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Denis P.A.,Computational Nanotechnology | Denis P.A.,Centro Interdisciplinario en Nanotecnologia y Quimica | Iribarne F.,Laboratorio Of Bioinformatica Y Farmacologia Molecular
International Journal of Quantum Chemistry | Year: 2011

The addition of oxygen-centered radicals to fullerenes has been intensively studied due to their role in cell protection against against hydrogen peroxide induced oxidative damage. However, the analogous reaction of sulfur-centered radicals has been largely overlooked. Herein, we investigate the addition of S-centered radicals to C50, C60, C70, and C100 fullerenes by means of DFT calculations. The radicals assayed were: S, SH, SCH3, SCH2CH3, SC 6H5, SCH2C6H5, and the open-disulfide SCH2CH2CH2CH2S. Sulfur, the most reactive species, prefers to be attached to a 66-bond of C 60 with a binding energy (Ebind) of 2.4 eV. For the SR radicals the electronic binding energies to C60 are 0.77, 0.74, 0.58, 0.67, and 0.35 eV for SH, SCH3, SCH2CH3, SCH2C6H5, and SC6H5, respectively. The reactivity of C60 toward SR radicals can be increased by lithium doping. For Li@C60, the Ebind is increased by 0.65 eV with respect to C60, but only by 0.33 eV for the exohedral doping. Fullerenes act like free radical sponges. Indeed, the C 60-SR Ebind can be duplicated if two radicals are added in ortho or para positions. The enhanced reactivity because of multiple additions is mostly a local effect, although the addition of one radical makes the whole cage more reactive. Therefore, as observed for hydroxylated fullerenes, they should protect cells from oxidative damage. However, the thiolated fullerenes have one advantage, they can be easily attached to gold nanoparticles. For the addition on pentagon junctions smaller fullerenes like C50 are more reactive than C60. Interestingly, C70 is as reactive as C60, even for the addition on the equatorial belt. For larger fullerenes like C100, reactivity decreases for the carbon atoms belonging to hexagon junctions. © 2010 Wiley Periodicals, Inc. Source

Denis P.A.,Computational Nanotechnology | Iribarne F.,Laboratorio Of Bioinformatica Y Farmacologia Molecular
International Journal of Quantum Chemistry | Year: 2010

Herein, we perform a comparative study on the addition of azomethine ylides to graphene, carbon nanotubes, C60, ethene, pyrene and a C 48H18 hydrocarbon. The calculated binding energies and free energy corrections suggest that the addition of azomethine ylide to perfect graphene is not spontaneous (ΔG > 0). However, the presence of Stones-Wales defects significantly increases reactivity: the binding energy between SW-defective graphene and the azomethine ylide is 0.83 eV, close to that determined for a (5,5) SWCNT. The electronic properties of the sheet are not modified by the 1,3 cycloaddition. The binding energies determined for the addition of an azomethine ylide to a (5,5) SWCNT are significantly lower than previously reported. © 2009 Wiley Periodicals, Inc. Source

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

The interaction between alkyl radicals and graphene was studied by means of dispersion-corrected density functional theory. The results indicate that isolated alkyl radicals are not likely to be attached onto perfect graphene. It was found that the covalent binding energies are low, and because of the large entropic contribution, ΔG° 298 is positive for methyl, ethyl, isopropyl, and tert-butyl radicals. Although the alkylation may proceed by moderate heating, the desorption barriers are low. For the removal of the methyl and tert-butyl radicals covalently bonded to graphene, 15.3 and 2.4 kcal mol -1 are needed, respectively. When alkyl radicals are agglomerated, the binding energies are increased. For the addition in the ortho position and on opposite sides of the sheet, the graphene-CH 3 binding energy is increased by 20 kcal mol -1, whereas for the para addition on the same side of the sheet, the increment is 9.4 kcal mol -1. In both cases, the agglomeration turns the ΔG° 298<0. For the ethyl radical, the ortho addition on opposite sides of the sheet has a negative ΔG° 298, whereas for isopropyl and tert-butyl radicals the reactions are endergonic. The attachment of the four alkyl radicals under consideration onto the zigzag edges is exergonic. The noncovalent adsorption energies computed for ethyl, isopropyl, and tert-butyl radicals are significantly larger than the graphene-alkyl-radical covalent binding energies. Thus, physisorption is favored over chemisorption. As for the ΔG° 298 for the adsorption of isolated alkyl radicals, only the tert-butyl radical is likely to be exergonic. For the phenalenyl radical we were not able to locate a local minimum for the chemisorbed structure since it moves to the physisorbed structure. An important conclusion of this work is that the consideration of entropic effects is essential to investigate the interaction between graphene and free radicals. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Denis P.A.,Computational Nanotechnology | Iribarne F.,Laboratorio Of Bioinformatica Y Farmacologia Molecular
Chemistry - A European Journal | Year: 2011

By means of first principle calculations we have investigated a set of molecules that are presumed to contain carbon-sulfur triple bonds, namely HCSOH, H 3SCH, cis-FCSF, F 3CCSF 3, and F 5SCSF 3. For HCSOH, FCSF, and H 3SCH we used the CCSD(T) methodology and the correlation-consistent basis sets. On the other hand, F 3CCSF 3 and F 5SCSF 3 were studied at the B3LYP, M06-2X, MP2, and G3 levels of theory. We found that none of these molecules display a carbon-sulfur adiabatic bond dissociation energy (ABDE) as strong as diatomic CS (170.5a kcalmol -1), or a diabatic bond dissociation energy (DBDE) larger than the one found in SCO (212.0a kcalmol -1), although the DBDE of FCSF comes quite close at 208.3a kcalmol -1. The CS ABDEs of F 3CCSF 3, F 5SCSF 3, and H 3SCH are comparable to that of a single C-S bond. In contrast with the experimental results, F 3CCSF 3 and F 5SCSF 3 are predicted to be linear with C 3v and C s symmetry, respectively, at the B3LYP/6-311+G(3df,2p) level. MP2/6-311+G(2df,2p) calculations support the C 3v symmetry for F 3CCSF 3, despite F 5SCSF 3 not having a perfect linear structure; the CSC angle is 174.6°, which is nearly 20° larger than the experimental value. The analysis of the carbene structures of HCSOH and H 3SCH revealed that they are not significant, because the triplet state is dissociative in these cases. However, for F 3CCSF 3 and F 5SCSF 3, the carbene triplet states lie 0.81 and 0.77a eV above the singlet state, respectively. In the same vein, our investigation supports the presence of a strong double bond for HCSOH. The conflicting evidence available for F 3CCSF 3 and F 5SCSF 3 makes it very difficult to determine the nature of the CS bonds. However, the bond dissociation energies and the singlet-triplet splittings clearly suggest that these compounds should be considered as masked sulfinylcarbenes. The analysis of the bond dissociation energies challenges the existence of a triple bond in these five molecules, but from a strictly thermodynamic standpoint, cis-FCSF is found to be the candidate most likely to exhibit triple-bond character. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

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. Source

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