José Manuel Guevara-Vela

Chemical bonding in excited states: Energy transfer and charge redistribution from a real space perspective

This work provides a novel interpretation of elementary processes of photophysical relevance from the standpoint of the electron density using simple model reactions. These include excited states of H2 taken as a prototype for a covalent bond, excimer formation of He2 to analyze non‐covalent interactions, charge transfer by an avoided crossing of electronic states in LiF and conical interesections involved in the intramolecular scrambling in C2H4. The changes of the atomic and interaction energy components along the potential energy profiles are described by the interacting quantum atoms approach and the quantum theory of atoms in molecules. Additionally, the topological analysis of one‐ and two‐electron density functions is used to explore basic reaction mechanisms involving excited and degenerate states in connection with the virial theorem. This real space approach allows to describe these processes in a unified way, showing its versatility and utility in the study of chemical systems in excited states.

Valence Shell Charge Concentration (VSCC) Evolution: A Tool to Investigate the Transformations within a VSCC Throughout a Chemical Reaction

Theoretical studies about reaction mechanisms are usually limited to the determination of the energetic paths that connect reactants, transition states, and products. Recently, our group proposed the structural evolution, which has provided insights about the molecular structure changes occurring along a reaction path. Structural evolution may be defined as the development of a chemical reaction system across the partitioning of the nuclear configuration space into a finite number of structural regions defined on account of the topology of a scalar field, e.g., the electron density. In this paper, we present a tool to investigate within the framework of the Quantum Theory of Atoms in Molecules the evolvement of the Valence Shell Charge Concentration, the VSCC evolution, which is the description of the changes of electron density concentrations and depletions around the bonding area of an atom. The VSCC evolution provides supplementary information to the structural evolution because it allows the analysis of valence shells within a structural region, i.e., a subset of R(Q) with the same connectivity among the atoms forming a molecule. This new approach constitutes also a complement to the Valence-Shell Electron Pair Repulsion (VSEPR) model because it gives an account of the adjustments of electron pairs in the valence shell of an atom across a chemical reaction. The insertion reaction in the hydroformylation reaction of ethylene, the reduction of cyclohexanone with lithium aluminum hydride, the oxidation of methanol with chlorochromate, and the bimolecular nucleophilic substitution of CH(3)F with F(-) are used as representatives examples of the application of the VSCC evolution. Overall, this paper shows how the VSCC evolution through an analysis of the modifications of local charge concentrations and depletions in individual steps of a chemical reaction gives new insights about these processes.

Where Does Electron Correlation Lie? Some Answers from a Real Space Partition

We discuss an intra- and interatomic partition of the electron correlation energy (Ecorr ) within the interacting quantum atoms (IQA) approach at the accurate coupled cluster level with single, double, and estimated triple excitations (CCSDT(T)). This division offers a privileged window into the spatial localization of this component of the molecular energy. We show that the total molecular Ecorr is dominated by the intra-atomic or intra-fragment terms and that interatomic contributions change smoothly from short- to long-range correlation (dispersion). The sign of these interatomic correlation terms differentiate between (i) mainly covalent or (ii) mainly ionic or nonbonded interactions. We propose that this feature may be used to define and examine intramolecular dispersion terms.

Hydrogen-Bond Weakening through π Systems: Resonance-Impaired Hydrogen Bonds (RIHB)

Redefining interactions: The concept of the resonance-impaired hydrogen bond (RIHB) as an interaction in which a conjugated π system strongly impairs the formation of a hydrogen bond (HB) is introduced. A typical HB involving charged species can have a formation energy of tens of kcal mol-1 , whereas the corresponding value for the examined RIHB is only 2.6 kcal mol-1 . Quantum chemical topology tools are used to analyse the low formation energy of the studied RIHBs.

Structural effects of trifluoromethylation and fluorination in gold(I) BIPHEP fluorothiolates

We synthesised and characterised two new BIPHEP-bridged digold(I) perfluorothiolates. Experimental and theoretical analyses through the Quantum Theory of Atoms in Molecules revealed that the long-range inductive effects are larger for CF3 than they are for F. Moreover, the aurophilic interaction and F–Au contacts were qualified mostly as covalent and closed-shell type respectively.

Performance of the RI and RIJCOSX approximations in the topological analysis of the electron density

The performance of the Møller–Plesset (MP2) method in its resolution of the identity (RI-MP2), and the chain of spheres exchange (RIJCOSX-MP2) variants within the Quantum Theory of Atoms in Molecules (QTAIM) wavefunction analyses is examined. We have obtained QTAIM descriptors at bond critical points for a series of small molecules and water clusters of different sizes. We also considered integrated properties, like QTAIM atomic charges or localization and delocalization indices. The performance of RI methods with respect to the plain MP2 benchmark results is excellent, with mean deviations for all the properties considered below 0.15%. However, in systems where electron delocalization plays a more important role, we found differences up to 5% (e.g.

Hydrogen‐Bond Cooperative Effects in Small Cyclic Water Clusters as Revealed by the Interacting Quantum Atoms Approach

\hbox {C}_6\hbox {H}_6

The nature of resonance-assisted hydrogen bonds: a quantum chemical topology perspective

C6H6) from a suitable reference. The account of RIJCOSX-HF results shows that the RIJCOSX approximation works better when electron correlation is included. Finally, a topological analysis of the electron density on the endofullerene complex