Interplanar Separation Research Articles

High Potential Electrochemical Synthesis of Thermally Reduced Graphene Nanomaterial

In this study, we provide electrochemical techniques for synthesizing thermally reduced graphene nanomaterial that have high potential, low defects, cost-effectiveness, and ecological sustainability. The electrochemical exfoliation is carried out by employing a 195 W DC (voltage = 60[Formula: see text]V and current = 3.25 A) power source at a maximum electrolyte temperature of about 92.5∘C within the aqueous suspension of 2[Formula: see text]M of sulfuric acid (H2[Formula: see text]. Thereafter, the synthesized nanomaterial was treated in the weak piranha [combination of sulfuric acid and hydrogen peroxide (H2[Formula: see text]] solution using an electrochemical technique inside the water bath sonicator at 80∘C. X-ray diffraction (XRD) analysis shows the peak of diffraction to the (002) plane of the reduced graphene oxide (RGO) samples emerges at around 2[Formula: see text] and 26.56∘ with an interplanar distance of 3.40 Å and 3.54 Å. According to the XRD data, after the high-temperature thermal reduction phase, the structure of the crystals and interplanar separation were recovered. The size of the crystallite of RGO produced under H2SO4 conditions was discovered to be greater than the crystallite size of graphene oxide produced under piranha solution conditions. The Raman analysis results show that the degree of disorder of the graphene synthesized within the H2O2 was higher than in comparison to the graphene synthesized in H2SO4. Field emission scanning electron microscopy (FE-SEM) results show that graphene synthesized in the presence of H2O2 has a thin and porous microstructure in comparison to H2SO4 with no significant effect on the presence of the availability of the C/O ratio. The atomic force microscopy (AFM) analysis indicates that the surface roughness of the graphene synthesized in the H2O2 was higher than that of the H2SO4. The Fourier transform infrared spectroscopy (FT-IR spectroscopy) analytical results show that the majority of the functional groups have been eliminated within the samples.

Pancake-Bonded Dimers of Semiquinone Radical Cations of N,N,N′,N′-Tetramethyl-p-phenylenediamine (Wurster’s Blue)

Pancake bonding in dimers of semiquinoid radical cations in a novel compound, TMPD chloride co-crystal with 2,5-dichlorohydroquinone, is studied by a combination of X-ray charge density and quantum chemical computations. The rings in a pancake-bonded dimer are parallel with a transversal offset, the interplanar separation distance is 3.1191(1) Å, and the centroid distance is 3.2022(2) Å. AIM analysis of experimental charge density revealed four symmetry-independent bonding critical points and a cage critical point between the TMPD cations in a dimer, with a maximum electron density of 0.055 e Å–3. A highest occupied molecular orbital extends between both rings of a dimer. The estimated energy of the covalent component is −3.24 kcal mol–1.

Stacks of Equidistant 5,6-Dichloro-2,3-dicyanosemiquinone (DDQ) Radicals under Variable-Temperature and High-Pressure Conditions

The effects of temperature (80–400 K) and pressure (up to 4.67 GPa) variations on stacks of equidistant semiquinone radicals were studied by single-crystal X-ray diffraction using a salt of 2-amino-5-methyl-N-methylpyridinium (MMA) and 5,6-dichloro-2,3-dicyanosemiquinone radical anion (DDQ). A temperature-driven phase transformation occurs in the temperature range 150–240 K. The nature of stacking changes upon cooling: equidistant radicals in the room-temperature phase are re-arranged into two-electron/multicenter (2 e/mc) bonded dimers of radicals at low temperature. The phase transition proceeds via a pair-exchange dynamic mechanism. By application of pressure, the interplanar separation in the stacks is reduced by 0.37 Å, from 3.25 Å at ambient pressure to 2.88 Å at 4.67(3) GPa. This may indicate that the covalent component of the interaction considerably increases, consistent with previously performed density functional theory (DFT) calculations.

