Simulation of Water Cluster Assembly on a Graphite Surface - The

Water clusters adsorbed on polycyclic aromatic hydrocarbons: Energetics and conformational dynamics. Aude Simon , Fernand Spiegelman. The Journal of ...
0 downloads 0 Views 321KB Size
Download PDF
J. Phys. Chem. B 2005, 109, 14183-14188

14183

Simulation of Water Cluster Assembly on a Graphite Surface C. S. Lin,† R. Q. Zhang,*,† S. T. Lee,† M. Elstner,‡ Th. Frauenheim,‡ and L. J. Wan§ Center of Super-Diamond and AdVanced Films (COSDAF) and Department of Physics and Materials Science, City UniVersity of Hong Kong, Hong Kong SAR, China, Fachbereich Physik, Theoretische Physik, UniVersita¨t Paderborn, 33098 Paderborn, Germany, and Nano Science and Technology Center and Institute of Chemistry, Chinese Academy of Science, Beijing 100080, China ReceiVed: January 26, 2005; In Final Form: May 20, 2005

The assembly of small water clusters (H2O)n, n ) 1-6, on a graphite surface is studied using a density functional tight-binding method complemented with an empirical van der Waals force correction, with confirmation using second-order Møller-Plesset perturbation theory. It is shown that the optimized geometry of the water hexamer may change its original structure to an isoenergy one when interacting with a graphite surface in some specific orientation, while the smaller water cluster will maintain its cyclic or linear configurations (for the water dimer). The binding energy of water clusters interacting with graphite is dependent on the number of water molecules that form hydrogen bonds, but is independent of the water cluster size. These physically adsorbed water clusters show little change in their IR peak position and leave an almost perfect graphite surface.

1. Introduction The structures and energetics of small water clusters have been the subject of intensive study that has sought to model the properties of bulk liquid water that acts as a universal solvent.1-9 The interaction between the water cluster and ions has been one key focus of study. It has been revealed that the water molecules are nearly symmetrically arranged around the ions in solution and show strong ionic attraction.10-12 In contrast, the interaction behavior is complicated when water clusters are adsorbed on a solid surface such as a graphite sheet, due to the weak van der Waals force between them. Although graphite is hydrophobic except when there are activated carbon sites, such as carboxyl, hydroxyl, quinone, and aldehyde groups, etc.,13 the electronic binding energy of a water molecule with single-layer graphite is comparable to the binding energy of the hydrogenbonded water dimer.8 It is believed that the flexible structure of small water clusters would change their molecular geometries when interacting with a graphite surface. Several theoretical studies on water-graphite systems that focus on a single water molecule8,14,15 or on water layers16-18 have been reported. Recently, Hamad and co-workers19,20 modeled a small water cluster adsorbed on a soot surface with active COOH groups. To the best of our knowledge, there is only one theoretical study21 conducted on bare graphite adsorbed with a small water cluster, (H2O)n (n ) 2-6), by using a manybody polarizable potential model. Such studies require the inclusion of van der Waals forces in the adopted theory, which is missing in most of the currently available computational tools. However, the study of such interaction systems is scientifically important as it may provide insight into the assembly of * To whom correspondence should be addressed. E-mail: aprqz@ cityu.edu.hk. † City University of Hong Kong. ‡ Universita ¨ t Paderborn. § Chinese Academy of Science.

