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Garcia-Borras M.,Institute Of Quimica Computacional | Osuna S.,Institute Of Quimica Computacional | Swart M.,Institute Of Quimica Computacional | Swart M.,Catalan Institution for Research and Advanced Studies | And 2 more authors.
Chemical Communications | Year: 2013

In this work we show that the regioselectivity of the Diels-Alder, 1,3-dipolar, and carbene cycloadditions to C60 changes from the usual [6,6] addition in neutral species to the [5,6] attack when the number of electrons added to the fullerenic cage increases. Changes in the aromaticity of the five- and six-membered rings of C60 during the reduction process provide a rationale to understand this regioselectivity change. This journal is © 2013 The Royal Society of Chemistry.

Osuna S.,Institute Of Quimica Computacional | Swart M.,Institute Of Quimica Computacional | Swart M.,Catalan Institution for Research and Advanced Studies | Sola M.,Institute Of Quimica Computacional
Physical Chemistry Chemical Physics | Year: 2011

The last two decades have witnessed major advances in the synthesis and characterization of endohedral fullerenes. These species have interesting physicochemical properties with many potential interesting applications in the fields of magnetism, superconductivity, nonlinear optical properties, radioimmunotherapy, and magnetic resonance imaging contrast agents, among others. In addition to the synthesis and characterization, the chemical functionalization of these species has been a main focus of research for at least four reasons: first, to help characterize endohedral fullerenes that could not be well described structurally otherwise; second, to generate materials with fine-tuned properties leading to enhanced functionality in one of their multiple potential applications; third, to produce water-soluble endohedral fullerenes needed for their use in medicinal sciences; and fourth, to generate electron donor-acceptor conjugates that can be used in solar energy conversion/storage. The functionalization of these species has been achieved through different types of reactions, the most common being the Diels-Alder reactions, 1,3-dipolar cycloadditions, Bingel-Hirsch reactions, and free-radical reactions. It has been found that the performance of these reactions in endohedral fullerenes may be quite different from that of the empty fullerenes. Indeed, encapsulated species have a large influence on the thermodynamics, kinetics, and regiochemistry of these reactions. A detailed understanding of the changes in chemical reactivity due to incarceration of atoms or clusters of atoms is essential to assist the synthesis of new functionalized endohedral fullerenes with specific properties. This Perspective seeks to highlight the key role played by computational chemistry in the analysis of the chemical reactivity of these systems. It is shown that the information obtained through calculations is highly valuable in the process of designing new materials based on endohedral fullerenes. © the Owner Societies 2011.

Feixas F.,Institute Of Quimica Computacional | Jimenez-Halla J.O.C.,Institute Of Quimica Computacional | Matito E.,University Of Szczecin | Poater J.,Institute Of Quimica Computacional | Sola M.,Institute Of Quimica Computacional
Journal of Chemical Theory and Computation | Year: 2010

As compared to classical organic aromatic compounds, the evaluation of aromaticity in all-metal and semimetal clusters is much more complex. For a series of these clusters, it is frequently found that different methods used to discuss aromaticity lead to divergent conclusions. For this reason, there is a need to evaluate the reliability of the different descriptors of aromaticity to provide correct trends in all-metal and semimetal aromatic clusters. This work represents the first attempt to assess the performance of aromaticity descriptors in all-metal clusters. To this end, we introduce the series of all-metal and semimetal clusters [XnY4-n] q± (X, Y = Al, Ga, Si, and Ge; n = 0-4) and [X nY5-n]4-n (X = P and Y = S and Se; n = 0-5) with predictable aromaticity trends. Aromaticity, in these series, is quantified by means of nucleus-independent chemical shifts (NICS) and electronic multicenter indices (MCI). Results show that the expected trends are generally better reproduced by MCI than by NICS. It is found that NICS(0)π is the kind of NICS that performs better among the different NICS indices analyzed. © 2010 American Chemical Society.

Pan S.,Indian Institute of Technology Kharagpur | Sola M.,Institute Of Quimica Computacional | Chattaraj P.K.,Indian Institute of Technology Kharagpur
Journal of Physical Chemistry A | Year: 2013

Hardness and electrophilicity values for several molecules involved in different chemical reactions are calculated at various levels of theory and by using different basis sets. Effects of these aspects as well as different approximations to the calculation of those values vis-à-vis the validity of the maximum hardness and minimum electrophilicity principles are analyzed in the cases of some representative reactions. Among 101 studied exothermic reactions, 61.4% and 69.3% of the reactions are found to obey the maximum hardness and minimum electrophilicity principles, respectively, when hardness of products and reactants is expressed in terms of their geometric means. However, when we use arithmetic mean, the percentage reduces to some extent. When we express the hardness in terms of scaled hardness, the percentage obeying maximum hardness principle improves. We have observed that maximum hardness principle is more likely to fail in the cases of very hard species like F-, H2, CH4, N2, and OH appearing in the reactant side and in most cases of the association reactions. Most of the association reactions obey the minimum electrophilicity principle nicely. The best results (69.3%) for the maximum hardness and minimum electrophilicity principles reject the 50% null hypothesis at the 2% level of significance. © 2013 American Chemical Society.

