Complexes with the same aromatic molecules

Figure 5.1 shows the structures of the most stable minima located for the complexes formed by guanidinium cation and two benzene molecules, whereas Table 5.1 lists the values obtained for their complexation energies with different levels of calculation.

Four different minima have been found for these complexes. Bz-Bz-1 corresponds to a structure with stacked aromatic rings and the guanidinium cation interacting with one of them, adopting a perpendicular arrangement which has been shown to be the most stable minimum in guanidinium···Bz complexes.[20, 37] Two N-H···π contacts are observed: one at 2.3 Å and a longer one at 2.9 Å. Bz-Bz-2 and Bz-Bz-3 correspond to structures where benzene rings display a perpendicular arrangement (T-shaped) with C-H···π contacts at around 2.5-2.6 Å to the center of the other aromatic ring. These arrangements roughly correspond to the typical structures found for benzene dimer (parallel displaced and C-H···).[5] Finally, Bz-Bz-4 corresponds to a doubly T-shaped structure, where the aromatic rings are far apart, both of them coordinating the

5. Guanidinium aromatic trimers

113

guanidinium cation in perpendicular arrangements, forming hydrogen bonds at 2.3 Å of the center of the ring.

Table 5.1 shows the complexation energies of Bz-Bz complexes as obtained with a variety of methods. Taking into account the values in Table 5.1 the results obtained at the M06-2X/6-31+G* and at the SCS-MP2/CBS levels of calculation are pretty similar, whereas MP2 gives in all cases overestimated complexation energies, especially with the larger basis set. Therefore, in the following we will mainly discuss values obtained with these two levels of calculation.

Table 5.1. Complexation energies (kcal/mol) obtained for complexes formed by guanidinium and two equal aromatic molecules as obtained with different methods, all employing the optimized M06-2X/6-31+G* geometry.

631+G* aug-cc-pVDZ CBS M062X MP2 SCS-MP2 MP2 SCS-MP2 Bz-Bz-1 -18.55 -21.03 -16.97 -22.79 -18.54 Bz-Bz-2 -21.03 -22.44 -18.58 -24.30 -20.27 Bz-Bz-3 -20.58 -21.41 -17.20 -23.25 -18.85 Bz-Bz-4 -25.62 -27.12 -23.52 -29.25 -25.48 Ph-Ph-1 -33.19 -31.93 -27.83 -34.05 -29.71 Ph-Ph-2 -31.21 -30.47 -25.76 -32.80 -27.85 Ph-Ph-3 -29.47 -29.93 -26.34 -31.88 -28.10 Ph-Ph-4 -28.73 -27.31 -22.36 -29.58 -24.36 Ph-Ph-5 -32.20 -31.05 -26.76 -33.20 -28.67 Ph-Ph-6 -32.21 -32.71 -29.05 -34.67 -30.81 Ph-Ph-7 -32.81 -30.63 -27.90 -32.17 -29.35 In-In-1 -29.61 -34.47 -27.43 -36.87 -29.53 In-In-2 -32.72 -36.01 -30.25 -38.37 -32.36 In-In-3 -31.54 -34.70 -28.94 -37.04 -31.04 In-In-4 -33.14 -36.39 -30.55 -38.79 -32.70 In-In-5 -38.16 -40.04 -34.70 -42.69 -37.09

Alba Campo Cacharrón

114

Figure 5.1. Most stable minima found for complexes of guanidinium with two benzene molecules as obtained at the M06-2X/6-31+G* level. Selected distances in Å.

