Some Pictures of Alcoholic Dancing: From Simple to Complex

Article pubs.acs.org/JPCC

Some Pictures of Alcoholic Dancing: From Simple to Complex Hydrogen-Bonded Networks Based on Polyalcohols Antonela C. Marele,† Inés Corral,*,‡ Pablo Sanz,‡ Rubén Mas-Ballesté,§ Félix Zamora,§ Manuel Yáñez,‡ and José M. Gómez-Rodríguez*,† †

Departamento de Física de la Materia Condensada, ‡Departamento de Química, and §Departamento de Química Inorgánica, Universidad Autónoma de Madrid, 28049, Madrid, Spain S Supporting Information *

ABSTRACT: The present study is aimed at elucidating the factors governing the organization on surfaces of some aromatic alcohol molecules. The ability to self-assemble on an Au (111) surface monolayer structures of three different polyalcohols with trigonal symmetry, 1,3,5-trihydroxybenzene (THB), 1,3,5-tris(4-hydroxyphenyl)benzene (THPB), and 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP), has been evaluated. The characterization has been performed in situ by means of ultrahigh vacuum scanning tunneling microscopy (STM), and the obtained networks have been rationalized by density functional theory (DFT) calculations. One single phase consisting of trimers has been identified for each of the trialcohols studied, whereas for HHTP two additional phases have been characterized. The use of HHTP molecules has shown more versatility in the interaction modes of the hydroxyl groups, leading to larger structural variety on Au (111).

■

INTRODUCTION One of the most challenging issues in nanotechnology is the design and fabrication of nanostructures that can perform molecular recognition with controlled functionality, size, shape, and position. To reach this goal and overcome the drawbacks of lithographic procedures, new bottom-up approaches have recently emerged. Often, these approaches are based on the concept of self-assembly of molecular building blocks to generate well-defined architectures based on noncovalent interactions. Such intriguing supramolecular assemblies often possess polymeric characteristics and are, therefore, referred to as “supramolecular polymers”.1 In contrast with covalent polymers, a quite common feature of structures assembled through noncovalent interactions is reversibility, which allows defect self-healing during the formation process in a similar way biological systems self-repair. The great diversity of structural motifs found in biological systems explains the large number of complex self-organized nanostructures, which can perform specific vital biofunctions. Inspired by nature, supramolecular chemistry has fascinated scientists over the past few decades, leading to the synthesis of a large number of sophisticated and elegant structures with dimensions close to the nanoscale. It has recently become possible to apply the concept of hierarchical self-assembly, that is, the noncovalent organization of (macro)molecules that takes places over distinct multiple levels. The generation of such organized structures, obtained by controlling supramolecular interactions, makes it possible to tune a large number of physicochemical properties of molecular-based materials. In particular, those leading to porous 2-D hydrogen-bonded networks on surfaces have attracted attention due © XXXX American Chemical Society

to their potential use to trap diffusing species as guests, in a similar way to porous materials.2 The scanning tunneling microscopy (STM) technique allows observing at the submolecular level 1D and 2D structures confined on surfaces. To study how intermolecular interactions direct the self-assembly of supramolecular structures, an inert crystalline surface is needed (e.g., gold). Thus, side reactions or strong interactions are prevented, and substrate−molecule interactions themselves do not significantly distort the structures. To gain a deeper understanding on how noncovalent interactions control and direct self-organization of molecules to form materials, strong hydrogen bonds facilitated by carboxylic acids have been extensively used to generate 2D supramolecular networks on surfaces. In this line, the most studied molecule has been trimesic acid (benzene-1,3,5tricarboxilyc acid, TMA). Depending on the surface and the deposition conditions, different phases have been observed for this molecule.3−5 In addition to short distance organization, mesoscopic arrays have also been observed.6 Design of molecular structure of TMA analogues and other polycarboxylic acids gave rise to a wide variety of ordered structures, demonstrating the versatility of hydrogen bonds to form 2D supramolecular materials. 7,8 Unusual examples of selfassembled structures on surfaces using tetra-carboxylic acids have been recently reported.9,10 In addition to carboxylic acids, Received: December 17, 2012 Revised: February 8, 2013

A

dx.doi.org/10.1021/jp312424q | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

other functional groups that facilitate weaker hydrogen bonds, such as amides, amines, and alcohols sitting on different/the same molecules have been used. In this respect, DNA bases, with their versatile hydrogen bonding, are ideal systems for selfassembly. Ultra-high-vacuum STM experiments have shown the ability of modified nucleobases to form nanopatterns stabilized by hydrogen bonding on surfaces.11−15 Much scarce is, however, the literature on supramolecular self-assemblies from alcohols on surfaces that has predominantly concentrated on the study of self-assembled monolayers of alkanols,16−18 with only a few works focused on supramolecular networks of polyalcohols.6,19,20 Supramolecular aggregation of polyfunctional molecules multiplies the number of possibilities for interaction of the building blocks leading to the formation of motifs, which could accommodate cavities of different sizes and which could be, in principle, used to discriminate between different size guest molecules.2,3 Furthermore, the possibility to oxidize alcohols into aldehydes and ketones opens the door to the transformation of weakly bound networks into other very stable and stronger structures through the control of the bonding state of the building blocks.20 In this article, we examined from both experimental and theoretical points of view the 2D supramolecular assembly of alcohol groups in aromatic molecules with trigonal symmetry. Deposition on Au(111) of 1,3,5-trihydroxybenzene (THB) and its analogue 1,3,5-tris(4-hydroxyphenyl)benzene (THPB) (Scheme 1) has been studied, obtaining robust networks with

Figure 1. STM images of the self-assembled monolayer formed upon adsorption of THB on Au(111) at RT. A well-ordered hexagonal lattice can be resolved. (a) 10 × 10 nm2 image measured at VS = −0.38 V and It = 80 pA. (b) High-resolution image (2.5 × 2.3 nm2) measured at VS = −0.35 V and It = 100 pA. Each protrusion corresponds to one THB molecule, as shown in the overlaid unit cell.

