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Denis P.A.,Computational Nanotechnology | Denis P.A.,Centro Interdisciplinario en Nanotecnologia
Journal of Physical Chemistry C | Year: 2011

Herein, we perform a comparative investigation on the reactivity of pristine and lithium doped monolayer and bilayer graphene. To study the addition of free radicals and 1,3 dipolar cycloadditions, we employed periodic DFT calculations. In all cases lithium increases the reactivity, even though the effect is weaker for the 1,3 dipolar cycloaddition as compared with that exhibited by free radicals. The effect of lithium doping is local and decreases as the functional group is attached far from the lithium atom. The increased reactivity can be explained by analyzing the band structure at the Fermi level. Lithium doped graphene is metallic thus showing an increased reactivity toward free radicals as observed for metallic nanotubes. However, the metallic character of lithium doped graphene is lost when free radicals like OH, SH, H, F, and CH3 are attached; the Fermi level is raised and a gap is opened. The reactivity of bilayer graphene was almost unaffected by the lithium doping. Therefore, to enhance the reactivity of graphene it is crucial to use the monolayer form. Also, it is important to eliminate defects to decrease lithium diffusion to the other side of the sheet because Li atoms can remove the functional groups attached to graphene, forming compounds like LiOH, LiF, etc. Finally, for comparative purposes, we performed M06-2X and MP2 calculations for the addition of free radicals to lithium doped benzene. The results showed similar trends to those observed for graphene. This work demonstrates that lithium doping can dramatically increase the reactivity of graphene to such an extent that chemical groups that do not react with graphene become bonded when lithium is underneath. Thus, lithium doping can open new avenues for investigating the chemistry of the rather unreactive sp2 framework of graphene. © 2011 American Chemical Society.


Denis P.A.,Computational Nanotechnology
Computational Materials Science | Year: 2013

First principle calculations were applied to study the electronic properties of S and P-doped graphene. In particular, the PBE and HSE06 density functionals were utilized. The comparison of the band gaps obtained with both functionals indicated that the band gaps at the PBE level are only slightly smaller than those obtained with HSE06. Specifically, the deviation variation was much smaller than that observed for carbon nanotubes or graphane. Phosphorus doping is somewhat more effective in opening larger optical gaps. The latter decreases very fast, upon lowering of dopant concentration. In the case of S-doping, for a doping concentration smaller than 0.5 at.%, the gaps are close to 0.1-0.2 eV, making the material not too attractive to develop graphene based electronics. However, for phosphorus doping, a dopant concentration of 0.5% is still useful as band gaps close to 0.3-0.4 eV are expected. Further work must be devoted to obtain larger band gaps by doping graphene with heteroatoms, which are necessary to develop graphene based electronics. © 2011 Elsevier B.V. All rights reserved.


Denis P.A.,Computational Nanotechnology
RSC Advances | Year: 2013

We have studied the ability of 26 receptors to catch the fullerenes C 60 and C70. The prediction of which host displays the largest affinity with fullerenes is complicated by the fact that some hosts are extremely flexible. For example, the cyclotriveratrylene (CTV) based host, with three 2-[9-(1,3-dithiol-2-ylidene)anthracen-10(9H)-ylidene]-1,3-dithiole (exTTF) pincers, has an interaction energy that is quite modest and even lower than that determined for the famous C60H28 buckycatcher. Notwithstanding this energetic difference, experimental results indicated that the exTTF-CTV host has an association constant comparable to those reported for metalloporphyrins. In line with the recent experimental results we found that when three corannulene pincers are attached to cyclotriveratrylene, the ability of the host to interact with fullerene is not improved with respect to the C60H28 buckycatcher, which has two corannulene pincers. Our theoretical calculations showed that the reason for such an outcome is that the corannulene pincers are stacked and thus a large amount of energy is required to break the intramolecular dispersion interactions that keep the structure stacked. A similar scenario was found when we attached one, two and three exTTF pincers to pentakis(1,4-benzodithiino)corannulene. Bearing these results in mind, and considering that the C60H28 buckycatcher is somewhat rigid, we alkylated the rim of the corannulene pincers. The alkylated buckycatcher is able to interact with C60 with an interaction energy that is larger than that corresponding to the unsubstituted host. Therefore, functionalization of the C60H28 buckycatcher seems to be the most promising road to the synthesis of new fullerene receptors that are not based on metalloporphyrins. The design of new hosts must be pursued with the aim of finding receptors whose most stable conformations are similar to that expected in the supramolecular complex. This journal is © The Royal Society of Chemistry.


