Comparative Theoretical Study of O- and S-Containing Hydrogen

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J. Phys. Chem. C 2008, 112, 17340–17350

Comparative Theoretical Study of O- and S-Containing Hydrogen-Bonded Supramolecular Structures N. Martsinovich* and L. Kantorovich Department of Physics, King’s College London, Strand, WC2R 2LS, U.K. ReceiVed: June 24, 2008; ReVised Manuscript ReceiVed: August 14, 2008

We report a computational density-functional theory (DFT) study of planar hydrogen-bonded supramolecular structures (dimers, chains, and networks) of cyanuric acid (CA) and trithiocyanuric acid (TTCA). We confirm that the electronegative atom (oxygen or sulfur) influences both the geometry and the strength of hydrogen bonds and therefore affects the order of stabilities of the two-dimensional networks of CA and TTCA. The N-H · · · O bonds are stronger and have the energy of ∼ -0.27 eV in a CA dimer, compared to ∼ -0.18 eV for N-H · · · S bonds. Our analysis, based entirely on available hydrogen bonding connections, predicts that six 2D network structures are possible both for CA and TTCA. Our DFT calculations show that all these network structures are stable for CA, however, only four out of the six networks are stable for TTCA. We compare our results with the available experimental data on the structure of self-assembled networks on some flat crystal surfaces and with X-ray data on the bulk crystal structure of these molecules. For CA, the calculated most stable network closely corresponds to the bulk crystal structure of CA, the two next most stable 2D structures have been observed as self-assembled networks on the Au(111) and graphite surfaces. For TTCA, however, only indirect comparison with the available bulk crystal data is possible, and we find that our most stable TTCA network does not match the layers in the observed crystal structure. 1. Introduction Hydrogen bonds (H-bonds) are ubiquitous and can be found in systems ranging from water to proteins and molecular crystals.1 Hydrogen-bonded two-dimensional (2D) self-assembled networks have attracted particular attention in recent years due to their potential use in building molecular-scale nanostructures.2-7 Most studies of hydrogen-bonded systems, both experimental and theoretical, concentrate on the systems involving electronegative oxygen (O) and nitrogen (N) atoms. However, weak H-bonds involving less electronegative atoms, such as sulfur (S), are also important for biological molecules8 and may be utilized to form extended supramolecular structures.9,10 Theoretical calculations showed that S-containing H-bonds are slightly less stable than O-containing bonds in analogous molecules.11-15 Theoretical studies suggested differences in the origin of S-containing and O-containing H-bonds: O-containing bonds are suggested to be mostly electrostatic, while Scontaining bonds involve dispersive charge-dipole and multipole interactions;11,14 different lone pairs of O and S were suggested to be involved in H-bonding,13 while charge transfer to the electronegative atom was shown to be important in both cases.13,15 Details of H-bond geometries and sometimes actual stable structures of H-bonded molecular pairs may differ from their oxygen counterparts.11-15 In this paper, we investigate the influence of the nature of the electronegative atom (O or S) on the energies and geometry of H-bonds and extend the previous studies, which were concerned mostly with H-bonded molecular dimers, to more complex 2D H-bonded structures. Specifically, we perform a comparative study of H-bonded structures formed by two molecules, cyanuric acid (CA), and trithiocyanuric acid (TTCA); see Figure 1. These two molecules are topologically identical * Corresponding author.

Figure 1. TTCA molecule (S is replaced with O in the CA molecule).

and differ only in that S in the TTCA molecule shown in Figure 1 is replaced with O in the CA molecule. Each molecule has three equivalent donor and three acceptor sites available for hydrogen bonding. The high symmetry of these molecules and their compact rigid shape make them a good simple model system to identify the effects of the electronegative atom on H-bonding. CA and its supramolecular structures have been a subject of many experimental studies.16-24 In particular, the observed CA-melamine network stabilized by triple H-bonds is an archetypal H-bonded network which has been studied experimentally.16,20-22,24 Self-assembled mixed networks of CA with other molecules have also been seen,17-19 as well as pure CA networks.21-24 Bulk CA molecular crystals also contain H-bonded layers of molecules, as known from X-ray and neutron diffraction measurements.25 There have been fewer theoretical studies concerned with structures formed by CA molecules: the CA-melamine network was modeled in refs 21, 22, and 24 while pure CA networks on graphite and Au(111) were considered in refs 21-23, and CA crystal structure was modeled in ref 26.

10.1021/jp805569f CCC: $40.75  2008 American Chemical Society Published on Web 10/10/2008

O- and S-Containing Supramolecular Structures

J. Phys. Chem. C, Vol. 112, No. 44, 2008 17341

The sulfur analogue, TTCA, has so far attracted fewer studies. We are not aware of any experimental reports of self-assembled TTCA networks, either pure or mixed. Bulk cocrystals of TTCA with a number of other molecules have been prepared and their structures established with X-ray diffraction,27-29 and onedimensional (1D) regular H-bonded structural motifs of TTCA have also been identified.28,29 Preparation of TTCA single crystals has been reported to be difficult due to its easy cocrystallization with solvents;28,30 however, powder diffraction experiments revealed the crystal structure of TTCA,30 which is different from that of CA crystals. As far as we are aware, the theoretical modeling of TTCA molecules has been concerned so far only with the thione-thiol tautomerism,31,32 and the structures of the anions and metal coordination compounds.33,31 Theoretical studies of H-bonded supramolecular structures of TTCA are still lacking. In this paper, we present a systematic comparative study of the H-bonded 2D structures that can be formed by CA and TTCA on flat crystal surfaces on which these molecules are sufficiently mobile. This is equivalent to considering surfaces for which the corrugation of the crystalline potential is much smaller than adsorption energies, as is the case for flat metal surfaces such as Au(111).34-36 Starting from dimers, we systematically construct all 1D chains and 2D networks with one and two molecules in the unit cell. Some of the possible structures with more molecules in the unit cells are also briefly discused. We find that both the geometry and the strength of H-bonds differ for the O and S analogues. Therefore, we find that the lowest-energy structures of CA and TTCA networks are not the same. We compare the results of our calculations with the available experimental and theoretical results on the structures of CA and TTCA surface networks and bulk structures and with reported theoretical studies of other H-bonded systems involving sulfur or oxygen. 2. Method Calculations were done using two density functional theory (DFT) codes, SIESTA37 and VASP.38,39 In both cases, the Perdew-Burke-Ernzerhof (PBE)40 generalized gradient approximation for exchange and correlation was used, which was found previously to be adequate in representing H-bonding.41 SIESTA uses localized numerical basis sets, specifically, double-ζ polarized (DZP) basis set was used in CA calculations and double-ζ (DZ) basis set in TTCA calculations. Basis set superposition error (BSSE) correction was applied to all binding energies obtained using SIESTA according to the Boys-Bernardi counterpoise method,42 which is also discussed in the context of H-bonded systems in ref 43. The BSSE-corrected energy ∆ECP(AB) of a system AB composed of components A and B is thus calculated as AB A ∆ECP(AB) ) [EAB (AB) - EAA(A) - EBB(B)] + [EAB (A) AB B AB (A) + EAB (B) - EAB (B)] EAB

