Régis T. Santiago

Orbital signatures as a descriptor of regioselectivity and chemical reactivity: the role of the frontier orbitals on 1,3-dipolar cycloadditions.

The FERMO concept emerges as a powerful and innovative implement to investigate the role of molecular orbitals applied to the description of breakage and formation of chemical bonds. In this work, Hartree-Fock (HF) theory and density functional (DFT) calculations were performed for a series of four reactions of 1,3-dipolar cycloadditions and were analyzed by molecular orbital (MO) energies, charge transfer, and molecular dynamics (ADMP) techniques for direct dynamics using the DFT method. The regioselectivity for a series of four 1,3-dipolar cycloaddition reactions was studied here using global and local reactivity indexes. We observed that the HOMO energies are insufficient to describe the behavior of these reactions when there is the presence of heteroatoms. By using the frontier effective-for-reaction molecular orbital (FERMO) concept, the reactions that are driven by HOMO, and those that are not, can be better explained, independent of the calculation method used, because both HF and Kohn-Sham methodologies lead to the same FERMO.

How computational methods and relativistic effects influence the study of chemical reactions involving Ru-NO complexes?

Two treatments of relativistic effects, namely effective core potentials (ECP) and all‐electron scalar relativistic effects (DKH2), are used to obtain geometries and chemical reaction energies for a series of ruthenium complexes in B3LYP/def2‐TZVP calculations. Specifically, the reaction energies of reduction (A‐F), isomerization (G‐I), and Cl− negative trans influence in relation to NH3 (J‐L) are considered. The ECP and DKH2 approaches provided geometric parameters close to experimental data and the same ordering for energy changes of reactions A‐L. From geometries optimized with ECP, the electronic energies are also determined by means of the same ECP and basis set combined with the computational methods: MP2, M06, BP86, and its derivatives, so as B2PLYP, LC‐wPBE, and CCSD(T) (reference method). For reactions A‐I, B2PLYP provides the best agreement with CCSD(T) results. Additionally, B3LYP gave the smallest error for the energies of reactions J‐L.

Quantum chemical topological analysis of hydrogen bonding in HX…HX and CH3X…HX dimers (X = Br, Cl, F)

We present a systematic investigation of the nature and strength of the hydrogen bonding in HX···HX and CH3X…HX (X = Br, Cl and F) dimers using ab initio MP2/aug-cc-pVTZ calculations in the framework of the quantum theory of atoms in molecules (QTAIM) and electron localisation functions (ELFs) methods. The electron density of the complexes has been characterised, and the hydrogen bonding energy, as well as the QTAIM and ELF parameters, is consistent, providing deep insight into the origin of the hydrogen bonding in these complexes. It was found that in both linear and angular HX…HX and CH3X…HX dimers, F atoms form stronger HB than Br and Cl, but they need short (∼2 Å) X…HX contacts.

New density functional parameterizations to accurate calculations of electric field gradient variations among compounds.

This research provides a performance investigation of density functional theory and also proposes new functional parameterizations to deal with electric field gradient (EFG) calculations at nuclear positions. The entire procedure is conducted within the four‐component formalism. First, we noticed that traditional hybrid and long‐range corrected functionals are more efficient in the description of EFG variations for a set of elements (indium, antimony, iodine, lutetium, and hafnium) among linear molecules. Thus, we selected the PBE0, B3LYP, and CAM‐B3LYP functionals and promoted a reoptimization of their parameters for a better description of these EFG changes. The PBE0q variant developed here showed an overall promising performance in a validation test conducted with potassium, iodine, copper, and gold. In general, the correlation coefficients found in linear regressions between experimental nuclear quadrupole coupling constants and calculated EFGs are improved while the systematic EFG errors also decrease as a result of this reparameterization.

Novas Perspectivas sobre o Papel dos Orbitais Moleculares de Fronteira no Estudo Da Reatividade Química: Uma Revisão

Molecular orbitals play a crucial role in the understanding of the chemical reactivity at the atomic level and are important descriptors for the rationalization of various chemical reactions. In this context, the molecular orbital concept stated by Fukui refers to the use of the frontier electron density, HOMO e LUMO, for predicting the most reactive position on chemical species: HOMO (highest occupied molecular orbital energy) and LUMO (unoccupied molecular orbital of lowest energy). However, the HOMO-LUMO approach has some limitations, and other concepts are needed to complement the HOMO-LUMO arguments. New perspectives on the chemical reactivity of organic and inorganic compounds can, easily, be rationalized in terms of the composition and location of the frontier molecular orbitals. Thus, in this review, we have analyzed our recent efforts to investigate the role of the Frontier Effective-for-Reaction Molecular, so-called FERMO, which emerged more recently as a powerful tool to describe the breaking and formation of new chemical bonds and, thus open new possibilities to better understand the mechanistic aspects that govern chemical reactivity of atoms and molecules. DOI: 10.5935/1984-6835.20160032