destroy the herringbone reconstruction of the pristine surface, apparent in the almost vertical stripes in the image corresponding to the original soliton walls of Au(111).21,22 As it has been pointed out for the adsorption of other aromatic molecular systems, the preservation of the Au(111) reconstruction can be a clear indication of a very low substrate− adsorbate interaction.23 The molecular layer should then be stabilized by molecule−molecule interactions as a result of the formation of hydrogen bonds. Figure 1b shows a higher resolution STM image of this phase. It is also shown as an overlay a model, illustrating the surface unit-cell resulting from such hydrogen bonds. According to the experiments, it is a hexagonal unit cell (γ = 120 ± 5°) with lattice parameters a = b = 7.5 ± 0.4 Å and one THB molecule per unit cell. This corresponds to a molecular density n = (2.0 ± 0.2) × 1014 molecules/cm2 (Table 1). Within the THB molecule, we identify three equivalent binding sites or OH moieties that can function at the same time as hydrogen donors and hydrogen acceptors. Figure 2 shows the structure of the three most stable arrangements of triads of THB molecules, as obtained from density functional theory (DFT) calculations. The stability and geometry of the three species, THB1, THB2, and THB3, in Figure 2, arising from the interaction of three THB molecules via three- (THB1) and four- (THB2) center ring like hydrogen bonds or alternatively three-center sequential hydrogen bonds (THB3), was analyzed to discern the conformation compatible with the experimental STM image recorded for the assembly of THB. Structure THB1 corresponds to the most stable arrangement; see Table 2. Here the THB molecules occupy the vertices of an equilateral triangle, and neighboring binding sites interact, leading to three identical hydrogen bond interactions, in which each OH group behaves simultaneously as hydrogen bond donor and hydrogen bond acceptor, very much like in water and methanol trimers.24−26 These hydrogen bonds are characterized by an electron density of 0.0281 e au−3 (Figure 2), organized around a central ring critical point (rcp), according to a C3h symmetry. Less stable is the molecular arrangement defined in structure THB2. In this case, the incorporation of an additional hydroxyl from one of the THB molecules of the triad to the intermolecular bonding breaks the C3h symmetry and leads to less effective hydrogen bond interactions that translate into smaller hydrogen bond electron density values (0.0216 e au−3/ 0.0143 e au−3/0.0207 e au−3 vs 0.0281 e au−3 in THB1, Figure

Scheme 1. Trigonal Polysubstituted Alcohols Considered in the Present Work

variable porosity determined by the election of the tecton. We have also increased the complexity of the phases obtained by using 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) (Scheme 1). Studies on the last compound deposited on Ag(111) have been recently reported, showing that depending on the substrate temperature during deposition different phases can be observed as a result of dehydrogenation reactions.20 Only at low temperatures can a phase, which can form well-ordered mesoscopic arrays, be assigned to assembly of unreacted HHTP. However, by using a less reactive surface (Au(111)), we demonstrate that the versatility of such molecule to generate 2D supramolecular networks is higher than expected.

■

RESULTS AND DISCUSSION Assembly of 1,3,5-Trihydroxybenzene. Figure 1 shows typical STM images measured after deposition of THB molecules on clean Au(111) surfaces at room temperature (RT) at coverage close to the monolayer. It is evident from this kind of images the formation of a well-ordered structure resulting from the self-assembly of THB molecules. Some hints on the low interaction of the molecules with the Au(111) substrate come from the fact that the adsorption does not B



Article

Table 1. Experimental Lattice Constants, Angles, Number of Molecules Per Unit Cell and Densities Extracted from the STM Images Measured on THB, THPB, and HHTP on Au(111) phase THB THPB HHTP phase 1 HHTP phase 2 (“bow tie”) HHTP phase 3 (“ribbons”)

a [Å] 7.5 15.5 11.5 69 12.8

± ± ± ± ±

0.4 0.8 0.6 3 0.6

γ [deg]

b [Å] 7.5 15.5 11.5 22 22

± ± ± ± ±

0.4 0.8 0.6 1 1

120 120 120 120 120

± ± ± ± ±

5 5 5 5 5

molecules per unit cell 1 1 1 12 2

density [molecules/cm2] (2.0 ± 0.2) × (0.48 ± 0.05) (0.87 ± 0.09) (0.91 ± 0.09) (0.82 ± 0.08)

1014 × 1014 × 1014 × 1014 × 1014

Figure 2. B3LYP/6-31G*-optimized structures for THB trimers, THB1, THB2, and THB3, and molecular graph for the most stable structure THB1. Red and yellow dots in the molecular graph denote bond and ring critical points, respectively. Bond lengths in Å and electron density in e au−3.

changes and energetic stabilizations. The weakening of the bond in the OH donor groups and the decrease in distance of two consecutive oxygen atoms upon the formation of a new hydrogen bond are among some of the signatures that identify cooperative effects in hydrogen bond clusters. Energetically, cooperative effects are quantified through the evaluation of the additive interaction energy index (ΔEadd), as defined in the Computational Details section. For the particular case of the most stable THB1 structure, the interaction of the three molecules is expected to result into positive and significant cooperative effects because the attachment of the third molecule to the dimer introduces two stabilizing effects per hydrogen bond that would result in their net reinforcement. In this respect, the additive interaction energy for THB1 amounts to 1.28 kcal/mol; see Table 2. Cooperative effects are necessarily smaller for THB2 because the incorporation of one more hydroxyl to the ring-like hydrogen bond leads to the elimination of at least one stabilizing interaction. Consequently, ΔEadd was calculated to amount to 0.67 kcal/mol. From its greater gas-phase stability and the good agreement between the experimental STM image and the optimized geometry calculated for this structure (Figure 2), we can conclude that THB1 structure accounts for the experimentally observed assembly of THB.

S1 in the Supporting Information) and larger H•••O distances (2.023 Å/2.192 Å/2.024 Å vs 1.888 Å in THB1) as compared with the THB1 structure. In THB3, the change in the conformation of one of the hydroxyl groups participating in the three-center hydrogenbond structure in THB1 annihilates one of the hydrogen bonds and, consequently, the intermolecular ring -like linkage characteristic of the THB1 arrangement. This fact introduces geometric flexibility in the cluster, allowing the optimization of the two remaining intermolecular hydrogen bonds, which become strengthened. The reinforcement of these bonds, however, is not enough to compensate the stabilization of the third hydrogen bond or the cooperative effects, explaining the 3.9 kcal/mol energy differences between THB3 and THB1 structures; see Table 2. Qualitatively, cooperative effects can be anticipated by estimating the enhancement or decrease in the acid/basic properties of the two units composing a hydrogen bonded dimer upon introducing one or more additional monomers in the cluster. In this respect, the hydrogen bond is expected to reinforce if the donor moiety of the dimer acts as an acceptor of the new molecule or when the acceptor behaves as hydrogen donor in the resulting trimer but to weaken if the two moieties exchange the previous roles.27 Quantitatively, cooperative effects manifest into structural and electron density topological C



Article

stabilized by hydrogen bonds. Figure 3b displays a highresolution image, on top of which a molecular model is overlaid. In high-resolution images as the one shown, four protrusions per molecule can be resolved at both bias polarities in a large range of sample biases. This leads to ascribe the three outermost protrusions per molecule to the three hydroxyphenyl moieties of the THPB molecule, as shown in the model. It corresponds to a cell with only one flat-lying molecule (that is, parallel to the surface) per unit cell. As extracted from a large set of images, as those shown in Figure 3, the lattice vectors are a = b = 15.5 ± 0.8 Å, and the angle between them is γ =120 ± 5°. The corresponding molecular density is n = (0.48 ± 0.05) × 1014 molecules/cm2. As expected, the pattern defining the STM image in the THPB trialcohol coincides with that described in THB, compare Figures 1 and 3. Also here, the stabilizing intermolecular hydrogen bond is described through the interaction of three neighboring molecules arranged according to a C3h symmetry, where the hydroxyls act simultaneously as donors and acceptors (Figure 4). Not surprisingly, the

Table 2. Stabilization, Absolute, and Additive Interaction Energy Index (ΔEadd) for the Clusters Optimized from the THB, THPB, and HHTP Speciesa cluster-type

stabilization energy (kcal/mol)b

THB1 THB2 THB3

13.8 (14.5) 12.6 (13.4) 9.9 (10.6)

THPB

13.8 (14.5)

HHTP phase 1 3OH-1 3OH-2 3OH-3 3OH-4 4OH-1 4OH-2 5OH-1 5OH-2 5OH-3 6OH-1 6OH-2 phase 2 HHTP2 phase 3 HHTP3 a

relative energy (kcal/mol) 0.0 2.1 3.9

ΔEadd (kcal/mol) 1.28 0.67 1.37 1.60

7.7 (9.9) 12.7 17.4 19.2 4.8 10.2 12.4 3.9 6.5 8.6 (9.2) 10.3

0.0 9.4 16.9 30.6 2.2 10.2 3.1 7.1 11.9 1.1 28.7

0.7

−0.2

27.4 (30.9) (tetramer)

1.0 (tetramer)

24.2 (28.8) (tetramer)

−0.4 (tetramer)

b

All values in kilocalories per mole. Values in parentheses correspond to stabilization energies calculated from the energies of the monomers as defined in the cluster.