(1)

where EZY(X) is the energy of system X at geometry Y with basis set Z. Thus, EAB AB(AB) is the total optimized energy of the system AB; EAA(A) and EBB(B) are the energies of isolated optimized components A and B with their own basis sets; EAAB(A), EBAB(B) AB AB and EAB (A), EAB (B) are the energies of components A, B in the geometries of the combined system with their own basis sets (superscripts A, B) or the combined basis set of AB. VASP uses plane waves, which eliminates the BSSE issue in this case. The periodic boundary conditions were used in all our calculations. In calculations of isolated molecules and dimers,

the cell size was chosen large enough to avoid interactions with images in neighboring cells. Geometry optimization was performed in VASP until forces on atoms were less than 0.01 eV/Å (0.02 eV/Å in SIESTA calculations of 2D networks). During geometry optimization with Z-matrix in SIESTA, the same convergence criterion of 0.01 eV/Å was used for the optimization of interatomic distances, while the criterion was 0.049 eV/rad for angles. In the calculations of extended periodic structures, i.e. molecular chains and networks, lattice vectors were optimized as well as the atomic positions. The weakness of DFT is that dispersion interactions are not included. This may affect the calculated energies of H-bonds. The error might be greater for S-containing systems due to a larger polarizability of S than O, and the H-bond energies involving S might be underestimated compared to the H-bonds involving O. However, we believe that the dispersion contribution is small compared to the electrostatic interaction which is well described by DFT. Earlier calculations of H-bonded systems showed that DFT, despite not including dispersion interaction, gives similar results to the more accurate MP2 calculations.13-15 DFT calculations reported in the literature mainly used the B3LYP functional;12-15 however, PBE0, a modification of the PBE functional, was shown to have a better agreement with MP2 results.14 Both functionals (PBE0 and B3LYP) include a certain amount of exact exchange. Typically, the H-bonding energies calculated with DFT were about 10% smaller than the MP2 calculated energies. Our DFT-PBE test calculations of the systems studied in ref 15 containing O-H · · · O or O-H · · · S bonds show a good agreement with their MP2 results (not more than 10% deviation). A comparison done in ref 41 also shows a good agrement between DFT-PBE and MP2 energies of H-bonded molecular pairs. Zero point energies (ZPE) were not calculated for H-bonded complexes due to such calculations being computationally expensive. This is a limitation of our study, since ZPE would modify the calculated H-bond energies. However, according to refs 14 and 15, ZPE is rather small, approximately 10% of the H-bond energy values. Electron density difference plots were used for a qualitative analysis of the strength of H-bonding. They were calculated by subtracting the electron densities of the isolated molecules from the electron density of the combined system, all in the optimized geometries of the combined system, in a similar way to ref 41. 3. Results 3.1. CA and TCA Dimers and Trimers. We begin the study of H-bonded CA and TTCA networks by considering homodimers of CA and TTCA molecules. Each of the molecules has three equivalent H-bond donor and three H-bond acceptor sites. Thus, we can use the approach similar to ref 41 and identify the following “binding sites”: (i) three donor sites, (ii) three acceptor sites, and (iii) six possible “double” donor-acceptor sites each composed of two adjacent donor and acceptor sites. Due to the symmetry of the molecules, all donor sites, all acceptor sites, and all double sites are equivalent. Two dimers can be constructed using these sites. Dimer 1 (Figure 2a) involves a donor site of one molecule and an acceptor site of another, thus it contains a single N-H · · · O or N-H · · · S intermolecular hydrogen bond. Note that the H-bond angle at the O (or S) atom is exactly φ ) 180° in this case; see Figure 2a. A second dimer is stabilized by two H-bonds between the neighboring donor and acceptor sites of one molecule and the acceptor and donor sites of the other molecule (dimer 2, Figure 2b). The H-bond angle in this dimer is close to 120°, as expected for sp2-hybridized O and S atoms.

17342 J. Phys. Chem. C, Vol. 112, No. 44, 2008

Martsinovich and Kantorovich

Figure 2. Two possible TTCA dimers (replace S with O for CA dimers): (a) dimer 1; (b) dimer 2. H-bonds are shown with dotted lines. Here, φ is the C-S · · · H (or C-O · · · H) angle, r1 and r2 are S · · · H (O · · · H) distances.

TABLE 1: Binding energies (eV), H-bond lengths (Å), and Angles (deg) of CA and TTCA Dimers (SIESTA and VASP Calculations)a dimer 1 CA binding energies: per dimer per H-bond X · · · H distance C-X · · · H angle X · · · H-N angle

dimer 2 TTCA

CA

TTCA

SIESTA

VASP

SIESTA

SIESTA

VASP

SIESTA

VASP

-0.22 -0.22 1.81 180 180

-0.24 -0.24 1.87 180 180

-0.01 -0.01 2.48 180 180

-0.55 -0.28 1.68; 1.68 126.6; 126.7 173.2; 173.2

-0.53 -0.27 1.77; 1.77 127.2; 127.2 172.5; 172.5

-0.39 -0.19 2.20; 2.21 113.8; 113.8 175.8; 176.0

-0.36 -0.18 2.28; 2.29 110.0; 110.0 171.7; 172.0

a X denotes either O or S atom. The CA and TTCA dimer 1 SIESTA results were obtained using constrained optimisation (Z-matrix) calculations.

The binding energies of the dimers calculated using SIESTA and VASP and the details of H-bond geometry are given in Table 1. Our DFT calculations show that dimer 2 is stable for both molecules, with the VASP calculated binding energies of -0.36 eV (-0.18 eV/H-bond) and -0.53 eV (-0.27 eV/Hbond) for TTCA and CA, respectively. An optimized structure of dimer 1 was obtained for CA with binding energy -0.24 eV. In the case of TTCA, geometry optimization resulted in dimer 1 transforming into dimer 2 by a rotation of both TTCA molecules. We will address the issue of stability of dimer 1 of CA and TTCA molecules later in this section. Table 1 also shows that O · · · H and S · · · H distances are of the order of 1.8 and 2.2 Å, respectively, and are longer than the typical O-H and S-H covalent bond lengths (0.97 and 1.35 Å in H2O and H2S, respectively44). The O · · · H distance in dimer 1 is ∼0.1 Å longer than in dimer 2, indicating a weaker bonding in dimer 1 in spite of the perfect linear arrangement of the donor and acceptor atoms. The S · · · H bonds are longer than O · · · H bonds in the corresponding dimer by 0.5 Å, in agreement with the earlier studies of H-bonding11-15 and with the larger radius of S. Notably, the C-S · · · H angles in dimer 2 of TTCA are smaller than the corresponding angles in dimer 2 of CA. The S(or O) · · · H-N angles are close or equal to 180° in all cases (the last line in the table). We point out that the data in Table 1 demonstrate that there is a good agreement between SIESTA and VASP geometries (bond lengths agree within 0.1 Å, angles within 4°) and very good agreement in energies. This is not surprising, since the

