Predicting an Antiaromatic Benzene Ring in the Ground State Caused by Hyperconjugation

Benzene, the prototype of aromatics, has six equivalent C?C bonds (1.397 Å), which are intermediate between a C?C double bond and a C?C single bond. For over 80 years, chemists have spent much effort on freezing a localized structure to obtain a distorted bond‐length alternating benzene ring in the ground state, leading to various localized trisannelated benzene rings. However, most of the central benzene rings are still aromatic or nonaromatic. Here we report an antiaromatic benzene ring caused by hyperconjugation. Specifically, symmetric annulation of 5,5‐difluorocyclopentadiene results in an antiaromatic benzene ring, which is supported by various aromaticity indices, including nucleus‐independent chemical shift, anisotropy of the induced current density, π‐separated electron‐localization function and heat of hydrogenation. Our findings highlight a strong power of hyperconjugation, a “weak” interaction in organic chemistry, paving the way for designing and realizing more novel (anti)aromatics.     

Probing the Most Aromatic and Antiaromatic Pyrrolium Rings by Maximizing Hyperconjugation and Push–Pull Effect

Hyperconjugation, a weak interaction in organic chemistry, can have a strong effect on aromaticity, leading to the concept of hyperconjugative aromaticity, which was first proposed by Mulliken in 1939. However, most studies are limited to main group chemistry. Here we report the most aromatic and antiaromatic pyrrolium ring by maximizing the hyperconjugation caused by transition metal fragments and the push–pull effect. Our calculations reveal that the origin of the outperformance of transition metal substituents over main group ones on hyperconjugative aromaticity could be attributed to their higher highest occupied molecular orbitals (HOMOs). Among the group 11 transition metals, a silver substituent results in the best performance. All these findings highlight the magic of the transition metal (silver) and could be particularly helpful for the design of other aromatic and antiaromatic counterparts based on a nonaromatic parent species.     

Ground- and excited-state aromaticity and antiaromaticity in benzene and cyclobutadiene

The aromaticity and antiaromaticity of the ground state (S 0), lowest triplet state (T 1), and first singlet excited state (S 1) of benzene, and the ground states (S 0), lowest triplet states (T 1), and the first and second singlet excited states (S 1 and S 2) of square and rectangular cyclobutadiene are assessed using various magnetic criteria including nucleus-independent chemical shifts (NICS), proton shieldings, and magnetic susceptibilities calculated using complete-active-space self-consistent field (CASSCF) wave functions constructed from gauge-including atomic orbitals (GIAOs). These magnetic criteria strongly suggest that, in contrast to the well-known aromaticity of the S 0 state of benzene, the T 1 and S 1 states of this molecule are antiaromatic. In square cyclobutadiene, which is shown to be considerably more antiaromatic than rectangular cyclobutadiene, the magnetic properties of the T 1 and S 1 states allow these to be classified as aromatic. According to the computed magnetic criteria, the T 1 state of rectangular cyclobutadiene is still aromatic, but the S 1 state is antiaromatic, just as the S 2 state of square cyclobutadiene; the S 2 state of rectangular cyclobutadiene is nonaromatic. The results demonstrate that the well-known "triplet aromaticity" of cyclic conjugated hydrocarbons represents a particular case of a broader concept of excited-state aromaticity and antiaromaticity. It is shown that while electronic excitation may lead to increased nuclear shieldings in certain low-lying electronic states, in general its main effect can be expected to be nuclear deshielding, which can be substantial for heavier nuclei.     

Dianion and dication of tetrabenzo[5.7]fulvalene. Greater antiaromaticity than aromaticity in comparable systems