Curutchet C.,Institute Of Quimica Computacional | Voityuk A.A.,Catalan Institution for Research and Advanced Studies | Voityuk A.A.,University of Girona
Angewandte Chemie - International Edition | Year: 2011

No will to wander: Triplet-triplet electronic energy transfer in polyA-polyT DNA sequences was studied by using semiempirical quantum-chemical methods coupled to classical molecular-dynamics simulations. Triplet excited states in DNA were found to be almost completely localized on single nucleobases; the characteristic time for their migration along the A-A and T-T stacks was found to be 0.8 and 6.4ns. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Blancafort L.,Institute Of Quimica Computacional | Robb M.A.,Imperial College London
Journal of Chemical Theory and Computation | Year: 2012

The permutational isomers of the prefulvene-like minimum energy conical intersection lie on an extended conical intersection seam, where they are connected by higher symmetry structures. Here, we present a VB analysis of the electronic states involved along this extended seam. The VB method produces a spin-exchange density (ie. a bonding pattern) that provides the basis to assign resonance structures to the states. The results show that in the high symmetry region of the seam, the character of the states is dominated by the positive and negative combination of the Kekulé structures, (A+B) and (A-B). The low energy parts of the seam, comprised of lower symmetry conical intersection structures, are stabilized by mixing with the Dewar resonance structures. This feature is responsible for the stability of the benzvalene-like conical intersections. The validity of the VB model is confirmed by calculating the branching space vectors at this level of theory, which are in good agreement with the CASSCF calculated vectors. The VB analysis has also allowed us to complete our picture of the global seam, since it has provided the clue to locate a conical intersection saddle point that interconverts two minima of the prefulvene conical intersection where the carbon bent out of the plane is inverted and rotated by 60°. This saddle point has a benzvalene-like geometry, in agreement with the VB picture. © 2012 American Chemical Society.

Van Duijnen P.Th.,Zernike Institute for Advanced Materials | Swart M.,Institute Of Quimica Computacional | Swart M.,Catalan Institution for Research and Advanced Studies
Journal of Physical Chemistry C | Year: 2010

Applying the classical discrete reaction field (DRF) approach, which includes a treatment for the solution of the many-body polarization in complex systems, we calculated the mean atomic polarizability for a Si atom from the known molecular polarizability of Si3. With only this parameter (6.16 Å3, i.e., close to the free atom value), and the geometries as input, the effective atomic mean polarizabilities and their averages α)n = (α)(n)/n) for the series Si4-Si 10 were calculated and found to be in excellent agreement with theoretical and experimental values. These α)n are larger than the bulk value of 3.7 Å3. We used the same input parameter for (by hand) constructed model systems up to n = 4950 with various geometries. For the larger clusters with the diamond lattice, we obtained the bulk value, implying that we "predicted" the dielectric constant of silicon almost from first principles. However, even the largest system is still too small for considering it as a real dielectric. In other lattices (primitive and face centered cubic), the α)n are significantly smaller than 3.7 Å3, which we attribute to the tighter packing in these lattices in comparison with that of the diamond structure. The behavior in all these systems can be easily understood by accounting properly for the local fields and for damping the interaction between induced dipoles. We show that there is no need for additional (e.g., "charge transfer") parameters. © 2010 American Chemical Society.

Poater J.,Institute Of Quimica Computacional | Sola M.,Institute Of Quimica Computacional | Vinas C.,CSIC - Institute of Materials Science | Teixidor F.,CSIC - Institute of Materials Science
Chemistry - A European Journal | Year: 2013

A clear link between the hydrocarbon and borohydride chemistries is established by showing that hydrocarbons and borohydrides have a common root regulated by the number of valence electrons in a confined space. Application of the proposed method to archetypal hydrocarbons leads to well-known borohydrides but, more importantly, it allows the design of new and interesting boron-containing molecules that can be a source of inspiration for synthetic chemists. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Neugebauer J.,Leiden University | Curutchet C.,University of Toronto | Curutchet C.,Institute Of Quimica Computacional | Munoz-Losa A.,University of Pisa | Mennucci B.,University of Pisa
Journal of Chemical Theory and Computation | Year: 2010

We present a QM/QM approach for the calculation of solvent screening effects on excitation-energy transfer (EET) couplings. The method employs a subsystem time-dependent density-functional theory formalism [J. Chem. Phys. 2007, 126, 134116] and explicitly includes solvent excited states to account for the environmental response. It is investigated how the efficiency of these calculations can be enhanced in order to treat systems with very large solvation shells while fully including the environmental response. In particular, we introduce a criterion to select solvent excited states according to their approximate contribution weight to the environmental polarization. As a model system, we investigate the perylene diimide dimer in a water cluster in comparison to a recent polarizable QM/MM method for EET couplings in the condensed phase [J. Chem. Theory Comput. 2009, 5, 1838]. A good overall agreement in the description of the solvent screening is found. Deviations can be observed for the effect of the closest water molecules, whereas the screening introduced by outer solvation shells is very similar in both methods. Our results can thus be helpful to determine at which distance from a chromophore environmental response effects may safely be approximated by classical models. © 2010 American Chemical Society.

Feixas F.,Institute Of Quimica Computacional | Matito E.,University Of Szczecin | Duran M.,Institute Of Quimica Computacional | Sola M.,Institute Of Quimica Computacional | Silvi B.,University Pierre and Marie Curie
Journal of Chemical Theory and Computation | Year: 2010

In this work we present a 2-fold approximation for the calculation of the electron localization function (ELF) which avoids the use of the two-particle density (2-PD). The first approximation is used for the calculation of the ELF itself and the second one is used to approximate pair populations integrated in the ELF basins. Both approximations only need the natural orbitals and their occupancies, which are available for most methods used in electronic structure calculations. In this way, methods such as CCSD and MP2 can be used for the calculation of the ELF despite the lack of a pertinent definition of the 2-PD. By avoiding the calculation of the 2-PD, the present formulation provides the means for routine calculations of the ELF in medium-size molecules with correlated methods. The performance of this approximation is shown in a number of examples. © 2010 American Chemical Society.

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