same exchange-correlation potential is used. The slight differences may be attributed to the fact that SIESTA geometries and electronic structures are affected by a limited localized basis set. The agreement is also good for 1D and 2D structures discussed in the next sections. Therefore, to save space, only VASP results will be quoted further in the paper, and SIESTA results will be mentioned only if there are differences from the VASP results. Electron densities of these dimers were also analyzed to obtain a qualitative picture of the changes in the charge distribution due to H-bonding; see Figure 3. The electron densities presented in this paper are obtained from VASP calculations and thus are not affected by BSSE. H-bonded structures are typically characterized by “kebab”-like patterns of electron density difference, with alternating regions of excess and depletion of electron density;41 the pronounced character and regularity of the kebab structure is a qualitative measure of the strength of the H-bonds. One can see that the electron density is drawn from the N and H atoms (green blobs indicating the electron density depletion) to the area between them as well as between H and O (or S); in addition, there is a clear excess of electron density around O (or S) atoms. The electron density difference plots of dimer 2 of CA and TTCA, Figure 3b and d, show a strong redistribution of electron density along the N-H · · · O and N-H · · · S bonds, characteristic of strong H-bonding. One can also see that the binding in the case of CA is stronger than for TTCA. In the case of dimer 1 of CA, there is a weaker electron density redistribution along the N-H · · · O bond,



Figure 4. (a) Binding energies and (b) O · · · H and S · · · H distances in CA and TTCA dimers for fixed values of C-O · · · H and C-S · · · H angles (Z-matrix calculations).

Figure 3. Electron density difference plot (VASP results) of (a) CA dimer 1; (b) CA dimer 2; (c) TTCA dimer 1; (d) TTCA dimer 2. Red color corresponds to the excess, and green to the lack of electron density in the dimers compared to two isolated molecules. Contours are drawn at (0.01 electrons/Å3.

indicating a weaker H-bondingssee Figure 3asthan in the case of CA dimer 2. In the case of dimer 1 of TTCA, shown in Figure 3c, the kebab structure is practically missing; there is only an excess of the density between S and H atoms clearly visible; we shall see later on that this dimer is practically not bound. To investigate further the issue of stability of dimer 1, we performed a series of constrained energy optimizations using the Z-matrix option in SIESTA, where the C-O · · · H and C-S · · · H angle (labeled φ in Figure 2) was constrained at a number of values between 180° (dimer 1) and 120° (dimer 2), while the rest of the coordinates of the two molecules were allowed to relax. The binding energies both for CA and TTCA are plotted in Figure 4. It can be seen that dimer 1 (the one with the angle of φ ) 180°) in both cases has the smallest binding energy, close to zero for TTCA. It is, in fact, a saddle point, as can be seen from the graph in Figure 4, and is only obtained in geometry optimization calculations of CA because of its high symmetry. The 130° and the 115° structures are the most stable for CA and TTCA, respectively. These are the geometries that closely correspond to the dimer 2 of CA and TTCA (see Table 1). Figure 4 also shows that the binding energies of CA dimers change very weakly when the angles are close to 180°, and then change abruptly, as the structure becomes dimer 2 like, when the O · · · H distance r2 abruptly

becomes shorter and we can speak about a double rather than a single H-bond in the CA dimer. The TTCA dimer bonding energies and S · · · H distances vary much more gradually. There is no tendency for the S · · · H distance to become short abruptly to lower the energy. The very weak binding at angles close to 180°, and the lack of plateau in the energy vs angle curve of TTCA probably explains why dimer 1 of TTCA is not easy to obtain in optimization calculations, unlike in the case of CA. We note here that supramolecular structures based on dimer 1 type bonding have been observed experimentally for CA;21-23 therefore, we cannot drop the dimer 1 structure from the further analysis for either of the molecules. We also considered two trimers of CA and TTCA, shown in Figure 5, which are relevant to the further analysis of 1D and 2D structures. In trimer 1, an S (or O) atom participates in two H-bonds simultaneously. Such bonding cannot be found in a dimer but can appear in a trimer. The binding energy is -0.92 eV (CA) and -0.66 eV (TTCA). The average binding energies are therefore -0.23 and -0.16 eV/H-bond, which is slightly less than a normal dimer 2 binding energy (-0.27 and -0.18 eV/H-bond) for CA and TTCA, respectively. This shows that if an O or S atom is shared between two H-bonds, such bonds are noticeably weaker (estimated energies of -0.20 and -0.15 eV/H-bond, respectively) than “normal” dimer 2 type H-bonds where each O, S, and H atom participates just in one H-bond. During relaxation of these trimers, the molecules rotate slightly, so that the distance between O or S atoms labeled A and B becomes larger. Another trimer, Figure 5b (starting structure), where an H atom is shared between two H-bonds, was found not stable both for CA and TTCA.


Figure 5. Trimers of TTCA. (a) Optimized structure of trimer 1. (b) Initial trial structure of the unstable trimer 2. CA trimer structures are similar and thus not shown. (c and d) Electron density difference plots for the stable trimer 1 of CA (c) and TTCA (d).

Figure 6. TTCA dimers 1 and 2 and their binding sites available for the chain formation: (ovals) double binding sites; (circles) single binding sites. Symmetrically equivalent sites in each dimer are shown as circles or ovals of the same color (the same line types).

