Magnetic Criteria Of Aromaticity Research Articles

Ag(Sn9-Sn9)]5- and [(η4-Sn9)Ag(η1-Sn9)]7-, as aggregates of spherical aromatic building blocks. Persistence of aromaticity upon cluster gathering.

Formation of cluster-based materials requires a fundamental understanding of the resulting cluster aggregation processes. The Sn94- Zintl-ion structure can be viewed as a building block featuring a spherical aromatic species, leading to a cluster gathering upon oxidative coupling and/or mediated by transition metals. Here, we evaluate the spherical aromatic properties of [Sn9-Sn9]6-, [Ag(Sn9-Sn9)]5- and [(η4-Sn9)Ag(η1-Sn9)]7-, as aggregates of two Sn9 building units held together via oxidative coupling and mediated by a Ag(I) transition metal center. Our results from magnetic criteria of aromaticity show that the inherent spherical aromatic characteristics of the parent Sn94- cluster are persistent in the overall aggregate where the enabled shielding cones ascribed to each Sn9 unit are able to interplay between them, leading to an overlap of the shielding regions. Hence, the two approaches for bringing cluster units together are able to retain the inherent spherical aromatic features for each Sn9 unit, leading to a cluster-based dimer where the parent properties remain. Thus, further cluster-based materials can be envisaged from aggregation upon oxidative coupling and/or mediated by transition metals, where the constituent building blocks retain their initial features, useful to guide the formation of more complex cluster-based aggregates.

Evaluation of Slight Changes in Aromaticity through Electronic and Density Functional Reactivity Theory-Based Descriptors.

Aromaticity is a useful tool to rationalize the structure, stability, and reactivity in several compounds. Although aromaticity is not directly an observable, it is well accepted that electronic delocalization around the molecular ring is a key stabilizing feature of aromatic compounds. This contribution presents a systematic evaluation of the capability of delocalization and reactivity criteria to describe aromaticity in a set of fluorinated benzenes. The aromaticity indices are compared with quantities obtained from the magnetic criteria of aromaticity, i.e., the strength of the ring current induced by an external magnetic field and the popular NICSzz(1) index. In this evaluation, the indices based on delocalization criteria used are aromatic fluctuation index (FLU), para-delocalization index (PDI), PDIπ, and the multicenter delocalization index (MCI). In addition, indices based on the bifurcation values of scalar functions are derived from electron density such as electron localization function (the π contribution, ELFπ) and the π contribution of the localized orbital locator (LOLπ). Furthermore, reactivity indices based on chemical reactivity and the information-theoretic (reactivity) approach are para-linear response (PLR), Shannon entropy, Fisher information, and Ghosh–Berkowitz–Parr (GBP) entropy. The results obtained show that the delocalization-based indicators present a high sensitivity to slight changes in aromaticity and that the reactivity criterion can be considered as a complementary tool for the study of this phenomenon, even when these changes are minimal. These results encourage the use of multiple indicators for a complete understanding of aromaticity in various chemical compounds.

Thiosquaramide-Based Supramolecular Polymers: Aromaticity Gain in a Switched Mode of Self-Assembly.

Despite a growing understanding of factors that drive monomer self-assembly to form supramolecular polymers, the effects of aromaticity gain have been largely ignored. Herein, we document the aromaticity gain in two different self-assembly modes of squaramide-based bolaamphiphiles. Importantly, O → S substitution in squaramide synthons resulted in supramolecular polymers with increased fiber flexibility and lower degrees of polymerization. Computations and spectroscopic experiments suggest that the oxo- and thiosquaramide bolaamphiphiles self-assemble into “head-to-tail” versus “stacked” arrangements, respectively. Computed energetic and magnetic criteria of aromaticity reveal that both modes of self-assembly increase the aromatic character of the squaramide synthons, giving rise to stronger intermolecular interactions in the resultant supramolecular polymer structures. These examples suggest that both hydrogen-bonding and stacking interactions can result in increased aromaticity upon self-assembly, highlighting its relevance in monomer design.

