Pristine Graphene-Based Catalysis: Significant Reduction of the

Apr 30, 2015 - Herein, we investigated the inversion of corannulene, sumanene, and dibenzo[a,g]corannulene when they are adsorbed onto graphene or int...
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Pristine Graphene-Based Catalysis: Significant Reduction of the Inversion Barriers of Adsorbed and Confined Corannulene, Sumanene, and Dibenzo[a,g]corannulene Pablo A. Denis* Computational Nanotechnology, DETEMA, Facultad de Química, UDELAR, CC 1157, 11800 Montevideo, Uruguay, Montevideo, Uruguay ABSTRACT: Herein, we investigated the inversion of corannulene, sumanene, and dibenzo[a,g]corannulene when they are adsorbed onto graphene or intercalated in bilayer graphene. The results obtained with the M06-L, M06-2X, TPPS-D3, TPSS-D3BJ, B3LYP-D3, and B3LYPD3BJ methods supported a significant reduction of the inversion barriers. In the case of corannulene adsorbed onto graphene, nonbonded interactions reduce the inversion barrier by at least a 50% with respect to the gas phase, whereas for adsorbed sumanene and dibenzo[a,g]corannulene the reductions are at least 39 and 67%, respectively. When the molecules are intercalated in bilayer graphene the lowering of the activation energy is more significant. In the particular case of dibenzo[a,g]corannulene the molecule is expected to display an almost planar structure, with its 0.83 Å bowl depth almost completely quenched. For intercalated corannulene and sumanene, the inversion barriers are at least 66 and 60% lower, respectively. It is our hope that these results can help to improve the design of receptors that can catalyze the inversion of buckybowls.

1. INTRODUCTION Since the first observation of buckminsterfullerene,1 the investigation of fullerene fragments has received considerable atention.2−24 In particular, corannulene is, perhaps, the mostly investigated bowl-shaped molecule.3,4 This molecule is the prototype pincer to interact with receptors,5−10 and for this reason one of the most interesting aspects of corannulene is the fast bowl-to-bowl inversion of corannulene.11−14 Scott et al.13 showed that the activation free energy is 10.2 ± 0.2 kcal/mol, at 209 K, which implies that corannulene inverts more than 200 000 times per second at room temperature.13 In the same vein, Seiders et al.14 estimated the inversion barrier of corannulene by employing substituted corannulenes in a combined experimental and theoretical analysis. The inversion barrier suggested was 11.5 kcal/mol, 1.3 kcal/mol larger than the one reported by Scott et al.13 The magnitude of the inversion barrier can be adjusted through chemical functionalization,12,15,16 or heteroatom substitution.17,18 For example, the addition of 10 aryl groups lowers the barrier of corannulene to 2.5 kcal/mol at 100 K, whereas for sym-1,3,5,7,9pentamanisylcorannulene the barrier is larger than the value corresponding to corannulene.16 Another way to modify the inversion barrier, without altering the chemical structure, is through the use of induced fit catalysis19 or by means of surface adsorption.20 Juricek et al.19 showed that when corannulene is inside the recently developed ExBox4+ receptor, the inversion barrier is reduced by 2.09 ± 0.72 kcal/mol. Using a different approach, Jaafar et al.20 found that the inversion barrier of © XXXX American Chemical Society

sumanene is reduced when it is adsorbed onto Ag(111). Also, they observed for the first time that both bowl-up and bowldown conformations can be stabilized. Taking into consideration the latter findings and having in mind that for wellcurved conjugated systems, π stacking is extremely important,21−25 we wondered how the inversion barriers will be affected when these systems are confined in a receptor that maximizes π stacking interactions. To that end we selected monolayer graphene and bilayer graphene. By means of the state of the art density functional calculations, we investigated the inversion of corannulene,4,5 sumanene,26,27 and dibenzo[a,g]corannulene28,29 when they adsorbed onto monolayer graphene or intercalated in bilayer graphene. Because graphene can be considered a receptor of almost infinite size, we expect that this work can help to gauge how much a receptor can influence the inversion barrier of a buckybowl, by placing a limit of reduction that can be achieved through the use of π stacking.