3.2. 1D Structures. Using dimer 1, dimer 2, and monomers as building blocks, one can construct 1D chains of molecules. The procedure is similar to that described in ref 41. CA and TTCA can be connected to each other either by single H-bonds (dimer 1 type) or by double H-bonds (dimer 2 type). Figure 6 shows the binding sites of dimers 1 and 2 (ovals mark double binding sites; circles mark single binding sites), and Figure 7 shows the resulting chains of CA and TTCA containing one (chain 5) or two (chains 1-4 and 6) molecules per unit cell. Dimer 2 has six double donor-acceptor binding sites available for additional dimer 2 type H-bonding (light and dark green ovals in Figure 6). Due to symmetry, these sites give rise to two molecular chains, which are chains 1 and 2 in Figure 7. There are two more “double” binding sites in dimer 2, which involve an O or S atom already participating in an H-bond (blue ovals in Figure 6). These sites give rise to only one chain labeled 3 in Figure 7. There are also two double binding sites which involve an H atom already participating in an H-bond (light blue ovals in Figure 6), which give rise to the unstable CA and TTCA trimers described above. Dimer 2 also has four single binding sites of each type (shown by circles in Figure 6): four H atoms and four electronegative atoms not yet involved in the H-bonding; however, only one chain, which is shown in Figure

Martsinovich and Kantorovich 7 (structure 4), is possible in this case due to symmetry. The sites shown by red and yellow circles in Figure 6 are used for this chain. Alternatively, one can start the construction process from dimer 1 structure instead, which has eight double binding sites, but they give rise to only one periodic chain which is the same as the one already shown in Figure 7 (structure 4). Dimer 1 also has 10 single binding sites, which give rise to 2 chains: one linear chain, Figure 7 (structure 5), based on a single molecule in the unit cell, and one zigzag chain, Figure 7 (structure 6), based on a two-molecule cell. We stress that these six chains are all possibilities corresponding to not more than two molecule cells; more possibilities, however, arise if more molecules per cell are assumed. Thus, chains 1 and 2 have dimer 2 as a repeat unit and are based purely on dimer 2 type bonds. Chain 1 is linear, while chain 2 is zigzag and, when repeated in the second dimension, can give rise to a hexagonal 2D structure. Linear chain 3 also has a dimer 2 repeat unit and is based on the trimer 1 structure, where an O or S atom is shared between two H-bonds. Chain 4 has the dimer 2 repeat unit as well, but these dimers are connected via dimer 1 type linear bonds, so this chain has a mixed bond character. Chain 5 is similar to chain 4 but has a monomer as a repeat unit and is therefore half as thick; these monomers are connected by dimer 1 type bonds. Both chains 4 and 5 are linear. Chain 6 also involves only dimer 1 type bonds, but the repeat unit is a dimer, and the geometry is zigzag. The energies of the CA and TTCA chains (both calculated and predicted using the dimer binding energies), and their lattice vectors are shown in Table 2. The table shows that the chains which involve dimer 2 type bonding (i.e., chains 1 and 2) have larger binding energies per cell than the chains which are based entirely on dimer 1 type bonding (chains 5 and 6) which have fewer bonds per cell. Surprisingly, chain 4 of CA, which involves both types of bonding, has the largest binding energy. Notably, chains 3 of both CA and TTCA, which involve an O or S atom participating in two hydrogen bonds at the same time, are less strongly bound than chains based on dimer 2 type bonding and also much less strongly bound than one could expect from the dimer results. Note also that the binding energies of TTCA chains are smaller than those of CA chains on average by one-third with TTCA chain 2 being the most stable. 3.3. 2D Structures. Then, 2D supramolecular structures, shown in Figure 8, can be all constructed from the chains considered above. Chains are highlighted as structural units of the 2D networks in Figure 8 with black (chain 1), blue (chain 2), green (chain 3), red (chain 4), orange (chain 5), or light blue (chain 6) lines. Lattice vectors are shown with bold arrows. Note that different chains, when positioned in parallel next to each other, may give rise to the same network, which is why in the figure for every 2D structure we highlight several chains which can be used to generate them. Chains 1, when positioned next to each other, give rise to three networks 1, 3, and 4, which essentially differ from each other by the amount of shift of each chain 1 with respect to each other, as shown in detail in Figure 9. Chain 2 gives rise to the hexagonal network 2. This chain may also be a structural unit of networks 3 and 4 (not shown in the figure to avoid cluttering). Chain 3 also leads to networks 3 and 4 mentioned above, while chain 4 leads to network 1 (which is alternatively derived from chain 1). Dimer 1 type chain 5 leads to only one network 5, and chain 6 leads to network 6. Thus, only 6 types of 2D networks are possible: one mixed bond network containing both dimer 1 and dimer 2 types of bonding (network 1), three networks containing only dimer 2 type



Figure 7. CA and TTCA chains 1-6. The repeat units of these chains are shown with dashed boxes, and the corresponding lattice vectors are shown by arrows. H-bonds are shown with dotted lines.

TABLE 2: Calculated Energies (eV and eV/H-bond), Predicted Energies, and Lattice Vectors (Å) of CA and TTCA Chainsa chain

energy/cell (calculated)

energy/cell (predicted)

energy/H-bond (calculated)

lattice vector

1 2 3 4 5 6

-0.97 -1.07 -0.51 -1.18 -0.55 -0.46

-1.06 -1.06 -0.92 -1.01 -0.48 -0.48

-0.24 -0.27 -0.13 -0.30 -0.28 -0.23

11.95 10.35 7.39 6.72 13.62 11.97

1 2 3 4 5 6

-0.67 -0.73 -0.33 -0.54 -0.19 unstable

TTCA -0.72 -0.72 -0.66 -0.38 -0.02 -0.02

-0.17 -0.18 -0.08 -0.14 -0.10

13.17 11.36 9.24 7.78 15.61

CA

a The results for chain 5 are presented for two unit cells (i.e. for two molecules) to make the comparison with other chains, containing two molecules in their unit cells, easier.

bonding (networks 2, 3, and 4), and two networks containing only dimer 1-type bonding (networks 5 and 6). Network 1 has a rectangular symmetry and is achiral. Each molecule is directly connected to four neighbors by two pairs of double H-bonds and two single H-bonds. The chiral honeycomb network 2 is based on chain 2: each molecule is connected to three neighboring molecules by double H-bonds. Networks 3 and 4 are similar to network 1, but chains 1 are offset differently with respect to each other, so that double rather than single H-bonds connect these chains (the offset can more clearly be seen in Figure 9). In network 4, each molecule participates in three sets of H-bonds (three H-bonds per molecule, six per cell), where one O (or S) atom of each molecule forms two H-bonds simultaneously, and one such atom does not participate in any H-bonds. This network contains 6-membered rings of molecules (which can be seen clearly in Figure 9c), where O (or S) atoms in the center of these rings do not participate in H-bonds and may interact with each other only via van der Waals interactions. Network 3 can also be constructed from chain 4. As in network 4, one O (or S) atoms of each molecule participates in two H bonds at the same time. The structure of network 3 is peculiar since some H atoms participate in two H-bonds simultaneously as well, which should

adversely affect the stability of the arrangement. While network 3 is achiral (see Figure 8), network 4 is chiral. Network 5 is an achiral hexagonal network with just one molecule in the unit cell, where each molecule is connected to six neighbors by single H-bonds. Network 6 is similar to network 5 in that it involves only single H-bonds, but one molecule in the center of each hexagon is missing, resulting in a more porous honeycomb structure, where each molecule is connected to three neighbors by three single H-bonds. Thus, only two networks (2 and 4) are chiral; all others are achiral. The energies of these networks calculated using VASP are shown in Table 3. In these calculations, all atomic positions and lattice vectors were optimized first. Then, the third lattice vector (perpendicular to the 2D sheets of molecules) was set as 12 Å, large enough to avoid interaction between the sheets, and the structures were relaxed again. The energies of the noninteracting planar 2D networks are presented in Table 3. The calculated energies are compared with the energies predicted using the dimer and trimer results of section 3.1. We used the trimer energies only for networks 3 and 4 where O (or S) atoms participate in two H bonds simultaneously, exactly as in trimer 1.


