Aromaticity and Homoaromaticity in Methano[10]annulenes

Aromaticity and neutral homoaromaticity have been evaluated in methano[10]annulenes systems, 1,4-methano[10]annulene (1), 1,5-methano[10]annulene (2), and 1,6-methano[10]annulene (3). C-C bond lengths indicate that 1 presents higher bond alternation than 2 and 3. The relative energies were determined at the B3LYP/6-311+G(d,p) level, and they pointed out that 3 is the most stable isomer. Strain energies, evaluated employing homodesmotic reactions, show the same order as the relative energies. Through a decomposition of strain energies, it could be concluded that the rings absorb more tension than the bridges. The changes in aromaticity were evaluated by magnetic susceptibilities, chiM, HOMA, NICS, and resonance energies, RE. HOMA, RE, and chiM indicate that 2 and 3 are strongly, and 1 is fairly, aromatic. NICS does not provide reliable results, due to interference of ring and bridge atoms. NBO analysis presents some interactions that suggest the existence of neutral homoaromaticity. GPA indices (evaluated at the B3LYP/6-31G* level) point out that homoaromaticity plays a relevant role only in 3. Moreover, this work is the first in the current literature that studies 1,4-methano[10]annulene (1).     

Multidimensionality of delocalization indices and nucleus independent chemical shifts in polycyclic aromatic hydrocarbons

The aromaticity and local-aromaticity of a large set of polycyclic aromatic hydrocarbons (PAHs) is studied using multicenter delocalization indices from generalized population analysis and the popular nucleus independent chemical shift (NICS) index. A method for the fast computation of the NICS values is introduced, using the so-called pseudo-pi-method. A detailed examination is made of the multidimensional nature of aromaticity. The lack of a good correlation between the NICS and the multicenter delocalization indices is reported and the grounds discussed. It is shown through a thorough statistical analysis that the NICS values arise not only from local aromaticity of the benzenoid rings, but also from other circuits. It is shown that the NICS indices do not reveal the individual aromatic nature of a specific ring, contrary to the delocalization indices.     

Spherical sila- and germa-homoaromaticity

Guided by the 2(N + 1)2 electron-counting rule for spherical aromatic molecules, we have designed various spherical sila- and germa-homoaromatic systems rich in group 14 elements. Their aromaticity is revealed by density-functional computations of their structures and the nucleus-independent chemical shifts (NICS). Besides the formerly used endohedral inclusion strategy, spherical homoaromaticity is another way to stabilize silicon and germanium clusters.     

Molecular geometry as a source of chemical information. 3. How H-bonding affects aromaticity of the ring in the case of phenol and p-nitrophenol complexes: a B3LYP/6-311+G study

Molecular geometries of phenol and p-nitrophenol (ArOH) interacting with fluoride were optimized at the B3LYP/6-311+G level of theory taking as constraints the planarity of the systems and the linearity of the O...H...F moiety. For p-nitrophenol complexes, the substituent effect stabilization energy (SESE) was computed, and for all systems aromaticity indices, HOMA, and out-of-plane components of NICS(1) and NICS(1)(zz)() were calculated. SESE values depend strongly on the O...F distance, the same as both aromaticity indices. Variation in HOMA values for the studied ArOH...F(-) complexes is within the range of 0.55 to approximately 1.0 and for NICS(1)(zz)() between -12 and -26 ppm. It was also found that a decrease in aromaticity is well correlated with the variations of C-O bond length.     

Aromaticity in cyclic alkali clusters

Density functional calculations on a hexagonal 1D sodium cluster and a 2D potassium cluster show that the M6 (M = Na, K) rings in the chain present in 3D [Na2MoO3L(H2O)2]n (1) and 2D [K2MoO3L(H2O)3]n (2) are aromatic in character according to the nucleus independent chemical shift (NICS) and multicenter bond indices (MCI) values. The NICS values at the center of the Na6 rings and at the cage center of the K6 rings are comparable to the corresponding values of their polyacene analogues in most cases. The stability and reactivity patterns of the M6 rings also follow a similar trend as their organic analogues.     

Influence of the H/F replacement on the homoaromaticity of homotropylium ion: a GIAO/DFT theoretical study

The problem of homoaromaticity in mono-, di- and polyfluorinated- homotropylium cations is addressed by the B3LYP/6-311++G** DFT method. The energetic, structural and magnetic criteria are used for this purpose. They convincingly show that the ground state equilibrium species are aromatic, or in other words that the homoaromaticity is preserved by the (poly)fluorination. In contrast, a considerable decrease in the aromatic stabilization is observed in the transition structures (TS). According to the NICS(0) index, they vary form strongly antiaromatic, via weakly and non-aromatic to slightly aromatic transition states. However, the hierarchy of the aromaticity in fluorinated homotropylium ions predicted by NICS(0) is completely unrelated to that obtained by using the energy criterion assuming a kinetic definition of aromaticity. On the other hand the latter is closely related to geometric parameters of the equilibrium and transition structures.     