The dianion, 5(2-), and dication, 5(2+), of tetrabenzo[5.7]fulvalene represent an aromaticity/antiaromaticity continuum in which the fluorenyl system changes from aromatic in 5(2-) to antiaromatic in 5(2+). Conversely, the antiaromatic dibenzotropylium system of 5(2-) becomes an aromatic system in 5(2+), allowing an examination of aromaticity/antiaromaticity within the same carbon framework. Dianion 5(2-) was prepared and characterized by (1)H NMR spectroscopy. The fluorenyl system of 5(2-) showed the downfield shifts expected for an aromatic system, while the dibenzotropylium system showed the paratropic shifts expected for an antiaromatic system. The conclusions from (1)H NMR spectroscopy were supported by NICS(1) zz calculations for each system. Comparison of the (1)H NMR spectrum and NICS(1) zz of 5(2-) with those of 5(2+) supported the assignments of aromaticity/antiaromaticity for each system. Aromaticity/antiaromaticity were further examined through comparison of the degree of bond length alternation, which showed that the bond length alternation was slightly greater for the antiaromatic ring systems than for the aromatic systems. However, when structures of 5(2-) and 5(2+) with no bond length alternation were examined, there was a dramatic increase in the degree of antiaromaticity for the antiaromatic ring systems as evaluated through NICS. This result suggests that a decrease in bond length alternation results in an increase in antiaromaticity as well as an increase in aromaticity. The magnitude of the antiaromaticity of the fluorenyl system in 5(2+) was greater than the magnitude of the aromaticity in the fluorenyl system of 5(2-), with similar effects shown by the analogous tropylium systems. This is consistent with the behavior of the antiaromatic dication of tetrabenzo[5.5]fulvalene, compared to that of its aromatic dianion, and also with the behavior of the cyclopentadienyl cation/anion and tropylium cation/anion.     

Interpreting Aromaticity and Antiaromaticity through Bifurcation Analysis of the Induced Magnetic Field

In all molecules, a current density is induced when the molecule is subjected to an external magnetic field. In turn, this current density creates a particular magnetic field. In this work, the bifurcation value of the induced magnetic field is analyzed in a representative set of aromatic, non-aromatic and antiaromatic monocycles, as well as a set of polycyclic hydrocarbons. The results show that the bifurcation value of the ring-shaped domain adequately classifies the studied molecules according to their aromatic character. For aromatic and nonaromatic molecules, it is possible to analyze two ring-shaped domains, one diatropic (inside the molecular ring) and one paratropic (outside the molecular ring). Meanwhile, for antiaromatic rings, only a diatropic ring-shaped domain (outside the molecular ring) is possible to analyze, since the paratropic domain (inside the molecular ring) is irreducible with the maximum value (attractor) at the center of the molecular ring. In some of the studied cases, i. e., in heteroatomic species, bifurcation values do not follow aromaticity trends and present some inconsistencies in comparison to ring currents strengths, showing that this approximation provides only a qualitative estimation about (anti)aromaticity.     

An energetic measure of aromaticity and antiaromaticity based on the Pauling-Wheland resonance energies

Various criteria based on geometric, energetic, magnetic, and electronic properties are employed to delineate aromatic and antiaromatic systems. The recently proposed block-localized wave function (BLW) method evaluates the original Pauling-Wheland adiabatic resonance energy (ARE), defined as the energy difference between the real conjugated system and the corresponding virtual most stable resonance structure. The BLW-derived ARE of benzene is 57.5 kcal mol(-1) with the 6-311+G** basis set. Kistiakowsky's historical experimental evaluation of the stabilization energy of benzene (36 kcal mol(-1)), based on heats of hydrogenation, seriously underestimates this quantity due to the neglect of the partially counterbalancing hyperconjugative stabilization of cyclohexene, employed as the reference olefin (three times) in Kistiakowsky's evaluation. Based instead on the bond-separation-energy reaction involving ethene, which has no hyperconjugation, as well as methane and ethane, the experimental resonance energy of benzene is found to be 65.0 kcal mol(-1). We derived the "extra cyclic resonance energy" (ECRE) to characterize and measure the extra stabilization (aromaticity) of conjugated rings. ECRE is the difference between the AREs of a fully cyclically conjugated compound and an appropriate model with corresponding, but interrupted (acyclic) conjugation. Based on 1,3,5-hexatriene, which also has three double bonds, the ECRE of benzene is 36.7 kcal mol(-1), whereas based on 1,3,5,7-octatetraene, which has three diene conjugations, the ECRE of benzene is 25.7 kcal mol(-1). Computations on a series of aromatic, nonaromatic, and antiaromatic five-membered rings validate the BLW-computed resonance energies (ARE). ECRE data on the five-membered rings (derived from comparisons with acyclic models) correlate well with nucleus-independent chemical shift (NICS) and other quantitative aromaticity criteria. The ARE of cyclobutadiene is almost the same as butadiene but is 10.5 kcal mol(-1) less than 1,3,5-hexatriene, which also has two diene conjugations. The instability and high reactivity of cyclobutadiene thus mainly result from the sigma-frame strain and the pi-pi Pauli repulsion.     