Martsinovich and Kantorovich

Figure 8. CA (or TTCA) network structures 1-6. The chains that make up these networks are highlighted.

Our calculations show that the most stable 2D structure for CA is the rectangular network 1, both according to VASP and SIESTA (not shown) calculations. Networks 2 and 5 are slightly less stable and have similar binding energies, although they contain different types of H-bonds. Networks 3 and 4, which involve O atoms participating in two H-bonds simultaneously, are less strongly bound. Their binding energies are smaller than one can expect from summing up the corresponding dimer energies, similar to what was found for chain 3. The possible explanation is that some of the O atoms participate in two H-bonds simultaneously and therefore cannot provide enough electron density for the formation of two strong H-bonds. Notably, network 3 based on H atoms shared between two H-bonds was also found stable, unlike trimer 2 with a similar bonding. This is probably due to the symmetry of the network and a difficulty of energy-lowering structural rearrangements. Network 6 has the smallest binding energy of all. In this structure, each molecule participates only in three H-bonds, and we can expect it to have the binding energy half as big as network 5. In fact, according to our calculations, network 5 is 2.5 times more strongly bound than network 6. One can also see from the data in Table 3 that the energies of H-bonded networks are not strictly additive. The predicted energies are smaller than the calculated ones for networks 1, 2, and 5. This must be a manifestation of the so-called Resonance Assisted Hydrogen Bonding (RAHB) effect (see ref 4 and references therein). When each molecule participates in several H-bonds at the same time, each of these bonds causes perturbations of electron density on nearby atoms and functional groups and affects their ability to form strong H-bonds. Often, these bonds work in concert to enhance the overall binding (for

Figure 9. CA (or TTCA) networks 1 (a), 3 (b), and 4 (c).

example, ref 4). For instance, in the case of CA, the electron density is drawn from the CdO area to the N-H and the RAHB effect is facilitated by the fact that the donor and the acceptor sites around each molecule (which acquire or lose some electron density due to H-bonding) alternate, making the electrostatics work better. The RAHB is not working in the case of network 6 since for each molecule only one type of binding sites is active, so that one molecule (where N-H groups take part in H-bonds) acquires additional electron charge due to its three H-bonds and the other one consequently loses some charge. The situation with networks 3 and 4 is more complicated: both types of atoms participate in the binding, and some of the atoms participate in two H-bonds simultaneously. The electron density difference plots (Figure 10) for CA networks 1, 2, 4, and 5 show very regular kebab patterns indicative of strong hydrogen bonding. The plot for network 3 (Figure 10 (structure 3)) illustrates the difference in H-bonding compared to network 4: the H-bonding where one H atom participates in two H-bonds simultaneously does exist in network 3. However, the amount of electron density redistribution, and therefore, the H-bonding is weaker for such H-bonds than for normal dimer 2 type bonds. Interestingly, network 5 also shows a strong electron density redistribution, despite containing only dimer 1 type bonds. This agrees with the large binding energy of this network. On the other hand, the electron density redistribution in the less strongly bound network 6 is weaker and reminds of the electron density of dimers 1 of CA, Figure 3a, connected together.



TABLE 3: Calculated (VASP) Binding Energies of CA and TTCA Networks (with the Lattice Vector Perpendicular to the Plane of the Network Fixed at 12 Å), Predicted Energies Based on the Binding Energies of CA and TTCA Dimers and Average Energies per H-bond (all in electronvolts)a network

energy/cell (calculated)

energy/cell (predicted)

energy/H-bond (calculated)

O · · · H or S · · · H distance

lattice vectors

lattice angle

cell area

CA 1 2 3 4 5 6

-1.71 -1.68 -1.02 -1.10 -1.68 -0.62

-1.54 -1.59 -1.46 -1.46 -1.43 -0.72

-0.29 -0.28 -0.17 -0.18 -0.28 -0.21

1.75; 1.71 1.72 2.26; 2.20; 1.92 1.98; 1.91; 1.79 1.78 1.90

11.99; 7.62 10.35; 10.38 12.68;7.39 12.05; 7.43 6.73; 13.62 11.99; 11.97

90.0 119.9 54.1 60.2 60.4 60.0

80.58 93.06 75.89 77.60 79.73 124.23

1 2 3 4 5 6

-0.81 -1.11 unstable -0.67 -0.52 unstable

-0.74 -1.08 -1.02 -1.02 -0.06b -0.03b

-0.14 -0.19

TTCA 2.32; 2.33; 2.54 2.27

13.17; 7.78 11.36; 11.36

96.3 119.9

101.75 111.83

-0.11 -0.09

2.30; 2.53; 2.95 2.66

13.39; 9.24 7.81; 15.61

58.1 60.0

105.06 105.52

a O · · · H and S · · · H distances (Å), lateral lattice vectors A1 and A2 (Å), and the angle between them (deg), as well as the area of the unit cell (Å2), are also shown. The results for network 5 are presented for two unit cells (i.e. for two molecules) to make the comparision with other networks easier. b Predicted using the Z-matrix SIESTA calculations for TTCA.

Figure 11. Optimized structures of TTCA networks: (a) network 1, (b) network 2, (c) network 4, and (d) network 5.

Figure 10. Electron density difference plots for CA networks. O and H atoms participating in two H-bonds simultaneously are shown with arrows in structures 3 and 4.

The binding energies of TTCA networks are in all cases smaller than for CA networks. The order of stabilities of these networks is also different. For TTCA, the honeycomb network

2 is the most stable one, with the binding energy of -1.11 eV/ cell (-0.19 eV/H-bond). It is followed by network 1 and then network 4 (note that SIESTA calculations always resulted in a transformation of network 4 into network 1, while this did not happen with the VASP calculations). Note that network 1 (Figure 11a) is slightly distorted and less symmetric compared to its starting structure equivalent to that of CA; Figure 8 (structure 1). Network 3 is not stable and always transforms into network 4 by a small bond rearrangement. Network 5 has the smallest binding energy of all stable TTCA networks, although it is more strongly bound than one could expect from the dimer 1 energy. Unlike in the CA case, it does not preserve the symmetry. Instead, each TTCA molecule is rotatedssee Figure 11bsso that the C-S · · · H angle becomes less than 180°, while the S · · · H bond is slightly elongated. Finally, network 6 is not stable since it always transforms into network 2.