Neutral bishomoaromatic semibullvalenes

Isolobal substitution of CH units by boron carbonyl groups (BCO) at C2,6 and C2,8,4,6 in semibullvalene favors the delocalized neutral bishomoaromatic systems substantially. The homoaromaticity is documented by the computed diatropic nucleus independent chemical shifts (NICS). In addition, BCO substitution can result in low-lying triplet and open-shell singlet states. In contrast, substitution at C1,5 more than doubles the related Cope rearrangement barrier. The C2,6 and C2,8,4,6 BCO-substituted barbaralanes, barbaralones, and bullvalenes have substantially reduced barriers.     

Resurrection of neutral tris-homoaromaticity

Neutral in-plane tris-homoaromaticity is evaluated in tris(bismethano)benzene (15) and modifications of this parent structure in which the pi-orbitals might interact in the plane established by the unsaturated carbon atoms (in-plane conjugation). On the basis of magnetic susceptibility exaltation, nucleus-independent shift (NICS), and aromatic stabilization energy (ASE, evaluated via homodesmotic and isodesmic equations using B3LYP/6-311+G + ZPVE energies, as well as by MM3 and MM4 force field computations), we identified triene 17, a triply bridged analogue of 15, as the system where homoaromaticity is most effective. The NICS(total) in the center of 17 is -30.1 ppm and the diatropic pi-contribution is -18.0 ppm. This structure possesses more than one-third of the aromatic stabilization of benzene and is the best candidate for neutral tris-homoaromaticity ever proposed. The previously described tris-(bismethano)-benzene (15) also shows homoaromaticity but to a smaller extent compared to 17. Structure 18, which is closely related to 17, also is significantly homoaromatic but, as evaluated by MM3, strain partially counteracts the stabilizing effects from homoconjugation. Such a counteracting increase in strain largely cancels or even overwhelms the stabilization from homoconjugation in all other species considered in this study.     

Chiral aromaticities. AIM and ELF critical point and NICS magnetic analyses of Mobius-type aromaticity and homoaromaticity in lemniscular annulenes and hexaphyrins

An atoms-in-molecules (AIM) and electron localization function (ELF) critical point analysis is reported for two types of lemniscular system, each of which exhibits double-half-twist Mobius topology. This reveals that this type of conformation for [14]annulene 1 has, in addition to the obvious bond critical points (BCPs), two weaker transannular points in the central cross-over region. These can be interpreted in terms of local rings showing single-half-twist Mobius homoaromaticity in addition to the double-half-twist aromaticity revealed by the annulene as a whole. Another example of a single-half-twist Mobius homoaromatic 9 is suggested here to show aromatic properties as strong as its nonhomoaromatic analogue 8. The AIM critical points in 1 are relatively insensitive to the ring size (varied from 12 to 16), and only small changes are seen in the critical point properties when the pi-electron count is incremented from 4n+2 to 4n by dianion formation. These results are discussed in terms of the reported transformation of the 14-pi-electron octalene 10 by reduction/alkylation into 12, an isomer of 1. Another class of molecule that exhibits lemniscular topology is the phyrins. A transannular BCP in the central cross-over region for the double-half-twist aromatic [26]hexaphyrin 3 is revealed, which is not present for the double-half-twist antiaromatic [28]hexaphyrin 2. The NICS(rcp) for the former indicates strong Mobius homoaromaticity.     

Determination of three-center bond indices from population analyses: a fuzzy atom treatment

This work proposes the use of the treatment referred to as fuzzy atoms to describe three-center bond indices within studies of electron population analysis. A simple manipulation of our algorithms reported previously for describing multicenter bondings enables us to introduce this methodology in our mathematical framework, providing suitable numerical determinations of three-center bond indices, two-center bond ones, and electron atomic populations. The results, obtained in selected systems, are discussed and compared to those arising from other procedures of population analysis.     

Is Free Cyclooctatetraene Dianion an Aromatic System? A Quantum Chemistry Study

To investigate whether free cyclooctatetraene dianion (COT^2?) is aromatic, quantum chemistry methods were used to optimize its structure. Based on the optimized structures, the natural population analysis (NPA) charge, bond order, delocalization energy, nucleus‐independent chemical shift (NICS), and harmonic oscillator model of aromaticity (HOMA) values were computed by DFT‐B3LYP method with basis set 6‐311++G**, which shows that COT^2? is not aromatic as it is not planar and has different bond lengths and bond orders, smallest delocalization energy and positive NICS values. To further confirm the finding, the changes of NICS and energy against ring distortion angle were scanned. The COT^2? has positive NICS values all along the angle from 180° to 120° while other aromatic systems always have negative values. The energy scanning suggests that COT^2? should have the weakest capability to maintain its planar structure. All the calculations strongly indicate that COT^2? is not aromatic. This study also suggests that NICS scan might be a good approach to judge aromaticity.     