Assembly of 1,3,5-Tris(4-hydroxyphenyl)benzene. Figure 3 shows typical STM images measured upon exposure of

Figure 4. B3LYP/6-31G*-optimized structures and molecular graph constrained to the intermolecular hydrogen bond region for the most stable THPB trimer. Red and yellow dots in molecular graphs denote bond and ring critical points, respectively. In parentheses, corresponding values for the dimers. Bond lengths in Å and electron density in e au−3.

Figure 3. STM images measured at low temperature (100 K) of the long-range ordered hexagonal phase formed upon adsorption of THPB on Au(111) at RT. (a) 10 × 10 nm2 STM image measured at VS = −1.6 V and It = 50 pA. (b) 4.4 × 4.4 nm2 STM image measured at VS = −1.6 V and It = 50 pA. A model with one molecule per unit cell is overlaid.

multicenter hydrogen bond in THPB presents identical bond distances and electron densities as those found in THB1, revealing the very little effect of the aromatic skeleton holding the hydroxyl groups in the intermolecular hydrogen bond linkages. For this system, we also find that trimerization induces sizable cooperative effects, which translate into typical geometric and electron density topologic changes relative to the dimer and into a significant additive interaction energy. Specifically, incorporating a third THPB molecule to the dimer leads to the shortening of the O−O distance (0.06 Å)

the Au(111) surface to THPB. A clear indication of the high mobility of THPB molecules in Au(111) is that at RT it is very difficult to obtain stable STM images of the THPB island boundaries. Indeed, stable enough images as those shown in Figure 3 are obtained only after cooling the sample temperature well below RT. For instance, the images in Figure 3 were measured with the STM while keeping the sample temperature at 105 K. A very low molecule−substrate interaction is then expected, as in the previous case, being the hexagonal layer D



Article

and to the stretching of the O−H bond from the donor moieties (0.01 Å). Trimerization also produces a reorganization of the electronic electron density, which increases in a 13% at the position of the hydrogen bond critical points (bcp) but decreases in 0.008 e/au−3 for the O−H bonds, as compared with the donor moiety of the dimer. Geometric changes, however, are not only restricted to the binding sites; upon trimerization, C−O bonds adopt an intermediate bond distance between those calculated for the donor and the acceptor units in the dimer, and the angle defined by two THPB molecules linked through a hydrogen bond slightly increases from 117° in the dimer to 120° in the trimer due to the C3h symmetry requirements of the network. These structural features are reflected in additive interaction energy of 1.60 kcal/mol; see Table 2. The calculated stabilization energy per hydrogen bond for THPB amounts to 4.6 kcal/mol; see Table 2. Also here, the three hydrogen bonds are strictly equivalent and therefore contribute to the same extent to the stabilization of the superstructure. This value exceeds by 0.8 kcal/mol the stabilization energy per hydrogen bond calculated for the dimer, which is again consistent with the cooperative effects that characterize these structures. Assembly of 2,3,6,7,10,11-Hexahydroxytriphenylene. When depositing close to one monolayer of HHTP on Au(111) at RT, no long-range order was obtained according to the STM images, in stark contrast with the situations encountered with THB or THPB on Au(111), as shown in the previous sections. Only after annealing the surface at temperatures in the range of 450−500 K were well-ordered layers obtained. Three distinct phases were indeed found that will be described in the following. The HHTP molecule doubles the number of OH binding sites, as compared with the former alcohols, increasing the number of interacting possibilities with neighboring molecules, as also in this case hydroxyl groups can behave simultaneously as hydrogen donors and acceptors. Attending to the arrangement of the alcohols in the STM images, we have evaluated the stability and analyzed the bonding of HHTP trimers (phase 1) and tetramers (phases 2 and 3) to determine the most likely geometry for the observed superstructures. To account for the geometric restrictions imposed by the surrounding hydrogen bonds, we have considered clusters with four molecules in phase 1, seven molecules in phase 2, and eight molecules in phase 3 and used diphenols to saturate the more external hydrogen bonds. HHTP-Phase1. From all HHTP phases here reported, this is the only one that has some similarity to the ordered layer found by Pawlak et al.20 when depositing HHTP on Ag(111) surfaces. It is a rather simple hexagonal lattice stabilized by hydrogen bonds and formed by just one molecule per unit cell, as for the cases of THB and THPB adsorption on Au(111). Figure 5 shows typical STM images measured at RT on this phase. They consist of an array of bright protrusions corresponding to the adsorption of a single HHTP molecule per cell. Figure 5b displays a model that will be described later, corresponding to this phase. The lattice vectors amounts to a = b = 11.5 ± 0.6 Å and γ = 120 ± 5°. This corresponds to a molecular density n = (0.87 ± 0.09) × 1014 molecules/cm2. The increase in the number of binding sites per monomer prevents HHTP from assembling into very symmetric arrangements as those observed for the trialcohols because such a disposition would result in the loss of compactness in the

Figure 5. STM images of phase 1 of HHTP adsorbed on Au(111). (a) 7 × 7 nm2 image measured at VS = −0.25 V and It = 80 pA. (b) 3.5 × 3.5 nm2 STM image measured under the same tunnelling conditions. An overlaid hexagonal unit cell with one molecule per cell is drawn.

network. In this respect and taking into account the conformational richness of the monomer, we have systematically explored the most stable arrangements of HHTP trimers considering each of the following four possible hydrogen bond assemblies as central motif: three-center hydrogen bond, fourcenter hydrogen bond, five-center hydrogen bond, and sixcenter hydrogen bond, as shown in Scheme 2. Scheme 2. Different Hydrogen-Bond Motifs Occurring in the Trimeric Structures Studieda

a

Inset represents the three possible conformations of the hydroxyls within each dihydroxyphenyl moiety.

