The nuclear electric quadrupole moment (NQM) of the (63)Cu nucleus was determined from an indirect approach by combining accurate experimental nuclear quadrupole coupling constants (NQCCs) with relativistic Dirac-Coulomb coupled cluster calculations of the electric field gradient (EFG). The data obtained at the highest level of calculation, DC-CCSD-T, from 14 linear molecules containing the copper atom give rise to an indicated NQM of -198(10) mbarn. Such result slightly deviates from the previously accepted standard value given by the muonic method, -220(15) mbarn, although the error bars are superimposed.

ABSTRACT:The Frontier effective-for-reaction molecular orbital (FERMO) concept emerges as a powerful and innovative implement to investigate the role of molecular orbitals (MOs) applied in the description of breakage and formation of chemical bonds. In this work, theoretical calculations were carried out for conjugated acids of 18 amines and their acid-base behavior was analyzed using MO energies. We observed that highest occupied MO (HOMO) energies are inadequate to describe the acid-base behavior of these compounds. By using the FERMO concept, the reactions that are driven by HOMO, and those that are not, can be better explained, independent of the calculation method used, as independent of the calculation method used, both HF and Kohn-Sham methodologies lead to the same FERMO.

The issue of reactivity differences of the ions Nb+ and Nb2+ toward methane (CH4 + Nb+ → NbCH2 + + H2 and CH4 + Nb2+ → NbCH2 2+ + H2) is addressed using CCSD(T)//MP2 and CCSD(T)//B3LYP calculations. A thorough description of the reaction mechanisms and analyses of the geometrical structures, chemical bonds, and molecular orbitals provide important insights into the chemistry of the niobium species. The overall results indicate highly labile kinetics and more favorable thermochemical conditions for Nb2+. The NBO, AIM, and FERMO analyses provide a better understanding of the reactivity differences in the niobium reactions.

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.

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. ResumoOrbitais moleculares tem um papel crucial no entendimento da reatividade química em nível atômico, e são importantes descritores para a racionalização de várias reações químicas. Neste contexto, o conceito de orbital de fronteira introduzido por Fukui, refere-se ao uso da densidade eletrônica dos orbitais de fronteira, HOMO e LUMO, para predizer a posição mais reativa sobre espécies químicas: HOMO (Orbital Molecular Ocupado de maior energia) e LUMO (Orbital Molecular desocupado de menor energia). No entanto, a abordagem HOMO-LUMO tem algumas limitações e outros conceitos são necessários para complementar os argumentos HOMO e LUMO. Novas perspectivas podem ser facilmente racionalizadas em relação á concepção da reatividade química de compostos orgânicos e inorgânicos, com base na composição e localização dos orbitais moleculares de fronteira. Assim, nesta revisão, analisamos os esforços recentes no sentido de investigar o papel do orbital molecular de fronteira efetivo para a reação, chamado de FERMO, que emergiu mais recentemente como uma poderosa ferramenta para descrever a quebra e formação de novas ligações químicas e, assim, abrir novas possibilidades para melhor compreender os aspectos mecanicistas que governam a reatividade química de moléculas.Palavras-chave: FMO; FERMO; HOMO-LUMO; cálculos teóricos e reatividade.

The concept FERMO emerges as a tool to investigate the role of molecular orbitals used to describe the chemical bonds. Thus, calculations were carried out with a set of phosphines. Our data suggest that different molecular orbitals, beyond HOMO, can be used to describe the behavior of acid-base reactions.

The nuclear electric quadrupole moment (NQM) of ruthenium was reevaluated by means of the molecular method. Therefore, four-component relativistic electronic structure calculations of the electric field gradient at the Ru nucleus in ruthenium monocarbide, which were done with the coupled cluster methodology and carefully augmented large basis sets, were combined with the respective nuclear quadrupole coupling constant from experiments. This provided a recommended NQM value of (425 ± 13) mbarn for 101Ru.

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 NH (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. © 2017 Wiley Periodicals, Inc.