2. METHODS We studied the inversion barrier of the title bowls by employing a methodology similar to the one selected by us to investigate the stacking interaction between buckybowls and the hosting of fullerenes in different receptors.7,8,17,21,25,29 In the first place, we Received: March 5, 2015 Revised: April 24, 2015

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The Journal of Physical Chemistry A performed periodic M06-L30,31 calculations, combined with the 6-31G and 6-31G* basis sets32 as implemented in Gaussian 09.33 For comparative purposes the 6-311G basis set was employed, but the results were in line with those obtained using the smaller sets. Specifically, we considered the adsorption of corannulene, sumanene, and dibenzo[a,g]corannulene onto 7 × 7 graphene unit cells, which were sampled by employing 100 K points, which is enough for energetics. The second periodic approach involved the calculation of the intercalation energies of the bowls inside a 7 × 7 unit cell of bilayer graphene, AB stacked. This size of the graphene unit cell prevents lateral interactions between adosorbed molecules. Because of the large size of the system, we optimized the geometries at the M06-L/ 6-31G level and performed single point M06-L/6-31G* calculations at the latter geometry. We note that for monolayer graphene, geometry optimizations were carried out with both basis sets. In a second approach to the problem, we analyzed the adsorption of the above-mentioned bowls by employing a 6 × 6 hydrogen-terminated graphene nanoflake, which is shown below. In this case, we used the M06-L functional, but we also employed the M06-2X one30,31 because it is more accurate than M06-L. In addition to this, selective calculations were performed for a 7 × 7 flake and the results showed that the interaction energies were converged with respect to cluster size. For the nonperiodic calculations, transition states and minima were confirmed to have one and zero imaginary frequency by the calculation of the vibrational frequencies, respectively. The inclusion of basis set superposition error did not alter the conclusions reached and for this reason it was calculated only for corannulene. Although the aim of the work is not to obtain very accurte inversion barriers, but just to gauge their variation for different buckybowls, we have undertaken TPSS-D3/631G,34,35 TPSS-D3BJ,36 B3LYP-D3,37,38 and B3LYP-D3BJ calculations to confirm the results obtained with the Minnesota functionals, employing a different approach to treat dispersion interactions. In all cases we used the ultrafine grid.

Table 1. Inversion Barrier (kcal/mol) Determined for Free Corannulene, Dibenzocorannulene, and Sumanene, at Different Levels ΔE Free Corannulene M06-L/6-31G* 10.3 M06-L/6-31G 10.5 M06-2X/6-31G 10.1 TPSS-D3/6-31G 9.6 TPSS-D3BJ/6-31G 10.3 B3LYP-D3/6-31G 10.3 B3LYP-D3BJ/6-31G 11.1 exp13 exp14 Free Dibenzocorannulene M06-L/6-31G* 7.6 M06-L/6-31G 7.7 M06-2X/6-31G 7.3 Free Sumanene M06-L/6-31G* 20.3 M06-L/6-31G 20.4 M06-2X/6-31G 19.5