As can be seen from Scheme 2, depending on the combination of conformations of the dihydroxyphenyl moieties, namely, in, out, or half, intervening in the intermolecular junction, three and two different five- and six-center hydrogen bond motifs are possible, but only one three-center and one four-center hydrogen bond structure can be conceived. As a general rule, all hydrogen-bonded structures optimized involve hydroxyl groups from three different monomers. The possibility to form three- and four-center hydrogen-bonded assemblies involving only two molecules was also considered, but they clearly correspond to unstable species or were incompatible with the patterns observed in the STM images. The conformation of the hydroxyl groups interacting with the E



Article

intermolecular bonding region but not participating in the former figures was also varied to account for all possible structures. In the following, we will refer to the clusters indicating the size of the central hydrogen bond structure, followed by a number indicating the stability of the cluster within each family. Altogether, we have optimized 11 clusters; see Figure S2 of the Supporting Information, whose relative energies are summarized in Table 2. Because our model neglects periodic boundary conditions, we have assumed the most stable half conformation for the external OH groups not participating or interacting with the intermolecular hydrogen bond network. This way, we avoid artificial repulsive interactions between hydroxyl groups sitting at the same benzene moiety, which are not allowed to stabilize via hydrogen bond, allowing the direct comparison of the stability of different clusters considered. The examination of Table 2 reveals that the cluster 3OH-1 corresponds to the most stable species, closely followed by 6OH-1, 4OH-1, and 5OH-1, defined in Figure 6. The clusters 6OH-2 and 3OH-4 were found to be the least stables, the maximum energy gap being ca. 31 kcal/mol. We identify two main factors responsible for introducing instability in the system: (i) the presence of three-center hydrogen bonds

consisting of two donors and a single acceptor (see for instance 3OH-2, 3OH-3, and 3OH-4 clusters in Figure S2 of the Supporting Information) and (ii) the coexistence of parallel and linear hydrogen bonds within the same hydrogen bond motif (see hydrogen bond d and e motifs in Scheme 2), which prevents specific donor−acceptor interactions to be effective and results into weak hydrogen bonds. Consistently, the change from the half to the in conformation of one of the dihydroxyphenyl moieties in the 3OH-1 trimer results in an energy increase of ca. 9 kcal/mol (3OH-2 cluster, Figure S2 of the Supporting Information), whereas a second and a third OH rotation imply a further increase of 8 kcal/mol (3OH-3 cluster, Figure S2 of the Supporting Information) and 14 kcal/mol (3OH-4 cluster, Figure S2 of the Supporting Information), respectively; see Table 2. In turn, the rearrangement undergone by the monomers in 5OH-2 and 5OH-3 (Figure S2 of the Supporting Information) to maximize the parallel interaction of particular pairs of hydroxyl groups leads to considerable large O····H distance(s) in other parts of the 5OH ring and therefore to weak hydrogen bonds, affecting the stability of the former systems. This, however, does not apply to the second most stable 6OH-1 cluster (Figure 6), for which the very symmetric arrangement of the monomers allows the simultaneous optimization of the three equivalent hydrogen bonds between hydroxyl groups lying parallel. Remarkably, our most stable calculated cluster, 3OH-1 (Figure 6a), consisting of a main three-center hydrogen bond arising from the interaction of three monomers in their most stable configuration, does not correspond to the lowest energy conformation predicted by the DFT calculations of ref 20, which proposes our fourth most stable cluster 5OH-1 (Figure 6b) as the deposited superstructure of HHTP molecules. Note that this structure lies ca. 3 kcal/mol above 3OH-1 and is characterized by a five-center hydrogen bond motif gathering two monomers in a half conformation and a third out monomer. Neither such a small energy difference, of the same order of magnitude as the error inherent to the methodology used, nor the comparison of the structural pattern defined in the STM image with the two cluster models as obtained from DFT calculations, is determinant to reject one of the structures in favor or the other, highlighting the importance of carrying a complete stability analysis for all of the possible conformers of the superstructure to interpret the STM patterns at molecular level. Structural or energetic criteria allow, however, definitively discarding the clusters 3OH-2, 3OH-3, 3OH-4, 4OH-2, 5OH-2, 5OH-3, 6OH-1, or 6OH-2 as the structures occurring in the experimentally observed monolayers because they either correspond to very unstable arrangements or they involve structures incompatible with the experiment; see Table 2 and Figure S2 of the Supporting Information. The same criteria point to the trimers 3OH-1, 4OH-1, and 5OH-1 as the most likely to describe the experimentally observed supramolecular assembly of HHTP molecules in phase 1. It is worth noting the absence of correlation between the stabilization energy and the relative stability of the different assemblies by comparing the stabilization and relative energies in Table 2. In this respect, the highest cohesion energy was obtained for the 3OH-4 cluster, which actually corresponds to the least stable structure, whereas the cohesion energies for the two most stable structures were calculated to be ca. half that of the former species. This unexpected lack of correlation is attributed to the inherent stability of the monomers composing

Figure 6. B3LYP/6-31G* optimized structures for the most stable isomers: (a) 3OH-1, (b) 5OH-1, (c) 4OH-1, and (d) 6OH-1 describing HHTP phase 1 and molecular graphs constrained to the hydrogen bond intermolecular region for 3OH-1 and 5OH-1. Red and yellow dots in molecular graphs of trimers 3OH-1 and 5OH-1 denote bond and ring critical points, respectively. In parentheses, corresponding values for the dimers. Bond lengths in Å and electron density in e au−3. F



Article

the cluster. Thus, the 3OH-4 cluster with the strongest intermolecular hydrogen bonds is built up from high -energy monomers, containing dihydroxylphenyl moieties in the in conformation. Therefore, although this cluster presents the strongest intermolecular interactions, these interactions are not enough to counterbalance the smaller stability of the constituting monomers. Hence, the most stable structure is that arising from the most stable monomers but not necessarily that containing the strongest hydrogen bonds. Consequently, it is possible to conclude that the relative stability of these clusters is governed by the interplay of the strength of the intermolecular interactions and the relative stabilities of the monomers forming the cluster. Cooperative effects were also evaluated for the two most stable gas-phase structures, 3OH-1 and 6OH-1. Contrary to the trialcohols, where cooperative effects are positive and sizable, for these two clusters the additive interaction energy index was found to be significantly smaller or even slightly negative in the case of 6OH-1. In this respect, the compensation within each monomer between stabilizing and destabilizing interactions, see Figure 6, explains the absence of cooperativity ΔEadd = −0.2 kcal/mol found for 6OH-1 upon trimerization. For the particular case of 3OH-1, the geometric constraints imposed by the extended aromatic skeleton translate into a small increase in electronic density at the hydrogen bond bcp’s on going from the dimer to the trimer that are consistent with the smaller (0.7 kcal/mol) calculated ΔEadd values as compared to the trialcohols that share the same hydrogen bond pattern. In summary, for this phase, the simplest structure that allows describing the intermolecular interaction is a trimer of HHTP molecules arranged according to the 3OH-1 optimized cluster, where the network of hydrogen bonds closely resembles that observed in water trimers. HHTP-Phase2 (“Bow Tie” Phase). The second phase found after depositing close to one monolayer HHTP on Au(111) and annealing is shown in Figure 7. At first sight, this seems a rather complex structure, where it is not easy to find a repeated unit cell. A careful inspection reveals that although some defects are detected a huge unit cell can be assigned to this long-range self-assembled layer. Figure 7b shows that the molecular layer forms an oblique lattice with a unit cell characterized by the parameters a = 69 ± 3 Å, b = 22 ± 1 Å, and γ = 120 ± 5°. The substrate−molecule interaction seems to be low enough to keep intact the herringbone reconstruction of the Au(111) substrate (apparent as the parallel lines running across the images from top left to bottom right). The structure must be, then, stabilized by hydrogen bonds between the hydroxyl moieties. The unit cell consists of 12 molecules; this corresponds to a molecular density n = (0.91 ± 0.09) × 1014 molecules/cm2, which is very close to that of the HHTP-phase 1. (See Table 1.) This structure can be described as the intercalation of headto-head and head-to-tail rows of HHTP molecules (see Scheme 3), where the direction of the head-to-tail rows is apparently random. Because of this peculiar geometry, this HHTP reconstruction has been called the “bow tie” phase. The singular arrangement of HHTP molecules in the STM image of phase 2 does not allow any other conformation for the hydroxyl groups participating in the hydrogen bond connecting the monomers than that shown in Figure 8, where the OH groups within the dihydroxyphenyl moieties are arranged in a half conformation (recall the inset in Scheme 2). Any other combination of conformations for the hydroxyl pairs in the

