Vincent Tognetti

On the physical role of exchange in the formation of an intramolecular bond path between two electronegative atoms

In this paper, we present a detailed energetic decomposition of intramolecular O···X interactions (X being O, S, or a halogen atom) based on the interacting quantum atoms approach of Pendás and co-workers. The nature of these interactions (repulsive or attractive, more or less electrostatic) is discussed in the framework of Baders atoms in molecules theory, a particular emphasis being put on delocalization (measured by delocalization indexes and in terms of the source function) and on the exchange contributions. Notably, the concept of exchange channels introduced by Pendás and collaborators provides means of rationalizing and predicting the presence of bond critical points, enhancing the physical meaning of bond paths.

Insights into the chemical meanings of the reaction electronic flux

Abstract The negative derivative of the chemical potential with respect to the reaction coordinate is called reaction electronic flux and has recently focused a wide interest to better understand chemical reactions at molecular level. After much consideration, it is now well accepted that positive REF values are associated with spontaneous processes, while negative REF ones translate unspontaneous phenomena. These characteristics of the REF are based on a thermodynamic analogy and have been shown right through computational results. In this paper, we develop two analytical expressions of the REF in both the canonical and the grand canonical ensembles. The connection between both equations is established. They are then analyzed, and some arguments are put forward to support the alleged characteristic of the REF and its ability to properly discriminate spontaneous from unspontaneous phenomena.

On the influence of density functional approximations on some local Bader's atoms-in-molecules properties.

In this article, we assess the ability of various density functionals to predict accurate values for some basic properties of the bond critical points of about 50 small molecules, including the recently proposed reduced gradient variation rates and involving typical ionic and covalent bonds, agostic interactions, and van der Waals complexes. The relation between the computed deviations and the geometric variations are discussed, as well as the topology variations. The possible correlation of these descriptors to atomization energies is considered, and the relevance of an accurate QTAIM analysis for correct descriptions of potential energy surfaces is addressed. Finally, we provide typical margins of error for the evaluation of these quantities and discuss their consequences for computational applications.

A new parameter-free correlation functional based on an average atomic reduced density gradient analysis

A new parameter-free correlation functional based on the local Ragot-Cortona approach [J. Chem. Phys. 121, 7671 (2004)] is presented. This functional rests on a single ansatz for the gradient correction enhancement factor: it is assumed to be given by a simple analytic expression satisfying some exact conditions and containing two coefficients. These coefficients are determined without implementing the functional and without using a fitting procedure to experimental data. Their values are determined by requiring that the functional gives a correct average reduced density gradient for atoms, which, to some extent, can be considered an intrinsic atomic property. The correlation functional is then coupled with the Perdew-Burke-Erzernhof (PBE) exchange and compared with the original PBE approach as well as with some other pure density or hybrid approaches. Standard tests for atomic and molecular systems show that our new functional significantly improves on PBE, showing very interesting properties.

Role of Cationization and Multimers Formation for Diastereomers Differentiation by Ion Mobility-Mass Spectrometry

AbstractStereochemistry plays an important role in biochemistry, particularly in therapeutic applications. Indeed, enantiomers have different biological activities, which can have important consequences. Many analytical techniques have been developed in order to allow the identification and the separation of stereoisomers. Here, we focused our work on the study of small diastereomers using the coupling of traveling wave ion mobility and mass spectrometry (TWIMS-MS) as a new alternative for stereochemistry study. In order to optimize the separation, the formation of adducts between diastereomers (M) and different alkali cations (X) was carried out. Thus, monomers [M + X]+ and multimers [2M + X]+ and [3M + X]+ ions have been studied from both experimental and theoretical viewpoints. Moreover, it has been shown that the study of the multimer [2Y + M + Li]+ ion, in which Y is an auxiliary diastereomeric ligand, allows the diastereomers separation. The combination of cationization, multimers ions formation, and IM-MS is a novel and powerful approach for the diastereomers identification. Thus, by this technique, diastereomers can be identified although they present very close conformations in gaseous phase. This work presents the first TWIMS-MS separation of diastereomers, which present very close collision cross section thanks to the formation of multimers and the use of an auxiliary diastereomeric ligand. Figureᅟ

