cage-like structures are energetically favorable for the (H2O)n clusters (n ) 12-16) confined in the cavity, whereas it was suggested that the gasphase water clusters prefer layered prism structures within the same size range.33,37 The formation of cage-like configurations instead of the layered ones in the medium-sized water clusters confined in nanoscale cavities can be attributed to
the limited confinement space and the water-wall interaction. As we will discuss below, encapsulation of (H2O)n clusters with n > 12 is exothermic; thus, the configuration of these endohedral complexes are not discussed in detail. The Cartesian coordinates for the first and second lowest-energy structures for the confined (H2O)n clusters (n ) 12-16) as well as the corresponding stabilization energies can be found in the Supporting Information. 3.2. Properties of (H2O)n (n ) 1-16) Clusters Encapsulated in a C180 Cage. In the study of water clusters, the stabilization energy (SE)25,33,34 was used to characterize the strength of intermolecular interaction that stabilizes the molecular clusters. For a gas-phase water cluster, one can calculate the stabilization energy via the following definition,
SE ) -(En - nE1) ⁄ n
(1)
where En is the total energy of the entire water cluster, E1 is the total energy of an individual water molecule, and n is the number of water molecules inside the cluster. According to the present definition, a positive denotes an exothermic process that is energetically favorable, whereas a negative means an endothermic process.
11782 J. Phys. Chem. C, Vol. 112, No. 31, 2008
Wang et al.
TABLE I: Stabilities and Dipole Moments of (H2O)n Clusters (n ) 1-16) in the Confined C180 Cage and in the Gas Phasea confined phase
gas phase
number of water molecules
SE (kcal/mol)
dipole moment (debye)
SE (kcal/mol)
dipole moment (debye)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
5.30 7.80 9.79 10.42 10.63 11.21 12.37 12.04 12.47 11.96 11.98 11.42 10.96 9.64 9.08
0.39 0.70 0.34 0.00 0.25 0.61 0.26 0.00 0.44 0.69 0.98 1.02 1.09 0.88 0.61 0.18
3.14 6.51 8.69 9.13 9.38 10.06 11.16 11.22 11.49 11.42 11.78 (11.76) 11.85 (11.73) 12.09 (12.02) 12.07 (11.86) 12.21
1.99 2.82 1.28 0.00 1.17 2.77 1.11 0.00 1.71 2.75 2.53 0.00 (3.87) 2.14 (2.38) 2.54 (3.35) 4.51 (2.59) 4.25
a Due to different lowest-energy structures for gas and confined phases for n g 12, in the parenthesis we present the gas-phase values of SE and dipole moments for the water isomers that are most stable in the confined phase.
Figure 2. Stabilization energy (SE) of water clusters in the gas phase and confined in the C180 cage.
Similar to the gas phase, SE for (H2O)n clusters confined in fullerene cages can be obtained using the following formula;
SE ) -(Ecomplex - Ecage - nEwater) ⁄ n
(2)
where Ecomplex is the total energy of the cluster-fullerene complex, Ecage is the total energy of the fullerene cage, Ewater is the total energy of an individual water molecule, and n is the number of water molecules. The computed stabilization energy of (H2O)n (n ) 1-16) clusters in both the gas phase and confined phase in the C180 cage are summarized in Table I and are plotted in Figure 2 as function of n. For both gas phase and confined phase, SE rises rapidly with n for the smallest clusters with n ) 2-8, and then the variation of SE becomes smaller between n ) 8 and n ) 12. As the cluster size further increases (n > 12), the SE for confined water clusters dramatically drops due to the stronger repulsion by the cage wall, whereas that for the gas phase keeps rising slowly. Comparing the gas phase and confined phase, we find that SE for confined phase is larger than that of gas phase for n e 12, and the SE of the confined cluster becomes less than the gas-phase data of the same size after the crossing point at n ) 12. These results clearly indicate that a C180 cage with the diameter of 1.23 nm can be filled with up to twelve water molecules exothermically. Moreover, the difference between the SE for water clusters in the confined phase and in the gas phases may reflect the strength of molecule-wall interaction. As shown in Table I and