Coupling effect of (SCN)x nanoribbons on PCN nanosheets in the metal-free 2D/1D Van der Waals heterojunction for boosting photocatalytic hydrogen evolution from water splitting

Hydrogen extraction from the water splitting driven by solar energy is considered to be a sustainable technology, but it is still a challenging issue to develop high-efficient and stable photocatalysts. Herein, a new metal-free polymeric carbon nitride (PCN)-based Van der Waals (VdW) heterojunction photocatalyst is exploited by controlling the micro-morphologies of another structural unit, the unique 2D/1D microstructure of which is fabricated by decorating polythiocyanogen (SCN)x nanoribbons on PCN nanosheets. The coupling effect of (SCN)x on PCN effectively propels interplanar charge separation through Z-scheme charge transfer path formed by the compact face-to-face π-π electronic interaction between them. Moreover, the 1D nanoribbon morphology of (SCN)x also can shorten one direction of charge migration distance in the plane to inhibit the intramolecular carrier recombination compared with traditional 2D structure. The resultant photocatalytic hydrogen evolution (PHE) rate on the optimal PCN/(SCN)x-3 % sample reaches about 4 times as large as pure PCN, and a high apparent quantum efficiency (AQE) of 18.5 % is also achieved at 420 nm. This work develops a new design approach based on spatial dimension regulation strategy of structure unit for developing PCN-based VdW heterojunction photocatalyst applied to hydrogen production from the water splitting.

Insight into symmetry matching heterogeneous facet junction photocatalysts via {211} faceted CeO2 nanobelts@δ-Bi2O3 nanowires

Photocatalysts with homogenous facet junctions exhibit amazing performance in photocatalytic redox reactions. More challenging and alluring is the construction of heterogeneous facet junctions with two of them. Herein, we show a self-assembly procedure to buildup co-exposing faceted δ-Bi2O3 nanowires on the edges of co-exposing {211} faceted CeO2 nanobelts from sub 10 nm nanoparticles. Directed by surface O-vacancy energy, the δ-Bi2O3 nanoparticles preferentially adsorbed on the {111} facets of CeO2 nanobelts and oriented growth into co-exposing faceted δ-Bi2O3 nanowires. The CeO2@δ-Bi2O3 hetero facet junction displays superior and stable hydrogen evolution activities, 5.2 times of the pure co-exposing faceted CeO2 nanobelts, ascribed to following key factors: First, heterogeneous facet junctions contribute to efficient interplanar and inter-junction separation of photocarriers. Moreover, symmetry matching leads to strong connection between the two facet junctions, facilitating the dynamic charge transfer. This work can provide a new strategy in the design of heterojunction photocatalytic systems for water splitting.

Doping-Induced Pre-Transformation to Extend Solid-Solution Regimes in Li-Ion Batteries

In many intercalation electrodes of Li-ion batteries, lithiation induces distortive structural transformations with substantial implications for stress accumulation, capacity loss, and degraded rate performance. Here, we dope a phase-transforming electrode to stabilize a structure bearing considerable similarities to the initially lithiated phase. In this “pre-transformation” approach, demonstrated based on Mo-doping of V2O5, the thermodynamic penalty associated with the phase transformation is paid in part during materials’ synthesis rather than the charge/discharge of a battery. Mo-doping alters Li–V2O5 thermodynamics to unlock extended solid-solution lithiation regimes with modulated interplanar separations. Specifically, Mo-doping reduces structural distortions during charge/discharge processes, diminishes coherency stresses, and yields a substantially modified intercalation phase diagram. Operando synchrotron X-ray diffraction and chemical lithiation results in conjunction with phase-field modeling demonstrate the promise of doping-induced structural distortions to entirely alter phase evolution and improve electrochemistry–mechanics coupling in phase-transforming cathodes.

Semiconductive 2D arrays of pancake-bonded oligomers of partially charged TCNQ radicals.

Multicentre two-electron (mc/2e or 'pancake bonding') bonding between 7,7,8,8-tetra-cyano-quinodi-methane (TCNQ) radical anions was studied on its 14 novel salts with planar organic cations. The formal charges of the TCNQδ- moieties are -1/2 and -2/3, and they form mc/2e bonded dimers, trimers and tetramers which are further stacked into extended arrays. Multicentre bonding within these oligomers is characterized by short interplanar separations of 2.9-3.2 Å; distances between the oligomers are larger, typically >3.3 Å. The stacks are laterally connected by C-H⋯N hydrogen bonding, forming 2D arrays. The nature of mc/2e bonding is characterized by structural, magnetic and electrical data. The compounds are found to be semiconductors, and high conductivity [10-2 (Ω cm)-1] correlates with short interplanar distances between pancake-bonded oligomers.

Indeno[2,1-a]fluorene-11,12-dione Radical Anions: Synthesis, Characterization, and Properties.