nanostructures on surfaces, thereby providing complementary information for experiments using, for example, a scanning tunneling microscope. In this work, we perform simulations using a density functional tight-binding (DFTB) approach to study water clusters (H2O)n (n ) 1-6) interacting with graphite surfaces, and evaluate their binding energies, geometry structures, and IR spectra. 2. Theoretical Methods and Models A water cluster adsorbed on a graphite surface is a prototypical weakly bound van der Waals (vdW) π-system that involves water-graphite and water-water interactions. To obtain qualitative information about the interaction between water molecules and a graphite surface, the effect of vdW interaction should be taken into account. The most widely used method for this purpose is the expensive second-order MøllerPlesset (MP2) perturbation theory. For the water-benzene system, with the complete-basis limit, the MP2-level binding energy is within 0.1 kcal/mol from the value predicted by coupled cluster theory with a perturbative estimate of connected triples, CCSD(T).22-24 Using the MP2 method and the fusedbenzene model, Feller and Jordan8 estimated the interaction energy between a water molecule and a single layer of graphite to be -5.8 ( 0.4 kcal/mol, which is larger than other reported values, which vary from -1.65 to -4.3 kcal/mol.25,26 The binding energy was estimated on the basis of a series of fragment models up to C96H24 in size with only one water molecule involved, as the computational cost for a larger system in the MP2 calculation would be too much to manage even for a moderately large basis set such as aug-cc-VDZ. Moreover, the recent work of Karapetian and Jordan21 using a many-body polarizable potential model, namely, the Dang-Chang (DC) potential model, predicted the binding energy of a water molecule and graphite to be -2.50 kcal/mol. Sanfelix and coworkers17 used DFT with the plane-wave basis set with ultrasoft pseudopotentials to evaluate the structure of water layers on

10.1021/jp050459l CCC: $30.25 © 2005 American Chemical Society Published on Web 07/06/2005

14184 J. Phys. Chem. B, Vol. 109, No. 29, 2005

Lin et al.

Figure 1. Fused-benzene ring model (fbz)m, m ) 1, 7, 19, and 37.

the close-packed surface of graphite under low and high coverage. The obtained minimum-energy structure showed that the water molecule was located at least 3.5 Å above the graphite surface. However, the vdW force was not taken explicitly into account. In this study, we use a computationally efficient approximation to density functional theory, the self-consistent charge density functional tight-binding (SCC-DFTB) scheme, complemented by the empirical London dispersion energy term (acronym DFTB-D), to study the energy and geometry structure of water clusters (H2O)n, n ) 1-6, adsorbed on a graphite surface. The DFTB-D method has unprecedented computational efficiency while giving results comparable to those of MP2. In this work, we investigate the detailed interactions between water clusters and a graphite surface using this less expensive computational approach. The SCC-DFTB model was derived from a second-order expansion of the DFT total energy functional with respect to the charge density fluctuations, and the Hamiltonian matrix elements are calculated with a two-center approximation, which are tabulated together with the overlap matrix elements with respect to the interatomic distance. A comprehensive description of the method can be found in the literature.27,28 To describe the vdW interaction between the two fragments, the water cluster and the graphite surface, an empirical dispersion term was added to the SCC-DFTB total energy (called the DFTB-D approach).29,30 The vdW interaction energy is defined as

Evdw ) -

f(RRβ) C6Rβ(RRβ)-6 ∑ Rβ

where f(R) is the damping function, and the C6 coefficient for a given atom a is calculated by

C6R ) 0.75(NRpR3)1/2 where NR is the Slater-Kirkwood effective number of electrons and pR is the polarizability of atom R. We use the same damping function f(R) and C6 parameters as those used in ref 29. To confirm the validity of the result obtained using DFTBD, the binding energies of (H2O)n (n ) 1-6) clusters and one water molecule adsorbed on centrosymmetric fused-benzene ring structures with increasing sizes, denoted as (fbz)m (m ) 1, 7, 19, 37) models (see Figure 1), the same as that used in ref 8, were calculated using the MP2 method, and compared with those predicted by the DFTB-D method and the results of the MP2 method and/or experimental data available in the literature. Then, the behavior of various small water clusters (H2O)n, n being up to 6, interacting with (fbz)m, m being up to 51, were investigated

Figure 2. Lowest energy structures of water clusters: (a) dimer, (b) trimer, (c) tetramer, and (d) pentamer.