Figure 2, within n ) 3-10 the SE difference between confined and gas phase is about 1.1 kcal/mol per molecule, which is a typical strength for vdW interaction. Using the same PW91/ DNP scheme, we have obtained an interaction energy of 1.67 kcal/mol between a H2O molecule and a C54H18 model of graphene sheet, compared to a previous theoretical value of 2.39 kcal/mol at the MP2/6-31G(d) level38 and an experimental value for water-graphite interaction energy of 1.82 kcal/mol.39 For comparison, previous GGA calculations predicted an adsorption energy of about 0.8 kcal/mol for a H2O molecule on a single-walled (5, 5) carbon nanotube.40 It is known that GGA approximation within DFT is usually insufficient for describing the intermolecular vdW interaction. Thus, the true magnitude for the interaction energy between water molecules and a fullerene wall should be somehow larger than our GGA predictions. To further discuss the interaction between the outer fullerene cage and the encapsulated water clusters, we have examined the frontier molecular orbitals such as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the entire endohedral complex. From our calculation the HOMO-LUMO gap of a bare C180 cage is 1.465 eV, whereas encapsulation of the (H2O)n clusters (n ) 1-16) only leads to the small changes of the HOMO-LUMO gap of less than 0.05 eV, similar to previous theoretical result on
[email protected] The spatial distribution of the HOMO and LUMO of (H2O)12@C180 are presented in Figure 3. It is clear that the delocalized HOMO and LUMO distribute only on the carbon cage and that there is no coupling between the molecular orbitals from carbon fullerene and water molecules. Analysis of the deformation density of electrons (defined by the total electron density of the system subtracted by the density of the isolated atoms) also indicates that there is no density distribution in the region between water and the fullerene wall. Therefore, in the model systems we studied, the water-wall interaction is physisorption of vdW type, without forming any covalent bonds via electron sharing or coupling. We further investigated the electronic density of states (DOS) and found that the DOS of (H2O)12@C180 is basically a simple superposition of the DOS of the constituents, that is, (H2O)12 and C180 cage. This finding is very similar to a recent electronic band structures calculation on hybrid systems of an ice nanotube inside a carbon nanotube.41
Water Clusters Confined in Nonpolar Cavities
Figure 3. Plots of HOMO and LUMO for the (H2O)12@C180 complex.
Figure 4. Dipole moments of water clusters in the gas phase (squares) and confined phase (dots) in the C180 cage.
The water monomer is a polar molecule carrying a dipole moment of about 1.85 debye.29 In the gas phase, most water clusters still possess substantial dipole moments,29 whereas bare carbon fullerenes are nonpolar molecules with zero moment. It is known that many drugs and biological molecules carry certain dipole moments and that the changes of dipole moments may reflect the intermolecular interaction,42–45 biomolecular solvation,46,47 as well as the biochemical activities.48–50 Thus, it would be interesting to explore the effect of nanoscale nonpolar confinement on the dipole moment of the encapsulated water clusters. The calculated dipole moments for the gas-phase (H2O)n (n ) 1-16) clusters and the (H2O)n@C180 endohedral complex are compared in Table I and Figure 4. For small (H2O)n (n ) 1-6) clusters in the gas phase, the dipole moments from our calculations reproduce well the high-level quantum chemistry results by Gregory et al.29 When the water clusters are encapsulated inside the fullerene cage, the dipole moments of the entire systems are all smaller than the moment of clusters in the gas phase, except that the moments are zero for both gas phase and confined phase at n ) 4 and 8, which reflect the symmetry of cluster configurations. Analysis of the on-site charge of systems revealed that the polarized water cluster encapsulated in a carbon cage induces an inversed dipole moment on the outer fullerene. The cancelation of the permanent dipole moment on the water cluster and the induced moment on the carbon cage lead to a relatively smaller dipole moment with regard to the moment of the gas-phase cluster. Similar screening effect on the dipole moments of the confined water clusters was observed in H2O@C6025 as well as in water chains inside carbon nanotubes.24 3.3. Water Molecules Confined in Fullerene Cages of Different Diameters. We have further considered the confinement of water molecules in four fullerene cages (C60, C140, C180, and C240) of different diameters (0.73, 1.09, 1.23, and 1.41 nm).
