Structure, reactivity and aromaticity of acenes and their BN analogues: a density functional and electrostatic investigation

Topology of electrostatic potential and electron density reveals a covalent to non-covalent carbon-carbon bond continuum

The covalent and non-covalent nature of carbon-carbon (CC) interactions in a wide range of molecular systems can be characterized using various methods, including the analysis of molecular electrostatic potential (MESP), represented as V(r), and the molecular electron density (MED), represented as ρ(r). These techniques provide valuable insights into the bonding between carbon atoms in different molecular environments. By uncovering a fundamental exponential relationship between the distance of the CC bond and the highest eigenvalue (λv1) of V(r) at the bond critical point (BCP), this study establishes the continuum model for all types of CC interactions, including transition states. The continuum model is further delineated into three distinct regions, namely covalent, borderline cases, and non-covalent, based on the gradient, , with the bond distance of the CC interaction. For covalent interactions, this parameter exhibits a more negative value than -5.0 a.u. Å-1, while for non-covalent interactions, it is less negative than -1.0 a.u. Å-1. Borderline cases, which encompass transition state structures, fall within the range of -1.0 to -5.0 a.u. Å-1. Furthermore, this study expands upon Popelier's analysis of the Laplacian of the MED, denoted as ∇2ρ, to encompass the entire spectrum of covalent, non-covalent, and borderline cases of CC interactions. Therefore, the present study presents compelling evidence supporting the concept of a continuum model for CC bonds in chemistry. Additionally, this continuum model is further explored within the context of C-N, C-O, C-S, N-N, O-O, and S-S interactions, albeit with a limited dataset.

Electrostatic Potential for Exploring Electron Delocalization in Infinitenes, Circulenes, and Nanobelts

The π-conjugation, aromaticity, and stability of the newly synthesized 12-infinitene and of other infinitenes comprising 8-, 10-, 14-, and 16-arene rings are investigated using density functional theory. The π-electron delocalization and aromatic character rooted in infinitenes are quantified in terms of molecular electrostatic potential (MESP) topology. Structurally, the infinitene bears a close resemblance of its helically twisted structure to the infinity symbol. The MESP topology shows that infinitene possesses an infinity-shaped delocalization of the electron density that streams over the fused benzenoid rings. The parameter ∑i=13Δλi, derived from the eigenvalues (λ_i) corresponding to the MESP minima, is used for quantifying the aromatic character of arene rings of infinitene. The structure, stability, and MESP topology features of 8-, 10-, 12-, 14-, and 16-infinitenes are also compared with the corresponding isomeric circulenes and carbon nanobelts. Further, the strain in all such systems is evaluated by considering the respective isomeric planar benzenoid hydrocarbons as reference systems. The 12-infinitene turns out to be the most aromatic and the least strained among all the systems examined.

Structural stability and the low-lying singlet and triplet states of BN-n-acenes, n = 1-7

The chemical stability and the low-lying singlet and triplet excited states of BN-n-acenes (n = 1-7) were studied using single reference and multireference methodologies. From the calculations, descriptors such as the singlet-triplet splitting, the natural orbital (NO) occupations and aromaticity indexes are used to provide structural and energetic analysis. The boron and nitrogen atoms form an isoelectronic pair of two carbon atoms, which was used for the complete substitution of these units in the acene series. The structural analysis confirms the effects originated from the insertion of a uniform pattern of electronegativity difference within the molecular systems. The covalent bonds tend to be strongly polarized which does not happen in the case of a carbon-only framework. This effect leads to a charge transfer between neighbor atoms resulting in a more strengthened structure, keeping the aromaticity roughly constant along the chain. The singlet-triplet splitting also agrees with this stability trend, maintaining a consistent gap value for all molecules. The BN-n-acenes molecules possess a ground state with monoconfigurational character indicating their electronic stability. The low-lying singlet excited states have charge transfer character, which proceeds from nitrogen to boron.