Diatropicity of 3,4,7,8,9,10,13,14-octadehydro[14]annulenes: a combined experimental and theoretical investigation

The synthesis and study of a series of octadehydro[14]annulenes is described. The aromaticity of these annulenes was investigated through examination of experimental data from arene-fused systems as well as calculated nucleus-independent chemical shifts (NICS) and bond lengths. Benzene ring fusion to the parent system results in a stepwise loss in aromaticity as the number of fused rings is increased from one to two to three. This decrease in annulenic ring current is manifested in the alkene proton chemical shifts (0-2 benzenes) as well as the NICS (0-3 benzenes). Comparison of isomeric thiophene-fused annulenes shows further evidence of ring current competition as these allow for observation of intermittent degrees of delocalization throughout the annulenic core. A consistent relationship between the magnitude of the NICS values and the degree of benzannelation is also observed.     

The influence of aza-substitution on azole aromaticity

A homogeneous set of values for the aromaticity indices ASE (Aromatic Stabilisation Energy), HOMA (Harmonic Oscillator Model of Aromaticity) and NICS(1) (Nucleus-Independent Chemical Shift) for azoles has been investigated using multiple linear regression analysis. Statistically-significant relationships were found between the aromaticity indices and the number of nitrogen atoms at positions 2/5 and 3/4 of the ring. Aza-derivatives of pyrrole, furan and thiophene all gave similar relationships. For all three indices aza-substitution at positions 2 and/or 5 increases aromaticity. However, aza-substitution at positions 3 and/or 4 decreases classical aromaticity (ASE and HOMA) but increases magnetic aromaticity (NICS(1)). These indices appear to be measuring different properties of the azoles. The influence of aza-substitution on these different aspects of aromaticity is tentatively rationalized in terms of either bond length equalization or uniformity of π electron distribution.     

Structure-resonance theory for homoaromatic bridged annulenes

The π systems of bridged annulenes can ben represented by π molecular graphs with homoconjugative interactions at the bridge positions. Nonbonding MO's of odd molecular graphs derived from the parent bridged annulene graphs can be used to carry out structure-resonance theory calculations. A general outline of possible applications is given. Specific comparisons are made between calculated bond orders and bond lengths, and between calculated and experimental ionization potentials. The concept of neutral homoaromaticity is supported by good agreement between calculated and experimental properties.     

Long-distance structural consequences of H-bonding. How H-bonding affects aromaticity of the ring in variously substituted aniline/anilinium/anilide complexes with bases and acids

The aromaticity of the ring in variously substituted aniline/anilinium/anilide derivatives in their H-bonded complexes with various Broensted acids and bases was a subject of an analysis based on 332 experimental geometries retrieved from the Cambridge Structural Database and geometries optimized at the B3LYP/6-311+G** and MP2/aug-cc-pVDZ levels of theory. Ab initio modeling was applied to the para-substituted aniline, anilinium cation, and anilide anion derivatives (X = NO, NO2, CN, CHO, H, CH3, OCH3, and OH) and their H-bonded complexes (only for X = NO, NO2, CHO, H, and OH) with B (B = F- and CN-) or HB (HB = HF and HCN). In both cases, the harmonic oscillator model of aromaticity index (HOMA) was used, whereas for computational geometries, additionally, the magnetism-based indices NICS, NICS(1), and NICS(1)zz were also applied (NICS = nucleus-independent chemical shift). There is an equivalent prediction of aromaticity by NICSs and HOMA and approximate monotonic dependences of HOMA and NICS on the C-N bond length. The strongest changes in aromaticity estimated by HOMA and NICSs were found for aniline derivatives with NH2...B and anilide derivatives without and with NH-...HB interactions. The changes observed for two other kinds of interactions, NH2...HB and NH3+...base (for anilinium cations), are much smaller. For all four kinds of interactions, the relationships between ipso-bond angle, mean ipso-ortho bond length, and C-N bond length follow the Bent-Walsh rule.     

Evidence for 5-center 4-electron bonding in (C...H...C...H...C) array

This paper brings new independent evidence for 5-center 4-electron bonding, whose existence in (C...H...C...H...C) fragment of the molecule I was anticipated in the recent study by Tantillo and Hoffmann. The evidence is based on the applications of the so-called multicenter bond indices, recently proposed as new efficient tool for the detection and localization of multicenter bonding in molecules.     

Theoretical study of structure, bonding, and aromaticity of borazyne and B-substituted borazynes

The density functional theory (DFT) is used to study the geometries, and electronic structures of triplet and singlet of borazyne and B-substituted of borazyne. The aromaticity of these systems is analyzed in the light of nucleus-independent chemical shift (NICS), average of two-center indices (ATI). These methods show increasing of aromaticity in deactivating groups. The relation between electron density in ring critical point (RCP) and NICS(1.0) is observed. The most important interaction in these molecules has been investigated by natural bonding orbital method (NBO).     

Electronic nature of carbonium ions and their silicon analogues

Nonclassical ions or carbonium ions have multi-center bonding from delocalized sigma or pi electrons. The 2-norbornyl cation, its derivative 6,6-difluoro-2-norbornyl cation, tris-homocyclopropenyl cation, 7-norbornenyl cation, and 4-cyclopentenyl cation and their corresponding silicon analogues were studied in this work. All carbocations have topologically different 3c-2e systems. The magnitude of all delocalization indexes between each atomic pair of the 3c-2e bond can be used to predict homoaromaticity. The silicon analogues have a topologically different 3c-2e bond from their corresponding carbocation.