ΔG 10.9 10.9 10.7 10.3 10.8 10.8 11.6 10.2 11.5 7.8 7.9 7.5 20.3 20.2 19.4

the UP → TS conversion and a modest one for the DN → TS path. The veracity of all the speculations made in the previous paragraph can be confirmed if the results presented in Table 2 are analyzed. When we consider the infinite 7 × 7 graphene model, the inversion barriers are 7.4 and 2.4 kcal/mol, for the DN → TS and UP → TS paths, respectively, at the M06-L/631G* level of theory. These values are 2.9 and 7.9 kcal/mol smaller than the energy required to invert free corannulene. Thus, we confirm a huge reduction of the inversion barrier for the UP → TS path, and a smaller one for DN → TS. At this point, some accuracy considerations must be discussed. In the first place, the performance of M06-L can be questioned. For this reason, we carried M06-2X/6-31G and M06-L/6-31G calculations employing a 6 × 6 graphene flake. The reason for not using a periodic approach is that periodic M06-2X calculations are extremely expensive. The inversion barriers obtained at the M06-2X/6-31G level are 6.2 and 1.3 kcal/mol, for the DN → TS and UP → TS paths, respectively. These values are 2.1 and 1.7 kcal/mol smaller, respectively, than those obtained with the same model and the M06-L/6-31G method. Thus, M06-L is overestimating the inversion barriers when corannulene is adsorbed onto graphene. A final proof of this overrating is obtained when the dispersion-corrected TPSS-D3, TPSS-D3BJ, B3LYP-D3, and B3LYP-D3BJ functionals are utilized. Although the adsorption energies are significantly larger than those computed with the Minnesota class functionals, the inversion barriers calculated with the four functionals support the results obtained at the M06-L and M062X levels. We note that the differences observed for the absolute value adsorption energies are not new, given that Alfè et al.39 reported a similar behavior when corannulene is hosted inside the famous C60H24 buckycatcher.40 In effect, these authors confirmed that almost all the dispersion-corrected approaches, SCS-MP2, and other perturbation theory methods, tremendously overestimate the encapsulation energy for C60@ buckycatcher.39 Fortunately, M06-2X was among the best performers.

3. RESULTS AND DISCUSSION 3.1. Inversion of Corannulene. In Table 1 we present the inversion barrier determined for corannulene at different levels of theory. In general, the results are in good agreement with the experimental values of 10.2 and 11.5 kcal/mol, obtained by Scott et al.13 and Seiders et al.,14 respectively. To reduce the inversion barrier of corannulene, we reasoned that the planar nature of the transition state would interact via π stacking with a graphene sheet better than the curved bowl. However, it is essential to consider that corannulene can adopt two orientations on graphene: (a) (UP configuration) the convex surface points to the sheet; (b) (DN configuration) the concave surface points to the sheet. These structures are shown in Figure 1. For the DN configuration we expect a significant reduction of the π stacking interaction with respect to the planar structure, because the rings of corannulene interact very weakly if the hydrogen atoms point to the sheet. Yet, this configuration benefits from 10 CH:::π interactions. If we consider that the strength of the CH:::π interaction between methane and benzene is 1.43 kcal/mol, we can estimate that this nonbonded interaction will contribute to the binding energy at least with 14.3 kcal/mol. The CH:::π contacts are also present for the planar structure but are absent when the convex side (UP) interacts with graphene. Therefore, we can hypothesize a significant reduction of the inversion barrier for B

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Figure 1. Optimized 7 × 7 graphene with a corannulene molecule adsorbed (clockwise, down (DN), transition state (TS), and up (UP) configurations), at the M06-L/6-31G* level.

Table 2. Adsoprtion Energies, Relative Energies and Inversion Barriers (kcal/mol) Determined for Corannulene When It Is Adsorbed on Graphene, Graphene Nanoflakes, and Bilayer Graphene ∞ graphene 7 × 7 ∞ graphene 7 × 7 nanographene 6 × 6 nanographene 6 × 6 nanographene 6 × 6 nanographene 7 × 7 nanographene 6 × 6 nanographene 6 × 6 nanographene 6 × 6 nanographene 6 × 6 nanographene 6 × 6 ∞ bilayer graphene 7 × 7f ∞ bilayer graphene 7 × 7g

M06-L/6-31G* M06-L/6-31G M06-L/6-31G M06-L/6-31G BSSE corrected M06-L/6-311G M06-L/6-31G M06-2X/6-31G TPSS-D3/6-31G TPSS-D3BJ/6-31G B3LYP-D3/6-31G B3LYP-D3BJ/6-31G M06-L/6-31G* M06-L/6-31G