Proton transfers in aromatic and antiaromatic systems. How aromatic or antiaromatic is the transition state? An ab initio study

An ab initio study of six carbon-to-carbon identity proton transfers is reported. They refer to the benzenium ion/benzene (C6H7(+)/C6H6), the 2,4-cyclopentadiene/cyclopentadienyl anion (C5H6/C5H5(-)), and the cyclobutenyl cation/cyclobutadiene (C4H5(+)/C4H4) systems and their respective noncyclic reference systems, that is, [structure: see text], [structure: see text] and [structure: see text]. For the aromatic C6H7(+)/C6H6 and C5H6/C5H5(-) systems, geometric parameters and aromaticity indices indicate that the transition states are highly aromatic. The proton-transfer barriers in these systems are quite low, which is consistent with a disproportionately high degree of transition-state aromaticity. For the antiaromatic C4H5(+)/C4H4 system, the geometric parameters and aromaticity indices indicate a rather small degree of antiaromaticity of the transition state. However, the proton-transfer barrier is higher than expected for a transition state with a low antiaromaticity. This implies that another factor contributes to the barrier; it is suggested that this factor is angle and torsional strain in the transition state. The question whether charge delocalization at the transition state might correlate with the development of aromaticity was also examined. No such correlation was found, that is, charge delocalization lags behind proton transfer as is commonly observed in nonaromatic systems involving pi-acceptor groups.     

Ring Currents in Benzo‐ and Benzocyclobutadieno‐Annelated Biphenylene Derivatives

The effect of benzo and benzocyclobutadieno annelation on the current density induced in a series of biphenylene derivatives is examined at the B3LYP/cc‐pVDZ level of theory, by using the CTOCD‐DZ method. Angular benzo annelation increases, whereas linear benzo annelation decreases the intensity of paratropic (antiaromatic) current density along the four‐membered ring of biphenylene. The opposite effect is found for benzocyclobutadieno annelation. It is shown that the extent of local aromaticity of the four‐membered ring in biphenylene congeners can vary from highly antiaromatic to nonaromatic, as a result of different modes of annelation.     

Interatomic magnetizability: a QTAIM-based approach toward deciphering magnetic aromaticity

Interatomic magnetizability provides insight into the extent of electronic current density between two adjacent atomic basins. By studying a number of well-known aromatic, nonaromatic, and antiaromatic molecules, it is demonstrated that interatomic magnetizability (bond magnetizability) not only is able to verify the exact nature of aromaticity/antiaromaticity among different molecules, but also can distinguish the correct aromaticity order among sets of aromatic/antiaromatic molecules. The interatomic magnetizability is a direct measure of the current flux between two adjacent atomic basins and is the first QTAIM-derived index that evaluates aromaticity based on a response property, that is, magnetizability. Bond magnetizability is easy to compute, straightforward to interpret, and can be employed to evaluate the pure π- or σ-orbital contributions to magnetic aromaticity.     

Aromaticity changes along the reaction coordinate connecting the cyclobutadiene dimer to cubane and the benzene dimer to hexaprismane

Aromaticity and reactivity are two deeply connected concepts. Most of the thermally allowed cycloadditions take place through aromatic transition states, while transition states of thermally forbidden reactions are usually less aromatic, if at all. In this work, we perform a numerical experiment to discuss the change of aromaticity that occurs along the reaction paths that connect two antiaromatic units of cyclobutadiene to form cubane and two aromatic rings of benzene to yield hexaprismane. It is found that the aromaticity profile along the reaction coordinate of the [4+4] cycloaddition of two antiaromatic cyclobutadiene molecules goes through an aromatic highest energy point and finishes to an antiaromatic cubane species. Up to our knowledge, this represents the first example of a theoretically and thermally forbidden reaction path that goes through an intermediate aromatic region. In contrast, the aromaticity profile in the [6+6] cycloaddition of two aromatic benzene rings show a slow steady decrease of aromaticity from reactants to the highest energy point and from this to the final hexaprismane molecule a plunge of aromaticity is observed. In both systems, the main change of aromaticity occurs abruptly near the highest energy point, when the distance between the centers of the two rings is about 2.2 Å.     