TABLE 1: Geometry (RO-O, Å) and Binding Energy (EB, kcal/mol) of Water Cluster (H2O)n, n ) 2-5a hexamer dimer trimer tetramer pentamer (ring S6) RO-O DFTB-D MP2/aug-cc-pVDZ CCSD(T)/aug-cc-pVDZ EB/H-bond

2.868 2.920b 2.919c 3.399

a

2.789 2.799b 2.837d 3.279

2.763 2.743b 2.765e 4.557 b

2.764 2.743b

2.763

4.767

4.852 c

A hexamer (ring S6) is also included. Reference 31. Reference 32. d Reference 33, CCSD method. e Reference 34.

in detail with DFTB-D. In the DFTB-D calculation, the force converge criterion was set at 10-5 au. The conjugate gradient method was used in the geometric optimization. 3. Results and Discussion 3.1. Water Clusters. The global minimum of the water dimer is a linear Cs structure composed of one hydrogen bond (see Figure 2a). Larger water clusters (H2O)n, where n ) 3-5, adopt stable cyclic structures. The three water molecules in the cyclic trimer (see Figure 2b) are bonded together with three hydrogen bonds. Two free OH bonds point above (denoted as u) the plane defined by the oxygen atoms, and the remaining OH bond points down from this plane (denoted as d). We refer to the structure as uud. Similarly, the lowest energy structures of the tetramer and the pentamer are denoted as udud and ddudu, respectively. Table 1 lists the binding energies and geometric parameters of the lowest energy structures of the dimer, the trimer, the tetramer, and the pentamer. A special case of (H2O)6, the ring, is also included. It is found that the mean O-O distance of these water clusters predicted by DFTB-D is in good agreement with those obtained with the high-level methods such as MP231 and CCSD(T)32-34 with the basis set aug-cc-pVDZ. The O-O distance of the dimer predicted by DFTB-D is 2.868 Å, which is slightly shorter than the corresponding MP2 and CCSD(T) results. However, the distances of the trimer and the tetramer are comparable with those from these calculations. The O-O

Water Cluster Assembly on a Graphite Surface

J. Phys. Chem. B, Vol. 109, No. 29, 2005 14185

Figure 4. Structure of (a) benzene-water, (b) (fbz)7-H2O, top view, and (c) (fbz)7-H2O, side view.

TABLE 2: Adsorption Energya (AE, kcal/mol) and Oxygen-Graphite Surface Distance (Å) of (fbz)37-(H2O)n, n ) 1-6 n)1 Figure 3. Binding energies of ring, booklike, cage, prism, and bag structures of (H2O)6: circles, DFTB-D; triangles, MP2 of ref 36; crosses, MP2 result of ref 34 (shifted 15.0 kcal/mol up).

n)3

n)4

n)5

n)6 (ring)

AE -2.900 -5.152 -7.287 -9.916 -11.608 -13.723 ref 21 -2.50 -5.57 -6.70 -7.80 -9.66 -11.37 Roxygen-fbz 3.043 3.029 3.076 3.026 3.007 3.010 a

distance decreases in the results determined with these methods when the water cluster size increases from the dimer to the tetramer. The binding energy per hydrogen bond increases with increasing size of the water cluster except that of the trimer, which is slightly smaller than that of the dimer. Although the vibrational entropy is not included in the DFTB-D calculation, the binding energy of the dimer determined in our calculation is 3.399 kcal/mol, which agrees well with a previous experimental result (3.34 kcal/mol).35 However, the binding energy (4.98 kcal/mol) previously predicted by MP234 is quite a bit larger than the present values. The overestimation in the MP2 calculation is probably due to the lack of zero-point vibrational energy. It was found that the ring structure of the water hexamer is not the most stable configuration. The hexamer is at the crossover region between the cyclic structure and the stereo structures such as cage, booklike, prism, and bag (see Figure 3). The relative energy between these structures predicted by DFTB-D is quite comparable with that from an MP2/ HZ4P(2fg,2gd)++ calculation,36 which has been corrected with the basis set superposition error (BSSE) and the zero-point energy (ZPE). Another MP2 calculation34 at the complete limit set estimated but without ZPE correction gives the same trend as these structures. It was found that the energy difference is less than 2 kcal/mol. Both MP2 and DFTB-D predict that the cage and prism structures have larger binding energies. 3.2. Adsorption of a Single Water Molecule on a FusedBenzene Surface. The geometric structure of (fbz)1-H2O, i.e., one water molecule interacting with a benzene molecule, has been intensively studied using different methods.37-40 Using different basis sets, the binding energy of the benzene-water system predicted with the MP2 method is between 2 and 3 kcal/ mol. The result of the DFTB-D method gives 2.28 kcal/mol in this work, which is in good agreement with the experimental data, 2.25 ( 0.28 kcal/mol39 and 2.44 ( 0.09 kcal/mol.40 The equilibrium geometric structure is shown in Figure 4a. Both the experimental and theoretical results show that one of the hydrogen-oxygen bonds of the water molecule points to a carbon atom of the benzene ring, and the other hydrogenoxygen bond is nearly parallel to the benzene ring. The distance between the oxygen and the center of mass of the benzene ring predicted by DFTB-D is 3.257 Å, while the high-level theoretical calculations at both the CCSD(T)/aug-cc-pVDZ and MP2/ aug-cc-pVDZ levels of theory yield distances of 3.235 and 3.211

