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Nanoscaled 2D and 3D carbon structures with closed curved π-surfaces are of relevance in the development of desirable building units for materials science, where the connection between small and larger molecular systems is revealed.

Orbital contributions to the magnetic response depend on the method used to compute them. Here, we show that dissecting nuclear magnetic shielding tensors using natural localized molecular orbitals (NLMOs) leads to anomalous core contributions. The arbitrariness of the assignment might significantly affect the interpretation of the magnetic response of nonplanar molecules such as C-60 or [14]helicene and the assessment of their aromatic character. We solve this problem by computing the core- and sigma-components of the induced magnetic field (and NICS) and the magnetically induced current density by removing the valence electrons (RVE). We estimate the core contributions to the magnetic response by performing calculations on the corresponding highly charged molecules, such as C6H630+ for benzene, using gauge-including atomic orbitals and canonical molecular orbitals (CMOs). The orbital contributions to nuclear magnetic shielding tensors are usually estimated by employing a natural chemical shielding (NCS) analysis in NLMO or CMO bases. The RVE approach shows that the core contribution to the magnetic response is small and localized at the nuclei, contrary to what NCS calculations suggest. This may lead to a completely incorrect interpretation of the magnetic sigma-orbital response of nonplanar structures, which may play a major role in the overall magnetic shielding of the system. The RVE approach is thus a simple and inexpensive way to determine the magnetic response of the core- and sigma-electrons.

Endohedral metallofullerenes are key species for expanding the range of viable fullerenes, their versatility, and applications. Here we report our computational evaluation on the formation of spherical aromatic counterparts of the C60 fullerene from relativistic DFT calculations, based on the inclusion of Cr, Mo and W endohedral atoms. The resulting M@C60 endohedral fullerenes are 66-π electron neutral species exhibiting bonding properties and electronic structure mimicking the aromaticity and diamagnetic insulator behavior of alkali-C606− phases. The resulting structures are interesting candidates for further experimental realization as novel neutral building blocks for more flexible nanostructured organic materials, highlighting truly spherical aromatic neutral species retaining the truncated icosahedral structure of the seminal Buckminster fullerene.

The induced magnetic field of C2N (N = 3-14) carbon rings was dissected to contributions from out-of-plane and in-plane π orbitals revealing two concurrent long range shielding or deshielding cones as a manifestation of the dual aromatic and antiaromatic character of C4n+2 and of C4n rings respectively. Aromaticity based on the magnetic criterion was evaluated with regard to the bonding pattern and geometrical characteristics that elucidate the influence of bond length and bond angle alteration on out-of-plane and in-plane magnetic responses. Ground state polyynic geometries of C4n+2 rings exhibit comparable shielding cones to annulenes, decreasing the magnetic response with regard to the ring size and similar πout and πin diatropicity. Transition state cumulenic rings display increased aromaticity expressed by a very strong constant magnetic response and augmented πout diatropicity with regard to πin. The variations of the induced magnetic field are explained on the basis of frontier orbital interactions through rotational excitations, which enable further rationalization of the aromatic/antiaromatic behavior.

International audience; Materials combining amplified nonlinear optical responses and wide optical gaps that ensure high laser-induced damage thresholds and broadband transparency are highly required for photonic applications. In this article, we use well-established quantum chemical methods to demonstrate that a special class of polyaromatic hydrocarbons, in which multiple borazine (B3N3) units substitute aromatic carbon sextets, are suitable for the design and synthesis of systems meeting both above requirements. Computations conducted in a wide assortment of purposely designed nanographene model systems exposed robust doping/property relations which point out that multiple B3N3 doping can be considered as an efficient strategy to enhance the quadratic nonlinear optical responses of a given polyaromatic hydrocarbon maintaining at the same time wide optical gaps. A detailed analysis of the underlying charge transfer mechanism revealed that the observed features stem from local in character electronic excitations occurring between the incorporated B3N3 units and neighboring aromatic sextets.