J. Phys. Chem. C, Vol. 112, No. 31, 2008 11783 These cages are all quasi-spherical with high point-group symmetries of Ih or I. The smallest fullerene considered here, that is, C60 with a diameter of 0.73 nm, can only contain one water molecule with stabilization energy of 0.75 kcal/mol. Filling two water molecules inside C60 fullerene is energetically unfavorable, with the calculated SE of -28.65 kcal/mol, which is close to a previous MP2 result of -24.5 kcal/mol by Sathyamurthy and co-workers.25 Experimentally, an individual water molecule has been successfully encapsulated into an open-cage C60 derivative through a series of carefully designed chemical reactions.51 The diameter of C140 fullerene with I symmetry is 1.09 nm. Filling eight water molecules in the C140 cage forms a cubic structure with the stabilization energy of 10.75 kcal/mol, which is comparable to the SE (10.7 kcal/mol) of the water octamer in the gas phase. Indeed, the saturation number for water molecule storage in a C140 cage is eight. As discussed above, the cavity by C180 fullerene of 1.23 nm in diameter can contain up to twelve water molecules exothermically. For the larger cavity, we considered a C240 cage with Ih symmetry and diameter of 1.41 nm. From our calculations, the maximum size for exothermic encapsulation of water clusters is 21. For both the gas phase and the confined phase, the equilibrium configuration of a (H2O)21 cluster is an endohedral cage formed by one water molecule inside a (H2O)20 cage.37 The SE for the (H2O)21@C240 complex is 12.88 kcal/mol, whereas the SE of the gas-phase (H2O)21 cluster is 12.67 kcal/ mol. When the (H2O)22 cluster was encapsulated into a C240 cavity, the hydrogen atom on one water molecule forms an O-H covalent bond with the oxygen atom from another water molecule due to the narrow space, making the system no longer a coherent water cluster. The optimized structures for the endohedral complexes with maximum number of water molecules, that is, H2O@C60, (H2O)8@C140, (H2O)12@C180, and (H2O)21@C240, are displayed in Figure 5. We can now discuss the relation between the volume of cavities and the maximum number of water molecules that can be confined in the cavity exothermically. Assuming a spherical shape for the fullerene, the volume of the fullerene cages can be directly computed from their diameters. In Figure 6, we plot the cavity volume V (in unit of nm3) versus storage capacity x for encapsulating water molecules. It is interesting to find that the four points from H2O@C60, (H2O)8@C140, (H2O)12@C180, and (H2O)21@C240 fall in a linear relationship as follows:
V ) 0.161 + 0.064x
(3)
On the right of the above equation, the first term (0.161 nm3) corresponds to a minimum space due to vdW repulsion from the cavity wall, and the coefficient of the second term (0.064 nm3) denotes the space required for each water molecule. Therefore, eq 3 from our simulation may provide a rough estimation for the encapsulation of water molecules in nonpolar cavities of different diameters. It is noteworthy that the effective molecular volume of water in its liquid phase is only 0.030 nm3. The larger molecular volume for the encapsulated water molecules implies that water in the confined phase behaves differently from the liquid phase. 4. Conclusions To summarize, water clusters confined in the cavities of C60, C140, C180, and C240 fullerene cages with different diameters (0.73-1.41 nm) were computationally investigated with ab initio approaches. The relationship between the volume of cavities
11784 J. Phys. Chem. C, Vol. 112, No. 31, 2008
Wang et al. leads to a screening effect on the dipole moment of cluster. Therefore, the dipole moments of the water clusters inside fullerene cages are significantly smaller than those in the gas phase. The present results on the water clusters confined in nearly spherical cavities of nanoscale is helpful for achieving a comprehensive understanding on the behavior of water clusters in the nonpolar cavities and is of great interests to better characterizing water in a biologically relevant environment. To further understand water clusters confined in hydrophobic protein cavities, interaction between the (hydrophobic or hydrophilic) chemical groups inside the cavities and the confined water clusters have to be taken into account. The research on this direction is still under way. Acknowledgment. This work was supported by the NCET Program provided by the Ministry of Education of China (NCET06-0281), National Natural Science Foundation of China (10774019), and Ph. D. Programs Foundation of Education Ministry of China (20070141026). Supporting Information Available: Additional information cited within the text is provided. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 5. Equilibrium structures of water clusters confined in different fullerene cages from C60 to C240.
