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University

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[1] Popelier, P. L. A.; Brémond, É. A. G. Int.J.Quant.Chem. 2009, 109, 2542.

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Dennis R. Salahub

University of Calgary, Department of Chemistry, Centre for Molecular Simulation, Institute for Quantum Science and Technology, Institute for Sustainable Energy, Environment and Economy, 2500 University Drive NW, Calgary, AB, Canada T2N 1N4 dennis.salahub@ucalgary.ca, xingliu@ucalgary.ca

Molybdenum carbide nanoparticles as catalysts for hydrogenation reactions, between clusters and the bulk

A gap exists in understanding heterogeneous catalysis between a cluster of a few atoms and a periodic slab model; reactions catalyzed by transition-metal-containing nanoparticles are still not well understood. In this presentation, we provide a multiscale modelling approach to study benzene hydrogenation on molybdenum carbide nanoparticles (MCNPs) in the process of in-situ heavy oil upgrading. The QM DFTB method is coupled with an MM force field to yield a quantum mechanical/molecular mechanical (QM/MM) model describing the reactants, the nanoparticles and the surroundings. Umbrella sampling (US) was employed to calculate the free energy profiles for benzene hydrogenation in a model aromatic solvent under realistic conditions. Comparisons are made with the traditional methodologies; the results reveal new features of the metallic MCNPs. Under working conditions, rather than being rigid, they are very flexible due to the entropic contributions of the MCNPs and the solvent, which greatly affect the free energy profiles.

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Mohamed Amaouch

Laboratoire de Chimie Théorique Université Pierre et Marie Curie and CNRS. F-75005, Paris, France

There is currently a growing interest in using relativistic methods as it has become clear that the study of chemistry involving elements across the entire periodic table requires that relativistic effects are taken into account. The most rigorous approaches are based on the four-component (4c) formalism (Dirac equation) but several alternative two- component (2c) methods were proposed. The 2c methods are notably able to account for spin-orbit (SO) effects. However, the study of SO effects inside the chemical bond appears hindered mainly by the exclusive use of molecular orbital (MO) theory and the related Mulliken populations. Alternative strategies can be proposed to analyze the breaking and the formation of bonds between atoms : atoms in molecules theory (AIM) which relies on the properties (topology) of the total electron density when atoms interact or the Electron Localization Function (ELF) which is a signature of the electronic pairs distribution [1, 2, 3].In this context, an original formulation of ELF and AIM descriptors and their topologies were extended to the framework of 2c quantum calculations [4].

[1] Axel D Becke and Kenneth E Edgecombe.
The Journal of chemical physics, 92(9) :5397-5403, 1990.

[2] Bernard Silvi and Andreas Savin. Nature, 371(6499) :683–686, 1994. 


[3] Richard FW Bader. Atoms in molecules, 1990. 


[4] Julien Pilmé, Eric Renault, Tahra Ayed, Gilles Montavon, and Nicolas Galland. 
Journal of Chemical Theory and Computation, 8(9) :2985–2990, 2012.

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Roberto A. Boto

1. Sorbonne Universités. Université Pierre et Marie Curie. Institut du Calcul et de la Simulation. 75005. Paris. France.

2. Laboratoire de Chimie Théorique Université Pierre et Marie Curie and CNRS. F-75005, Paris. France.

On the interpretation of the reduced density gradient

Non-covalent interactions (NCIs) play a key role in many areas of science, ranging from biochemistry to condensed matter. In this regard, the NCI index has been proposed as a new tool to identify and characterise weak interactions of various strengths as chemical intuitive reduced density gradient (RDG) isosurfaces, that reveal both stabilising(hydrogen bonding and van der Waals interactions) and destabilising interactions.[1] Beyond qualitative analysis, the integration of both volume and charge within NCI isosurfaces are in good agreement with the hydrogen-bonding potential-energy surfaces. [2] Recently Saleh et al. found a good linear correlation between stabilisation energies and kinetic energies integrated on the main NCI isosurfaces. [3] These results connect the topology of RDG with energetics. In the present work we analyse the origin of these correlations through a set of molecular systems representing covalent, ionic and non-covalent interactions. A chemically sound interpretation of RDG is also presented.