Role of heteroatoms and substituents on the structure, reactivity, aromaticity, and absorption spectra of pyrene: a density functional theory study

Effect of heteroatoms viz. BN and substituents viz. –Me (methyl), –OH (hydroxyl), –NH2 (amine), –COOH (carboxyl), and –CN (cyano) on the structural parameters, global reactivity, aromaticity, and UV-visible spectra of pyrene are studied with the help of density functional theory (DFT). Global reactivity parameters such as global hardness (η) and electrophilicity (ω) are calculated using density functional reactivity theory (DFRT). Time dependent density functional theory (TD-DFT) is explored for interpreting the UV-visible absorption spectra. Aromaticity of the pyrene rings are predicted from the nucleus independent chemical shift (NICS) values. Presence of BN unit and substituent induces reasonable impact on the studied parameters. The observed absorption spectra lie predominantly within the UV-region (both blue and red shifts are observed in presence of BN and substituent). HOMO energy and absorption spectra are affected nominally in solvent phase.

Electrostatic Potential Topology for Probing Molecular Structure, Bonding and Reactivity

Following the pioneering investigations of Bader on the topology of molecular electron density, the topology analysis of its sister field viz. molecular electrostatic potential (MESP) was taken up by the authors’ groups. Through these studies, MESP topology emerged as a powerful tool for exploring molecular bonding and reactivity patterns. The MESP topology features are mapped in terms of its critical points (CPs), such as bond critical points (BCPs), while the minima identify electron-rich locations, such as lone pairs and π-bonds. The gradient paths of MESP vividly bring out the atoms-in-molecule picture of neutral molecules and anions. The MESP-based characterization of a molecule in terms of electron-rich and -deficient regions provides a robust prediction about its interaction with other molecules. This leads to a clear picture of molecular aggregation, hydrogen bonding, lone pair–π interactions, π-conjugation, aromaticity and reaction mechanisms. This review summarizes the contributions of the authors’ groups over the last three decades and those of the other active groups towards understanding chemical bonding, molecular recognition, and reactivity through topology analysis of MESP.

Synthesis of bis-BN-Naphthalene-Fused Oxepins and Their Photoluminescence Including White-Light Emission

A series of novel bis-BN-naphthalene-fused oxepin derivatives were synthesized via a Pd-catalyzed tandem reaction from brominated 2,1-borazaronaphthalenes and cis-bis(boryl)alkenes. X-ray crystallographic analysis revealed that bis-BN-naphthalene-fused oxepins feature a planar framework. The electronic and photophysical properties of the novel BN-naphthalene-fused oxepins were investigated by UV-vis and fluorescence spectroscopies and density functional theory (DFT) calculations, which disclosed the distinct electronic and photophysical properties of the analogous hydrocarbon system. Interestingly, dual-fluorescent emissions were observed upon dissolving N-substituted derivatives 10-14 in dimethyl sulfoxide. Tunable emission colors especially for white-light emissions can be achieved by controlling the ratio of solvents, concentration, or temperature using only a single-molecule compound.

Electrostatic Topographical Viewpoint of π-Conjugation and Aromaticity of Hydrocarbons