The one-electron reduction of indeno[2,1-a]fluorene-11,12-dione (IF) with various alkali metals prepare the radical anion salts. The data about different structures, properties, and characterization was obtained by single-crystal X-ray diffraction, electron paramagnetic resonance (EPR) spectroscopy, superconducting quantum interference device (SQUID) measurements, and physical property measurement system (PPMS). Compound IF.- K+ (18-c-6) is regarded as a one-dimensional magnetic chain through C-H⋅⋅⋅C interaction. Theoretical calculations and magnetic results showed that [IF.- K+ (15-c-5)]2 is a dimer with an open-shell ground state. Compounds IF.- Na+ (15-c-5) and IF.- K+ (cryptand) are monoradical anion salts: IF2 .- Li+ possesses unique π-stack structure with an interplanar separation less than 3.46 Å, making it a semiconductor (δRT =1.9×10-4 S ⋅ cm-1 ). This work gives insights into multifunctional radical anions, and describes the design and development of different functional radicals.

Investigating the oxidative reactivity and nanostructural characteristics of diffusion flame generated soot using methyl crotonate and methyl butyrate blended diesel fuels

Combustion-generated particulate matter has a consequential fallout on climate, environment, and human health. Thus, comprehension of soot formation and mitigation processes using biodiesel additives is a focal point of modern combustion research. This study presents an intricate investigation of enhanced oxidative reactivity and its dependence on the soot nanostructural properties induced by distinctive blending of a saturated and an unsaturated biodiesel surrogates namely, methyl butyrate (MB) and methyl crotonate (MC) respectively with pure diesel. 25% MB-75% diesel fuel resulted in the lowest sooting propensity and lowest activation energy (155.4 ± 2 kJ/mol) for soot oxidation in comparison to pure diesel fuel and diesel soot (175.1 ± 2 kJ/mol). The soot was collected using a smoke point apparatus involving a wick-fed laminar diffusion flame at atmospheric pressure after meticulous observation of the smoke points. The detailed nanostructural characterization of the flame generated soots were performed using high-resolution transmission electron microscopy, X-ray diffraction, elemental analysis and Raman spectroscopy. BET surface area analysis, and thermogravimetric analysis were carried out for surface area distinction and activation energy calculations respectively. It revealed that, addition of MB to diesel resulted in improved fuel combustion with reduction in primary soot particle diameter, greater inter-planar separation, increased fringe tortuosity and greater crystal structure disorder resulting in its enhanced reactivity with O2. The 25% MC-75% diesel blend resulted in relatively greater sooting propensity while its soot exhibited lower crystal structure disorder and greater activation energy (165.0 ± 2 kJ/mol) for O2 induced oxidation compared to 25% MB-75% diesel, possibly due to the resonance stabilized radical (RSRs) formation that altered the fuel combustion chemistry. Therefore, this study successfully depicts that the structural differences in the surrogate fuels do influence the soot formation and oxidation kinetics. These structural effects therefore need to be considered when formulating the global multiphase kinetic models for biodiesel-diesel fuel combustion.11Dedicated to Prof. Purnakala Samant on the occasion of her 50th birthday celebration.

Intra- and Inter-Molecular Spin Coupling in Phenalenyl Dimeric Systems.

Phenalenyl is a triangular aromatic molecule made of three fused benzene rings, carrying an unpaired electron, and many of its derivatives show crystal structures with stacked radicals. Here, we investigate the inter-molecular binding in phenalenyl dimers by state-of-the-art computational methods and phenomenological models. Aside from being important for the supramolecular assembly of such radical molecules, the theoretical insight is relevant in methodological aspects, due to the interplay of long-range exchange coupling effects and van der Waals forces. We used comparative wave function-based and density functional theories. Drawing the potential energy surfaces as a function of inter-planar separation and mutual rotation of the monomer units, we found an interesting pattern which is not discovered in previous computational reports on the title systems. The dependence can be nicely interpreted by a transparent phenomenological model based on an orbital overlap paradigm of exchange coupling. We also brought forth a simplified phenomenological valence bond (VB) model of inter-molecular coupling, which is realized on the background of the VB spin model inside of the aromatic monomers and calibrated with the corresponding ab initio data. As the systems can be considered good candidates with potential applications in spintronics and organic magnetism, the theoretical rationalization opens up prospective ways to realize such promises.