[1] R. Johnson, S. Keinan, P. Mori-Sanchez, J. Contreras-Garcia, A. J. Cohen, and W. Yang, “Revealing Noncovalent Interactions”, J. Am. Chem. Soc. 132 , 6498 (2010)

[2] J. Contreras-Garcia, E. R. Johnson, W. Yang, “Analysis of Hydrogen-Bond Interaction Potentials from the Electron Density: Integration of Noncovalent Interaction Regions”, J. Phys. Chem. A 115, 12983 (2011)

[3] G. Saleh, C. Gatti, L. L. Presti, “Energetics of non-covalent interactions from electron energy density distributions”. Comp. Theo. Chem., 1053, 53 (2015)

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Frank de Proft

Eenheid Algemene Chemie (ALGC), Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussels, Belgium.

Density Functional Theory Reactivity Indices: Theoretical Developments, Application in Transition Metal Chemistry and Inverse Design of Molecules with Optimal Reactivity Properties

Conceptual Density Functional Theory (sometimes also called DFT based reactivity theory or Chemical DFT) has proven to be an ideal framework for the introduction of chemical reactivity descriptors. These indices are defined as response functions of the energy E of the system with respect to either the number of electrons N, the external potential ν(r) or both. These definitions have afforded their non- empirical calculation and applications in many fields of chemistry have been performed.

In a first part of the talk, attention will be focused on the so-called linear response function (or polarizability kernel) defined as the second derivative of the energy with respect to the external potential. We will give an overview of the chemistry that can be extracted from this quantity: the kernel is shown to provide a measure of electron delocalization and a connection is made to the concept of aromaticity.

In a second part, an investigation will be presented of structural and electronic requirements of ligand non-innocent behavior in transition metal complexes.

Finally, we will highlight an application of the so-called inverse design methodology, using the reactivity indices to design molecules with optimal reactivity properties. More specifically, this approach will be used to construct radicals with unprecedented stabilities. The design of molecules with optimal properties is an important challenge in chemistry because of the astronomically large number of possible stable structures that are accessible in chemical space. The thiadiazinyl radical was chosen as case study of this new approach, and the optimized property of this compound was the intrinsic radical stability. The resulting optimal structure was found to be highly stable, intrinsically more so than other well-known stable radicals.

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Henry Chermette,^a Walid Lamine,^a,b Christophe Morell^a

^a Université de Lyon, Université Claude Bernard Lyon 1, Institut des Sciences Analytiques, UMR CNRS 5280, 69622 Villeurbanne Cedex, France

^b Université de Tunis El Manar, Faculté des Sciences de Tunis, UR11ES19 Unité de recherche Physico-Chimie des Matériaux condensés, El-Manar II, 2092, Tunis, Tunisie

Ill-advised self-interaction contribution in modeling anionic attack in a reaction path.

The catalytic role of an anion such as iodide in the insertion of CO2 into an epoxide over a Zn complex is presented.

It is shown how the contribution of the iodide ion may be incorrectly modeled in a reaction path, and how the ill-advised self-interaction contribution may be approximately cured.

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Philippe C. Hiberty, Wei Wu, Huaiyu Zhang, Benoît Braida, Sason Shaik

V state of ethylene: from myriads of MO-CI configurations to just four valence bond structures

