JosE E. Barquera-lozada

Group

Research

Aromaticity and electron delocalization are key concepts in chemistry, serving as central themes within our research group. Despite their widespread application, they are not physical observables and do not have a unique mathematical definition, which complicate the evaluation and comparison of these properties across different molecules. To address this challenge, we employ two measurable molecular properties closely linked to aromaticity and electron delocalization: electron density and magnetically induced current density. Electron density characterizes the spatial distribution of electrons within a molecule, whereas magnetically induced current density describes electron movement under the influence of a magnetic field. Our group employs a combination of theoretical and experimental methodologies to analyze electron and current densities, particularly focusing on organic and inorganic compounds where aromaticity or electron delocalization are pronounced. From the theoretical side we use ab-initio computational methods, while from the experimental side our two main tools are x-ray diffraction and NMR spectroscopy.


Some of our specific research interest are:

Video (Español)

[n]Cumulenes are known for the linearity of their carbon chains, but when the terminal groups are N-heterocyclic carbenes, this situation changes. The even cumulenes of this family can be bent depending on the π acidity of the carbene terminal group. Poor π-back-donation from the carbon chain to the terminal group promotes a significant bending in the carbon chain.

The triangulenes are a family of polycyclic aromatic hydrocarbons (PAH) that have possible applications in fields as different as spintronics or catalysis. However, the electron delocalization in such systems is not well understood because there are several differences to classical PAHs. Contrary to any other PAHs, the π e− of triangulenes and their ions are delocalized throughout the whole molecule and are even more delocalized than acenes. The π sextet aromaticity does not play a central role in the stabilization of triangulenes like for other PAHs. Interestingly, neither the radicals nor the ions follow Clar's rule, which makes them a unique type of PAH.

The “Trace of the Vorticity of the Current Density” tensor condenses the most relevant features of the current density (J(r)) in a simpler scalar field that does not depend in the direction of the magnetic field. It shows the importance of currents perpendicular to the molecular plane that are normally discarded and it can differentiate between 3D non-aromatic molecules and 3D aromatic molecules. With a simpler structure than the induced current density, its vorticity or more specifically its triple product is able to classify archetypic aromatic, antiaromatic, and nonaromatic molecules. Moreover, it clearly reproduces the Clar's structure of polycyclic aromatic hydrocarbons and can easily quantify the electronic circulation on each individual ring.

π-π interactions are unique dispersive interactions that are important for biological systems and materials, but the study of this interaction is not trivial. It is well-known that density functional theory (DFT) does not describe these interactions correctly. Our studies shows that the intermolecular electron density extracted from DFT calculation describes correctly the energetic trends in  π-π interactions, although the densities are underestimated. Unexpectedly, sign⁡(λ2)⁢ρ, used in the Non-Covalent Interaction (NCI) method to estimate attraction or repulsion, shows the wrong trend for π-π stacking systems. 

 

Metallacyclopentadienes (MCPs) are among the most common metallacycles, but their electron delocalization (aromaticity) has received far less attention than other metallacycles, such as metallabenzenes. We systematically studied the aromaticity of MCPs with energetic (isomerization stabilization energy), density (delocalization index) and magnetic (current density) aromaticity indices. The 18𝑒− complexes showed the expected weak aromaticity, and only the 𝑑8 molecules are somewhat anti-aromatic. However, the theoretical account of the aromaticity of the 16𝑒− MCPs is more convoluted. We find that the aromatic criteria for a 16𝑒− 𝑑4 ruthenacyclopentadiene disagree. The lack of agreement shows that significant electron delocalization is not always related to great stability or to strong diatropic currents.



The azulene's zwitterionic character gives it the ability to coordinate to a metallic centre on any of its two rings depending on the spectator ligands π-acidity nature. These ligands can control the haptotropic preferences in metal-Azulene complexes. QTAIM shows that π back-donation affects the coordination preference of the metal.