A molecular electrostatic potential (MESP) topographical study has been conducted for a variety of conjugated hydrocarbons at B3LYP/6-311+G(d,p) level of theory to understand their π-conjugation features and aromaticity. The value of MESP minimum (V_m) is related to the localized and delocalized distribution of π-electron density. The V_m values are located interior to the rings in polycyclic benzenoid hydrocarbons (PBHs), whereas they lie outside the boundary of the rings in antiaromatic and in fused systems consisting of aromatic and antiaromatic moieties. The V_m points lie on top and bottom of the π-regions in linear polyenes and annulenes, while a degenerate distribution of CPs around the midpoint region of triple bonds is observed in alkynes. The eigenvalues λ₁, λ₂, and λ₃ of the Hessian matrix at V_m(MESP minima) are used to define the aromatic character of the cyclic structures. The eigenvalues follow the trend λ₁ ≫ λ₂ > λ₃ ≅ 0 in PBHs, λ₁ > λ₂ > λ₃ ≅ 0 in linear polyenes, and λ₁ > λ₂ > λ₃ ≠ 0 in antiaromatic systems. The difference in the aromatic character of PBHs from that of benzene is related to the deviations Δλ₁, Δλ₂, and Δλ₃. The total deviation ∑_i=1³Δλ_i is found to be ≤ 0.011 au for all aromatic systems and lies between 0.011 and 0.035 au for all nonaromatic systems. For antiaromatic systems, its value is found to be above 0.035 au. Further, ∑_i=1³Δλ_i gives a direct interpretation of Clar's aromatic sextet structures for PBHs. In summary, MESP topography mapping is a powerful technique to quantify the localized and delocalized π-electron distribution in a variety of unsaturated hydrocarbon systems.

Regioselective Functionalization of Stable BN-Modified Luminescent Tetraphenes for High-Resolution Fingerprint Imaging

A series of novel BN tetraphene derivatives have been prepared successfully for the first time via a post-functionalization strategy. The optical and electronic properties of these derivatives could be tuned systematically by the incorporation of different substituents on the main skeleton. The functionalized BN-containing luminogens have been explored for the detection of latent fingerprints (LFPs) on different substrates, including glass, aluminum foil, plastic, and ironware. This strategy provides great versatility in LFP imaging and good potential in elucidating the chemical information within LFPs, making the strategy valuable in forensic investigations.

Mono BN-substituted analogues of naphthalene: a theoretical analysis of the effect of BN position on stability, aromaticity and frontier orbital energies

An insight into the electronic structure changes driven by CC → BN substitution in a naphthalene system has been given by quantum chemical calculations.

Electrostatics for probing lone pairs and their interactions

The value of the molecular electrostatic potential minimum (V_min ) and its topographical features (position, as well as the eigenvalues and eigenvectors of the corresponding Hessian matrix) are recently proposed as the criteria for characterizing a lone pair (Kumar A. et al., J. Phys. Chem. 2014, A118, 526). This electrostatic characterization of lone pairs is examined for a large number of small molecules employing MP4/6-311++G(d,p)//MP2/6-311++G(d,p) theory. The eigenvector of the Hessian matrix corresponding to its largest eigenvalue (λ_max ), is found to be directed toward the lone pair-bearing-atom, with λ_max showing a strong linear correlation with V_min . Large magnitudes of V_min and λ_max indicate a charge-dense lone pair. The topographical features of V_min are seen to provide insights into the interactive behavior of the molecules with model electrophiles, viz. HF, CO₂ , and Li⁺ . In all the complexes of HF and majority of the other complexes, the interaction energy (E_int ) correlates well with the respective V_min, value, but for some deviations occurring due to other competing secondary interactions. The electrostatic interactions are found to be highly directional in nature as the orientation of interacting atom correlates strongly to the position of lone pair. In summary, the present study on a large number of test molecules shows that electrostatics is able to probe lone pairs in molecules and offers a simple interpretation of chemical reactivity. © 2017 Wiley Periodicals, Inc.

A DFT study on 1,4-dihydro-1,4-azaborinine annulated linear polyacenes: Absorption spectra, singlet-triplet energy gap, aromaticity, and HOMO-LUMO energy modulation