Figure 7. STM images of the “bow tie” phase (phase 2) of HHTP adsorbed on Au(111). (a) 20 × 20 nm2 STM image measured at VS = −0.98 V and It = 40 pA. (b) Zoom-in STM image (9.3 × 8.0 nm2) measured under the same tunnelling conditions. The unit cell of the “bow tie” reconstruction is overlaid, as well as a molecular model, containing 12 flat-lying molecules per unit cell.

Scheme 3. Head-to-Head and Head-to-Tail Interactions of HHTP Molecules in Phase2

Figure 8. B3LYP/6-31G*-optimized structure for the HHTP heptamer describing the assembly of phase 2 (a) and its corresponding cross-shape arrangement (b). In parentheses, corresponding values for the dimers. Bond lengths in Å.

dihydroxyphenyl moieties would result in a structure incompatible with the STM image. G



Article

In the present phase, the basic cluster defining the intermolecular hydrogen bond corresponds to a tetramer, composed of a head-to-head HHTP central dimer and two monomers belonging to an upper and lower row pointing in the same or opposite directions; see the dashed square in Figure 8a. The only intermolecular interaction identified within this HHTP monolayer involves six hydroxyl groups arranged into two adjacent three-center ring-like hydrogen bonds that in turn define a central four-center ring like hydrogen bond, with the two additional OH groups of the tetramer not participating in the intermolecular bonding. Interestingly, a simultaneous 45° counter-clockwise rotation of the upper and lower subunit in the tetramer of Figure 8a would favor the participation of the two other OH groups leading to a stable cluster; see Figure 8b. However, this crossshape arrangement is not compatible with the experimental STM image. To discern whether the experimentally observed assembly for HHTP is a consequence of the interaction of the molecules with the surface or it rather corresponds to the inherently most stable structure, we have compared the stability of the two tetramers defining these two previous arrangements. (See dashed blue squares in Figure 8.) Indeed, we find that the cross-shape tetramer is 19 kcal/mol less stable than the rotated structure shown in Figure 8a. Therefore, we conclude that the observed superstructure reflects the lowest energy arrangement of the monomers within the tetramer and does not arise from the interaction of the monolayer with the surface. Finally, it is interesting to compare the size of cooperative effects in the tetramer with respect to the trialcohol and hexalcohol trimers. In principle, and taking into account the similarities with the intermolecular hydrogen bond in the trialcohol trimers, one would expect the reinforcement of all hydrogen bonds relative to the dimers because the hydrogen bond corresponds to the most favorable situation where each hydroxyl behaves at the same time as a donor and as an acceptor. However, cooperative effects for the tetramer were calculated to be positive but smaller than in the trialcohols (ΔEadd 1.0 kcal/mol, see Table 2). Indeed, the comparison of the geometry of the optimized dimers with that of the tetramer reveals that although a slight decrease in two O−O distances (0.03 Å) is observed in the tetramer, the third O−O distance, also taking part in the central four-center hydrogen bond, does not decrease in the tetramer but has the same value as in the dimer. HHTP-Phase3 (“Ribbons” Phase). In the present work, a third phase has been found experimentally after RT adsorption of HHTP molecules on Au(111), followed by annealing. This phase is shown in Figure 9. Because of its appearance, it will be named the “ribbons” phase. As for all of the other phases, the Au(111) herringbone reconstruction is preserved below the long-range ordered molecular layer, pointing again toward a hydrogen bonded structure, as shown in Figure 9b. The “ribbons” phase corresponds to an oblique lattice with two molecules per unit cell and the following lattice parameters: a = 12.8 ± 0.6 Å, b = 22 ± 1 Å, and γ = 120 ± 5°. The nominal density corresponding to this phase, n = (0.82 ± 0.08) × 1014 molecules/cm2, is also very close to the molecular densities of the other two HHTP phases. (See Table 1.) From the analysis of the experimental STM image, it is possible to extract a clear picture of the assembly of HHTP molecules on the surface. Within this phase, two different hydrogen bonds define the network of intermolecular interactions: (i) the first that defines a tetramer of HHTP

Figure 9. STM images of the “ribbons” phase (phase 3) of HHTP adsorbed on Au(111). (a) 10 × 10 nm2 STM image measured at VS = −0.22 V and It = 34 pA. (b) Model of the “ribbons” phase, with two molecules per unit cell, is overlaid on a 4.5 × 5.0 nm2 STM image, measured under the same tunneling conditions.

molecules and (ii) a second one responsible for the lateral interaction of tetramers; see Figure 10.

Figure 10. B3LYP/6-31G*-optimized structure for the cluster of HHTP molecules describing phase 3.

The complexity of the intermolecular bonding in this phase due to the large number of centers participating in the hydrogen bonds prevented a systematic analysis of the stability of the different hydrogen bonded clusters arising from all possible combinations of conformations of the dihydroxyphenyl moieties, as the one performed for HHTP phase 1. Thus, an alternative approach for determining the monolayer structure at atomic level was adopted, which consisted of a preliminary analysis of the stability of HHTP dimeric structures. For this, we specifically considered the interaction of a pair of HHTP molecules adopting an in, out, or half conformation (recall the inset of Scheme 2) for the dihydroxyphenyl moieties participating in the lateral hydrogen bond. All possible relative orientations for the dihydroxyphenyl moieties participating in the hydrogen bonds with a half conformation were considered, and the resulting structures were included in the stability analysis. A summary of all possible HHTP dimer lateral H