n)2

Adsorption energy ) total energy - energies of isolated fragments.

Figure 5. (fbz)37-(H2O)2: (a) side view and (b) top view.

Å.41 These results are in good agreement with the experimentally determined values of 3.347 ( 0.005 Å.42 In the case of (fbz)7-H2O, the optimized structure by the DFTB-D calculation shows that both the two H-O bonds point to a neighboring carbon atom of (fbz)7, as shown in Figure 2b,c. To verify the reliability of the DFTB-D result, we further performed MP2/aug-cc-pVDZ full geometry optimizations with the Gaussian-03 package.43 The so-determined equilibrium structure involves an O atom laid above (fbz)7 at a distance of 3.062 Å, almost the same as the DFTB-D result (3.057 Å). Moreover, both methods respectively predict the two hydrogen atoms pointing to (fbz)7. For (fbz)m-H2O with a larger m, the main character of the structure, i.e., both H-O bonds pointing down to (fbz)m, was not changed. The binding energy of water interacting with fbz will level off to a constant when the size of (fbz)m increases from (fbz)7 to (fbz)51. The binding energies of both (fbz)37-H2O and (fbz)51-H2O are 2.90 kcal/mol. Hence, in the following, only (fbz)37 is used to model a graphite surface since it is large enough to simulate a graphite sheet. 3.3. Adsorption of the Water Cluster on a Fused-Benzene Surface. As the water cluster size increases from a monomer to a hexamer, the binding energy between (H2O)n and the graphite surface increases monotonically (Table 2). The distance between one of the oxygen atoms of the water dimer and the graphite surface is 2.938 Å, and the other is 3.120 Å, showing that the linear dimer is nearly parallel to the graphite surface (Figure 5). The hydrogen bond length between the two water molecules is 2.858 Å, which is 0.01 Å shorter than the bond length of the isolated water dimer. Although in the case of a single water both OH bonds point to the graphite surface, the optimized structure of (fbz)m- (H2O)2 shows that only one OH bond points to the graphite surface. It should be mentioned that no matter