The induced magnetic field of B40 was dissected into contributions from π, σ and core electrons revealing the origins for the formation of the strong long range shielding response characterizing the spherical aromatic nature of the cavernous D2d structure. Our analysis showed the complementary role of π and σ orbitals for the formation of the global shielding cone, with weak π contributions at a long range and strong σ contributions inside the cage, supporting the molecule as double aromatic with weak π and strong σ delocalization. Similar local variations of both π and σ magnetic responses were identified portraying peripheral diatropic and local paratropic currents. The weak π aromaticity is explained on the basis of symmetry rules pertaining to its electronic structure which forbid small gap paratropic rotational excitations.

Dications of cycloparaphenyles ([n]CPPs) are known to exhibit in-plane global aromaticity, contained in a nanobelt structure. Recently synthesized ortho and meta isomers of [n]CPPs break the radial symmetry of π structure incorporating perpendicular oriented π orbitals. Herein we set to explore the aromaticity of neutral and dicationic ortho and meta isomers of [8]CPP by dissecting the induced magnetic field to contributions of the twofold radial/perpendicular π system using delocalized canonical molecular orbitals (CMO), and introducing the natural localized molecular orbitals (NLMO) analysis with DFT methods. The dications sustain a reduced global aromatic character of the radial π system under a perpendicular orientation of the external field which declines from ortho to meta isomer and reinforces local aromaticity of ortho ring while it destroys aromaticity of meta ring. Aromaticity variations are determined by symmetry governed rotational excitations of frontier π orbitals. The parallel orientation reveals a substantial reduction of local aromaticity verified with NICSπ analysis and electron delocalization indices.

Endohedral metallofullerenes are key species for expanding the range of viable fullerenes, their versatility, and applications. Here we report our computational evaluation on the formation of spherical aromatic counterparts of the C60 fullerene from relativistic DFT calculations, based on the inclusion of Cr, Mo and W endohedral atoms. The resulting M@C60 endohedral fullerenes are 66-π electron neutral species exhibiting bonding properties and electronic structure mimicking the aromaticity and diamagnetic insulator behavior of alkali-C606- phases. The resulting structures are interesting candidates for further experimental realization as novel neutral building blocks for more flexible nanostructured organic materials, highlighting truly spherical aromatic neutral species retaining the truncated icosahedral structure of the seminal Buckminster fullerene.

The induced magnetic field of fullerenes is strongly dependent on the charge state, where C60 is depicted as a nonaromatic species, in contrast to C6010+ which exhibits a strong spherical aromatic character. Here, we account for the response of relevant charged stable building blocks for novel extended networks with variable applications, as observed in A12C60 and A6C60 phases (A = alkali metal), given by, Li12C60 and Na6C60, as well as four different charge states of C60q (q = +10, 0, −6, −12), to an external magnetic field is studied in detail, focusing on the contributions from the π and σ systems to the induced magnetic field. C60, C606–, and C6012– accounts for the variation of their isolated species upon addition of charge, whereas C6010+ is a hypothetical highly aromatic counterpart. Our results show that each spherical shell and each canonical molecular orbital exhibit characteristic patterns, revealing the direct dependence of the magnetic response, and therefore of spherical aromatic character, ...

The induced magnetic field of B40 was dissected into contributions from π, σ and core electrons revealing the origins for the formation of the strong long range shielding response characterizing the spherical aromatic nature of the cavernous D2d structure. Our analysis showed the complementary role of π and σ orbitals for the formation of the global shielding cone, with weak π contributions at a long range and strong σ contributions inside the cage, supporting the molecule as double aromatic with weak π and strong σ delocalization. Similar local variations of both π and σ magnetic responses were identified portraying peripheral diatropic and local paratropic currents. The weak π aromaticity is explained on the basis of symmetry rules pertaining to its electronic structure which forbid small gap paratropic rotational excitations.

The induced magnetic fields originating from the π system of planar conjugated polycyclic hydrocarbons and spherical fullerenes are accurately reproduced by their corresponding hydrogen skeletal models (HSMs). Moreover, the individual contribution per molecular orbital is also reproduced unraveling simple symmetry rules related to canonical molecular orbitals. Hence, fast, handy and accurate 3D visualization of shielding and deshielding cones is realized, enabling the interpretation of global and local π aromaticity and antiaromaticity of PAHs and spherical species in a simple and concise manner to facilitate further interpretations of large sized hydrocarbon systems.

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.

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 benzene's 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.

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.

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. © 2017 Wiley Periodicals, Inc.