Resolving the Geometry/Charge Puzzle of the c(2 × 2)-Cl Cu(100) Electrode.

Potential-induced changes in charge and surface structure are significant drivers of the reactivity of electrochemical interfaces but are frequently difficult to decouple from the effects of surface solvation. Here, we consider the Cu(100) surface with a c(2 × 2)-Cl adlayer, a model surface with multiple geometry measurements under both ultrahigh vacuum and electrochemical conditions. Under aqueous electrochemical conditions, the measured Cu-Cl interplanar separation (dCu-Cl) increases by at least 0.3 Å relative to that under ultrahigh vacuum conditions. This large geometry change is unexpected for a hydrophobic surface, and it requires invoking a negative charge on the Cl-covered surface which is much greater than expected from the work function and our capacitance measurements. To resolve this inconsistency we employ ab initio calculations and find that the Cu-Cl separation increases with charging at a rate of 0.7 Å/e- per Cl atom. The larger Cu-Cl bond distance increases the surface dipole and, therefore, the work function of the interface, contributing to the negative charge under fixed potential electrochemical conditions. Interactions with water are not needed to explain either the large charge or large Cu-Cl interplanar spacing of this surface under electrochemical conditions.

High surface area Nanoflakes of P-gC3N4 photocatalyst loaded with Ag nanoparticle with intraplanar and interplanar charge separation for environmental remediation

The photocatalytic performance of gC3N4 is majorly restricted by insufficient collection of photogenerated charges on the surface during reaction due to highly dense stacking of lamellar structures with lateral size ranging in microns. This deficiency can be overcome by forming thin nanoflakes by systematically breaking the weak bonds that hold the gC3N4 framework without destroying the basic heptazine unit. With this aim, herein, a combination of three different strategies was implemented to design and develop, Ag-loaded and P-doped gC3N4 nanoflakes (Ag3-P1-NF-gC3N4). Using a systematic synthesis method, bulk gC3N4 was first converted into thin nanosheets, followed by fragmentation into nanoflakes, with a planar size up to 100 nm. P doping to replace the corner C atoms in the gC3N4 matrix (forming PN bonds) and intercalation of plasmonic Ag nanoparticles within the interlayers also assists in the bifurcation of the stacked layers and formation of nanoflake morphology. These strategies result in a significant increase in BET surface area to ∼196 m2/g from 12 m2/g of bulk gC3N4. Improved inter-planar and intra-planar charge mobility was recorded as a result of the reduced sizes. Doping with P also causes higher absorption of the visible spectrum in gC3N4 while the formation of heterojunction with Ag nanoparticles induces efficient separation of photo-generated charges. All these promoting photo-physical properties lead to an outstanding photocatalytic activity towards degradation of aqueous pollutants with reaction rates ∼20 times higher than bulk gC3N4. Complete mineralization of the pollutant and formation of non-toxic byproducts was also confirmed with suitable chromatography techniques.

Pressure-Induced Polymerization of Polycyclic Arene-Perfluoroarene Cocrystals: Single Crystal X-ray Diffraction Studies, Reaction Kinetics, and Design of Columnar Hydrofluorocarbons.

Pressure-induced polymerization of aromatic compounds leads to novel materials containing sp3 carbon-bonded networks. The choice of the molecular species and the control of their arrangement in the crystal structures via intermolecular interactions, such as the arene-perfluoroarene interaction, can enable the design of target polymers. We have investigated the crystal structure compression and pressure-induced polymerization reaction kinetics of two polycyclic 1:1 arene-perfluoroarene cocrystals, naphthalene/octafluoronaphthalene (NOFN) and anthracene/octafluoronaphthalene (AOFN), up to 25 and 30 GPa, respectively, using single-crystal synchrotron X-ray diffraction, infrared spectroscopy, and theoretical computations based on density-functional theory. Our study shows the remarkable pressure stability of the parallel arene-perfluoroarene π-stacking arrangement and a reduction of the interplanar π-stacking separations by ca. 19-22% before the critical reaction distance is reached. A further strong, discontinuous, and irreversible reduction along the stacking direction at 20 GPa in NOFN (18.8%) and 25 GPa in AOFN (8.7%) indicates the pressure-induced breakdown of π-stacking by formation of σ-bonded polymers. The association of the structural distortion with the occurrence of a chemical reaction is confirmed by a high-pressure kinetic study using infrared spectroscopy, indicating one-dimensional polymer growth. Structural predictions for the fully polymerized high-pressure phases consisting of highly ordered rods of hydrofluorocarbons are presented based on theoretical computations, which are in excellent agreement with the experimentally determined unit-cell parameters. We show that the polymerization takes place along the arene-perfluoroarene π-stacking direction and that the lateral extension of the columns depends on the extension of the arene and perfluoroarene molecules.