14186 J. Phys. Chem. B, Vol. 109, No. 29, 2005 what configuration of the initial structure is assumed, the final equilibrium structure always adopts the same configuration as shown in Figure 5, as long as the initial two water molecules are arranged with a distance of less than 6.0 Å. This result shows that the existence of a graphite plane does not affect the water dimer structure significantly. From the water dimer to the hexamer, a common character of these water clusters adsorbed on the graphite sheet is that the mean distance between the oxygen atoms in the nearest water and the graphite surface is in a narrow range, from 3.01 to 3.08 Å (Table 2), which is slightly shorter than the typical oxygen-benzene distance of 3.257 Å of the water-benzene system. The interaction of the trimer with the graphite sheet is similar to that of the dimer. The initial uud or udd structure which is placed parallel to the graphite surface will lead to exactly the same udd structure with two hydrogen atoms pointing to the graphite sheet and with the third hydrogen atom pointing away from the surface. In the optimization procedure, one of the hydrogen atoms pointing up in the uud gradually turns down, leading to a udd structure. If a higher energy ddd trimer in which all three hydrogen atoms point to the graphite surface is used as the initial structure, the final optimized geometry of ddd has a binding energy slightly lower than that of the udd structure by 0.114 kcal/mol. It is interesting to note that if the initial structure is placed with the oxygen plane of the trimer perpendicular to the graphite surface, the final result is the same as that with the parallel one. The cyclic structures of the tetramer and the pentamer also remain the same as their isolated configurations when they interact with the graphite surface, similar to those of the dimer and the trimer. The plane of the cyclic water tetramer and pentamer is nearly parallel to the graphite surface. Two hydrogen atoms of the water tetramer point down to the graphite, while three point down from the pentamer, no matter what is assumed about the starting orientation of the water cluster on the graphite surface. Except for the water monomer, the structures predicted by DFTB-D are in good agreement with those predicted by the DC potential model.21 The structures of the largest binding energy of the dimer, trimer, tetramer, and pentamer with graphite are the ones that use one, two, two, and three OH bonds, respectively, pointing to the graphite surface. However, the binding energy of DFTB-D is slightly larger than that of the DC potential model. The larger binding energy in the DFTB-D calculations may relate to its shorter predicted oxygen-graphite surface distance. While the minimum energy structures of (fbz)m- (H2O)n are not dependent on their initial structures for n ) 1-5, the water hexamer behaves differently on the graphite surface. The interaction between the water hexamer and the graphite is more complicated than with the smaller cluster due to the presence of its five energetically competitive cluster structures and the orientation between the cluster and the graphite sheet. For the cyclic-type hexamer with S6 symmetry, when the initial orientation of the oxygen plane is placed parallel to the graphite sheet, the optimized structure is similar to that of the lower energy cyclic water clusters with three hydrogen atoms pointing down to the graphite surface and the other three pointing up. However, when the initial oxygen plane of the hexamer is placed perpendicular to the graphite surface, one pair of oxygen atoms in the position opposite the hexagon will bond to each other, resulting in a hydrogen bond being presented and leading to a change of the cyclic hexamer to the “booklike” structure, as is observed during the geometric optimization. Compared to the

Lin et al.

Figure 6. Geometry structures of a booklike water hexamer (a) and that of different orientations (b, d) on the graphite surface, as well as the optimized adsorption structures (c, e, and f).

isolated configuration of the booklike structure, the adsorbed booklike structure is more planar and is nearly parallel to the graphite surface. In the booklike structure, there are two types of water molecules with respect to their bonding features. One is the water labeled ABDEF (Figure 6a), which uses one OH bond to form hydrogen bonds with its neighboring water, leaving the other OH bond free. The second is water C, which uses both its OH bonds to form hydrogen bonds with its neighboring waters A and D, as shown in Figure 6a. The free OH bonds of waters B and F point in one direction, while those of waters A, E, and D point in the opposite direction. There are three free OH bonds of the booklike edge BDF and two of ACE. If we assume the initial structure is booklike and that it stands perpendicularly on the graphite surface with the edge BDF on the lower side (Figure 6b), in the final structure (Figure 6c), the water cluster still maintains the main character of the booklike structure. But the OH bond orientation of water A is changed to the same direction as that of waters B and F, which are pointing to the graphite surface. Furthermore, the adsorbed booklike structure is more planar then the isolated state. The ∠BDF and ∠ACE angles of the isolated booklike structure are 118.7° and 124.8°, respectively, and they change to 158.0° and 166.7° upon adsorption on the graphite surface. If the initial structure is constructed with the edge ECA on the lower side (Figure 6d), the final structure (Figure 6e) shows that, except for waters A, D, and E with one of their OH bonds pointing to the graphite surface, water F that originally points upward also has its OH bond pointing down. The binding energy of the water cluster with this confirmation (12.033 kcal/mol) is

Water Cluster Assembly on a Graphite Surface

J. Phys. Chem. B, Vol. 109, No. 29, 2005 14187 TABLE 3: Adsorption Energya (AE, kcal/mol) of (fbz)37-(H2O)6 ring

book

cage

prism

bag

AE

-13.723 -14.143b

-9.265 -9.591

-11.37

-6.716 -7.890 -9.309 -9.548 -9.94

-10.662c -10.730

ref 21

-11.403 -11.825 -12.020 -12.033 -10.96

-7.49

a Adsorption energy ) total energy - energies of isolated fragments. The structure changes to a booklike one. c The structure changes to a prism.