Denis P.A.,Computational Nanotechnology | Iribarne F.,Laboratorio Of Bioinformatica Y Farmacologia Molecular
Journal of Materials Chemistry | Year: 2012

Herein, we study [2 + 2] cycloadditions reactions onto graphene. We have found that owing to stacking, CH-π interactions and steric hindrance existing between the aromatic rings, the addition of benzyne molecules follows a characteristic pattern. For a 4 × 4 graphene unit cell, the optimum level of functionalization is achieved when one benzyne group per 4.0 carbon atoms is attached. Although the addition of benzyne molecules does not result in unpaired electrons (as observed for free radicals), the attachment of benzyne molecules in pairs on opposite sides of the sheet and on neighboring carbon atoms dramatically increases binding energies. We observed that reaction energies were increased by more than three times, as compared with the addition of an isolated benzyne molecule. The preferred structure has a band gap close to 1.5 eV. The uniformity of the properties found for aryne modified graphene, the ease whereby this is achieved (due to non bonded interactions, cooperative effects and steric hindrance between the benzyne molecules) and the fact that the reaction occurs in solution, turns the nanomaterial into a very attractive species for electronics. Lastly, we have shown that the addition of larger benzyne molecules as well as the addition of biscyclopropyl alkenes is favored from a thermodynamical stand point. © The Royal Society of Chemistry 2012.


Denis P.A.,Computational Nanotechnology
Journal of Physical Chemistry C | Year: 2013

The chemical reactivity of electron-doped and hole-doped graphene was studied by means of first principles calculations, on the basis of dispersion corrected density functional theory. To model hole-doped graphene, the widely known electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) was utilized, while the electron donor tetrathiafulvalene (TTF) was selected for the electron-doped case. The results demonstrate that the reactivity of graphene can be modified by the adsorption of electron donating/withdrawing molecules. The reactions considered were the addition of fluorine atoms and hydroxyl radicals. In both cases, it was observed that the adsorption of F4-TCNQ and TTF increased the reactivity of graphene. This outcome was expected for electron-doped graphene because we have recently shown that lithium increases the reactivity of graphene. Yet, for F4-TCNQ, the finding is surprising given that this molecule accepts 0.4 e- from graphene. The gas phase free energies of association are calculated to be negative for F4-TCNQ and TTF, but for the latter is only -2.5 kcal/mol. The results obtained employing infinite models and using the VDW-DF and M06-L functionals are supported by cluster model calculations performed with the M06-2X method. When TTF was adsorbed onto graphene, a charge transfer from TTF to graphene was not observed. However, when TTF and F4-TCNQ are simultaneously adsorbed on opposite sides of the graphene sheet, the amount of charge accepted by F4-TCNQ and donated by TTF is increased. This work clearly suggests that dual doping is a useful tool to expand, even to a greater extent, the possibilities to tune the properties of graphene. Further work must be devoted to synthesize better electron donors and acceptors and thus allow for larger charge transfer. © 2013 American Chemical Society.


Denis P.A.,Computational Nanotechnology | Iribarne F.,Computational Nanotechnology
Journal of Physical Chemistry C | Year: 2013

We have applied dispersion corrected density functional theory to gauge the reactivity of the most common defects found in graphene. Specifically, we investigated single vacancies, 585 double vacancies, 555-777 reconstructed double vacancies, Stone-Wales defects, and hydrogenated zigzag and armchair edges. We found that the extent to which defects increase reactivity is strongly dependent on the (a) functional group to be attached and (b) number of functional groups attached. For the addition of one H, F, and phenyl groups to defective graphene, we found the following decreasing order of reactivity: single vacancy > hydrogenated zigzag edge > 585 double vacancy > 555-777 reconstructed double vacancy > Stone-Wales > hydrogenated armchair edge > perfect graphene. However, when two phenyl groups are attached, the Stone-Wales defect becomes more reactive than the 585 double vacancy and 555-777 reconstructed double vacancy. The largest increase of reactivity is observed for the functional groups whose binding energy onto perfect graphene is small. In contrast with recent experimental results, we determined that the reactivity of edges in comparison with perfect graphene is much higher than the reported value. When two groups are attached onto a 585, 555-777, or Stones-Wales defect, they prefer to be paired on the same CC bond on opposite sides of the sheet. However, for the single vacancy, this is not the observed behavior as the preferred addition sites are those carbon atoms that were previously bonded to the missing carbon. © 2013 American Chemical Society.


Denis P.A.,Computational Nanotechnology
Chemical Physics Letters | Year: 2014

Herein, we studied the interaction between the fullerenes C60 and C70 with pentaindenocorannulene (P), chrysaorole (C) and two new buckycatchers. The P and C bowls interact with the fullerenes with an interaction-energy (IE) that is twice the value determined for corannulene. The new receptors designed include a cyclooctatetraene core which has two P or C pincers attached. Notwithstanding the fact that the proposed hosts prefer stacked conformations at equilibrium, the IE determined are extremely large and close to the ones computed for the dimeric metalloporphyrins. It is our hope that this work stimulates the synthesis of these receptors. © 2013 Elsevier B.V. All rights reserved.