DOWNa

UPb

TSc

DOWN → TSd

UP → TSe

UP → DOWN

19.9 20.7 21.2 17.5

14.9 16.1 15.9 12.4

12.5 11.9 12.9 8.8

7.4 8.8 8.3 8.7

2.4 4.2 3.0 3.6

5.0 4.6 5.3 5.1

25.5 21.4 22.4 31.0 33.3 33.0 36.6 36.0 37.4

20.9 16.0 17.5 24.8 29.1 25.6 30.6

18.7

6.8

2.2

4.6

16.2 22.6 26.3 23.2 28.1 32.5 33.5

6.2 8.4 7.0 9.8 8.5 3.5 3.8

1.3 2.2 2.8 2.4 2.5 3.5 3.8

4.9 6.2 4.2 7.4 6.0

a

DOWN: adsorption energy determined when corannulene is adsorbed with the H atoms pointing to the graphene sheet. bUP: adsorption energy determined when corannulene is adsorbed with the H atoms pointing away from the graphene sheet. cTS: adsorption energy determined when the planar transition state is adsorbed onto the graphene sheet. dEnergy barrier for the path DOWN → TS. eEnergy barrier for the path UP → TS. fFor bilayer graphene, UP and DOWN structures are the same. gSingle point calculation performed at the M06-L/6-31G geometry.

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Table 3. Adsoprtion Energies, Relative Energies, and Inversion Barriers (kcal/mol) Determined for Sumanene When It Is Adsorbed on Graphene, Graphene Nanoflakes, and Bilayer Graphene ∞ graphene 7 × 7 ∞ graphene 7 × 7 nanographene 6 × 6 nanographene 6 × 6 ∞ bilayer graphene 7 × 7g ∞ bilayer graphene 7 × 7h

M06-L/6-31G* M06-L/6-31G M06-2X/6-31G M06-L/6-31G M06-L/6-31G* M06-L/6-31G

DOWNa

UPb

TSc

DOWN → TSd

UP → TSe

UP → DOWNf

19.8 19.3 20.5 20.0 33.2 34.4

15.9 15.7 15.3 15.4

3.6 3.1 7.7 3.4 25.5 26.2

16.2 16.2 12.8 16.6 7.7 8.2

12.3 12.6 7.6 12.0 7.7 8.2

3.9 3.6 5.2 4.6

34.4

a

DOWN: adsorption energy determined when sumanene is adsorbed with the H atoms pointing to the graphene sheet. bUP: adsorption energy determined when sumanene is adsorbed with the H atoms pointing away from the graphene sheet. cTS: adsorption energy determined when the planar transition state is adsorbed onto the graphene sheet. dEnergy barrier for the path DOWN → TS. eEnergy barrier for the path UP → TS. f Relative energy between the DOWN and UP configurations. gFor bilayer graphene UP and DOWN structures are the same. hSingle point calculation performed at the M06-L/6-31G geometry.

Figure 2. Optimized 7 × 7 graphene with a sumanene molecule adsorbed (clockwise, down (DN), transition state (TS), and up (UP) configurations), at the M06-L/6-31G* level.

The M06-2X result can be combined with the outcome of the periodic approach to make an improved estimation of the inversion barriers. The results obtained using the 6 × 6 graphene flake indicated that the barriers computed with M06L are too high by 2.1 and 1.7 kcal/mol for the DN → TS and UP → TS paths, respectively. Using the latter values we can correct the periodic M06-L/6-31G* result, to obtain a better

estimation of the inversion barriers. By doing so, the inversion barriers are 5.3 and 0.7 kcal/mol, for the DN → TS and UP → TS paths, respectively. Therefore, the adsorption of corannulene onto graphene reduces the inversion barrier from 10.3 kcal/mol (M06-L/6-31G*) to 5.3 (DN → TS) and 0.7 (UP → TS) kcal/mol. Two final considerations are mandatory to support the accuracy of the latter estimation: (a) Is the size of D

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Figure 3. Optimized 7 × 7 graphene with a dibenzo[a,g]corannulene molecule adsorbed (clockwise, down (DN), transition state (TS), and up (UP) configurations), at the M06-L/6-31G* level.

