Nickolas D. Charistos

Interpretation of electron delocalization in benzene, cyclobutadiene, and borazine based on visualization of individual molecular orbital contributions to the induced magnetic field.

The magnetic response of the valence molecular orbitals (MOs) of benzene, cyclobutadiene, and borazine to an external magnetic field has been visualized by calculating the chemical shielding in two-dimensional grids of points on the molecular plane and on a plane perpendicular to it, using gauge-including atomic orbitals (GIAOs). The visualizations of canonical MO contributions to the induced magnetic field (CMO-IMF) provide a clear view of the spatial extension, the shape, and the magnitude of shielding and deshielding areas within the vicinity of the molecule, originating from the induced currents of each valence orbital. The results are used to investigate the delocalization of each valence MO and to evaluate its contribution to the aromatic character of systems under study. The differentiation of the total magnetic response among the three molecules originates exclusively from π-HOMO orbitals because the magnetic response of the subsets of the remaining MOs is found to be almost identical. Borazine is classified as nonaromatic as the four electrons that occupy the π-HOMO are found to be strongly localized on nitrogen centers. CMO-IMF can clarify the interpretation of various NICS indexes and can be applied for the investigation of various types of electron delocalization.

Study of Electron Delocalization in 1,2-, 1,3-, and 1,4-Azaborines Based on the Canonical Molecular Orbital Contributions to the Induced Magnetic Field and Polyelectron Population Analysis

The electron delocalization in 1,2-azaborine, 1,3-azaborine, and 1,4-azaborine is studied using canonical molecular orbital contributions to the induced magnetic field (CMO-IMF) method and polyelectron population analysis (PEPA). Contour maps of the out-of-plane component of the induced magnetic field (Bz(ind)) of the π system show that the three azaborines, in contrast with borazine, sustain much of benzenes π-aromatic character. Among them, 1,3-azaborine exhibits the strongest π delocalization, while 1,4-azaborine is the weakest. Contour maps of Bz(ind) for individual π orbitals reveal that the differentiation of the magnetic response among the three isomers originates from the π-HOMO orbitals, whose magnetic response is governed by rotational allowed transitions to unoccupied orbitals. The low symmetry of azaborines enables a paratropic response from HOMO to unoccupied orbitals excitations, with their magnitude depending on the shape of interacting orbitals. 1,3-Azaborine presents negligible paratropic contributions to Bz(ind) from HOMO to unoccupied orbitals transitions, where 1,2- and 1,4-azaborine present substantial paratropic contributions, which lead to reduced diatropic response. Natural bond orbital (NBO) analysis employing PEPA shows that only the 1,3-azaborine contains π-electron fully delocalized resonance structures.

Magnetic Response of Aromatic Rings Under Rotation: Aromatic Shielding Cone of Benzene Upon Different Orientations of the Magnetic Field

The induced shielding cone is one of the most characteristic aspects of aromatic species. Herein, we explore its behavior under different orientations of the applied magnetic field by evaluating the overall and dissected π- and σ-electron contributions. Our results shed light onto the orientation dependence behavior of the shielding cone, unraveling a characteristic pattern upon rotation of the aromatic ring. This pattern decreases the long range of the magnetic response, such that it resembles the behavior under constant molecular tumbling in solution.

On the role of heteroatoms in aromatic rings. Insights from 10π main group elements heterorings [(EH)2S2N4]q (E = C, P, B, Si, Al and q = 0, −2)

Inclusion of heteroatoms into a ring skeleton obtained unique electronic features, which differ from the respective isoelectronic organic counterparts, increasing the versatility of aromatic molecules. Herein, we evaluate the role of heteroatoms on the electronic and magnetic properties in a number of inorganic 10π-electron eight-member aromatic rings involving the isoelectronic [(EH)2S2N4]q (E = C, P, B, Si, Al) series using density functional methods. The inclusion of different heteroatoms with increasing electronegativity increased the aromatic behavior in relation to the representative 10π-electron [C8H8]2− organic ring. A deeper analysis on the magnetic response to an applied magnetic field, in terms of individual π-orbitals contributions, revealed that the differentiation in aromaticity originates from orbitals with major contributions from pz of heteroatom E, whereas the diatropic contributions that arise from the S2N4 core remained similar throughout the series. Therefore, the effect of including a certain type of heteroatom can be addressed in terms of the variation and contribution of each individual π-orbital, starting from the respective organic counterpart, which appears to be a convenient approach. The similar π-aromatic character observed suggests the proposed hypothetical rings are feasible structures to explore synthetically. The less aromatic counterpart given by the Al counterpart should lead to a less stable ring in this series.

Design and Development of a Multimedia Educational Tool for Interactive Visualization and Three-Dimensional Perception of Vibrational Spectra Data of Molecules

In this paper the design and development of 3DNormalModes, an educational tool for interactive visualization and three dimensional perception of vibrational spectra data of molecules is presented. The details of the architecture of the tool and its functionality are described. Means of application in chemical education at university level are discussed. A pilot study summarizes the strengths, the educational value and the possible extensions of the system.

Canonical orbital contributions to the magnetic fields induced by global and local diatropic and paratropic ring currents

The induced magnetic field (IMF) of naphthalene, biphenyl, biphenylene, benzocyclobutadiene, and pentalene is dissected to contributions from the total π system, canonical π‐molecular orbitals (CMO), and HOMO→π* excitations, to evaluate and interpret relative global and local diatropicity and paratropicity. Maps of the IMF of the total π system reveal its relative strength and topology that corresponds to global and local diatropic and paratropic ring currents. The total π magnetic response is determined by this of canonical HOMOs and particularly by paratropic contributions of rotational excitations from HOMOs to unoccupied π* orbitals. Low energy excitations and similar nodal structure of HOMO and π* induce strong paratropic fields that dominate on antiaromatic rings. High energy excitations and different nodal structures lead to weak paratropic contributions of canonical HOMOs, which are overwhelmed by diatropic response of lower energy canonical orbitals in aromatic rings. CMO‐IMF analysis is found in agreement with ring current analysis.

Aromaticity variation in BN substituted triphenylene: A theoretical study

In this work we present a theoretical study of the response of BN substituted triphenylene analogues to an external magnetic field in order to evaluate the aromaticity of these systems. Based on the nucleus-independent chemical shifts (NICS) concept, we performed DFT calculations of the chemical shielding tensor in a fine three dimensional grid surrounding the molecule and visualized the results as vectors of the induced magnetic field Bind and as contour maps and isosurfaces of the z component of Bind. These visualizations provide a clear view of the shape and magnitude of shielding and deshielding areas around molecular space originating from electron delocalization. The variation of local aromaticity of triphenylene and its BN substituted analogues is profoundly revealed.