b

Figure 7. Geometry structures of a prism water hexamer and a bag water hexamer adsorbed on the graphite surface.

slightly larger than the one in the previous case (see Figure 6c, 11.825 kcal/mol). If one of the short edges of the booklike structure, i.e., AB or EF, is set perpendicular to the surface in the initial structure, the final structure upon optimization shows that there are only two OH bonds pointing to the graphite surface, with the binding energy (11.403 kcal/mol) smaller than those of the above two cases. The other original orientation of the water cluster is set such that one of the planes such as ABDC is parallel to the graphite surface. It is shown that, during the optimization procedure, no OH orientation is changed, and the number of OH bonds pointing to the graphite surface of the final optimized structure is the same as that of the initial one. In other words, if the booklike structure is placed at the beginning facing down to the graphite surface with the OH bonds of water B and F pointing to the surface, the final structure will be equivalent to the one shown in Figure 6f. In contrast, if it is placed facing up to the surface with the OH bonds of waters A, E, and D pointing down, the final structure will still have these three OH bonds pointing to the surface with a binding energy of 12.020 kcal/ mol, which is larger than the previous one (Figure 6f, 11.599 kcal/mol). From the above description, it is clear that, during the adsorption procedure, the booklike structure will keep its backbone structure unchanged, although there are some changes to the OH bond orientation and the spine angle. However, as a special case, the ring structure will change its structure to a booklike structure. Our study on the adsorption behaviors of the most stable water hexamer structures, the cage and the prism, on a graphite surface shows that they also do not change their skeleton structures significantly during the optimization procedure. The binding energy of the cage on a graphite surface is nearly the same no matter what original orientation was assumed on the graphite surface. But for the lowest energy local structure of the prism adsorbed on a graphite surface, the binding energy will vary considerably from 6.792 kcal/mol (Figure 7a) to 9.811 kcal/ mol (Figure 7b), depending on the final orientation of the prism with reference to the graphite surface. The lowest binding energy

is found when there is only one OH bond pointing to the graphite surface, while the highest one has two OH bonds pointing to the graphite surface. Figure 7a is at the same level of binding energy as the water trimer with graphite. The prism structure standing on the graphite surface can be considered as being composed of two cyclic trimers: one adsorbed on the graphite surface and the other stacked on the first one. This is also true for Figure 7b, which has a tetramer cyclic water cluster above the graphite surface and which also has nearly the same binding energy as the tetramer. When the bottom of the bag structure is assumed to face the graphite surface like the starting structure (Figure 7c), water molecules A and C bond together via a hydrogen bond, as does the paired waters B and D, as observed during the geometric optimization. The final optimized structure turns out to be a prismlike structure (Figure 7b). The initial bag structure when adopting other orientations will lead to a structure more tightly bound with the graphite, as shown in Figure 7d. The planar part of the bag adsorbed on the surface has a similar pentamer ddpuu (p stands for in-plane) structure. Its binding energy is comparable to that of the pentamer, as it also has only five water molecules close to the graphite surface. The binding energy is mainly dependent on the number of water molecules which are close to the graphite surface (see Table 3), as was also revealed earlier using the DC potential model.21 In the above conformations of differently sized water clusters adsorbed on the graphite surface, no matter whether the water cluster sizes are large or small, the graphite sheet is always left perfectly unchanged. The binding energy is mainly dependent on the number of nearest water molecules, and is independent of the water cluster size. 3.4. IR Spectra of the Water Cluster on a Fused-Benzene Surface. Parts a and b of Figure 8 show the IR spectra of the (fbz)37-water system calculated using the DFTB-D method. We can see that all of the peaks in the IR spectra originate from the water cluster except the peak at 3043 cm-1, which is the (fbz)37 C-H stretching mode. The peaks lower than 1000 cm-1 are attributed to the water cluster’s interwater molecular vibration, such as the torsion, twist, bend, stretch, and wag modes. The mode near 1500 cm-1 is the water intramolecular H-O-H bend. The O-H stretching mode corresponds to a peak larger than 3100 cm-1. There is no significant peak shift in Figure 8a,b. However, the relative intensities of some peaks are changed at frequencies ranging from 3100 to 3900 cm-1. Parts c and d of Figure 8 show the IR spectra of the bagtype water hexamer and its prismlike structure (Figure 7b) when it is adsorbed on (fbz)37. In this case, both the peak position and the relative intensities were changed because the water cluster structure was changed. The 3275 cm-1 peak in Figure 8d corresponds to one of the prism border O-H stretching modes of the prism hexamer, similar to that in Figure 8a,b. However, such a peak is absent in the IR spectrum of the bagtype hexamer (Figure 8c). This subtle difference can be used