Nanostructural Disorder and Reactivity Comparison of Flame Soot and Engine Soot Using Diesel and Jatropha Biodiesel/Diesel Blend as Fuels

Soot is a major anthropogenic air pollutant that affects human health and contributes to global warming. To understand its formation pathways and reduce emission, several flame and engine studies exist in the literature, though the fundamental differences in the characteristics of engine and flame soots are not well understood. This study presents a detailed comparative investigation of soot nanostructural properties and their relationship with the oxidative reactivity of soots from an engine and a diffusion flame using diesel and 20% Jatropha biodiesel/diesel blend fuels. X-ray diffraction, Raman spectroscopy, high-resolution transmission electron microscopy, electron energy loss spectroscopy, and thermogravimetric analyses confirm that engine soot has greater primary particle diameter, higher concentration of loosely held aliphatics, greater degree of graphitized nanocrystallites with lower interplanar separation, longer fringe lengths, lower tortuosity, and greater resistance for oxidation than the flame soot, though the differences in several properties were minor. The effects of biodiesel addition to diesel on soot properties and sooting tendency were predicted very well with both flame and engine setup. Moreover, the enhanced soot oxidation in the combustor catalyzed by fuel-bound oxygen in biodiesel further reduces the nanostructural and reactivity differences between engine and flame soots. Though engine soot properties have more relevance to anthropogenic particulate matter, flame setups appear to be suitable for screening and studying the effect of fuel additives on the sooting propensity and physicochemical properties of soot prior to their testing and utilization in engines.

Anisotropy of fracture in hexagonal metals

When plastic deformation in hexagonal metals with lower symmetry than in cubic metals cannot support the external load, the crystal can break along a certain plane. Using a simple criterion based on the surface energy, Young modulus and interplanar separation, the anisotropy of fracture is discussed for three typical hexagonal metals, magnesium, titanium and zinc. The obtained results are generally in agreement with the experimental observations.

Highly Stable Radical Cations of N,N'-Diarylated Tetrabenzotetraaza[8]circulene.

N,N'-Diarylated tetrabenzotetraaza[8]circulenes 3 a and 3 b were synthesized in good yields by a reaction sequence involving oxidation of tetrabenzodiazadithia[8]circulene 5-Oct and SN Ar reaction with aniline derivatives. The obtained aza[8]circulenes 3 a and 3 b were easily oxidized to give their radical cations 3 a+ and 3 b+ , which are highly stable under ambient conditions. X-ray diffraction analysis of radical cation 3 a+ showed a face-to-face dimer arrangement with an interplanar separation of 3.320 Å. The spin density of 3 a+ was calculated to be delocalized over the whole circulene π-systems with spin-spin exchange integral (J=-144 cm-1 ) in the dimeric part. These radical cations displayed far red-shifted absorption bands reaching to 2000 nm. Thus this study has proved the hetero[8]circulene scaffold to be a new entry of promising electronics and spin materials.

Pancake-bonding of semiquinone radicals under variable temperature and pressure conditions.

The effects of temperature (100-370 K) and pressure (0-6 GPa) on the non-localized two-electron multicentric covalent bonds (`pancake bonding') in closely bound radical dimers were studied using single-crystal X-ray diffraction on a 4-cyano-N-methylpyridinium salt of 5,6-dichloro-2,3-dicyanosemiquinone radical anion (DDQ) as the sample compound. On cooling, the anisotropic structural compression was accompanied by continuous changes in molecular stacking; the discontinuities in the changes in volume and b and c cell parameters suggest that a phase transition occurs between 210 and 240 K. At a pressure of 2.55 GPa, distances between radical dimers shortened to 2.9 Å, which corresponds to distances observed in extended π-bonded polymers. Increasing pressure further to 6 GPa reduced the interplanar separation of the radicals to 2.75 Å. This may indicate that the covalent component of the interaction significantly increased, in accordance with the results of DFT calculations reported elsewhere [Molčanov et al. (2019), Cryst. Growth Des. 19, 391-402].