the flake enough to study this problem? The answer to this question is given by adsorption energies computed using a larger graphene flake. When a 7 × 7 flake is employed, the adsorption energies are increased by 0.2 and 0.1 kcal/mol for the DN and UP, configurations, respectively, so increasing the flake size unlikely change the results. (b) Should we include the basis set superposition error? In our previous works7,21 we showed that it is better to not include BSSE for the M06-L and M06-2X functionals because, in this way, we can compensate for the underestimation of the dispersion interactions that affect these two functionals (see the detailed discussion in ref 7). This being said, we have calculated BSSE and there is not a qualitative change in the results obtained. The final evidence of the lack of influence of BSSE is obtained when a larger basis set is employed. At the M06-L/6-311G level, the inversion barriers are 6.8 and 2.2 kcal/mol, for the DN → TS and UP → TS paths, respectively, so again these new pieces of evidence confirm the observed trend. Given that the adsorption of corannulene onto graphene reduces the inversion barrier by at least a 50%, we reasoned that if corannulene is intercalated between two graphene sheets, the reduction will be even more significant. At the M06-L/6-31G* level, we determined that the inversion barrier of corannulene is 3.5 kcal/mol, when it is inside bilayer graphene. Bearing in

mind that we have showed that M06-L overestimates the inversion barriers, we suggest that the true values should be 1.7−2.1 kcal/mol lower, as we observed for the inversion onto monolayer graphene. In this way, for corannulene@bilayer graphene we estimate an inversion barrier equal to 3.5−1.9 = 1.6 kcal/mol. Thus, monolayer and bilayer graphene catalyze the inversion of corannulene by making use of the stacking interaction. The intercalation of corannulene distorts the structure of the bowl. The deformation energies are 0.84 and 0.03 kcal/mol for the bowl and transition state, respectively. As regards the curvature of the bowl, it is reduced by 0.13 Å, if we considered only the carbon atoms and 0.32 Å if the H atoms are also included to measure the bowl depth. Finally, it is important to mention that when corannulene is inside bilayer graphene, the interlayer distance is between 6.8 and 7.3 Å, so there is no stacking interaction of the layers that contributes to the encapsulation, but on the contrary, the sheets are slightly deformed because of the inclusion of corannulene, the deformation energy of the sheets being equal to 0.6 kcal/mol. Interestingly, this value is only 0.2 kcal/mol lower than the difference between the intercalation energy of corannulene (36.0 kcal/mol, at the M06-L/6-31G level) and the sum of the adsorption energies of UP (16.1 kcal/mol) and DN (20.7 kcal/ mol) corannulene. Once we have established that it is possible E

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Table 4. Adsoprtion Energies, Relative Energies, and Inversion Barriers (kcal/mol) Determined for Dibenzo[a,g]corannulene When It Is Adsorbed on Graphene, Graphene Nanoflakes, and Bilayer Graphene ∞ graphene 7 × 7 ∞ graphene 7 × 7 nanographene 6 × 6 nanographene 6 × 6 ∞ bilayer graphene 7 × 7g

M06-L/6-31G* M06-L/6-31G M06-2X/6-31G M06-L/6-31G M06-L/6-31G

DOWNa

UPb

TSc

DOWN → TSd

UP → TSe

UP → DOWNf

29.2 28.3 30.6 29.3 56.4

25.7 25.1 29.0 25.8 56.4

23.8 23.5 28.4 24.6

5.4 4.8 1.7 4.7

1.9 1.6 0.6 1.2

3.5 3.2 1.6 3.5

a DOWN: adsorption energy determined when dibenzo[a,g]corannulene is adsorbed with the H atoms pointing to the graphene sheet. bUP: adsorption energy determined when dibenzo[a,g]corannulene is adsorbed with the H atoms pointing away from the graphene sheet. cTS: adsorption energy determined when the planar transition state is adsorbed onto the graphene sheet. dEnergy barrier for the path DOWN → TS. eEnergy barrier for the path UP → TS. fRelative energy between the DOWN and UP configurations. gFor bilayer graphene UP and DOWN structures are the same.