Bz-Bz-2

Bz-Bz-1

Bz-Bz-3

Bz-Bz-4

2.262

2.911

2.172

3.107

2.331

2.632

3.652

_2.293

2.292

2.930

2.962

5. Guanidinium aromatic trimers

115

As expected, Bz-Bz-4, with a doubly T-shaped structure is the most stable complex with a complexation energy of -25.5 kcal/mol because guanidinium can coordinate at the same time the two benzene rings, establishing two simultaneous cation··· interactions. Bz-Bz-2, with one T-shaped contact, is about 5 kcal/mol less stable (-20.3 kcal/mol). Here, one of the benzene rings coordinates to the guanidinium cation through a cation··· interaction and by means of a CH··· contact to the other benzene ring. Bz- Bz-1 corresponds to a parallel-displaced benzene dimer coordinated to guanidinium by one of the phenyl rings. The lack of direct contact between guanidinium and one of the benzene moieties makes this structure less stable, reaching only -18.5 kcal/mol. Finally, in Bz-Bz-3 the benzene ring that does not interact directly with the guanidinium cation moves away from the perpendicular arrangement. This structure could be considered as a combination of two of the minima of guanidinium···benzene complexes; one T-shaped and one parallel displaced with guanidinium parallel to the ring.[20, 37] This is the only structure which shows significant differences between M06-2X (-20.6 kcal/mol) and SCS- MP2/CBS (-18.9 kcal/mol) results.

Contrary to benzene, phenol possesses two different regions where a cation can establish a stabilizing interaction: the aromatic cloud and the lone pairs of the hydroxyl oxygen. Therefore, even when phenol is very similar to benzene, the presence of the hydroxyl group introduces a greater complexity on the potential energy surface of the clusters, allowing for a greater variety of stable structures. Figure 5.2 shows the structures of the most stable minima located for the complexes formed by guanidinium cation and two phenol molecules. It can be observed that phenol complexes cannot be classified into prototypical structures as easily as benzene ones, since due to the presence of the hydroxyl group there is a tendency to form O-H···O hydrogen bond contacts. Ph-Ph-6 and Ph-Ph-7 correspond to doubly T-shaped structures. In Ph-Ph-6 guanidinium cation coordinates the phenyl ring and the hydroxyl group whereas in Ph- Ph-7 it is coordinated only to the hydroxyl groups. It is worth noting that similar structures but with different disposition of the rings have been located, though only the most stable ones are included in Figure 5.2. In most cases the structures show O-H···O short hydrogen bonds at 1.7-1.9 Å, whereas guanidinium is also hydrogen-bonded to the hydroxyl group by means of N-H···O hydrogen bonds at around 1.8-1.9 Å. The contacts between guanidinium and the phenyl ring show similar behavior as that observed in benzene complexes.

No typical stacked structure has been found in complexes with phenol because the hydroxyl group tends to interact establishing OH···π or OH···O hydrogen bonds as it can be observed in all minima in Figure 5.2 except the doubly T-shaped ones. As regards T- shaped structures, only Ph-Ph-3 could be considered within this arrangement because it is the only one that presents both aromatic rings interacting in a perpendicular disposition, even though the interaction takes place by means of a O-H···O contact.

Alba Campo Cacharrón

116

Figure 5.2. Most stable minima found for complexes of guanidinium with two phenol molecules as obtained at the M06-2X/6-31+G* level. Selected Distances in Å.

2.676 2.507 1.874 2.457 2.571 1.897 Ph-Ph-1 Ph-Ph-2 Ph-Ph-3 Ph-Ph-5 Ph-Ph-4 Ph-Ph-6 Ph-Ph-7 2.650 1.886 1.954 1.979 3.161 1.807 2.384 2.151 2.444 1.998 2.341 1.929 1.897 2.021 2.021

5. Guanidinium aromatic trimers

117

Figure 5.3. Most stable minima found for complexes of guanidinium with two indole molecules as obtained at the M06-2X/6-31+G* level. Selected Distances in Å.

In-In-1

In-In-2

In-In-3

In-In-4

In-In-5

Alba Campo Cacharrón

118

Complexation energies are listed in Table 5.1. In these complexes more discrepancies are found between M06-2X/6-31+G* and the SCS-MP2/CBS values though, overall, M06- 2X tends to overestimate the interaction whereas the order of stability of the complexes is almost unaffected. As expected, the doubly T-shaped minima Ph-Ph-6 and Ph-Ph-7 are among the most stable ones with both methods, though it can be observed that M06-2X favors the contact with the hydroxyl oxygens, a trend that can be observed in the rest of the complexes with phenol. The interaction energy is larger than those observed in benzene complexes, reaching -30 kcal/mol or more. Therefore, an increment in stability of around -5 kcal/mol is obtained with respect to benzene complexes. This happens when comparing doubly T-shaped structures, but differences are even larger for other arrangements, as a consequence of the interaction of guanidinium with the hydroxyl group and the formation of O-H···O or O-H···π contacts.

