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

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. Source

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

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. Source

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

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. Source

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

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. Source

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

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. Source

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