Aromaticity and Möbius antiaromaticity in monocyclic [11]annulenium cations

Monocyclic [11]annulenium cations, which are experimentally unknown, have been studied primarily via DFT methods but also with some CCSD(T) validation. We have located six minima: two doubly trans (26, 27), one triply trans (28), one singly trans (29), one quintuply trans (trannulene-type, 33), and one all-cis (31). The first three are aromatic, 33 is modestly aromatic, 29 is nonaromatic, and the last is a M?bius antiaromatic species. We also investigated the fusion of various numbers of three-membered rings (3MRs) to the central 11-membered ring (11MR). We found several planar, all-cis-[11]annulenium ion derivatives as well as another M?bius antiaromatic species (52b); for comparison, we also found planar, antiaromatic all-cis-[12]annulene (60) and [15]annulenium cation (61) derivatives. The (anti)aromatic characterization of these compounds is based mainly on calculated magnetic data for the ground singlet and vertical triplet states, although aromatic stabilization energies (ASE) are also considered. Data for optimized triplets, several of which are M?bius aromatic systems (31t, 52t, 63t, 64t), are also included. Several of these cations are reasonable synthetic targets.     

Unique Role of Heterole-Fused Structures in Aromaticity and Physicochemical Properties of 7,8-Dehydropurpurins

Porphyrins with a fused five-membered ring, such as 7,8-dehydropurpurins, have appeared as an emerging class of unique porphyrinoids. Their altered absorption spectra, relatively short lifetimes of excited states, and small HOMO–LUMO gaps arise from the harmony of the antiaromatic 20π-circuit and the aromatic 18π-circuit. In this regard, the electronic properties of 7,8-dehydropurpurins are expected to be controlled by modulating the contribution of the antiaromatic π-circuit to the whole aromaticity. Here the comparison of pyrrole- and phosphole-fused 7,8-dehydropurpurins is reported in terms of their aromaticity and physicochemical properties. The spectroscopic investigation revealed the larger contribution of the antiaromatic 24π-circuit in pyrrole-fused 7,8-dehyrdopurpurins than in phosphole-fused 7,8-dehydropurpurins. The DFT calculations also supported the feasibility of tuning the aromaticity of 7,8-dehydropurpurins by heterole-fused structures. Thus, the introduction of heterole-fused structures into porphyrinoids is a universal strategy to get new insight into aromaticity and their intrinsic properties in cyclic π-conjugated molecules.     

Downfield proton chemical shifts are not reliable aromaticity indicators

[structure: see text] The downfield chemical shifts of arene hydrogens (delta (1)H) are due only in part to the pi ring current contribution [sigma(//)(pi)]; local framework effects are equally important. Neither proton chemical shifts nor even sigma(||)(pi) tensor elements, per se, are reliable aromaticity indicators. Unsaturated polycyclic hydrocarbons with nonaromatic quinoid structures have delta (1)H and sigma(//)(pi) values in the "aromatic range". Conversely, numerous aromatic protons, including those in five-membered ring heterocycles, resonate in the "nonaromatic range".     

Aromaticity and antiaromaticity in fulvenes, ketocyclopolyenes, fulvenones, and diazocyclopolyenes

The structures, energies, natural charges, and magnetic properties of 3-, 5-, 7-, and 9-membered cyclic polyenes 1-4, respectively, with exocyclic methylene, keto, ketenyl, and diazo substituents (a-d, respectively) were computed at the B3LYP/6-311G+ **//B3LYP/6-311+G** level to elucidate their aromatic and antiaromatic properties. The corresponding conjugated cyclic cations le and 3e were also studied. The criteria used are isomerization energies (ISE), magnetic susceptibility exaltations (lambda), aromatic stabilization energies (ASE), nucleus independent chemical shifts (NICS), and bond length alternation (deltaR). Planar C2v structures were found to be the lowest energy minima with the exceptions of diazocyclopropene (1d), cycloheptafulvenone (3c), diazocycloheptatriene (3d), and all of the cyclononatetraene derivatives (4). The fulvenes (1a-4a) have modest aromatic or antiaromatic character, and are used as standards for comparison. By these criteria the ketenylidene and diazo cyclopropenes and cycloheptatrienes 1,3-c,d and oxo cyclopentadiene and cyclononatetraene 2,4b are antiaromatic, while the 5- and 9-ring ketenyl and diazo compounds and 3- and 7-ring ketones are aromatic. The degree of aromatic/antiaromatic character decreases with ring size. The consistent agreement with Hückel rule predictions for all the criteria shows their utility for the evaluation of the elusive properties of aromaticity and antiaromaticity.     