Figure 12. Electron density difference plots of TTCA networks 1 (a), 2 (b), 4 (c), and 5 (d).

The electron density difference plots in Figure 12 confirm that only network 2 has strong H-bonding. On the other hand, dimer 1 type bonds in network 1 and H-bonds involving shared S atoms in network 4 are considerably weaker, as seen from the smaller electron density redistribution. In network 5, the electron density differences are very small, similar to the picture for dimer 1 of TTCA, in agreement with the small binding energy of this network. 4. Discussion In this section, we will compare the behavior of O-containing and S-containing H-bonded structures and will show parallels between our findings and earlier available computational work on H-bonded systems. Then, we will compare our calculated 2D structures with the available experimental data on CA and TTCA networks and/or bulk crystals. 4.1. S vs O in H-Bonded Dimers and 2D Networks. The results presented above show that the energy of H-bonding depends strongly on the type of the electronegative atom involved. Although both O and S are able to form strong H-bonds, the relative strength and geometries of bonds are different: the H-bonds involving O, the more electronegative atom, are stronger than the equivalent bonds involving sulfur (as can be expected, given the lower electronegativity and nucleophilicity of S). The N-H · · · O H-bond in the simplest case of the CA dimer 2 has the energy of -0.27 eV. The energy of the N-H · · · S bond in the TTCA dimer 2 is one-third less, -0.18 eV. These values can be compared and are of the same order of magnitude as the energies of H-bonds available in the literature calculated for different molecular pairs using DFT (B3LYP functional) and quantum chemistry methods: -0.30 to -0.45 eV for O-H · · · O bonds, -0.30 to -0.37 eV for O-H · · · S bonds and up to -0.10 eV for S-H · · · O bonds,14,15 -0.30 to -0.40 eV for F-H · · · O, and -0.16 to -0.30 eV for F-H · · · S bonds.11,13 The spread of the energies for each of the H-bond types shows again that the strength of H-bonds is affected not only by the chemical nature of the atoms directly involved in H-bonding

Martsinovich and Kantorovich but also by the neighboring groups. These literature values of H-bond energies are slightly larger than our N-H · · · O and N-H · · · S bond energies; this can be explained by a low electronegativity of nitrogen (in our system) than oxygen or fluorine (as in refs 11 and 13-15). Both our calculations and refs 11 and 13-15 show S-containing bonds to be slightly (by 8 to 30%, depending on the system) weaker than O-containing bonds. We also note that since S-containing H-bonds are strong, they can give rise to strongly bound dimers and robust supramolecular structures. We find that the geometries of H-bonds with O and S in CA and TTCA are also somewhat different. The O · · · H bonds in dimer 2 are shorter (1.77 Å) than S · · · H bonds (2.28 Å). The optimized bond angle C-O · · · H in CA is ∼127° (a double H-bond is formed), while the optimum C-S · · · H angle is slightly smaller, ∼110°. This agrees with the trend reported in the earlier studies of H-bonded molecular pairs, where S was shown to form H-bonds with more “perpendicular” angles (i.e., closer to 90°), while O formed more “linear” bonds (124°-140°) both for sp2 and sp3 hybridized O and S atoms. Notably, the 180° structure (dimer 1) is almost not bound in the TTCA case, while in the CA case this saddle-point structure was obtained in calculations with a binding energy of -0.24 eV, only slightly less than the energy of dimer 2 per H-bond. The analysis of the CA and TTCA binding sites and possibilities of H-bonding shows that these molecules, in principle, should be able to form the same dimers and 1D and 2D periodic structures. Our DFT calculations of these structures show, however, that the order of stabilities of the periodic structures is different for the S and O analogues. It is the ability of CA and inability of TTCA to form strong dimer 1-type bonds that gives rise to a difference in stabilities of 1D chains and 2D networks. The structures based on dimer 1 type bonding (networks 1 and 5) are stable for CA and are comparable to pure dimer 2-based structures (network 2). On the contrary, for TTCA, dimer 1-based networks 1 and 5 are disfavored; although some additional structural deformation can make these networks more stable. In particular, the distortion of network 5 of TTCA can be understood if we look at the angle dependence of TTCA dimer energies, Figure 4. As the C-S · · · H angle deflects from 180°, the energy goes down. Thus, a small rotation of the molecules in the network with respect to each other is favorable, even though the S · · · H distance is increased due to this rotation. A similar effect operates in TTCA network 1. For CA, for comparison, the change of binding energies due to little changes of the angle is smaller, but the effect of the increased O · · · H distance can be expected to be stronger. Thus, it is favorable to keep the O · · · H distances in network 5 short, rather than rotate the molecules. We note that we considered only networks based on dimers and monomers as repeat units. Networks having large unit cells, e.g., tetramers, can also be constructed systematically in the same way. To do this, we should first consider all possible unit cells formed by four molecules; then, we should identify available binding sites in the cells and use this information to construct all possible chains as 1D periodic repeats of the unit cell (taking into account that different cells may result in equivalent chains). Then, at the final stage, we should put the chains parallel to each other to construct all possible 2D monolayers. It is easy to see that as the number of molecules in the cell becomes larger, the number of possible networks constructed out of each particular cell will grow very fast and will be much larger than the number of dimer-based networks considered in the previous section. However, we know from



TABLE 4: Comparison of the Calculated (VASP) Binding Energies (eV/cell) of the CA Networks Using Two Methods: (i) Complete Relaxation Including the Third Lattice Vector, A3, Connecting the Parallel Layers and (ii) Using the Fixed Third Lattice Vector so That the Separation between Layers is 12 Å network