observed overestimation of the barriers computed with M06-L, but at least, the barrier is reduced 60%, a result in line with that obtained for corannulene. Employing the average of the overestimations found for monolayer graphene (4.1 kcal/ mol), we estimate the inversion barrier inside bilayer graphene as 7.7 − 4.1 = 3.6 kcal/mol. Finally, we note that sumanene is slightly deformed inside bilayer graphene, as indicated by a deformation energy of 1.3 kcal/mol and a lowering of the bowl depth by 0.11 Å, as measured from the carbon atoms. 3.3. Inversion of Dibenzo[a,g]corannulene. The last bowl assayed was dibenzo[a,g]corannulene, which has a bowl depth 0.82 Å long. This value is in good agreement with that reported by Filatov et al.,28 i.e., 0.83 Å. At the M06-L/6-31G level, corannulene is 0.07 Å deeper than dibenzo[a,g]corannulene. The smaller depth is accompanied by a smaller inversion barrier, which is between 7.3 and 7.6 kcal/mol, about 3 kcal/mol smaller than the one corresponding to corannulene. In Figure 3 we show the structure adopted by dibenzo[a,g]corannulene when it is adsorbed onto graphene. The results gathered in Table 4 indicate that, when dibenzo[a,g]corannulene is adsorbed onto infinite graphene, the inversion barrier is reduced by 2.2 and 5.7 kcal/mol for the DN → TS and UP → TS paths, respectively, at the M06-L/6-31G* level of theory. Therefore, the results follow the same trend as for corannulene and sumanene. The reductions are smaller (in absolute values) with respect to the values observed for corannulene and sumanene. However, if we consider the percentage of diminution, the net reduction is similar to that observed for corannulene, 28−29% for the DN → TS path and 75−77% for the UP → TS path. The results obtained by employing the more accurate M06-2X/6-31G method and the finite graphene flake indicated a more significant reduction of the barrier heights, which are 5.6 and 7 kcal/mol smaller than that computed in the gas phase. Using the same finite model and the M06-L/6-31G method, we found that the inversion barriers are 3.0 and 0.6 kcal/mol larger than those computed with M06-2X/6-31G. This finding is not new given that we observed the same overestimation for corannulene and sumanene. Employing the latter differences, we can correct the results computed for the 7 × 7 infinite graphene model and the M06-L/6-31G* method. In such a way, our best estimation for the inversion barriers of dibenzo[a,g]corannulene adsorbed on monolayer graphene are 2.4 and 1.3 kcal/mol, for the DN → TS and UP → TS paths, respectively. As in the previous two cases, we also studied the intercalation of dibenzo[a,g]corannulene in bilayer graphene. The optimization of the transitions state and minima proved to be extremely difficult as the molecule displayed an almost planar structure in both cases. For this reason, we confirmed this finding by employing two 6