Article

motif that clearly leads to the energetic minimum. We found that the most stable arrangement consists of three hydroxyl groups forming a three-membered ring where each OH group behaves simultaneously as hydrogen bond donor and hydrogen bond acceptor, very much like in water and methanol trimers. However, the situation is different for tectons based on orthodiphenols because this structural motif does not correspond to the most stable assembly, and a more complex scenario is found. Thus, HHTP molecules can form several arrangements that are practically isoenergetic. This is in good agreement with the experimental evidence that shows that three ordered phases can coexist under similar preparation conditions. This structural versatility lies on the degrees of freedom allowed on the eight OH groups from four different molecules that simultaneously interact to form various stable arrangements. The understanding of such complex structures required a complete stability analysis for the different conformers, which revealed that the relative stability of many of these structures lies within the inherent error of the theoretical method employed. In fact, even more than one plausible structure has been found for a same phase experimentally observed. The insights presented in this work are especially valuable in the field of designing 2D nanomaterials. On one hand, recent studies have demonstrated the ability of weakly bound hydrogen bond networks of alcohols to evolve to more stable and strongly bound structures upon partial oxidation of the hydroxyl groups. Thus, hydrogen-bonded assemblies can act as templates for new materials. On the other hand, a deeper understanding of the weak interactions resulting in ordered structures offers the possibility of engineering new supramolecular polymers with unique features. One of these features exclusive of supramolecular polymers is the so-called selfhealing processes, which imply easy repair of defects thanks to the structural versatility and reversibility of networks based on hydrogen bonding. In a further twist in the quest of control over structural diversity of hydrogen-bonding-based supramolecular materials, studies on hybrid structures generated by the self-assembly of the alcohols reported in this work combined with polyacids are in progress.

connections and their relative energies is presented in Figure S3 of the Supporting Information. This analysis reveals that the most stable dimers carry a half conformation in the dihydroxyphenyl moieties participating in the hydrogen bonds, and among these structures, those with the lowest energies are the ones where the relative orientation of the interacting hydroxyls allows the smallest distance between the two HHTP moieties leading to the strongest hydrogen bonds (dimers 1 and 2). From these findings, tetramers fixing the conformation of the external hydrogen bonds as in the most stable dimers were constructed. In these clusters, the four dihydroxyphenyl moieties participating in the central hydrogen bond were assumed in the most stable half conformation, and different orientations were considered to find the most stable tetramer that better reproduces the observed STM image. The geometries of the optimized tetramers and their relative energies are collected in Figure S4 of the Supporting Information. Among these clusters, the most stable one fitting the experimental STM image is tetram4, in which two HHTP molecules act simultaneously as hydrogen donors and acceptors whereas the other two remaining monomers constitute a double hydrogen donor and a double hydrogen acceptor, respectively. Finally, the relative orientation of the dihydroxyphenyl moieties lying at the interface of two consecutive tetramers (Figure 10) was modified so as to maximize the number of hydrogen bonds between the two pairs of tetramers, leading to the final structure in Figure 10. Also, in this case, the neglect of periodic conditions urged the use of phenol molecules to saturate external hydrogen bonds to obtain a suitable final model for the HHTP phase 3. Interestingly, the central hydrogen bond governing the cluster used for the modeling of phase3, tetram4, recalls the intermolecular bonding in the cross-shape arrangement initially proposed to describe the HHTP-phase 2 assembly that was calculated 19 kcal/mol over the most stable structure describing this phase (compare Figures 8b and 10). However, even if the central hydrogen bond involves the same number of centers in the two clusters, the anticomplementary arrangement of the hydroxyls in the head-to-head central dimer in tetram4 prevents the interaction between these two HHTP monomers conferring more flexibility to the cluster and allowing the rotation of these two moieties that can now establish two new hydrogen bonds with the immediately adjacent HHTP molecules. These additional hydrogen bonds bring stability to the cluster (in fact tetram4 is calculated 7 kcal/mol below the cross-shape-arranged tetramer) and would explain why tetram4 corresponds to the repeated structure observed in one of the experimental phases registered for the assembly of HHTP molecules. Overall, we have described the formation of self-assembled monolayer structures of three different polyalcohols with trigonal symmetry. In particular, THB, THPB, and HHTP, absorbed on an Au (111) surface have been studied by STM experiments and analyzed by means of theoretical calculations. The analysis presented herein shows that hydrogen-bonding interactions govern the organization of the building blocks on Au(111), being the interaction with the substrate very weak. From a fundamental perspective, several interacting polyalcohols can adopt multiple conformations; therefore, a considerable geometric flexibility is expected. However, for simple alcohols, such as THB and THPB, a unique phase on Au(111) has been characterized. These observations indicate that from all potential geometric possibilities there is a unique

■

EXPERIMENTAL SECTION STM Images. The experiments were carried out in an ultrahigh vacuum (UHV) chamber (base pressure below 1 × 10−10 Torr) equipped with a homemade variable-temperature scanning tunneling microscope (VT-STM)28 and typical surface preparation and characterization techniques as ion sputtering, sample annealing by electron bombardment, STM tips conditioning by field emission, exchangeable evaporation cells, low-energy electron diffraction (LEED)-Auger electron spectroscopy (AES) combined device, and quadrupole-based mass spectrometer (QMS). The VT-STM is connected to a continuous-flow liquid-helium cryostat allowed to vary the sample temperature between 40 and 400 K. The three molecular species studied in the present work were deposited under UHV conditions on clean Au(111) surfaces kept at RT. Pristine Au(111) surfaces were obtained routinely by means of repeated cycles of sputtering with Ar+ ions (typically at 600 eV kinetic energy and 5 μA sample current), followed by annealing at 870 K. After this procedure, the samples were always checked by STM and they displayed clean and well-reconstructed surfaces presenting the standard Au(111)-(22 × √3)-rect, or herringbone, reconstruction. I



Article

different systems/clusters, we have used the geometry of the monomers as occurring in the nmer, eliminating in this way the geometric deformation contribution to the additive interaction energy index.

The three molecular species were deposited from home-built evaporators consisting of tantalum crucibles heated by irradiation from a tungsten filament. Depending on the molecule, the crucibles were heated at different temperatures: 345 K for THP, 430 K for THPB, and 550 K for HHTP. In all cases, the deposition rate and purity was controlled by the QMS. STM images were measured at RT and at low temperature (∼100 K) in the constant current mode. The tunneling conditions were, typically, sample bias voltages between −2 and +2 V and tunnel currents below 100 pA. All STM data were acquired and processed with the WSxM software.29 Computational Details. The superstructures observed in the STM images were theoretically investigated through the characterization of the possible 2-D arrangements of the three alcohols THP, THPB, and HHTP by means of DFT calculations. In view of the different types of 2D networks experimentally observed, we have restricted our investigation to the optimization of minimum size clusters, which allow the description of the linkage between monomers, that is structures where the stabilizing intermolecular hydrogen bond involves three molecules in the case of THB and THPB and three or four molecules in the case of the hexalcohol HHTP. Assuming, as in other studies investigating the self-assembly of the former20 or other aromatic molecules on Au(111) surfaces,30−34 that intermolecular interactions dominate over molecule−surface interactions, all of the calculations have been performed in the gas phase, neglecting surface−molecule interactions and enforcing Cs symmetry constraints to mimic the parallel disposition of the molecules with respect to the substrate arising from the charge transfer between the delocalized π* molecular orbitals of the aromatic alcohols and the flat, noncorrugated Au(111)surface. Moreover, geometry optimizations were performed on clusters with the number of molecules necessary to accurately describe intermolecular interactions; therefore, no periodic boundary conditions were applied, which is consistent with the local character of the multicenter hydrogen bond in these systems, as previously pointed by Pawlak et al. for HHTP.20 All calculations were performed using Becke’s threeparameter exchange potential,35 and the correlation functional of Lee et al.,36 B3LYP, as implemented in Gaussian 09 suite of programs,37 and the basis sets 6-31G* and 6-311+G(d,p) for geometry optimizations38−40 and final energies,41,42 respectively. The B3LYP/6-311+G(d,p)//B3LYP/6-31G* protocol has been proven to provide optimized geometries and binding energies for large hydrogen bonded systems in fairly good agreement with the experiment.43−48 Hydrogen bonding in the optimized gas-phase clusters was investigated through the atoms in molecules theory of Bader.49 For this, we have constructed molecular graphs by locating the bond critical points, that is, points where the electron density is a minimum in the direction of the bond and a maximum in the other two directions. There, we have evaluated the electron density, which provides a measure of the strength of the linkages. The analysis of the topology of the electron density additionally allows identifying ring critical points, connected to the concept of ring-like structures, characteristic of multicenter hydrogen bonds. Finally, cooperative effects were estimated using the additive interaction energy index (ΔEadd) calculated by subtracting from the stabilization energy of the nmer (n = 3, 4) that of all possible dimers we can define in the cluster. To allow a one-to-one comparison of the ΔEadd values obtained for