A3 optimized

A3 fixed

1 2 3 4 5 6

-1.87 -1.81 -1.08 -1.16 -1.70 -0.76

-1.71 -1.68 -1.02 -1.10 -1.68 -0.62

the studies of melamine H-bonded 2D structures done in our group45 that tetramer-based structures are usually more porous, have fewer H-bonds per molecule, and therefore have smaller binding energies. Consequently, such networks are less likely to be formed on surfaces and observed in experiment. Disregarding possible structures with a unit cell larger than two molecules is, however, a limitation of our analysis, as some strongly bound structures may be still omitted. For example, Kannappan et al.23 reported an experimentally observed network of CA with a large hexamer unit cell, which we will discuss in the next section. A similar scheme can be used to construct possible mixed CA-TTCA networks. However, since N-H · · · O bonding is much stronger than N-H · · · S, phase segregation into CA and TTCA domains may be more energetically favorable than formation of mixed networks. 4.2. Comparison with Experiment. Our calculations of 2D H-bonded structures can be compared both with molecular networks that can be formed on surfaces and, partially, with bulk molecular crystal structures. Indeed, a comparison with experimental 2D networks (monolayers) is justified, if the interaction between the molecules and the crystal surface is very weak or the interaction energy has a negligible corrugation across the surface, i.e. much smaller than their adsorption energies, as is the case for flat metal surfaces such as Au(111).34-36 There are examples of theoretical modeling of networks in the literature.6,21,22,36 Comparison with the bulk crystal structures is also possible, since CA and TTCA crystals are known to have a layered structure, with H-bonded layers of molecules and weak interlayer interactions. Thus, our calculations can be applicable to these H-bonded layers. We should be cautious, however, when comparing our calculated 3D structures with experimental results on bulk crystals: the interaction between molecular layers in crystals is of the van der Waals type and is therefore not properly described by DFT. The overlap of the π-like basis orbitals of the molecules in the adjacent layers causes some attractive interaction between the layers in our calculations, which brings an additional 0.03-0.09 eV/molecule interaction energy compared to the case of large interlayer separations where there is no interlayer interaction (see Table 4). However, our calculations do not reproduce the experimentally observed shift of each layer with respect to the neighboring layers above and below it. Thus, the comparison with bulk crystal data can only be done with respect to the intermolecular structure within the layers. 4.2.1. CA. Network 1, the most stable H-bonded structure of CA, agrees well with the experimentally found structure of CA layers in the bulk crystal.25 Note also that, perhaps fortuitously, our fully relaxed VASP calculations (with the third lattice vector, connecting two adjacent layers, relaxed as well) give the interlayer distance of 7.5 Å for network 1 of CA which

is close to the interlayer distances of 7.75 Å found by X-ray and neutron diffraction. Networks 2 and 5, according to our calculations, have very similar stabilities and are both slightly less stable than network 1. Network 2 has been experimentally observed in CA monolayers on the Au(111) 21,22 and graphite (0001) surface,23 and network 5 on the graphite surface,23 both under ultrahigh vacuum (UHV) conditions. It is notable that, although network 1 is shown in our calculations to be the most stable one, it has not been observed on crystal surfaces. We suggest that the reason is that both the Au(111) and graphite surfaces used in experiments21-23 have a hexagonal symmetry. Both networks 2 and 5 also have a hexagonal symmetry, while network 1 does not. The ratio of the network 5 lattice vectors to the Au(111) surface lattice vectors is approximately 2.5:1, while to those of graphite the ratio is approximately 5:1; for network 2, the ratios are ∼2.1:1 and ∼4.2:1, respectively. Then, when the energy differences between the structures are very small, as it is in the case of the three most stable CA networks (at most 0.1 eV/molecule difference), the interaction with the substrate is likely to play a crucial role and promote the formation of hexagonal networks 2 and 5. It is also possible that van der Waals interaction between molecules and the surfaces play a role in stabilizing particular networks. Lattice vectors of the networks may also be adaptable in order to achieve commensurability with the substrate. For instance, according to the STM measurements,21-23 the measured lattice vectors of the hexagonal network 5 are slightly different on the graphite (7.5 Å 23) and Au(111) substrate (6.9 Å,21 7.0-7.3 Å 22) and in both cases slightly larger than our calculated value (6.73 Å). It was suggested in ref 23 that the cause of stability of network 1 in the bulk is due to the enhanced interlayer interaction. However, our calculations with the large interlayer separation reported in the previous section and with the optimized interlayer distance as mentioned above give the same ordering of the monolayers with very small energy differences (see Table 4). Therefore, we conclude that the order of stabilities of the networks is mainly due to their intraplanar H-bonds, rather than due to interplanar interactions. None of the networks 3, 4, or 6 have been so far observed experimentally, which is probably due to them being much less stable than the networks discussed above. Interestingly, Kannappan et al.23 reported a more complex “flower” structure with 6 molecules in the unit cell. It has both single and double H-bonds between CA molecules, like network 1, but has a hexagonal symmetry which is likely to be favored on hexagonal surfaces. Our calculations of this network gave the binding energy of -0.81 eV/molecule, which makes this structure only slightly less stable than the lowest-energy networks 1, 2, and 5. Another intriguing CA structure was observed by Zhang et al.24 on the Au(111) surface in water. The STM images are topologically similar to the flower structure of ref 23 but show a smaller unit cell size. The authors of ref 24 attribute it to molecules standing upright rather than lying flat on the surface. The repeat unit, according to ref 24, is a H-bonded cyclic hexamer with molecules bound to each other by double hydrogen bonds (like six molecules of chain 1 rolled in a ring), with (presumably) Van der Waals or π-π interactions between the hexamers, and attached to the surface alternately via the O atoms and N-H groups of CA. This seems only possible for CA anions, where the NH group has lost its H atom and can form a chemical bond to Au. The water environment may be

17350 J. Phys. Chem. C, Vol. 112, No. 44, 2008 responsible for the formation of such structures by acting as a sink for protons. We have not attempted to check the stability of this proposed structure with our method here. 4.2.2. TTCA. There is less experimental data on TTCA intermolecular structures; in particular, there have been no reports of TTCA surface monolayers. However, there is X-ray data on the structures of TTCA polycrystals30 and mixed cocrystals.28,29 1D chains 1, 2, and 3 have been observed as structural units of TTCA cocrystals with organic solvents capable of H-bonding28,29 and TTCA salts with water and organic solvents.27 The particular type of chain depends on the size and chemical properties of the cocrystallizing species. According to experimental reports,28,30 attempts to obtain TTCA single crystals always resulted in cocrystallization with the solvent, but polycrystalline pure TTCA has been analyzed by powder X-ray diffraction.30 The TTCA crystal structure found in the latter experiments corresponds to a layered structure with network 4 as its layers, with the interlayer distance of 8.8 Å (note that our relaxed interlayer distance was found to be close to this value: 8.2 Å). However, according to our calculations, network 4 is not the most stable one. Instead, network 2 has the lowest energy. At the same time, it has a lower density (55.9 Å2/molecule in the plane of the H-bonds), whereas networks 1 and 4 allow a more dense packing of TTCA molecules (50.9 and 52.5 Å2/molecule). This inefficient packing may be the reason why network 2 is not formed in the bulk. Another reason may be related to the interlayer interactions which are significantly underestimated in our DFT-PBE calculations. 5. Conclusions We compare H-bonding involving O and S atoms by comparing two model molecules, CA and TTCA. We show that both O and S give rise to strong H-bonds; however, the details of the H-bond geometries are different, and this results in different stabilities of both dimers and larger 1D and 2D structures of CA and TTCA. Six 2D networks are predicted (two of them found unstable for TTCA). One of the networks found has an unexpected structure (network 3) in which some hydrogen atoms participate in two H-bonds at the same time. Two of the networks match experimentally observed selfassembled CA network structures. The analysis of 2D structures can be extended to 3D structures (i.e., bulk crystals) based on H-bonded molecular layers with weak interlayer interaction. The molecular layer structure in the experimentally observed bulk crystal of CA matches the most stable CA network found in our calculations. Thus, there is good agreement between experimental results and our calculations of CA assemblies. The situation is more difficult with TTCA, where there is no experimental data on networks available for comparison with our calculations, while the bulk crystal data point to the structure which is not the most stable one in our calculations. In addition to this, more structures may be possible to generate using our method by adopting more molecules in the unit cell as was exemplified here by the flower structure containing six CA molecules in the unit cell. Thus, more experiments and theoretical calculations are desirable to clarify the situation. This study also paves the ground for future studies of mixed networks of CA and TTCA with other molecules. Acknowledgment. We would like to acknowledge the computer time on the HPCx supercomputer via the Materials Chemistry Consortium. N.M. would also like to acknowledge the financial support from the EPSRC, Grant GR/S97521/01.