Linear polyacene (LPA) mimics containing multiple heterocycles have been computationally designed by annulating 1,4-dihydro-1,4-azaborinine moieties to benzene (aB₁ -aB₅ ), naphthalene (aN₁ -aN₅ ), anthracene (aA₁ -aA₅ ), and tetracene (aT₁ -aT₅ ) cores. DFT studies conducted on them using M06L/6-311++G(d,p) method reveal a perfect planar structure for all and suggest the utilization of nitrogen lone pairs for aromatic π-electron delocalization. The computed values of aromaticity indices such as HOMA, NICS, and dehydrogenation energy (E_dh ) of heterocycles support strong aromatic character for each six-membered ring in the LPA mimics. On the basis of the minimum value of the molecular electrostatic potential (V_min ) observed on each LPA unit in the LPA mimics, the extended delocalization of π-electrons is verified. The energetic parameter E_dh showed strong linear correlation with HOMA, NICS and V_min parameters, which strongly supports the multidimensional character of aromaticity in LPA mimics. The electronic property modification is shown by the theoretical absorption spectra data and singlet-triplet energy gap (ΔE_ST ). The bandgap and ΔE_ST tunings are achieved for LPA mimics by selecting appropriate number of azaborinine type units and the size of LPA core used for annulation. © 2017 Wiley Periodicals, Inc.

Does borazine-water behave like benzene-water? A matrix isolation infrared and ab initio study

Borazine is isoelectronic with benzene and is popularly referred to as inorganic benzene. The study of non-covalent interactions with borazine and comparison with its organic counterpart promises to show interesting similarities and differences. The motivation of the present study of the borazine-water interaction, for the first time, stems from such interesting possibilities. Hydrogen-bonded complexes of borazine and water were studied using matrix isolation infrared spectroscopy and quantum chemical calculations. Computations were performed at M06-2X and MP2 levels of theory using 6-311++G(d,p) and aug-cc-pVDZ basis sets. At both the levels of theory, the complex involving an N-H⋯O interaction, where the N-H of borazine serves as the proton donor to the oxygen of water was found to be the global minimum, in contrast to the benzene-water system, which showed an H-π interaction. The experimentally observed infrared spectra of the complexes corroborated well with our computations for the complex corresponding to the global minimum. In addition to the global minimum, our computations also located two local minima on the borazine-water potential energy surface. Of the two local minima, one corresponded to a structure where the water was the proton donor to the nitrogen of borazine, approaching the borazine ring from above the plane of the ring; a structure that resembled the global minimum in the benzene-water H-π complex. The second local minimum corresponded to an interaction of the oxygen of water with the boron of borazine, which can be termed as the boron bond. Clearly the borazine-water system presents a richer landscape than the benzene-water system.

The synthesis of BN-embedded tetraphenes and their photophysical properties

A series of intensely blue fluorescent BN-embedded tetraphene derivatives have been synthesized by the catalytic cyclization of BN-naphthalenes with alkynes.

The Clar Structure in Inorganic BN Analogues of Polybenzenoid Hydrocarbons: Does it Exist or Not?

The Clar structure of polybenzenoid hydrocarbons (PBHs) have attracted considerable interest of both theoretical and experimental chemists since it was proposed in the 1950s. However, it remains unclear whether the Clar structure could exist in inorganic PBHs, the boron nitride (BN) analogues where the alternate boron and nitrogen atoms are used to replace the carbon atoms of PBHs. Here, we carry out thorough density functional theory (DFT) calculations to probe the possibility of Clar structures in BN analogues of PBHs. A strong correlation (r(2) =0.975) between the ring number (n=3-10) of BN analogues of [n]acenes and energy differences between the most and least stable isomers is identified, suggesting the existence of Clar structures in inorganic PBHs. In addition, the slightly weaker correlations in comparison to that (r(2) =0.989) of the organic PBHs is rationalized by the reduced aromaticity, which is revealed by two aromatic indices: ELFπ and SCI.

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 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.

