HuLiS, a program to teach mesomerism and more

International audienceThe HuLiS program is presented as a tool to decompose a delocalized wave function as a linear combination of localized electronic structures. The principles of the energy based HL-CI and the overlap based schemes HL-P are developed. The results obtained for a set of 10 relevant organic molecules are shown to compare very well with high level quantum chemistry calculations. A trust factor τ is introduced and its use is shown on the allyl radical case where symmetry must be taken into account

Exhaustive Generation of Benzenoid Structures Sharing Common Patterns

Benzenoids are a subfamily of hydrocarbons (molecules that are only made of hydrogen and carbon atoms) whose carbon atoms form hexagons. These molecules are widely studied both experimentally and theoretically and can have various physicochemical properties (mechanical resistance, electronic conductivity, ...) from which a lot of concrete applications are derived. These properties can rely on the existence or absence of fragments of the molecule corresponding to a given pattern (some patterns impose the nature of certain bonds, which has an impact on the whole electronic structure). The exhaustive generation of families of benzenoids sharing the absence or presence of given patterns is an important problem in chemistry, particularly in theoretical chemistry, where various methods can be used to better understand the link between their shapes and their electronic properties. In this paper, we show how constraint programming can help chemists to answer different questions around this problem. To do so, we propose different models including one based on a variant of the subgraph isomorphism problem and we generate the desired structures using Choco solver

Mesomerism, Ring & Substituent Effects, A Computational Chemistry Experiments

International audienceThe introduction of computational chemistry experiments for undergraduate students in sciences can really facilitate knowledge acquisition. It can make consistent various concepts in chemistry without a lengthy experimental protocol involving synthesis and physical chemistry mesurements. In the present case the students use a very simple molecular modeling program that we implemented: HuLiS. The computational chemistry experiment described herein is devoted to an important concept in chemistry: resonance between Lewis structures (mesomerism). In this context, conjugated ring species, as herein, are somewhat specific in the sense that aromaticity can play an important role. Hence, we discuss here a rather advanced resonance effect, where the usual qualitative rules do not suffice by themselves, but aromaticity and the ―4n+2 electron‖ rule must be considered. This Hückel rule can be numerically displayed with an appropriate computational experiment. We use our free java applet (HuLiS) designed to evaluate the weights of resonance structures. As it is based on Hückel theory, the computations are fast, and the theoretical background is much simpler than with ab initio programs, hence the teachers are likely to master the tool used by the students. Upon substitutions, the weights of the zwitterionic Lewis structures adapt to the electronegativity. This is an opportunity to discuss the role of electronegativity in mesomerism, and strengthen the knowledge of the students

Bonding of Gold with Unsaturated Species

International audienceInteractions of gold(I) catalysts with alkenes and alkynes are analyzed. Neutral chlorido, and cationic phosphine and N-heterocyclic carbene complexes are studied. High-level ab initio calculations are performed to benchmark the accuracy of popular DFT methods. Donation and backdonation contributions in the bond between the gold fragment and the alkene/alkyne substrate are discussed. These contributions depend on the nature of the gold fragment, but also on the substituents on the alkene/alkyne

Potential energy surfaces of the electronic states of Li2F and Li2F−

International audienceThe potential energy surfaces of the ground and low-lying excited states for the insertion reaction of atomic fluorine (F) and fluoride (F−) into the dilithium (Li2) molecule have been investigated. We have carried out explicitly correlated multi-reference configuration interaction (MRCI-F12) calculations using Dunning’s augmented correlation-consistent basis sets. For the neutral system, the insertion of F into Li2 proceeds via a harpoon-type mechanism on the ground state surface, involving a covalent state and an ionic state which avoid each other at long distance. A detailed analysis of the changes in the dipole moment along the reaction coordinate reveals multiple avoided crossings among the excited states and shows that the charge-transfer processes play a pivotal role for the stabilization of the low-lying electronic states of Li2F. For the anionic system, which is studied for the first time, the insertion of F− is barrierless for many states and there is a gradual charge transfer from F− to Li2 along the reaction path. We also report the optimized parameters and the spectroscopic properties of the five lowest states of the neutral and seven lowest states of the anionic systems, which are strongly stabilized with respect to their respective Li2 + F/F− asymptotes. The observed barrierless insertion mechanisms for both systems make them good candidates for investigation under the ultracold regime

Visualizing electron delocalization in contorted polycyclic aromatic hydrocarbons

International audienceElectron delocalization in contorted polycyclic aromatic hydrocarbon (PAH) molecules was examined through 3D isotropic magnetic shielding (IMS) contour maps built around the molecules using pseudo-van der Waals surfaces. The resulting maps of electron delocalization provided an intuitive, yet detailed and quantitative evaluation of the aromatic, non aromatic, and antiaromatic character of the local and global conjugated cyclic circuits distributed over the molecules. An attractive pictural feature of the 3D IMS contour maps is they are reminiscent of the Clar π-sextet model of aromaticity. The difference in delocalization patterns between the two faces of the electron circuits in contorted PAHs was clearly visualized. For π-extended contorted PAHs, some splits of the π system resulted in recognizable patterns typical of smaller PAHs. The differences between the delocalization patterns of diastereomeric chiral PAHs could also be visualized. Mapping IMS on pseudo-van der Waals surfaces around contorted PAHs allowed visualizing their superimposed preferred circuits for electron delocalization and hence their local and global aromaticity patterns.La délocalisation des électrons dans les hydrocarbures aromatiques polycycliques (HAP) déformés a été analysée à l'aide de cartes de contour de blindage magnétique isotrope (IMS pour isotropic magnetic shielding) en 3D construites autour des molécules à l'aide de surfaces pseudo-van der Waals. Les cartes de délocalisation électronique qui en résultent fournissent une évaluation intuitive, mais détaillée et quantitative du caractère aromatique, non aromatique et antiaromatique des circuits conjugués locaux et globaux distribués sur les molécules. Une caractéristique attrayante des cartes de contour IMS 3D est qu'elles rappellent le modèle du sextet de Clar. La différence des schémas de délocalisation entre les deux faces des circuits électroniques dans les HAP déformés a été clairement visualisée. Pour les HAP déformés π-étendus, certaines scissions du système π ont donné lieu à des motifs reconnaissables typiques de plus petits HAP. Les différences entre les schémas de délocalisation des HAP chiraux diastéréoisomères ont également pu être visualisées. La cartographie de l'IMS sur les surfaces de pseudo-van der Waals autour des HAP déformés a permis de visualiser leurs circuits superposés privilégiés pour la délocalisation des électrons et donc leurs modèles d'aromaticité locaux et globaux

Hückel-Lewis Projection Method: A “Weights Watcher” for Mesomeric Structures †

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Multidimensional Isotropic Magnetic Shielding Contour Maps for the Visualization of Aromaticity in ortho-Arynes and Their Reactions

Abstract Visualization of electron delocalization and aromaticity in some selected arynes, including nonplanar examples, and their Diels–Alder or dimerization reactions was achieved through multidimensional isotropic magnetic shielding contour maps. These maps showed that arynes are generally less aromatic than the corresponding arenes, and that aromaticity peaks during their reactions when approaching the transition state

Localized Structures at the Hückel Level, a Hückel-Derived Valence Bond Method

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HuLiS : Simple HÜckel and LewIS embedded in the Hückel theory

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