Figure 6. Relationship between the volume of fullerene cavities and the maximum number of water molecules that can be confined in the cavity exothermically.
and the maximum number of exothermically confined water molecules was discussed, and a linear relationship has been obtained. The structures and properties of water clusters containing up to 16 molecules in a C180 cage of 1.23 nm in diameter were discussed in detail. The smallest water clusters, such as monomer and dimer, sit off-center inside the C180 cage. A crossover from cyclic to cage-like configurations at (H2O)6 were observed. For smaller (H2O)n clusters with n e 11, the guest-host interaction only leads to little changes on the cluster geometries. For the medium-sized (H2O)n clusters (n > 12) confined in the C180 cage, the equilibrium geometries tend to adopt cage-like configurations instead of layered structures, even the latter ones are energetically favorable in the gas phase. The interaction between encapsulated water clusters and outer fullerene cages was identified to be of vdW type (about 1.1 kcal/mol per water molecule), which has nearly no effects on the electronic structures (characterized by the molecular orbitals) of both constituents. The outer cage is inversely polarized by the confined water clusters with permanent dipole moment and
(1) Teeter, M. M. Proc. Natl. Acad. Sci. U.S.A. 1984, 81, 6014. (2) Royer, W. E.; Pardanani, A.; Gibson, Q. H.; Peterson, E. S.; Friedman, A. J. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 14526. (3) Ernst, J. A.; Clubb, R. T.; Zhou, H.-X.; Gronenborn, A. M.; Clore, G. M. Science 1995, 267, 1813. (4) Yu, B.; Blaber, M.; Gronenborn, A. M.; Clore, G. M.; Caspar, D. L. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 103. (5) Infantes, L.; Motherwell, S. Cryst. Eng. Commun. 2002, 4, 454. (6) Supriya, S.; Das, S. K. J. Cluster Sci. 2003, 14, 337. (7) Supriya, S.; Manikumari, S.; Raghavaiah, P.; Das, S. K.; New, J. Chem. 2003, 27, 218. (8) (a) Moorthy, J. N.; Natarajan, R.; Venugopalan, P. Angew. Chem., Int. Ed. 2002, 41, 3417. (b) Mukhopadhyay, U.; Bernal, I. Cryst. Growth Design 2005, 5, 1687. (9) (a) Park, K. M.; Kuroda, R.; Iwamoto, T. Angew. Chem., Int. Ed. 1993, 32, 884. (b) Michaelides, A.; Skoulika, S.; Bakalbassis, E. G.; Mrozinski, J. Cryst. Growth Design 2003, 3, 487. (c) Ghosh, S. K.; Bharadwaj, P. K. Inorg. Chem. 2004, 43, 5180. (10) Blanton, W. B.; Gordon-Wylie, S. W.; Clark, G. R.; Jordan, K. D.; Wood, J. T.; Geiser, U.; Collins, T. J. J. Am. Chem. Soc. 1999, 121, 3551. (11) de Groot, B. L.; Grubmu¨ller, H. Science 2001, 294, 2353. (12) Tajkhorshid, E.; Nollert, P.; Jensen, M. O.; Miercke, L. J. W.; O’Connell, J.; Stroud, R. M.; Schulten, K. Science 2002, 296, 525. (13) Fujiyoshi, Y.; Mitsuoka, K.; de Groot, B. L.; Philippsen, A.; Grubmu¨ller, H.; Agre, P.; Engel, A. Curr. Opin. Struct. Biol 2002, 12, 509. (a) Liu, K.; Kozono, D.; Kato, Y.; Agre, P.; Hazama, A.; Yasui, M. Proc. Nat. Acad. Sci. 2005, 102, 2192. (b) Agre, P.; King, L. S.; Yasui, M.; Guggino, W. B.; Ottersen, O. P.; FujiyoshiY.; Engel, A.; Nielsen, S. J. Physiol. 2002, 542, 3. (14) Hummer, G.; Rasaiah, J. C.; Noworyta, J. P. Nature 2001, 414, 188. (a) Rasaiah, J. C.; Garde, S.; Hummer, G. Annu. ReV. Phys. Chem. 2008, 59, 713. (b) Waghe, A.; Rasaiaha, J. C.; Hummer, G. J. Chem. Phys. 2002, 102, 10789. (15) Mashl, R. J.; Joseph, S.; Aluru, N. R.; Jakobsson, E. Nano. Lett. 2003, 3, 589. (16) Wan, R. Z.; Li, J. Y.; Lu, H. J.; Fang, H. P. J. Am. Chem. Soc. 2005, 127, 7166. (17) Mann, D. J.; Halls, M. D. Phys. ReV. Lett. 2003, 90, 195503. (18) Dellago, C.; Naor, M. M.; Hummer, G. Phys. ReV. Lett. 2003, 90, 105902. (19) Li, J. Y.; Gong, X. J.; Lu, H. J.; Li, D.; Fang, H. P.; Zhou, R. H. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 3687. (20) Koga, K.; Gao, G. T.; Tanaka, H.; Zeng, X. C. Nature 2001, 412, 802. (21) Gong, X. J.; Li, J. Y.; Lu, H. J.; Wan, R. Z.; Li, J. C.; Hu, J.; Fang, H. P. Nature Nanotech. 2007, 2, 709. (22) Vaitheeswaran, S.; Yin, H.; Rasaiah, J. C.; Hummer, G. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 17002. (23) Yin, H.; Hummer, G.; Rasaiah, J. C. J. Am. Chem. Soc. 2007, 129, 7369.