to reduce the inversion barrier of buckybowls using graphene, we analyze this phenomenon for other bowls. In particular, we selected sumanene, which presents an inversion barrier larger than that of corannulene and dibenzo[a,g]corannulene, which exhibits an inversion barrier smaller than that of corannulene. 3.2. Inversion of Sumanene. The inversion barriers computed for adsorbed sumanene are gathered in Table 3, and the values corresponding to free sumanene are shown in Table 1. At the M06-L/6-31G* level, the energetic cost to invert sumanene is 20.3 kcal/mol. This value is 10.0 kcal/mol larger than the one computed for corannulene. The main differences between corannulene and sumanene are that (i) sumanene is composed of four hexagons and three pentagons, wheras corannulene has five hexagons and one pentagon and (ii) sumanene has three CH2 units that are absent for corannulene. In Figure 2 we display the structures adopted by sumanene when it is adsorbed onto graphene. At the M06L/6-31G* level, and using the 7 × 7 infinite graphene unit cell, the inversion barriers of sumanene are 16.2 and 12.3 kcal/mol for the DN → TS and UP → TS paths, respectively. These values are 4.1 and 8.0 kcal/mol smaller than the energy required to invert free sumanene. Thus, in line with the results obtained for corannulene, we appreciate a reduction of the barriers. When the M06-2X/6-31G method is used to study the adsorption of sumanene onto the 6 × 6 graphene flake, we also appreciate a diminution of the barriers, but the effect is more significant given that they are 6.7 and 11.9 kcal/mol lower than the value determined in the gas phase for sumanene (19.5 kcal/ mol), at the M06-2X, level of theory. When the latter values are compared with those calculated at the M06-L/6-31G level using the same flake, we found that again M06-L overestimates the DN → TS and UP → TS barriers by 3.8 and 4.4 kcal/mol, respectively. Although the differences are important, it must be stressed that the error is similar to that observed for corannulene; i.e., M06-L gives larger barriers than M06-2X. Employing the same procedure as for corannulene, we can combine the finite model M06-2X values and the periodic M06L ones to obtain improved inversion barriers. In this way, using the M06-L-M06-2X difference, we estimate that the inversion barriers of sumanene adsorbed onto graphene are 12.4 and 7.9 kcal/mol for the DN → TS and UP → TS paths, respectively. Therefore, graphene reduces the energy required to invert sumanene by 7.1 and 11.6 kcal/mol, respectively. Finally, we investigated the conversion of sumanene when it is intercalated in bilayer graphene. At the M06-L/6-31G* level, we found that the inversion barrier of confined sumanene is 7.7 kcal/mol. The latter value is 11.8 kcal/mol lower than that computed in the gas phase. Again, we expect that the real inversion barrier inside bilayer graphene should be lower than 7.7 kcal/mol due to the F

DOI: 10.1021/acs.jpca.5b02181 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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the inversion barriers for the three curved conjugated systems studied when they are adsorbed and intercalated inside bilayer graphene. These results clearly indicate the strong effect of π stacking on the inversion barriers. For this reason we expect that receptors like ExCage6+ 41 would have a stronger effect on the inversion barriers of buckybowls than ExBox4+.19 Such studies are under investigation at our laboratory.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 59899714280. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author thanks the Uruguayan institutions CSIC, ANII, and PEDECIBA Quimica for financial support.

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REFERENCES

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Figure 4. Optimized structure for dibenzo[a,g]corannulene intercalated inside two 6 × 6 graphene nanoflakes, at the M06-2X/6-31G level.

that, they became curved, and the dibenzo[a,g]corannulene adopts a planar structure with real vibrational frequencies. Thus, the intercalation of dibenzo[a,g]corannulene is expected to planarize the molecule, a striking difference with respect to corannulene.

4. CONCLUSIONS By means of dispersion-corrected density functional theory, we have studied how the inversion barriers of corannulene, sumanene, and dibenzo[a,g]corannulene are affected when they are adsorbed on monolayer graphene or intercalated in bilayer graphene. The results obtained with the M06-L, M062X, TPPS-D3, TPSS-D3BJ, B3LYP-D3, and B3LYP-D3BJ methods indicated that π stacking reduces the inversion barrier of corannulene by at least a 50% with respect to the gas phase. Similar trends were found for adsorbed sumanene and dibenzo[a,g]corannulene. In the same line, the intercalation inside bilayer graphene makes the inversion barrier of corannulene to be smaller than 3.5 kcal/mol. For sumanene and dibenzo[a,g] corannulene the results followed the same trend and significant reductions of the inversion barrier were observed. In the particular case of dibenzo[a,g]corannulene intercalated in bilayer graphene, the bowl depth of the molecule (0.83 Å gas phase) is almost zero, displaying an almost planar structure. Finally, in Table 5 we present our best estimations for

Table 5. Best Estimations (kcal/mol) of the Inversion Barriers Determined for Dibenzo[a,g]corannulene When It Is Adsorbed on Graphene

corannulene sumanene dibenzo[a,g]corannulene

monolayer graphene DOWN → TS

monolayer graphene UP → TS

bilayer graphene

gas phase

5.3 12.4 2.4

0.7 7.9 1.3

1.6 3.6 ≈0

10.1 19.5 7.3

G

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DOI: 10.1021/acs.jpca.5b02181 J. Phys. Chem. A XXXX, XXX, XXX−XXX