A DFT Study of the Modulation of the Antiaromatic and Open-Shell Character of Dibenzo[a,f]pentalene by Employing Three Strategies: Additional Benzoannulation,BN/CC Isosterism,and Substitution

Dibenzo[a,f]pentalene ( [a , f ]DBP ) is a highly antiaromatic molecule having appreciable open-shell singlet character in its ground state. In this work, DFT calculations at the B3LYP/6-311+G(d,p) level of theory were performed to explore the efficiency of three strategies, that is, BN/CC isosterism, substitution, and (di)benzoannulation of [a , f ]DBP , in controlling its electronic state and (anti)aromaticity. To evaluate the type and extent of the latter, the harmonic oscillator model of aromaticity (HOMA) and aromatic fluctuation (FLU) indices were used, along with the nucleus-independent chemical shift NICS-XY-scan procedure. The results suggest that all three strategies could be employed to produce either the closed-shell system or open-shell species, which may be in the singlet or triplet ground state. Triplet states have been characterized as aromatic, which is in accordance with Baird's rule. All the singlet states were found to have weaker global paratropicity than [a , f ]DBP . Additional (di)benzo fusion adds local aromatic subunit(s) and mainly retains the topology of the paratropic ring currents of the basic molecule. The substitution of two carbon atoms by the isoelectronic BN pair, or the introduction of substituents, results either in the same type and very similar topology of ring currents as in the parent compound, or leads to (anti)aromatic and nonaromatic subunits. The triplet states of all the examined compounds are also discussed.     

Interplay of para- and diatropic ring currents [(anti)aromaticity] of macrocyclic rings subject to conformational influences,further annelation and hydrogenation of aromatic ring moieties

The spatial magnetic properties (Through Space NMR Shieldings—TSNMRS) of a variety of porphyrins, hemiporphyrazines and tetraoxo[8]circulenes have been computed, visualized as Iso-chemical Shielding Surfaces (ICSS) of various size and direction, and were examined subject to the interplay of present (para)-diatropic ring currents [(anti)aromaticity] and influences on the latter property originating from the macrocyclic ring conformation, further annelation and partial to complete hydrogenation of aromatic ring moieties. Caution seems to be indicated when concluding from a single NICS parameter to present (para)diatropic ring currents [(anti)aromaticity].     

Dications of 3-phenyl-indenylidene dibenzo[a.d]cycloheptene: the role of charge in the antiaromaticity of cationic systems

Dications of 9-(3-phenyl-1H-inden-1-ylidene)-5H-dibenzo[a,d]cycloheptene, 5(2+), were prepared by oxidation with SbF(5) in SO(2)ClF, and their magnetic behavior was compared to dications of 9-(3-phenyl-1H-inden-1-ylidene)-9H-fluorene, 2(2+). The good correlation between the experimental (1)H NMR shifts for the dications that were oxidized cleanly and the chemical shifts calculated by the GAIO method supported the use of the nucleus independent chemical shifts, NICS, to evaluate the antiaromaticity of the indenyl systems of 2(2+)/5(2+) and their unsubstituted parent compounds, 6(2+) and 7(2+), as well as the antiaromaticity of the fluorenyl system of 2(2+)/7(2+) and the aromaticity of the dibenzotropylium system of 5(2+)/6(2+). Antiaromaticity was shown to be directly related to the amount of charge in the antiaromatic systems, with the antiaromatic systems more responsive to changes in the calculated NBO charge than the aromatic systems. The antiaromaticity was also shown to be directly related to the amount of delocalization in the ring system. The aromaticity of the dibenzotropylium system was much less responsive to changes in the amount of charge in the tropylium system, because the aromatic system was much more completely delocalized. Thus, antiaromatic species are more sensitive probes of delocalization than aromatic ones.     

Effect on ring current of the Kekulé vibration in aromatic and antiaromatic rings

Derivative current-density maps are used to follow the changes in ring-current (and hence, on the magnetic criterion, the changes in aromaticity) with the Kekulé vibrations of the prototypical aromatic, antiaromatic, and nonaromatic systems of benzene, cyclooctatetraene (COT), and borazine. Maps are computed at the ipsocentric CHF/6-31G**//RHF/6-31G** level. The first-derivative map for benzene shows a growing-in of localized bond currents, and the second-derivative map shows a pure, paratropic "antiring-current", leading to the conclusion that vibrational motion along the Kekulé mode will reduce the net aromaticity of benzene, on average. For planar-constrained D(4h) COT, the Kekulé mode (positive for reduction of bond-length alternation) increases paratropicity at both first and second order, indicating an average increase in antiaromaticity with zero-point motion along this mode. On the ring-current criterion, breathing expansions of benzene and D(4h) COT reduce aromaticity and increase antiaromaticity, respectively.