Nicolás Otero

A Computational Study on the Acidity Dependence of Radical-Scavenging Mechanisms of Anthocyanidins

On the basis of quantum chemical calculations, the radical-scavenging property attributed to anthocyanidins was analyzed considering three mechanisms: hydrogen atom transfer (HAT), stepwise electron-transfer-proton-transfer (ET-PT), and sequential proton loss electron transfer (SPLET). We found that the activity of anthocyanidins and the mechanism through which they react are pH-dependent, because the diverse colorful forms in which anthocyanidins may exist in prototropic equilibria (cationic, neutral, anionic) are susceptible to experience each of the mechanisms proposed. According to redox parameters calculated, we can conclude that HAT is always the most favored of the generally accepted mechanisms to scavenge reactive oxygen species (ROS) by the three colored forms. Nevertheless, only neutral and anionic forms are found to be able to scavenge 1,1-diphenyl-2-picrylhydrazyl (DPPH.) radical through HAT and SPLET mechanisms from a thermodynamical point of view, whereas ET-PT is only feasible for anions. Sequential proton loss hydrogen atom transfer (SPL-HAT) is proposed as the only pathway for the reaction between anthocyanidin cations and the DPPH. radical. It should be viable according to our quantum mechanical calculations and even competitive with typical HAT, ET-PT, and SPLET.

Computational Study on the Stacking Interaction in Catechol Complexes

The stability and electron density topology of catechol complexes (dimers and tetramer) were studied using the MPW1B95 functional. The QTAIM analysis shows that both dimers (face to face and C-H/pi one) display a different electronic origin. The formation of the former is accompanied by a significant change in the values of atomic electron dipole and quadrupole components, flattening the most diffuse part of the electron density distribution toward the molecular plane. A small electron population transfer is observed between catechol monomers connected by C-H/pi interactions, whose QTAIM characterization does not differ from that of a weak hydrogen bond. Cooperative effects in the tetramer on binding energies are small and negligible for bond properties and charge transfer. Nevertheless, they are significant on atomic electron populations.

How does aromaticity rule the thermodynamic stability of hydroporphyrins

Several measures of aromaticity including energetic, magnetic, and electron density criteria are employed to show how aromatic stabilization can explain the stability sequence of hydroporphyrins, ranging from porphin to octahydroporphin, and their preferred hydrogenation paths. The methods employed involve topological resonance energies and their circuit energy effects, bond resonance energies, multicenter delocalization indices, ring current maps, magnetic susceptibilities, and nuclear-independent chemical shifts. To compare the information obtained by the different methods, the results have been put in the same scale by using recently proposed approaches. It is found that all of them provide essentially the same information and lead to similar conclusions. Also, hydrogenation energies along different hydrogenation paths connecting porphin with octahydroporphin have been calculated with density functional theory. It is shown by using the methods mentioned above that the relative stability of different hydroporphyrin isomers and the observed inaccessibility of octahydroporphin both synthetically and in nature can be perfectly rationalized in terms of aromaticity.

Revisiting the calculation of condensed Fukui functions using the quantum theory of atoms in molecules

The analysis of previously reported shortcomings of the condensed Fukui functions obtained making use of the quantum theory of atoms in molecules indicates these drawbacks are due to the inadequacy of the definition employed to compute them and not to the partitioning. A new procedure, which respects the mathematical definition and solves these problems, is presented for the calculation of condensed Fukui functions for atomic basins defined according to the quantum theory of atoms in molecules. It is tested in a set of 18 molecules, which includes the most controversial reported cases.

A QTAIM‐based energy partitioning for understanding the physical origin of conformational preferences: Application to the Z effect in O=C‐X‐R and related units

A quantum theory of atoms in molecules‐based energy partitioning was carried out for Z and E conformers of a series of O=C‐X‐R containing compounds. The results obtained for the simplest compound (formic acid) indicate that the attraction of the electron density within carbonyl oxygen by the nucleus of the acid hydrogen is the most important energy term for Z preference. This conclusion can be extended (mutatis mutandis) to larger carboxylic acids, esters, sulfur derivatives, secondary amides, and carbonyl isocyanates, and even explains the sequence of relative conformational energies in the HCXOH series (X = O, S, Se). In contrast, although the hyperconjugative model has been traditionally employed to explain this preference, we observe it is incompatible with: (i) relative values of diverse QTAIM atomic populations for the Z/E conformational equilibrium; (ii) conformational energies in the HCXOH series.

Anion-π Aromatic Neutral Tweezers Complexes: Are They Stable in Polar Solvents?

The impact of the solvent environment on the stabilization of the complexes formed by fluorine (T-F) and cyanide (T-CN) substituted tweezers with halide anions has been investigated theoretically. The study was carried out using computational methodologies based on density functional theory (DFT) and symmetry adapted perturbation theory (SAPT). Interaction energies were obtained at the M05-2X/6-31+G* level. The obtained results show a large stability of the complexes in solvents with large dielectric constant and prove the suitability of these molecular tweezers as potential hosts for anion recognition in solution. A detailed analysis of the effects of the solvent on the electron withdrawing ability of the substituents and its influence on the complex stability has been performed. In particular, the interaction energy in solution was split up into intermonomer and solvent-complex terms. In turn, the intermonomer interaction energy was partitioned into electrostatic, exchange, and polarization terms. Polar resonance structures in T-CN complexes are favored by polar solvents, giving rise to a stabilization of the intermonomer interaction, the opposite is found for T-F complexes. The solvent-complex energy increases with the polarity of the solvent in T-CN complexes, nonetheless the energy reaches a maximum and then decreases slowly in T-F complexes. An electron density analysis was also performed before and after complexation, providing an explanation to the trends followed by the interaction energies and their different components in solution.