Martsinovich and Kantorovich We thank E. Anglada and K. Rezouali for sharing with us the sulfur pseudopotentials needed for the SIESTA calculations.46 References and Notes (1) Vinogradov, S. N.; Linnell, R. H. Hydrogen bonding; Van Nostrand Reinhold Company: New York, London, 1971. (2) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029. (3) Chen, Q.; Richardson, N. Nat. Mater. 2003, 2, 324. (4) Otero, R. Angew. Chem., Int. Ed. 2005, 44, 2270. (5) Lackinger, M.; Griessl, S.; Heckl, W. A.; Hietschold, M.; Flynn, G. W. Langmuir 2005, 21, 4984. (6) Kelly, R. E. A.; Kantorovich, L. N. J. Mater. Chem. 2006, 16, 1894. (7) Barth, J. V. Annu. ReV. Phys. Chem. 2007, 58, 375. (8) Franc¸ois, S.; Rohmer, M.-M.; Benard, M.; Moreland, A. C.; Rauchfuss, T. B. J. Am. Chem. Soc. 2000, 122, 12743. (9) Steiner, T. Acta Crystallogr. C 2000, 56, 876. (10) Hernandez-Halindo, M. D.; Jancik, V.; Moya-Cabrera, M. M.; Toscano, R. A.; Cea-Olivares, R. J. Organomet. Chem. 2007, 692, 5295. (11) Platts, J. A.; Howard, S. T.; Bracke, B. R. F. J. Am. Chem. Soc. 1996, 118, 2726. (12) Rablen, P. R.; Lockman, J. E.; Jorgensen, W. L. J. Phys. Chem. B 1998, 102, 3782. (13) Salai Cheettu Ammal, S.; Venuvanalingam, P. Chem. J. Faraday Trans. 1998, 94, 2669. (14) Wennmohs, F.; Staemmler, V.; Schindler, M. J. Chem. Phys. 2003, 119, 3208. (15) Wierzejewska, M.; Saldyka, M. Chem. Phys. Lett. 2004, 391, 143. (16) Seto, C. T.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 905. (17) Huo, Q.; Russell, K. C.; Leblanc, R. M. Langmuir 1999, 15, 3972. (18) Barnett, S. A.; Blake, A. J.; Champness, N. R. Cryst. Eng. Commun. 2003, 5, 134. (19) Berl, V.; Schmutz, M.; Krische, M. J.; Khoury, R. G.; Lehn, J. M. Chem.sEur. J. 2002, 8, 1227. (20) Perdiga˜o, L. M. A.; Champness, N. R.; Beton, P. H. Chem. Commun. 2006, 538. (21) Staniec, P. A.; Perdig Rogers, L. M. A.; Champness, N. R.; Beton, P. H. J. Phys. Chem. C 2007, 111, 886. (22) Xu, W. Small 2007, 5, 854. (23) Kannappan, K.; Werblowsky, T. L.; Rim, K. T.; Berne, B. J.; Flynn, G. W. J. Phys. Chem. B 2007, 111, 6634. (24) Zhang, H.-M.; Xie, Z.-X.; Long, L.-S.; Zhong, H.-P.; Zhao, W.; Mao, B.-W.; Xu, X.; Zheng, L.-S. J. Phys. Chem. C 2008, 112, 4209– 4218. (25) Coppens, P.; Vos, A. Acta Crystallogr. B 1971, 27, 146. (26) Lewis, T. C.; Tocher, D. A.; Price, S. L. Cryst. Growth Des. 2005, 5, 983. (27) Mahon, M. F.; Molloy, C. F.; Venter, M. M.; Haiduc, I. Inorg. Chim. Acta 2003, 348, 75. (28) Dean, P. A. W. Cryst. Eng. Commun. 2004, 6, 543. (29) Ahn, S. Chem.sEur. J. 2005, 11, 2433. (30) Guo, F.; Cheung, E. Y.; Harris, K. D. M.; Pedireddi, V. R. Cryst. Growth Des. 2006, 6, 846. (31) Armstrong, D. R. J. Mol. Modell. 2000, 6, 234. (32) Rostkowska, H.; Lapinski, L.; Khvorostov, A.; Nowak, M. J. J. Phys. Chem. A 2005, 109, 2160. (33) Kopel, P.; Travnicek, Z.; Zboril, R.; Marek, J. Polyhedron 2004, 23, 2193. (34) Xu, W. Small 2007, 3, 2011. (35) Otero, R. Science 2008, 319, 312. (36) Silly, F. J. Phys. Chem. C 2008112, 11476. (37) Soler, J. M. J. Phys.: Condens. Matter 2002, 14, 2745. (38) Kresse, J. M.; Furthmuller, J. Phys. Sci. 1996, 6, 15. (39) Kresse, J. M.; Furthmuller, J. Phys. ReV. B 1996, 54, 11169. (40) Perdew, J.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (41) Kelly, R. E. A.; Lee, Y. J.; Kantorovich, L. N. J. Phys. Chem. B 2005, 109, 11933. (42) Boys, S.; Bernardi, F. Mol. Phys. 1970, 19, 553. (43) Daza, M. C.; Dobado, J. A.; Molina Molina, J.; Salvador, P.; Duran, M.; Villaveces, J. L. J. Chem. Phys. 1999, 110, 11806. (44) Pauling, L. The nature of the chemical bond and the structure of molecules and crystals; Cornell University Press: Ithaca, NY, 1960. (45) Mura, M.; Martsinovich, N.; Kantorovich, L. Nanotechnology, in press. (46) SIESTA pseudopotentials can be obtained from the SIESTA website http://www.uam.es/departamentos/ciencias/fismateriac/siesta/ or by directly contacting E. Anglada ([email protected]).

JP805569F

Comparative Theoretical Study of O- and S-Containing Hydrogen

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