Swift Heavy Ion Irradiated Low-Density Polyethylene: Structural and Optical Investigations

The structural and optical effects of copper (50 MeV) and carbon (70 MeV) ion irradiation of low density polyethylene (LDPE) are described. X-ray diffraction (XRD) and differential scanning calorimetric (DSC) techniques were used to explore the structural behavior, while Fourier transform infrared (FTIR) and UV-visible spectroscopies were used to collect the optical data. Structural parameters, such as interplanar separation, distortion factor, crystallite size and degree of crystallinity, were measured to understand the change in the structural behavior of LDPE caused by the swift heavy ion irradiation. In the irradiated polymeric samples, the formation and growth of new carbon enriched species were revealed by the FTIR and UV-visible analysis, which led to the decrease of their optical bandgap energy with the increase of ion fluence.

Orientational Dependence of Cofacial Porphyrin–Quinone Electronic Interactions within the Strong Coupling Regime

We examine the relative magnitudes of electronic coupling HDA in two face-to-face rigid and diastereomeric (porphinato)zinc(II)-quinone (PZn-Q) assemblies, 1β-ZnA and 1β-ZnB, in which the six quinonyl carbon atoms lie in virtually identical arrangements relative to the PZn plane at sub-van der Waals donor-acceptor (D-A) interplanar separations. Steady-state and time-resolved transient optical data and computational studies show that minor differences in relative D-A cofacial orientation give rise to disparate HDA magnitudes for both photoinduced charge separation (CS) and thermal charge recombination (CR). Time-dependent density functional theory (TDDFT) computations illuminate the nature of direct charge transfer states and the electronic structural factors that give rise to these differential HDAs. These data show more extensive mixing of locally excited (LE) and CS states in 1β-ZnA relative to 1β-ZnB and that these HDA differences track the magnitudes of electronic coupling matrix elements determined from steady-state electronic spectral data and thermal CR rate constants measured via pump-probe spectroscopy. Collectively, this work shows that electron transfer dynamics may be manipulated in cofacial D-A systems, even at sub-van der Waals contact, provided that conformational rigidity precludes structural fluctuations that modulate D-A interactions on the charge transfer time scale.

Multicentric two-electron covalent bonding (pancake bonding) between semiquinone radicals determines bulk properties

Fine details of stacking interactions in three different types of π-stacked tetrachlorosemiquinone radical anions (Cl4Q) were studied by a combination of X-ray charge density, quantum chemical computation and atoms-in-molecules (AIM) analysis: 1) stacks of pancake-bonded radical dimers in triclinic polymorph of N-MePy·Cl4Q (N-MePy = N-methylpyridinium cation), 2) stacks of trimers of partially charged semiquinones in [4-damp])2[Cl4Q]3 (4-damp = 4-dimethylamino-N-methylpyridinium) and 3) stacks of equidistant radicals in orthorhombic polymorph of N-MePy·Cl4Q. For the first time, we provide experimental evidence (based on X-ray charge density) of two-electron multicentric covalent bonding (pancake bonding) between the radicals. Typical pancake-bonded radical dimers in 1) are characterised by short interplanar distance (2.86 A) and multiple bonding (3, -1) critical points between the rings with maximum electron density exceeding 0.095 e A-3 ; in addition, a (3, +3) critical point (local minimum of electron density) was also found, indicating a cage-like electronic structure [1]. The covalent contribution to total interaction in a dimer was calculated to be -9.4 kcal mol-1. Between the dimers, interplanar separation is 3.60 A and only negligible electron density is found. In trimers there are two electrons shared between three closely interacting rings (interplanar separations are ca. 2.84 A), and (3, -1) bonding critical points are found with maximum electron density of 0.077 e A-3 ; there are also two (3, +3) local minima [2]. The calculated covalent contribution is -6.8 kcal mol-1. Maximum electron density between the rings in a stack of equidistant radicals (interplanar separation of 3.17 A) is much lower, 0.050 e A-3, and there is no local minimum of electron density [1]. However, the HOMO orbitals extend between the rings, and the calculated covalent contribution is -2.9 kcal mol-1. This compound is a 1D semiconductor [3, 4], and its semiconductivity is explained by pancake bonding extending along the stack.