Denis P.A.,Computational Nanotechnology
ChemPhysChem | Year: 2013

Dispersion-corrected density functional theory is utilized to study the addition of aryl radicals to perfect and defective graphene. Although the perfect sheet shows a low reactivity against aryl diazonium salts, the agglomeration of these groups and the addition onto defect sites improves the feasibility of the reaction by increasing binding energies per aryl group up to 27 kcal mol-1. It is found that if a single phenyl radical interacts with graphene, the covalent and noncovalent additions have similar binding energies, but in the particular case of the nitrophenyl group, the adsorption is stronger than the chemisorption. The single vacancy shows the largest reactivity, increasing the binding energy per aryl group by about 80 kcal mol-1. The zigzag edge ranks second, enhancing the reactivity 5.4 times with respect to the perfect sheet. The less reactive defect site is the Stone-Wales type, but even in this case the addition of an isolated aryl radical is exergonic. The arylation process is favored if the groups are attached nearby and on different sublattices. This is particularly true for the ortho and para positions. However, the enhancement of the binding energies decreases quickly if the distance between the two aryl radicals is increased, thereby making the addition on the perfect sheet difficult. A bandgap of 1-2 eV can be opened on functionalization of the graphene sheets with aryl radicals, but for certain configurations the sheet can maintain its semimetallic character even if there is one aryl radical per eight carbon atoms. At the highest level of functionalization achieved, that is, one aryl group per five carbon atoms, the bandgap is 1.9 eV. Regarding the effect of using aryl groups with different substituents, it is found that they all induce the same bandgap and thus the presence of NO2, H, or Br is not relevant for the alteration of the electronic properties. Finally, it is observed that the presence of tetrafluoroborate can induce metallic character in graphene. Getting into position: Dispersion-corrected density functional theory is utilized to study the addition of aryl radicals to perfect and defective graphene. The single vacancy shows the largest reactivity, followed by the zigzag edge, and the Stone-Wales defect site (see picture). Arylation is favored if the groups are attached nearby and on the same sublattice, particularly in the ortho and para positions. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Denis P.A.,Computational Nanotechnology
Journal of Physical Chemistry C | Year: 2011

The effect of lithium doping on the reactivity of single-wall carbon nanotubes (SWCNTs) was studied by means of first principle calculations. Two prototype reactions were considered, the 1,3 dipolar cycloaddition and the attachment of free radicals. The results obtained employing the PBE, M06L, and M06-2X functionals indicated that lithium significantly increases the reaction energies. Although this effect is slightly increased upon diameter enlargement, it decreases when the distance between the functional group and lithium is augmented. We observed a stronger enhancement of reactivity for semiconducting SWCNTs as compared with the metallic ones. Regarding the endohedral and exohedral lithium dopings, both increase the binding energies, although for some functional groups the latter doping is far less effective because exohedral lithium can remove the fluorine atoms or hydroxyl or thiol groups from the tube walls. Therefore, endohedral lithium is expected to be more useful to enhance the reactions performed onto SWCNTs, whereas in some cases exohedral lithium may have a negative effect depending on the affinity that lithium has with the functional groups attached to the SWCNTs. If the lithium-functional group interaction is stronger than the binding energy between the nanotube and the functional group, the perfect sp 2 framework of the nanotube will be restored. All of the functionals employed gave the same results from a qualitative standpoint, but important quantitative differences were observed in the magnitude of the reactivity enhancement. © 2011 American Chemical Society.


Herein, the thermochemical properties of five-membered rings heterocycles were studied employing the CCSD(T) methodology coupled with the correlation consistent basis sets and including corrections for relativistic and core-valence effects as well as anharmonicities of the potentials. For pyrrole, furan, imidazole, pyrazole, 1H-1,2,4-triazole, and 1H-tetrazole, the mean absolute deviation (MAD) of the Δ H°f, 298, computed at the CCSD(T) level, is 0.5 kcal/mol with respect to the experimental values. In the case of 1H-1,2,3-triazole, 2H-1,2,3-triazole, 4H-1,2,3-triazole, 4H-1,2,4-triazole, 2H-tetrazole, and pentazole, we propose the following Δ H°f, 298: 62.6, 59.2, 85.0, 54.2, 77.7, and 107.5 kcal/mol, respectively. For thiophene, we revisit our previous result and propose a value of 26.0 kcal/mol. The theoretical estimations were used to study the performance of the M06-2X and B2PLYP functionals. Also, the convergence toward the complete basis set limit (CBS) was analyzed. M06-2X did not show a smooth convergence toward the CBS limit. Particularly, for the cc-pVTZ and cc-pVQZ basis sets, some problems were detected. Yet, along the cc-pVQZ, cc-pV5Z, and cc-pV6Z basis sets, the TAE smoothly decreased. The diminution of the TAE upon increase in basis set was not expected because the opposite behavior is more frequently observed. The MAD of the total atomization energies determined at the M06-2X level was 0.42 kcal/mol, with respect to the CCSD(T) results. In the case of the double hybrid B2PLYP functional, a smooth convergence toward the CBS limit was detected, even though the performance seriously degradated when the basis set was increased. At the CBS limit, the MAD with respect to the CCSD(T) TAEs was 8.26 kcal/mol. © 2011 Springer-Verlag.

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