■

ASSOCIATED CONTENT

* Supporting Information S

Additional figures of some optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Phone: (34) 91 4976417. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the MICINN (projects MAT201020843-C02-01, ACI2009-0969, CTQ2009-13129-C02-01, CSC2007-00010, MAT2010-14902, and CSD2010-00024), Comunidad de Madrid (S-0505/MAT/0303, S2009PPQ/ 1533, and S2009/MAT-1467). A generous allocation of computing time at the CCC of the UAM is acknowledged.

■

REFERENCES

(1) Brunsveld, L.; Folmer, B. J. B.; Meijer, E. W.; Sijbesma, R. P. Supramolecular Polymers. Chem. Rev. 2001, 101, 4071−4097. (2) Slater, A. G.; Beton, P. H.; Champness, N. R. Two-Dimensional Supramolecular Chemistry on Surfaces. Chem. Sci. 2011, 2, 1440− 1448. (3) Griessl, S.; Lackinger, M.; Edelwirth, M.; Hietschold, M.; Heckl, W. M. Self-Assembled Two-Dimensional Molecular Host-Guest Architectures from Trimesic Acid. Single Mol. 2002, 3, 25−31. (4) Nath, K. G.; Ivasenko, O.; Miwa, J. A.; Dang, H.; Wuest, J. D.; Nanci, A.; Perepichka, D. F.; Rosei, F. Rational Modulation of the Periodicity in Linear Hydrogen-Bonded Assemblies of Trimesic Acid on Surfaces. J. Am. Chem. Soc. 2006, 128, 4212−4213. (5) Ye, Y. C.; Sun, W.; Wang, Y. F.; Shao, X.; Xu, X. G.; Cheng, F.; Li, J. L.; Wu, K. A Unified Model: Self-Assembly of Trimesic Acid on Gold. J. Phys. Chem. C 2007, 111, 10138−10141. (6) Clair, S.; Abel, M.; Porte, L. Mesoscopic Arrays from Supramolecular Self-Assembly. Angew. Chem., Int. Ed. 2010, 49, 8237−8239. (7) Gutzler, R.; Sirtl, T.; Dienstmaier, J. F.; Mahata, K.; Heckl, W. M.; Schmittel, M.; Lackinger, M. Reversible Phase Transitions in SelfAssembled Monolayers at the Liquid-Solid Interface: TemperatureControlled Opening and Closing of Nanopores. J. Am. Chem. Soc. 2010, 132, 5084−5090. (8) Blunt, M. O.; Russell, J. C.; Gimenez-Lopez, M. D.; Garrahan, J. P.; Lin, X.; Schroder, M.; Champness, N. R.; Beton, P. H. Random Tiling and Topological Defects in a Two-Dimensional Molecular Network. Science 2008, 322, 1077−1081. (9) Blunt, M.; Lin, X.; Gimenez-Lopez, M. D.; Schroder, M.; Champness, N. R.; Beton, P. H. Directing Two-Dimensional Molecular Crystallization Using Guest Templates. Chem. Commun. 2008, 2304−2306. (10) Zhou, H.; Dang, H.; Yi, J. H.; Nanci, A.; Rochefort, A.; Wuest, J. D. Frustrated 2D Molecular Crystallization. J. Am. Chem. Soc. 2007, 129, 13774−13775. (11) Xu, W.; Wang, J. G.; Jacobsen, M. F.; Mura, M.; Yu, M.; Kelly, R. E. A.; Meng, Q. Q.; Laegsgaard, E.; Stensgaard, I.; Linderoth, T. R.; et al. Supramolecular Porous Network Formed by Molecular Recognition between Chemically Modified Nucleobases Guanine and Cytosine. Angew. Chem., Int. Ed. 2010, 49, 9373−9377. J