Hirshfeld‐based intrinsic polarizability density representations as a tool to analyze molecular polarizability

In this work, a general scheme to visualize polarizability density distributions is proposed and implemented in a Hirshfeld‐based partitioning scheme. This allows us to obtain easy‐to‐interpret pictorial representations of both total and intrinsic polarizabilities where each point of the density is formed by the contribution of any atom or group of atoms in the molecule. In addition, the procedure used here permits the possibility of removing the size dependence of the electric‐dipole polarizability. Such a development opens new horizons in exploring new applications for the analysis of the molecular polarizability tensor. For instance, this visualization shows which atoms or regions are more polarizable distinguishing, moreover, the fine structure of atoms affected by the vicinity, and might extend the dipole polarizability as a tool for aromaticity studies in polycyclic aromatic hydrocarbons. Additionally, this approach can serve us to assess the methods performance in describing the interaction of electric fields with a molecule and local electron correlation effects in intrinsic polarizabilities.

A Computational Study of the Interaction and Polarization Effects of Complexes Involving Molecular Graphene and C60 or a Nucleobases.

A systematic analysis of the molecular structure, energetics, electronic (hyper)polarizabilities and their interaction-induced counterparts of C60 with a series of molecular graphene (MG) models, CmHn, where m = 24, 84, 114, 222, 366, 546 and n = 12, 24, 30, 42, 54, 66, was performed. All the reported data were computed by employing density functional theory and a series of basis sets. The main goal of the study is to investigate how alteration of the size of the MG model affects the strength of the interaction, charge rearrangement, and polarization and interaction-induced polarization of the complex, C60-MG. A Hirshfeld-based scheme has been employed in order to provide information on the intrinsic polarizability density representations of the reported complexes. It was found that the interaction energy increases approaching a limit of -26.98 kcal/mol for m = 366 and 546; the polarizability and second hyperpolarizability increase with increasing the size of MG. An opposite trend was observed for the dipole moment. Interestingly, the variation of the first hyperpolarizability is relatively small with m. Since polarizability is a key factor for the stability of molecular graphene with nucleobases (NB), a study of the magnitude of the interaction-induced polarizability of C84H24-NB complexes is also reported, aiming to reveal changes of its magnitude with the type of NB. The binding strength of C84H24-NB complexes is also computed and found to be in agreement with available theoretical and experimental data. The interaction involved in C60 B12N12H24-NB complexes has also been considered, featuring the effect of contamination on the binding strength between MG and NBs.

Establishing the pivotal role of local aromaticity in the electronic properties of boron-nitride graphene lateral hybrids

Within an attempt to unravel the conundrum of irregular bandgap variations in hybrids of white-graphene (hBN) and graphene (G) observed in both experiment and theory, strong proofs about the decisive role of aromaticity in their electronic properties are brought to light. Sound numerical experiments conducted on zero-, one- and two-dimensional hBNG hybrids demonstrate that upon structural and/or electronic perturbation caused by foreign doping agents, the uniformity in local cyclic electron delocalization of ideal graphene restructures locally creating carbon hexagons of contrasting cyclic electron delocalization (c.c. local aromatic patterns) which may dominate the bandgap size of the resulting systems. In addition, relying on the quantum chemical aspect of aromaticity in terms of quantitative computations of cyclic electron delocalization together with pictorial intrinsic polarizability density representations, this work provides a solid and handy rule-of-thumb to be used in qualitative and intuitive predictions. According to this empirical rule, the origin of any nonmonotonic bandgap variation observed in stoichiometric 0D (BN)n/graphene hybrids with increasing hBN segment lies in instabilities caused by partially substituted benzenoid rings formed locally at the hBNG interfaces. This relationship, established in 0D graphene flakes and extended to 1D periodic ribbons, can be used to understand and qualitatively predict conflicting bandgap variations of vacancy-free 2D periodic lattices, pointing at the property of aromaticity as the missing link needed to solve the puzzle of conflicting bandgap variations in hBNG hybrids observed in experiment.

Chemical reactivity in the framework of pair density functional theories.

Chemical reactivity descriptors are derived within the framework of the pair density functional theory. These indices provide valuable information about bonding rearrangements and activating mechanisms upon electrophilic or nucleophilic reactions. Indices derived and tested in this work represent nonlocal counterparts of the local reactivity indices derived in the context of conceptual density functional theory (CDFT) and frequently used in reactivity studies; the Fukui function, the local softness and the dual descriptor. In this work, we show how these nonlocal indices provide a quantum chemical basis to explain the success of qualitative resonance models in chemical reactivity predictions. Also, local information is implicitly contained as CDFT indices are obtained by simple integration. As illustrative examples, we have considered in this work the Markovnikovs rule, the reactivity of enolate anion, the nucleophilic conjugate addition to α,β‐unsaturated compounds and the electrophilic aromatic substitution of benzene derivatives. The densities used in this work were obtained with Hartree‐Fock, Kohn‐Sham DFT, and singles and doubles configuration interaction (CISD) approaches.