Water Clusters Confined in Nonpolar Cavities (24) Li, Y.; Lu, D.; Schulten, K.; Ravaioli, U. J. Comput. Electron. 2005, 4, 161. (25) (a) Ramachandran, C. N.; Sathyamurthy, N. Chem. Phys. Lett. 2005, 410, 348. (b) Shameema, O.; Ramachandran, C. N.; Sathyamurthy, N. J. Phys. Chem. A 2006, 110, 2. (26) (a) Delley, B. J. Chem. Phys. 1990, 92, 508. (b) J. Chem. Phys. 2000, 113, 7756. (27) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671. (28) Johnson, B. G.; Gill, P. M. W.; Pople, J. A. J. Chem. Phys. 1993, 98, 5612. (29) Gregory, J. K.; Clary, D. C.; Liu, K.; Brown, M. G.; Saykally, R. J. Science 1997, 275, 814. (30) NIST Chemistry WebBook, http://webbook.nist.gov/chemistry/, and references herein. (31) Dyke, T. R.; Mack, K. M.; Muenter, J. S. J. Chem. Phys. 1976, 66, 498. (32) Gonzalez, B. S.; Hernandez-Rojas, J.; Breton, J.; Gomez Llorente, J. M. J. Phys. Chem. C 2007, 111, 14862. (33) Maheshwary, S.; Patel, N.; Sathyamurthy, N.; Kulkarni, A. D.; Gadre, S. R. J. Phys. Chem. A 2001, 105, 10525. (34) Wales, D. J.; Hodges, M. P. Chem. Phys. Lett. 1998, 286, 65. (35) Ludwig, R. Angew. Chem., Int. Ed. 2001, 40, 1808, and references therein. (36) Kabrede, H.; Hentschke, R. J. Phys. Chem. B 2003, 107, 3914. (37) James, T.; Wales, D. J.; Herna´ndez-Rojas, J. Chem. Phys. Lett. 2005, 415, 302.
J. Phys. Chem. C, Vol. 112, No. 31, 2008 11785 (38) Sudiarta, I. W.; Wallace Geldart, D. J. J. Phys. Chem. A 2006, 110, 10501. (39) Helmy, A. K.; Ferreiro, E. A.; Bussetti, S. G. de. Appl. Sur. Sci. 2007, 253, 4966. (40) Pati, R.; Zhang, Y.; Nayak, S. K.; Ajayan, P. M. Appl. Phys. Lett. 2002, 81, 2638. (41) Kurita, T.; Okada, S.; Oshiyama, A. Phys. ReV. B 2007, 75, 205424. (42) Schro¨der, C.; Rudas, T.; Boresch, S.; Steinhauser, O. J. Chem. Phys. 2006, 124, 234907. (43) Cordone, L.; Cottone, G.; Giuffrida, S. J. Phys.: Condens. Matter 2007, 19, 205110. (44) Takashima, S.; Yamaoka, K. Biophys. Chem. 1999, 80, 153. (45) Dekhtyar, M. L.; Ishchenko, A. A.; Rozenbaum, V. M. J. Phys. Chem. B 2006, 110, 20111. (46) Smith, P. E.; Pettitt, B. M. J. Phys. Chem. 1994, 98, 9700. (47) Boresch, S.; Ringhofer, S.; Ho¨chtl, P.; Steinhauser, O. Biophys. Chem. 1999, 78, 43. (48) Taskin, T.; Sevin, F. J. Mol. Struct. 2007, 803, 61. (49) Mershin, A.; Kolomenski, A. A.; Schuessler, H. A.; Nanopoulos, D. V. BioSystems 2004, 77, 73. (50) Kaminski, G. A.; Friesner, R. A.; Zhou, R. H. J. Comput. Chem. 2003, 24, 267. (51) Iwamatsu, S.; Uozaki, T.; Kobayashi, K.; Re, S.; Nagase, S.; Murata, S. J. Am. Chem. Soc. 2004, 126, 2668.
JP8048185