14188 J. Phys. Chem. B, Vol. 109, No. 29, 2005

Lin et al. References and Notes

Figure 8. Calculated IR spectra of a free water hexamer and its adsorbed cases on a graphite surface: (a) prism (H2O)6, (b) (fbz)37(H2O)6 prism a, (c) bag (H2O)6, and (d) (fbz)37-(H2O)6 prism b.

to distinguish the different water structures and goes some way to explaining the relevant experimental data. 4. Conclusion A comparative study of the use of the DFTB-D and MP2 methods on small water clusters (H2O)n, n ) 2-6, and (fbz)mH2O, reveals that the DFTB-D method is reasonably reliable and efficient. The calculations using DFTB-D reveal that the water dimer will maintain its original structure when adsorbed on a graphite surface. The cyclic trimer, tetramer, and pentamer also maintain their cyclic geometries when they interact with a graphite surface, regardless of their assumed starting orientations. For the hexamer, it is interesting that, with the influence of a graphite sheet, the highest energy of the local minimum S6 structure will change to that of the more stable booklike structure when its starting orientation is perpendicular to the graphite surface. With a special orientation, a bag structure will also change to a prism structure. However, most of the starting orientations relative to the graphite surface will keep the water clusters as their original skeleton structure but with a slight distortion. The graphite surface always keeps a perfect planar structure. The binding energy of the water cluster with a graphite surface is only dependent on the number of water molecules that are in the hydrogen bond length range, about 3.0 Å, but it is independent of the water cluster size. These physically adsorbed water clusters show little change in their IR peak position and leave an almost perfect graphite surface. Acknowledgment. The work described in this paper is supported by the Research Grants Council of the Hong Kong SAR (Project Nos. CityU 1138/02P and CityU 1/02C) and the Chinese Academy of Sciences, China.