DFT calculations of structures, (13)C NMR chemical shifts, and Raman RBM mode of simple models of small-diameter zigzag (4,0) carboxylated single-walled carbon nanotubes

Linearly conjugated benzene rings (acenes), belt-shaped molecules (cyclic acenes), and models of single-walled carbon nanotubes (SWCNTs) with one carboxylic group at the open end were fully optimized at the B3LYP/6-31G* level of theory. These models were selected to obtain some insight into the nuclear isotropic changes resulting from systematically increasing the basic building units of open-tip-monocarboxylated SWCNTs. In addition, the position of radial breathing mode (RBM), empirically correlated with the SWCNT diameter, was directly related with the radius of model cyclic acene rings. A regular convergence of selected structural, NMR, and Raman parameters with the molecular system size increase was observed, and a simple two-parameter mathematical formula enabled their estimation in infinity. The predicted (13) C NMR chemical shifts of carbon atoms close to the substituted rim of carboxylated models of zigzag (4,0) SWCNTs differed significantly from the pristine nanotubes.

DFT calculation of structures and NMR chemical shifts of simple models of small diameter zigzag single wall carbon nanotubes (SWCNTs)

Linearly conjugated benzene rings (acenes), belt-shape molecules (cyclic acenes) and model single wall carbon nanotubes (SWCNTs) were fully optimized at the unrestricted level of density functional theory (UB3LYP/6-31G*). The models of SWCNTs were selected to get some insight into the potential changes of NMR chemical shift upon systematic increase of the molecular size. The theoretical NMR chemical shifts were calculated at the B3LYP/pcS-2 level of theory using benzene as reference. In addition, the change of radial breathing mode (RBM), empirically correlated with SWCNT diameter, was directly related with the radius of cyclic acenes. Both geometrical and NMR parameters were extrapolated to infinity upon increase in the studied systems size using a simple two-parameter mathematical formula. Very good agreement between calculated and available experimental CC bond lengths of acenes was observed (RMS of 0.0173 Å). The saturation of changes in CC bond lengths and (1)H and (13)C NMR parameters for linear and cyclic acenes, starting from 7-8 conjugated benzene rings, was observed. The (13)C NMR parameters of individual carbon atoms from the middle of ultra-thin (4,0) SWCNT formed from 10 conjugated cyclic acenes differ by about 130 ppm from the corresponding open end carbon nuclei.

On the applicability of fragmentation methods to conjugated pi systems within density functional framework

For the accurate ab initio treatment of large molecular systems, linear scaling methods (LSMs) have been devised and successfully applied to covalently bonded systems as well as to those involving weak intra/intermolecular bonds. Very few attempts to apply LSM to highly conjugated molecules, especially to two-dimensional systems, have so far been reported in the literature. The present article examines the applicability of a LSM, viz., molecular tailoring approach (MTA), to pi-conjugated systems within density functional theory. A few test cases within second order Møller-Plesset framework are also reported. MTA is applied to some one-dimensional pi-conjugated molecules, for which the difference between MTA energy and actual energy is found out to be less than 1 mhartree and also reduced computation time as well as hardware requirements. The method is also extended to some small/medium-sized two-dimensional pi-conjugated molecules by developing a systematic algorithm for tailoring such systems. However, for such systems, although the energies are in error by a few millihartrees, gradients are found to match reasonably well their actual counterparts. Hence, geometry optimization of these systems within MTA framework is attempted. The geometries thus generated are found to be in good agreement with their actual counterparts, with the actual single point energies matching within 1 mhartree, along with reduced computational effort. These results point toward the potential applicability of MTA to large two- and three-dimensional pi-conjugated systems.

Laterally extended spiral graphite analogue boron-nitrogen helices

Ab initio self-consistent field molecular orbital and density functional theory calculations have been performed on a series of extended helical boron-nitrogen analogues of a "spiral graphite", the [N]polymethylenylnaphthalenes (N = 6, 8, and 12), with the molecular formula N(x)B(y)H(z) (where x = 28, 37, and 55, y = 27, 36, and 54, z = 23, 29, and 41). Interchanging the positions of the boron and nitrogen atoms in the helix leads to very similar structures N(x-1)B(y+1)H(z) in all three studied cases. The electronic structure and the optimum geometries of these helices were investigated at the HF/6-31G(d,p) and B3LYP/6-31G(d,p) levels of theory. Electron density contours were calculated for the largest helices at the B3LYP/6-31G(d,p) level of theory.