The relevance of the hydrogen bonds can be observed in Ph-Ph-1, which is as stable as the doubly T-shaped structures. This minimum consists of a guanidinium cation coordinated to the hydroxyl group of one phenol unit which establishes a hydrogen bond to the hydroxyl group of the other phenol unit, reaching complexation energies around -30 kcal/mol (-33 kcal/mol with M06-2X, the most stable minimum). The rest of minima are less stable, differing mainly in the orientation of the guanidinium cation with respect to the rings, whereas both hydroxyl groups form a hydrogen bond.

Complexes of guanidinium cation with two indole molecules are shown in Figure 5.3. As in Bz-Bz complexes, the complexes with indole can be classified in the established arrangements: doubly T-shaped, T-shaped and parallel displaced. Guanidinium cation interacts with the two rings of indole by means of N-H···π hydrogen bonds, the one formed with the phenyl ring being always shorter than that formed with the pyrrol ring. Again, like in Bz-Bz complexes, the stability order of the structures does not change with the level of calculation employed as it can be observed in Table 5.1. As expected, the doubly T-shaped In-In-5 minimum where guanidinium cation coordinates both aromatic rings is the most stable complex with -37.1 kcal/mol. Comparing with complexes formed with benzene and phenol, it can be observed an increment in the stability of the complexes of around -5 kcal/mol when going from benzene to phenol, and a further -6 kcal/mol when passing from phenol to indole. In any case, the energy gain in phenol is mainly related to the presence of the hydroxyl group, whereas in indole complexes has its origin in the larger size of the aromatic systems leading lo larger dispersion and induction contributions to the stability. The increase in stability as the size of the aromatic unit grows is in line with previous work where the size of the interacting units was identified as one of the most important factors controlling the interaction.[50, 51]

5. Guanidinium aromatic trimers

119

As in phenol complexes, the structures of indole complexes are conditioned by the tendency of the N-H group of indole to establish hydrogen bonds with the aromatic rings of the other indole moiety at around 2.3 Å of the center of the other ring. C-H···π contacts at around 2.6 Å are also observed in T-shaped minima. The only true stacked structure found is In-In-1, where both indole molecules are rotated by about 90o, thus avoiding the formation of N-H···π contacts. The complexation energy for his structure reaches -29.5 kcal/mol, about -10 kcal/mol more stable than the corresponding stacked structure in benzene complexes Bz-Bz-1. The rest of minima show in some way or another a N-H···π contact which contributes to the stability of the complexes, so all of them are more stable than the stacked one. Thus, in In-In-4 indole rings have a quasi- perpendicular disposition, forming an angle of 67º, being the structure among those found which closest resembles a T-shaped contact. However, the indole ring that interacts directly with guanidinium also establishes a N-H···π hydrogen bond with the phenyl ring of the second indole unit. The combination of stacking and hydrogen bond makes this structure the second most stable one, reaching -32.7 kcal/mol. As in benzene complexes, there is a large energy gap between the doubly T-shaped structure and the rest of minima.

However, when the aromatic units are part of the side chain of amino acids they could rarely be so free as to adopt this doubly T-shaped structure, so T-shaped or stacked ones can be of importance to the stability of the system. In-In-2 and In-In-3 are structures closely related to In-In-4, the main difference being that the N-H···π contact takes place with the pyrrol ring of the second indole unit. In any case, the stability of the structures is pretty similar. Therefore, in all cases the most stable arrangement corresponds to the doubly T-shaped structures, with the stability increasing from benzene to phenol and to indole. The next stable structures are T-shaped ones, especially in phenol and indole complexes, where O-H···X or N-H···π hydrogen bonds can be formed. Finally, the least favorable arrangement is the parallel displaced stacked structure, with differences of about 1.5-2.5 kcal/mol with respect to T-shaped ones. Trends are not so clear in phenol complexes due to the great tendency of phenol to form O-H···X hydrogen bonds. As a consequence, no true stacked structure has been located and the T-shaped one has a O-H···O hydrogen bond. Also, the possibility of forming hydrogen bonds gives rise to other structures as Ph-Ph-1, where a chain of N- H···O-H···O hydrogen bonds is formed leading to a structure with the same stability as the doubly T-shaped ones.