(1) Reimers, J. R.; Watts, R. O. Chem. Phys. 1984, 85, 83. (2) Belford, D.; Campbell, E. S. J. Chem. Phys. 1987, 86, 7013. (3) Honegger, E.; Leutwyler, S. J. Chem. Phys. 1988, 88, 2582. (4) Knochenmuss, R.; Leutwyler, S. J. Chem. Phys. 1992, 96, 5233. (5) Xantheas, S. S.; Dunning, T. H., Jr. J. Chem. Phys. 1993, 98, 8037. (6) Pribble, R. N.; Zwier, T. S. Science 1994, 265, 75. (7) Gregory, J. K.; Clary, D. C.; Liu, K.; Brown, M. G.; Saykally, R. J. Science 1997, 275, 814. (8) Feller, D.; Jordan, K. D. J. Phys. Chem. A 2000, 104, 9971. (9) Nagy, G. J. Electroanal. Chem. 1996, 409, 19. (10) Curtiss, L. A.; Jurgens, R. J. Phys. Chem. 1990, 94, 5509. (11) Bauschlicher, C. W.; Langhoff, S. R., Jr.; Partridge, H. J. Chem. Phys. 1991, 94, 2068. (12) Bauschlicher, C. W.; Sodupe, M., Jr.; Partridge, H. J. Chem. Phys. 1992, 96, 4453. (13) Muller, E. A.; Rull, L. F.; Vega, L. F.; Gubbins, K. E. J. Phys. Chem. 1996, 100, 1189. (14) Ruuska, H.; Pakkanen, T. A. Carbon 2003, 41, 699. (15) Pertsin, A.; Grunze, M. J. Phys. Chem. B 2004, 108, 1357. (16) Werder, T.; Walther, J. H.; Jaffer, R. L.; Halicioglu, T.; Koumoutsakos, P. J. Phys. Chem. B 2003, 107, 1345. (17) Sanfelix, P. C.; Holloway, S.; Kolasinski, K. W.; Darling, G. R. Surf. Sci. 2003, 532, 166. (18) Semashko, O. V.; Brodskaya, E. N. Colloid J. 2004, 66, 71. (19) Hamad, S.; Mejias, J. A.; Lago, S.; Picaud, S.; Hoang, P. N. M. J. Phys. Chem. B 2004, 108, 5405. (20) Picaud, S.; Hoang, P. N. M.; Hamad, S.; Mejias, J. A.; Lago, S. J. Phys. Chem. B 2004, 108, 5410. (21) Karapetian, K.; Jordan, K. D. Properties of Water Clusters on a Graphite Sheet. In Water in Confined EnVironments; Devlin, J. P., Buch, V., Eds.; Springer: New York, 2003; p 139. (22) Purvis, G. D., III; Bartlett, R. J. J. Chem. Phys. 1982, 76, 1910. (23) Raghavachari, K.; Trucks, G. W.; Pople, J. A.; Head-Gordon, M. Chem. Phys. Lett. 1989, 157, 479. (24) Watts, J. D.; Gauss, J.; Bartlett, R. J. J. Chem. Phys. 1993, 98, 8718. (25) Vernov, A.; Steele, W. A. Langmuir 1992, 8, 155. (26) Gale, R. L.; Beebe, R. A. J. Phys. Chem. 1964, 68, 555. (27) Seifert, G.; Porezag, D.; Frauenheim, Th. Int. J. Quantum Chem. 1996, 58, 185. (28) Elstner, M.; Porezag, D.; Jungnickel, G.; Elsner, J.; Haugk, M.; Frauenheim, T.; Suhai, S.; Seifert, G. Phys. ReV. B 1998, 58, 7260. (29) Elstner, M.; Hobza, P.; Frauenheim, T.; Suhai, S.; Kaxiras, E. J. Chem. Phys. 2001, 114, 5149. (30) Elstner, M.; Frauenheim, T.; Suhai, S. J. Mol. Struct.: THEOCHEM 2003, 632, 29. (31) Xantheas, S. S.; Dunning, T. H., Jr. J. Chem. Phys. 1993, 99, 8774. (32) Halkier, A.; Koch, H.; Jørgensen, P.; Christiansen, O.; Nielsen, I. M. B.; Helgaker, T. Theor. Chem. Acc. 1997, 97, 150. (33) Nielsen, I. M. B.; Seidl, E. T.; Janssen, C. L. J. Chem. Phys. 1999, 110, 9435. (34) Xantheas, S. S.; Burnham, C. J.; Harrison, R. J. J. Chem. Phys. 2002, 116, 1493. (35) Curtiss, L. A.; Frurip, D. L.; Blander, M. J. Chem. Phys. 1979, 71, 2703. (36) Kim, J.; Kim, K. S. J. Chem. Phys. 1998, 109, 5886. (37) Kim, K. S.; Tarakeshwar, P.; Lee, J. Y. Chem. ReV. 2000, 100, 4145. (38) Tarakeshwar, P.; Choi, H. S.; Kim, K. S. J. Am. Chem. Soc. 2001, 123, 3323. (39) Cheng, B.-M.; Grover, J. R.; Walters, E. A. Chem. Phys. Lett. 1995, 232, 364. (40) Courty, A.; Mons, M.; Dimicoli, I.; Piuzzi, F.; Gaigeot, M.-P.; Brenner, V.; Pujo, P. d.; Millie´, P. J. Phys. Chem. A 1998, 102, 6590. (41) Feller, D. J. Phys. Chem. A 1999, 103, 7558. (42) Suzuki, S.; Green, P. G.; Bumgarner, R. E.; Dasgupta, S.; Goddard, W. A.; Blake, G. A. Science 1992, 257, 942. (43) Frisch, M. J.; et al. Gaussian 98; Gaussian, Inc.: Pittsburgh, PA, 1998.