B-N as a C-C substitute in aromatic systems

The substitution of isoelectronic B–N units for C–C units in aromatic hydrocarbons produces novel heterocycles with structural similarities to the all-carbon frameworks, but with fundamentally altered electronic properties and chemistry. Since the pioneering work of Dewar some 50 years ago, the relationship between B–N and C–C and the wealth of parent all-carbon aromatics has captured the imagination of organic, inorganic, materials, and computational chemists alike, particularly in recent years. New applications in biological chemistry, new materials, and novel ligands for transition-metal complexes have emerged from these studies. This review is aimed at surveying activity in the area in the past couple of decades. Its organization is based on ring size and type of the all-carbon or heterocyclic subunit that the B–N analog is derived from. Structural aspects pertaining to the retention of aromaticity are emphasized, along with delineation of significant differences in physical properties of the B–N compound as compared to the C–C parent.Key words: boron-nitrogen heterocycles, aromaticity, organic materials, main-group chemistry.

An ab initio study of 15N-11B spin-spin coupling constants for borazine and selected derivatives

Ab initio equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have been performed to investigate substituent effects on coupling constants for borazine and selected substituted borazines. For molecules in which F atoms are not bonded to adjacent atoms in the ring, F substitution increases the one-bond (11)B-(15)N coupling constants involving the atom at which substitution occurs but leaves the remaining one-bond B-N coupling constants essentially unchanged. For these molecules, the magnitudes of one-bond B-N coupling constants are only slightly dependent on the number of F atoms present. Fluorine substitution at adjacent B and N atoms in the borazine ring further increases the one-bond B-N coupling constant involving the substituted atoms and has the same effect on the other one-bond coupling constants as observed for corresponding molecules in which substitution occurs at alternate sites. In contrast to the effect of F substitution, substitution of Li at either N or B decreases one-bond B-N coupling constants relative to borazine. The effects of F and Li substitution on one-bond B-N coupling constants for borazine are similar to F and Li substitution effects on (13)C-(13)C coupling constants for benzene.

Theoretical Study of Reaction Pathways to Borazine

Four reaction pathways from diborane and ammonia to borazine, (HBNH)3, have been studied computationally at the density functional level (B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d)). The cycloaddition of H2BNH2 to 1,3-diaza-2,4-diborabuta-1,3-diene and subsequent elimination of two molecules of H2 was found to be the lowest-energy pathway to (HBNH)3. In the other pathways, the formation and conversion of the intermediates 1,3,5-triaza-2,4,6-triborahexatriene, cyclotriborazane, and 1,3,5-triaza-2,4,6-triborahexa-1,5-diene into (HBNH)3 were investigated. The formation of 1,3-diaza-2,4-diborabuta-1,3-diene and, subsequently, the formation and electrocyclization of 1,3,5-triaza-2,4,6-triborahexatriene and the cycloaddition of H2BNH2 to 1,3-diaza-2,4-diborabuta-1,3-diene are predicted to be the kinetically favored pathways to (HBNH)3 in the gas phase. At low concentrations of 1,3-diaza-2,4-diborabutene, high concentrations of H2BNH2, and a temperature of 298.15 K, the formation of the polyolefins H3BNH2(H2BNH2)nNHBH2 (n=1,2) is predicted to be competitive with the formation of 1,3-diaza-2,4-diborabuta-1,3-diene.

Aromaticity in Polyacene Analogues of Inorganic Ring Compounds

The aromaticity in the polyacene analogues of several inorganic ring compounds (BN-acenes, CN-acenes, AlN-acenes, BO-acenes, BS-acenes, and Na6-acenes) is reported here for the first time. Conceptual density functional theory-based reactivity descriptors and the nucleus-independent chemical shift (NICS) values are used in this analysis. The nature of the site selectivity is understood through the charges and the philicities.