Article

(12) Otero, R.; Xu, W.; Lukas, M.; Kelly, R. E. A.; Laegsgaard, E.; Stensgaard, I.; Kjems, J.; Kantorovich, L. N.; Besenbacher, F. Specificity of Watson-Crick Base Pairing on a Solid Surface Studied at the Atomic Scale. Angew. Chem., Int. Ed. 2008, 47, 9673−9676. (13) Mamdouh, W.; Dong, M. D.; Xu, S. L.; Rauls, E.; Besenbacher, F. Supramolecular Nanopatterns Self-Assembled by Adenine-Thymine Quartets at the Liquid/Solid Interface. J. Am. Chem. Soc. 2006, 128, 13305−13311. (14) Xu, S. L.; Dong, M. D.; Rauls, E.; Otero, R.; Linderoth, T. R.; Besenbacher, F. Coadsorption of Guanine and Cytosine on Graphite: Ordered Structure Based on GC Pairing. Nano Lett. 2006, 6, 1434− 1438. (15) Otero, R.; Lukas, M.; Kelly, R. E. A.; Xu, W.; Laegsgaard, E.; Stensgaard, I.; Kantorovich, L. N.; Besenbacher, F. Elementary Structural Motifs in a Random Network of Cytosine Adsorbed on a Gold(111). Surf. Sci. 2008, 319, 312−315. (16) Buchholz, S.; Rabe, J. P. Molecular Imaging of Alkanol Monolayers on Graphite. Angew. Chem., Int. Ed. 1992, 31, 189−191. (17) Wang, G.; Lei, S.; De Feyter, S.; Feldman, R.; Parker, J. E.; Clarke, S. M. Behavior of Binary Alcohol Mixtures Adsorbed on Graphite Using Calorimetry and Scanning Tunneling Microscopy. Langmuir 2008, 24, 2501−2508. (18) Zhang, H. M.; Yan, J. W.; Xie, Z. X.; Mao, B. W.; Xu, X. SelfAssembly of Alkanols on Au(111) Surfaces. Chem.Eur. J. 2006, 12, 4006−4013. (19) Abel, M.; Oison, V.; Koudia, M.; Porte, L. Conformational Change of Tetrahydroxyquinone Molecules Deposited on Ag(111). Phys. Rev. B 2008, 77, 085410. (20) Pawlak, R.; Clair, S.; Oison, V.; Abel, M.; Ourdjini, O.; Zwaneveld, N. A. A.; Gigmes, D.; Bertin, D.; Nony, L.; Porte, L. Robust Supramolecular Network on Ag(111): Hydrogen-Bond Enhancement through Partial Alcohol Dehydrogenation. Chem. Phys. Chem. 2009, 10, 1032−1035. (21) Harten, U.; Lahee, A. M.; Toennies, J. P.; Woll, C. Observation of a Soliton Reconstruction of Au(111) by High-Resolution HeliumAtom Diffraction. Phys. Rev. Lett. 1985, 54, 2619−2622. (22) Barth, J. V.; Brune, H.; Ertl, G.; Behm, R. J. Scanning Tunneling Microscopy Observations on the Reconstructed Au(111) SurfaceAtomic-Structure, Long-Range Superstructure, Rotational Domains, and Surface-Defects. Phys. Rev. B 1990, 42, 9307−9318. (23) Nicoara, N.; Roman, E.; Gomez-Rodriguez, J. M.; Martin-Gago, J. A.; Mendez, J. Scanning Tunneling and Photoemission Spectroscopies at the PTCDA/Au(111) Interface. Org. Electron. 2006, 7, 287− 294. (24) Mó, O.; Yánez, M.; Elguero, J. Study of the Methanol Trimer Potential Energy Surface. J. Chem. Phys. 1997, 107, 3592−3601. (25) Mó, O.; Yáñez, M.; Elguero, J. Cooperative (nonpairwise) Effects in Water Trimers: An Ab Initio Molecular Orbital Study. J. Chem. Phys. 1992, 97, 6628−6638. (26) Mó, O.; Yáñez, M.; Elguero, J. Cooperative Effects in the Cyclic Trimer of Methanol. An Ab-Initio Molecular Orbital Study. J. Mol. Struct.: THEOCHEM 1994, 314, 73−81. (27) Frank, H. S.; Wen, W. Y. Structural Aspects of Ion-Solvent Interaction in Aqueous Solutions- A Suggested Picture of Water Structure. Discuss. Faraday Soc. 1957, 24, 133−140. (28) Custance, O.; Brochard, S.; Brihuega, I.; Artacho, E.; Soler, J. M.; Baro, A. M.; Gomez-Rodriguez, J. M. Single Adatom Adsorption and Diffusion on Si(111)-(7 × 7) Surfaces: Scanning Tunneling Microscopy and First-Principles Calculations. Phys. Rev. B 2003, 67, 235410. (29) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. WSXM: A Software for Scanning Probe Microscopy and a Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705. (30) Kelly, R. E. A.; Lukas, M.; Kantorovich, L. N.; Otero, R.; Xu, W.; Mura, M.; Laegsgaard, E.; Stensgaard, I.; Besenbacher, F. Understanding the Disorder of the DNA Base Cytosine on the Au(111) Surface. J. Chem. Phys. 2008, 129, 184707.

(31) Lukas, M.; Kelly, R. E. A.; Kantorovich, L. N.; Otero, R.; Xu, W.; Laegsgaard, E.; Stensgaard, I.; Besenbacher, F. Adenine Monolayers on the Au(111) Surface: Structure Identification by Scanning Tunneling Microscopy Experiment and Ab-initio Calculations. J. Chem. Phys. 2009, 130, 024705. (32) Mura, M.; Martsinovich, N.; Kantorovich, L. Theoretical Study of Melamine Superstructures and Their Interaction with the Au(111) Surface. Nanotechnology 2008, 19, 465704. (33) Mura, M.; Silly, F.; Briggs, G. A. D.; Castell, M. R.; Kantorovich, L. N. H-Bonding Supramolecular Assemblies of PTCDI Molecules on the Au(111) Surface. J. Phys. Chem. C 2009, 113, 21840−21848. (34) Otero, R.; Schock, M.; Molina, L. M.; Laegsgaard, E.; Stensgaard, I.; Hammer, B.; Besenbacher, F. Guanine Quartet Networks Stabilized by Cooperative Hydrogen Bonds. Angew. Chem., Int. Ed. 2005, 44, 2270−2275. (35) Becke, A. D. Density-Functional Thermochemistry. 3. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (36) Lee, C. T.; Yang, W. T.; Parr, R. G. Development of the ColleSalvetti Correlation-Energy Formula into a Functional of the ElectronDensity. Phys. Rev. B 1988, 37, 785−789. (37) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09; Gaussian, Inc.: Wallingford, CT, 2009. (38) Ditchfield, R.; Hehre, W. J.; Pople, J. A. Self-Consistent Molecular-Orbital Methods. 9. Extended Gaussian-Type Basis for Molecular-Orbital Studies of Organic Molecules. J. Chem. Phys. 1971, 54, 724−728. (39) Hariharan, P. C.; Pople, J. A. The Influence of Polarization Functions on Molecular Orbital Hydrogenation Energies. Theor. Chim. Acta 1973, 28, 213−222. (40) Hehre, W. J.; Ditchfield, R.; Pople, J. A. Self-Consistent Molecular Orbital Methods. XII. Further Extensions of Gaussian-Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules. J. Chem. Phys. 1972, 56, 2257−2261. (41) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. V. Efficient Diffuse Functional-Augmented Basis-sets for Anion Calculations. 3. The 3-21+G Basis set for 1st-Row Elements, Li-F. J. Comput. Chem. 1983, 4, 294−301. (42) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Selfconsistent Molecular-Orbital Methods. 20. Basis Set for Correlated Wave-Functions. J. Chem. Phys. 1980, 72, 650−654. (43) Boyd, S. L.; Boyd, R. J. A Density Functional Study of Methanol Clusters. J. Chem. Theor. Comput. 2007, 3, 54−61. (44) Riley, K. E.; Op’t Holt, B. T.; Merz, K. M., Jr. Critical Assessment of the Performance of Density Functional Methods for Several Atomic and Molecular Properties. J. Chem. Theor. Comput. 2007, 3, 407−433. (45) González, L.; Mó, O.; Yáñez, M. High Level Ab Initio and Density Functional Theory Studies on Methanol-Water Dimers and Cyclic Methanol(water)2 Trimer. J. Chem. Phys. 1998, 109, 139−150. (46) González, L.; Mó, O.; Yáñez, M. Density Functional Theory Study on Ethanol Dimers and Cyclic Ethanol Trimers. J. Chem. Phys. 1999, 111, 3855−3861. (47) González, L.; Mó, O.; Yáñez, M. High-Level Ab-Initio versus DFT Calculations on (H2O2)2 and H2O2-H2O Complexes as Prototypes of Multiple Hydrogen Bond Systems. J. Comput. Chem. 1997, 18, 1124. (48) González, L.; Mó, O.; Yáñez, M.; Elguero, J. Cooperative Effects in Water Trimers. The Performance of Density Functional Approaches. J. Mol. Struct.: THEOCHEM 1996, 371, 1. (49) Bader, R. F. W. Atoms in Molecules - A Quantum Theory; Oxford University Press: Oxford, U.K., 1990.

K


Some Pictures of Alcoholic Dancing: From Simple to Complex

Recommend Documents