A proposal for an extended dual descriptor: a possible solution when Frontier Molecular Orbital Theory fails

In this paper, we introduce new local descriptors in the framework of Conceptual Density Functional Theory. They can be considered as an extension of the dual descriptor [Morell et al., J. Phys. Chem. A, 2005, 109, 205]. These indices are particularly suited for the discrimination between electrophilic and nucleophilic sites inside a molecule. They are computed using the densities of the electronic excited states, giving a picture of the polarization of the electron density induced by the approach of a reactant. Links with the linear-response function are discussed, and the first examples of applications are given, highlighting how these new descriptors can be used in practice for reactivity studies. It has been found that this extension of the dual descriptor can handle tricky cases, such as nitrobenzene or isoquinoline, for which Frontier Molecular Orbital Theory fails.

Characterizing agosticity using the quantum theory of atoms in molecules: bond critical points and their local properties.

In this paper, we extend the work of Popelier and Logothetis [J. Organomet. Chem. 1998, 555, 101] on the characterization of agosticity by considerably enlarging the set of the studied organometallic molecules. To this aim, 23 representative complexes have been considered, including all first line transition metals at various oxidation states and exhibiting four types of agosticity (α, β, γ, and δ). From these examples, the concepts of agostic atom, agostic bond, and agostic interaction are defined and discussed, notably by advocating Baders analysis of the electron density. The nature and the local properties of the bond critical points are then investigated, and the relationships with the main geometric parameters of the complexes are particularly examined. Moreover, new local descriptors based on kinetic energy densities are developed in order to provide new tools for bond characterization.

Toward a Combined DFT/QTAIM Description of Agostic Bonds: The Critical Case of a Nb(III) Complex

A detailed analysis concerning the effect of the exchange-correlation functional on a prototypical agostic niobium complex has been carried out, with particular attention to a fundamental property of the functional, namely, the recovering of the uniform electron gas limit. The obtained results allow for revisiting the role of this limit for a proper description of the beta-H agostic interaction. Starting from these results, a new criterion for the bond analysis based on the electron density behavior is proposed. Indeed, the density homogeneity between the metal and the involved hydrogen has been evaluated at the bond critical point, as defined in the framework of Baders atoms in molecules theory, by calculating the average variation rates of the (reduced) density gradients. Such descriptors not only provide useful insights on the nature of such an interaction but also could be used as a starting point for a deep (and new) analysis of the chemical bond.

Quantifying electro/nucleophilicity by partitioning the dual descriptor

Translating local electro/nucleophilicities into the language of reactive sites is an appealing theoretical challenge that could be conducive to strengthen the collaborative dialogue between experimentalists and quantum chemists. The usual schemes for such condensation, relying on atomic charges, may however lead to important information loss, due to a sometimes inappropriate averaging of the reactivity anisotropy. In this article, we present instead an approach based on the dual descriptor Δf, which aims at partitioning real space into nonoverlapping reactive domains that feature a constant Δf sign. This strategy enables not only to identify the nucleo/electrophilic regions inside a molecule but also to quantify meaningful properties (mean value, volume, electron population…). Its interest is then illustrated on two specific chemical problems: the measure of σ‐holes in the context of halogen bonds, and of the electrophilicity of organic carbocations, casting the light on the versatility of this method.

Dual descriptor and molecular electrostatic potential: complementary tools for the study of the coordination chemistry of ambiphilic ligands

In this paper, we show that the ambiphilic properties of some organic ligands in organometallic complexes may be retrieved readily from simple calculations in the framework of conceptual density functional theory (C-DFT): namely, the dual descriptor (DD) and the molecular electrostatic potential (MEP) of the ligands afford a rather straightforward interpretation of experimental trends such as the bonding geometry and the electronic properties of complexes in terms of σ-, π- and back-bonding. The studied ligands were chosen to be representative of the wide variety organometallic chemistry offers, ranging from neutral to charged systems and from diatomic to polyatomic molecules. The present approach is general since all relevant parameters are retrieved from the electron density, obtained either from a DFT or post-Hartree-Fock calculation. It is believed to be helpful for organometallic chemists, since it allows a deep understanding and may be used as a predictive tool of the coordinating properties of ligands.