Synthesis and structure of 1-D Na6cluster chain with short Na–Na distance: Organic like aromaticity in inorganic metal cluster

A unique 1-D chain of sodium cluster containing (Na6) rings stabilized by a molybdenum containing metalloligand has been synthesized and characterized and the DFT calculations show striking resemblance in their aromatic behaviour with the corresponding hydrocarbon analogues.

Direct estimate of conjugation and aromaticity in cyclic compounds with the EDA method

The nature of the interatomic interactions in cyclic conjugated molecules has been investigated with the Energy Decomposition Analysis (EDA). The focus of this work is on the strength of the pi bonding and pi conjugation in carbocyclic and heterocyclic molecules. The calculated deltaE(pi) values suggest that the EDA may be used directly for estimating the strength of pi conjugation in aromatic, homoaromatic, homoantiaromatic and antiaromatic compounds. The theoretical data show a pattern which agrees with the 4n + 2 rule. The extra aromatic stabilization energy has been obtained by comparing cyclic conjugation with the strength of pi conjugation in acyclic reference compounds. The deltaE(pi) values indicate that benzene is significantly stabilized by aromatic stabilization, but the total pi bonding contribution to the carbon-carbon bonding becomes even more stabilizing when the geometry is distorted toward a D(3h) form which has three long and three short C-C bonds. The D(6h) equilibrium structure is enforced by the sigma orbital interactions and by the quasiclassical electrostatic attraction. The VRE and ASE values suggest that pyridine is more strongly stabilized by aromatic conjugation than benzene. The EDA data of the six-membered cyclic 6pi species C5H5E (E = CH, N-Bi) and (HB = NH)3 predicts strength of aromatic stabilization in the order N > CH > P > As > Bi >> borazine. The ASE values of the five-membered heterocyclic systems C4H4E (E = NH, O, S) are smaller than for the benzene analogues showing the order NH approximately S > O. The ASE value for Cp is very small which probably comes from the choice of the reference system. The VRE results indicate that Cp is strongly stabilized by cyclic conjugation. The ASE values for the cyclic singlet carbenes indicate weak aromatic stabilization in the 2pi system cyclopropenylidene which is much stronger in the 6pi compound cycloheptatrienylidene while the 4pi molecule cylopentadienylidene is clearly antiaromatic. The triplet states of all three cyclic carbenes are antiaromatic. The strength of the pi conjugation in the homoconjugated cyclic compounds suggests weak aromatic stabilization in the 6pi compounds 1,3-cyclobutene and 1,3-cyclopentadiene but weak antiaromatic destabilization in the 2pi compound cyclopropene and in the 8pi molecules 1,3-cyclohexadiene and 1,3,5-cycloheptatriene. The 4n + 2 rule thus holds also for homoconjugated systems. A large negative value for the ASE is calculated for 1,3-cyclobutadiene.

Small Fullerenes with BN Belts: A Density Functional Theory Investigation

Density functional calculations have been carried out on a series of BCN hybrid fullerenes with certain substitution patterns in comparison with their parent compounds Cn (n = 30, 32, 36, 38, 40, 44, 48, 50, 52). The substitutional structures, energy gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, ionization potentials, electron affinities, as well as molecular electrostatic potentials have been systematically investigated. The following important points of BCN hybrid fullerenes are stressed: The present studied fullerenes, comprising tubular "belt" and polar "cap", could be divided into three types of structure; each has different indexes of tubular structure and terminal caps. The properties of BCN hybrid fullerenes depend on the type of "tubular belt + polar cap" structures, especially, the HOMO and LUMO characters and MEPs of BCN fullerene are strongly governed by their structure types.

Stable planar six-pi-electron six-membered N-heterocyclic carbenes with tunable electronic properties

Carbene analogues of borazines are highly thermally stable. Keeping quasi-identical steric demands, the electronic properties of the carbene can be precisely tuned by varying the nature of the substituents at the boron centers.