Are Multicentre Bond Indices and Related Quantities Reliable Predictors of Excited-State Aromaticity?

Systematic scrutiny is carried out of the ability of multicentre bond indices and the NOEL-based similarity index dAB to serve as excited-state aromaticity criteria. These indices were calculated using state-optimized complete active-space self-consistent field wavefunctions for several low-lying singlet and triplet states of the paradigmatic molecules of benzene and square cyclobutadiene and the inorganic ring S2N2. The comparison of the excited-state indices with aromaticity trends for individual excited states suggested by the values of magnetic aromaticity criteria show that whereas the indices work well for aromaticity reversals between the ground singlet and first triplet electronic states, addressed by Baird’s rule, there are no straightforward parallels between the two sets of data for singlet excited states. The problems experienced while applying multicentre bond indices and dAB to singlet excited states are explained by the loss of the information inherently present in wavefunctions and/or pair densities when calculating the first-order density matrix.

Can Anti‐Aufbau DFT Calculations Estimate Singlet Excited State Aromaticity? Correspondence on “Dibenzoarsepins: Planarization of 8π‐Electron System in the Lowest Singlet Excited State”

AbstractThe simple anti‐aufbau DFT approach for estimating singlet excited state aromaticity suggested in a recent Communication published in this journal is shown to produce incorrect results because it targets a linear combination of the singlet and triplet configurations involving the HOMO and LUMO rather than the first singlet excited state. If the S1 state of a molecule is dominated by the HOMO→LUMO excitation, a comparably simple but theoretically consistent and qualitatively correct approximation to the S1 wavefunction can be achieved by performing a small “two electrons in two orbitals” CASSCF(2,2) calculation which can be followed by the evaluation of magnetic aromaticity criteria such as NICS.

Searching for double σ- and π-aromaticity in borazine derivatives.

Inspired by the double-aromatic (σ and π) C6H3+, C6I62+, and C6(SePh)62+ ring-shaped compounds, herein we theoretically study their borazine derivative analogues. The systems studied are the cation and dications with formulas B3N3H3+, B3N3Br62+, B3N3I62+, B3N3(SeH)62+, and B3N3(TeH)62+. Our DFT calculations indicate that the ring-shaped planar structures of B3N3H3+, B3N3I62+, and B3N3(TeH)62+ are more stable in the singlet state, while those of B3N3Br62+ and B3N3(SeH)62+ prefer the triplet state. Besides, exploration of the potential energy surface shows that the ring-shaped structure is the putative global minimum only for B3N3I62+. According to chemical bonding analysis, B3N3H3+, B3N3I62+, and B3N3(TeH)62+ have σ and π delocalized bonds. The number of delocalized σ/π electrons is 2/6 for the first, and 10/6 for the second and third, similar to what their carbon analogs exhibit. Finally, the analysis of the magnetically induced current density allows B3N3H3+, B3N3I62+, and B3N3(TeH)62+ to be classified as strongly σ aromatic, and poorly π aromatic compounds.

Effect of the meso/beta halogenation in the photoelectronic properties and aromaticity of expanded porphyrins

A systematic analysis of the photoelectronic properties and aromaticity in meso and beta halogenated expanded porphyrins (Sapphyrin and Orangarin), have been carried out by means of Time Dependent Density Functional Theory (TD-DFT) and magnetic criteria of aromaticity. Results indicate that there is a notorious redshift in the Soret-type (Q-type) bands. The electronic absorption of meso-substituted compounds showed a strong redshift in aromatic porphyrins compared to its beta counterpart and can be attributed to the lower energy gap. For antiaromatic derivatives an inverse trend is observed where beta compounds have a greater redshift than meso compounds. Additionally, major redshifts are observed for the Q bands in iodine-derived compounds. Magnetic aromaticity analysis showed that there is a systematic decrease for aromatic derivatives, following a periodical trend (F to I), whereas a decrease in antiaromaticity for orangarin derivatives is found. Finally, correlations have been found between the energy gaps and Q-type bands of the aromatic porphyrins studied. Halogenation strategy for enhancing the properties in aromatic/antiaromatic porphyrins could be useful in the design of chromophores with potential application in dye sensitized solar cells.

