RESUMEN

The understanding of the molecular- and colloidal-structure of asphaltenes has seen a major progress; however, there are still issues that require answer. One of them is the location of the heteroatoms in the polycyclic aromatic hydrocarbon (PAH) fused aromatic ring (FAR) region of asphaltenes. Therefore, the effect on the frontier molecular orbitals (HOMO-LUMO) energy-gap due to the addition of a heteroatom (N or S) to PAHs, which are candidates of the PAH region in asphaltenes, has been systematically analyzed by placing S or N in various sites of the PAH molecule. The S is introduced as a thiophenic ring in a bay region, while the N is introduced as a pyridinic-N, which are prevalent forms in the asphaltene-PAH. 174 PAHs are studied with five fused aromatic rings (5FAR) to 10FAR. The π-electron allocation in resonant π-sextets and isolated double bonds is obtained using the Y-rule. The frontier orbitals optical transition is calculated with the ZINDO/S method. Within a FAR family an increment of π-sextets produces and increase of the HOMO-LUMO energy-gap. There is a linear relationship between the Y-rule mapping (percentage of fraction of π-sextet bond divided by nFAR) and the HOMO-LUMO energy-gap. In addition, the effect on the frontier orbitals energy-gap and on the π-electronic allocation due to the presence of N and S is negligible; therefore, to reach conclusions related to the asphaltene-PAH based on conclusions reached for PAH systems, with no heteroatoms, is a reasonable approach.

RESUMEN

In the present contribution, we have developed a database, called the FAR-database, where the acronym FAR stands for Fused Aromatic Rings, which presents the results of nuclear independent chemical shifts calculations, NICS(0), NICS(1), NICS(0)ZZ, and NICS(1)ZZ, of 660 neutral benzenoid-PAHs and cyclopenta-fused PAHs. The FAR-database provides NICS data of aromaticity of PAHs that could be used in data science and machine learning. To the best of our knowledge, no such database is available in the literature. The importance of this database lies in its potential to transform data into insight and knowledge. Additionally, a new visual representation of the NICS aromaticity pattern, based on the magnitude of the NICS value, is presented. Nowadays calculations of NICS(0)ZZ or NICS(1)ZZ have become popular methods to evaluate aromaticity of systems. By looking at all the 660 systems in the FAR-database, it becomes evident that NICS(0), NICS(1), and NICS(1)ZZ present similar NICS aromaticity patterns for most of the systems. But the NICS aromaticity patterns found with NICS(0)ZZ in many cases do not agree with the NICS aromaticity patterns found with NICS(0), NICS(1), and NICS(1)ZZ. There are cases where the NICS(0)ZZ aromaticity pattern does not show an aromatic character at all. From XY NICS scan at planes from Z = 0 to Z = 1, it is found that as the Z-height is increased, the π-electron ring current effects are dominant, and the σ-bonding contributions are diminished. Therefore, it is recommended here to compute NICS(1)ZZ instead of NICS(0)ZZ when analyzing NICS of PAHs.

Asunto(s)

Bases de Datos Farmacéuticas , Hidrocarburos Policíclicos Aromáticos/química , Espectroscopía de Resonancia Magnética , Modelos Moleculares , Conformación Molecular , Teoría Cuántica

RESUMEN

The first critical micelle concentration (CMC) of the ionic surfactant sodium dodecyl sulfate (SDS) in diluted aqueous solution has been determined at room temperature from the investigation of the bulk viscosity, at several concentrations of SDS, by means of coarse-grain molecular dynamics simulations. The coarse-grained model molecules at the mesoscale level are adopted. The bulk viscosity of SDS was calculated at several millimolar concentrations of SDS in water using the MARTINI force field by means of NVT shear Mesocite molecular dynamics. The definition of each bead in the MARTINI force field is established, as well as their radius, volume, and mass. The effect of the size of the simulation box on the obtained CMC has been investigated, as well as the effect of the number of SDS molecules, in the simulations, on the formation of aggregates. The CMC, which was obtained from a graph of the calculated viscosities versus concentration, is in good agreement with the reported experimental data and does not depend on the size of the box used in the simulation. The formation of a spherical micelle-like aggregate is observed, where the dodecyl sulfate tails point inward and the heads point outward the aggregation micelle, in accordance with experimental observations. The advantage of using coarse-grain molecular dynamics is the possibility of treating explicitly charged beads, applying a shear flow for viscosity calculation, and processing much larger spatial and temporal scales than atomistic molecular dynamics can. Furthermore, the CMC of SDS obtained with the coarse-grained model is in much better agreement with the experimental value than the value obtained with atomistic simulations.