The use of two-dimensional (2D) symbolic representations for describing, communicating and reasoning about phenomena and concepts is a common practice in Chemistry. Many researchers have shown that there is a positive correlation between visuospatial ability and fluency with chemical representations. The aim of this study is the exploration of chemistry students’ skills to visualize and mentally manipulate molecular structures. In this paper the results of the implementation to chemistry students of the computerized version of the Purdue Spatial Visualization Test: Rotation and two purposely designed and developed prototype «visualization» instruments dealing with rotation and reflection of 2D symbolic molecular representations are presented.

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.

A hybrid course of Molecular Symmetry and Group Theory which combines traditional face-to-face instruction with an online web enhanced learning environment within a Course Management System was designed, developed, and implemented with a purpose to establish an active and student-centred educational setting. Multi-representational educational material enclosing 3D interactive molecular visualization applets was designed, developed, and effectively integrated in the course in order to enhance students' learning and enable practice with molecular symmetry concepts. Moodle was utilized for content delivery in study blocks, formative online assessment, final examination, student tracking, communication and feedback. The hybrid instructional model provides an efficient mode of integration of visualization tools in the teaching and learning process. The findings over three years of implementation of the hybrid course reveal its positive impact on students' involvement with the course content during the whole semester, on students' performance as well as on their satisfaction with their educational experience.

Electronic structure calculations (B3LYP/6-311+G**) predict that nucleophilic attack of the aromatic cyclo- Cu4(μ-�)4 ring yields ligand-stabilized tetranuclear Cu4 clusters formulated as cyclo-Cu4(μ-�)4Nucn (n = 1-4; Nuc = N2, CO, H2O, NH3 and PH3). Depending on the number of added nucleophiles, the tetranuclear Cu4 clusters adopt planar, bent or 3D tetrahedral geometries. These molecules exhibit aromatic character, which is primarily due to 4s and 3d cyclic elec- tron delocalization over the Cu4 framework (s and d-orbital aromaticity). The aromaticity of the novel ligand stabilized tetranuclear Cu4 clusters was verified by a number of established criteria of aromaticity. In particular, the nucleus- independent chemical shift, NICS(0) and NICS(1) and their out-of-plane components NICSzz(0), NICSzz(1) and the NICS scan pictures are indicative for the aromaticity of the planar, bent and tetrahedral Cu4 clusters. The effect of the substitu- ents on the aromatic character of the Cu4 clusters is also a main concern of the present work. It was found that increasing the number of the attacking nucleophiles increases the diatropic response of the aromatic tetranuclear Cu4 clusters. Moreover, the aromaticity of the 3D structures (spherical aromaticity) is higher than the aromaticity of the planar and bent structures. The effect of the coordinated nucleophiles, Nuc, on the stability, geometry, electronic structure and bonding mode of the cyclo-Cu4(μ-�)4Nucn molecules is also thoroughly discussed.

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.

One of the ongoing challenges in teaching chemistry is helping students explore various structural features of molecules. It can be difficult for students to visualize the three-dimensional shape of a molecule from a two-dimensional printed image. Using a molecular modelling kit in the classroom helps, but such models can be difficult for students to see when demonstrated by an instructor. These difficulties are more pronounced in teaching symmetry, especially recognition of various symmetry elements within a molecule and conceptualization of the symmetry operations.

3D Normal Modes is a Web application for interactive visualization and three-dimensional perception of the normal modes of molecular vibration, suitable for undergraduate students in chemistry.

Students’ understanding of molecular structure through the use of two dimensional diagrammatic representations is essential for chemistry learning. However, several researches suggest that students exhibit difficulties in achieving representational competence with molecular diagrams. This study is focused on three forms of diagrammatic representations: Newman projections (NP), Fisher projections (FP) and Dash-Wedge diagrams (DW). Three computer based diagram matching tests were designed, developed and implemented to 33 undergraduates from our chemistry department. Students’ visuospatial skills and prior chemistry knowledge were also assessed. The findings of this pilot study were derived from the analysis of participants’ performance (scores and response times) in the matching diagrams tests, the spatial tests and the Chemistry test. Statistical analysis was performed in order to investigate the relationships between the examined cognitive factors and students’ performance in matching diagrams.