An Additivity Scheme for Aromaticity: The Heteroatom Case.

The nucleus-independent chemical shift (NICS)-XY-Scan is a simple and easy tool for the quantitative measurement of the aromaticity of polycyclic aromatic hydrocarbons and identification of the existence of local and global ring currents. We recently introduced an additivity scheme that uses the NICS-XY-Scans of smaller building blocks to predict the aromatic profiles of larger polycyclic aromatic hydrocarbon systems. We now report on an expansion of the methodology to include systems of varying aromatic natures containing the heteroatoms B, N, O, and S. The additivity approach allows for rapid and resource-efficient generation of NICS-XY-Scans of large, complex systems. Moreover, it reveals that the magnetic criterion of aromaticity behaves in an additive manner, and that the ring currents of multi-ring systems appear to be mostly localized within subunits of up to three rings.

Effects on the aromatic character of DNA/RNA nucleobases due to its adsorption onto graphene

AbstractIn the last years, experimental/theoretical studies have shown that graphene has a strong affinity toward nucleobases, serving as a promising nanomaterial for self‐assembly, sensing, and/or sequencing of DNA/RNA constituents. Then, a complete picture of the properties of the nucleobase–graphene systems is required for its use in technological applications. This work describes a detailed quantum chemical analysis of the aromaticity of adsorbed nucleobases on graphene, comparing between aromaticity indexes based on magnetic, geometry, electron density, and electron delocalization properties of graphene–nucleobase systems. Contrary to the stated by magnetic‐based aromaticity criteria (such as nucleus‐independent chemical shifts), it is proved that the aromatic character of nucleobases is not increased/decreased upon binding on graphene. Therefore, magnetic aromaticity criteria are not recommended to analyze aromaticity in related systems, unless a fragmented scheme be adopted. Finally, these results are expected to expand the knowledge about the understanding of biomolecules‐graphene interactions.

Graph Theory of Ring-Current Diamagnetism

Abstract Various magnetic criteria of aromaticity have been proposed so far, because they are easily calculated and applicable to a variety of cyclic π-systems. Many researchers, however, are reluctant to accept all or some of them. Our graph theory of ring-current diamagnetism revealed serious flaws in familiar magnetic criteria of aromaticity. Physically meaningful information on aromaticity can nevertheless be extracted from the ring-current diamagnetism. In particular, magnetic resonance energy (MRE), derived from the ring-current diamagnetic susceptibility, is interpretable as a kind of aromatic stabilization energy (ASE) and helps in consistently interpreting energetic and magnetic criteria of aromaticity. MRE and related quantities can be determined without reference to any hypothetical polyene-like structure but with a reasonable assumption that, when a magnetic field is applied to a cyclic π-system, a current is induced in each circuit in proportion to the ASE arising from the circuit. Unlike other conventional magnetic indexes, they are independent of the size and shape of the π-system.

Redox-Switchable 20π-, 19π-, and 18π-Electron 5,10,15,20-Tetraaryl-5,15-diazaporphyrinoid Nickel(II) Complexes.

The first examples of air-stable 20π-electron 5,10,15,20-tetraaryl-5,15-diaza-5,15-dihydroporphyrins, their 18π-electron dications, and the 19π-electron radical cation were prepared through metal-templated annulation of nickel(II) bis(5-arylamino-3-chloro-8-mesityldipyrrin) complexes followed by oxidation. The neutral 20π-electron derivatives are antiaromatic and the cationic 18π-electron derivatives are aromatic in terms of the magnetic criterion of aromaticity. The meso N atoms in these diazaporphyrinoids give rise to characteristic redox and optical properties for the compounds that are not typical of isoelectronic 5,10,15,20-tetraarylporphyrins.