The first singlet excited state of ethylene (so-called the V state) is a notoriously difficult test case that has necessitated elaborate strategies and extensive configuration interaction in the molecular orbital (MO) framework. By contrast, the description of this electronic state and its transition energy from the ground state (so-called the N state), becomes very simple with valence bond methods. It is shown that extremely compact wave functions, made of three VB structures for the N state and four structures for the V state (1-4 in the scheme below), provide an N→V transition energy of 8.01 eV, in good agreement with experiment (7.88 eV for the N→V transition energy estimated from experiments). Further improvement to 7.96/7.93 eV is achieved at the variational and diffusion Monte Carlo (MC) levels, using a Jastrow factor coupled with the same compact VB wave functions. Furthermore, the measure of the spatial extension of the V state wave function, 19.14 a₀², is in the range of accepted values obtained by large-scale state-of-the-art MO-based methods. The σ response to the fluctuations of the π electrons in the V state, known to be a crucial feature of the V state, is taken into account using the breathing-orbital valence bond method (BOVB), which allows the VB structures to have different sets of orbitals, as is made apparent in 1 and 2. Further valence bond calculations in a larger space of configurations confirm the results of the minimal structure-set, yielding an N→V transition energy of 7.97 eV and a spatial extension of 19.16 a₀² for the V state. Both types of valence bond calculations show that the V state of ethylene is not fully ionic as usually assumed, but involving also a symmetry-adapted combination of VB structures,3 and 4, each with asymmetric covalent π bonds. The latter VB structures have cumulated weights of at least 18-26%, and stabilize the V state by about 0.9 eV. It is further shown that these latter VB structures, rather than the commonly considered zwitterionic ones, are the ones responsible for the spatial extension of the V state, known to be ca. 50% larger than the V state.

Scheme — The compact VB representation of the V state of ethylene

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Aurélien de la Lande

Laboratoire de Chimie Physique, Université Paris Sud, CNRS, Université Paris Saclay. 15, avenue Jean Perrin, 91405 Orsay, Cedex. France

Robust, basis-set independent method for the evaluation of charge-transfer energy in nonconvalent complexes

Separation of the energetic contribution of charge transfer to interaction energy in noncovalent complexes would provide important insight into the mechanisms of the interaction. However, the calculation of charge-transfer energy is not an easy task. It is not a physically well-defined term and the results might depend on how it is described in practice. Commonly, the charge transfer is defined in terms of molecular orbitals; in this framework, however, the charge transfer vanishes as the basis set size increases towards the complete basis set limit. This can be avoided by defining the charge transfer in terms of the spatial extent of the electron densities of the interacting molecules, but the schemes used so far do not reflect the actual electronic structure of each particular system and thus are not reliable. We propose a novel approach – spatial partitioning of the system which is based on a charge transfer-free reference state, namely superimposition of electron densities of the non-interacting fragments. We show that this method, employing constrained DFT for the calculation of the charge-transfer energy, yields reliable results and is robust with respect to the strength of the charge transfer, the basis set size and the DFT functional used. Because it is based on DFT, the method is applicable to rather large systems.

[1] Řezáč, J; de la Lande, A. J. Chem. Theor. Comput. 2015, 11, 528-537.

A. Martín Pendás

Universidad de Oviedo, Julian Claveria, 33006, Oviedo, Spain

Electron distribution functions from academic models: towards a real space Aufbau principle

Electron number distribution functions (EDFs) allow for the computation of the weights of real space resonance structures. To obtain them we need a partition of space into chemically meaningul regions, i.e. through QTAIM, ELF, Hirshfeld, or any other exhaustive or fuzzy decomposition available in the literature.

With such a decomposition we may compute the probability of distributing the N electrons of a molecular system into the m regions in which we have divided space, in every possible way. EDFs provide valuable insight into chemical bonding, and here we show that they may be successfully approximated by very simple models, giving rise to an interesting interpretation of the standard Aufbau principle in real space. This is obtained from academic models of the wave functions of simple systems and a Mulliken-like condensation.

P. Reinhardt, P. Chaquin

LCT, UPMC, Paris, France

Realistic molecular frequencies from a simple atomic-density model

Based on the Hellmann-Feynman theorem we construct a simple model for calculating force constants in molecular, poly-nuclear frameworks. We show the vibrational frequencies for homonuclear diatomics are obtained with an astonishing precision, and for larger molecules domains of frequencies can be identified.

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