Redox‐Switchable 20π‐, 19π‐, and 18π‐Electron 5,10,15,20‐Tetraaryl‐5,15‐diazaporphyrinoid Nickel(II) Complexes

AbstractThe first examples of air‐stable 20π‐electron 5,10,15,20‐tetraaryl‐5,15‐diaza‐5,15‐dihydroporphyrins, their 18π‐electron dications, and the 19π‐electron radical cation were prepared through metal‐templated annulation of nickel(II) bis(5‐arylamino‐3‐chloro‐8‐mesityldipyrrin) complexes followed by oxidation. The neutral 20π‐electron derivatives are antiaromatic and the cationic 18π‐electron derivatives are aromatic in terms of the magnetic criterion of aromaticity. The meso N atoms in these diazaporphyrinoids give rise to characteristic redox and optical properties for the compounds that are not typical of isoelectronic 5,10,15,20‐tetraarylporphyrins.

A critical approach toward resonance-assistance in the intramolecular hydrogen bond interaction of 3,5-diiodosalicylic acid: a spectroscopic and computational investigation.

The photophysics of a prospective drug molecule, 3,5-diiodosalicylic acid (3,5-DISA), having a wide spectrum of biological and medicinal applications, have been investigated using spectroscopic techniques and computational analyses. The remarkably large Stokes' shifts in various solvents from 3,5-DISA has been intertwined with the occurrence of an excited-state intramolecular proton transfer (ESIPT) reaction. Concurrently, the emergence of an intriguing dual emission feature in less interacting solvents is also reported and the spectral response of 3,5-DISA toward the variation of medium acidity/basicity has been exploited to decipher the nature of various species present in different solvents. Our experimental results, unveiling the occurrence of an ESIPT reaction in 3,5-DISA, have been aptly substantiated from computational studies in which the operation of ESIPT has been explored from structural as well as energetics (analysis of potential energy surface (PES)) perspectives. A major focus of the present study is on the evaluation of the intramolecular hydrogen bond (IMHB) interaction in 3,5-DISA, including the application of various methodologies to estimate the IMHB energy and subsequently, an in-depth analysis of the IMHB interaction reveals its partially covalent nature through the application of advanced quantum chemical tools, e.g., the natural bond orbital (NBO) method. In this context, the interplay between the aromaticity of the benzene nucleus and the IMHB energy has been rigorously explored, showing indications for the occurrence of resonance-assisted hydrogen bonding (RAHB) in 3,5-DISA. To this end, the geometric as well as magnetic criteria of aromaticity have been analyzed.

Magnetic criteria of aromaticity.

This review describes the current state of magnetic criteria of aromaticity. The introduction contains the fundamentals of ring currents in aromatic and antiaromatic systems, followed by a brief description of experimental and computational tools: NMR, diamagnetic susceptibility exaltation, current density analyses (CDA) and nucleus independent chemical shifts (NICS). This is followed by more comprehensive chapters: NMR - focusing on the work of R. Mitchell - NICS and CDA - describing the progress and development of the methods to their current state and presenting some examples of representative work.

Aromaticity of Closed-Shell Charged Polybenzenoid Hydrocarbons

The aromatic stabilization of closed-shell charged polybenzenoid hydrocarbons (PBHs) has been scrutinized by means of energetic and magnetic aromaticity criteria and by direct measures of electron delocalization. Thus, topological resonance energies and their circuit contributions, ring current maps, and multicenter delocalization indices have been calculated for a series of 18 polybenzenoid cations containing from 3 to 10 benzene rings. All calculations indicate that the closed-shell cations have a similar degree of aromaticity compared to that of the corresponding closed-shell neutral PBHs. All cations investigated display a large degree of electronic delocalization in the ring, accompanied by significant aromatic stabilization and a strong diatropic peripheral electron current. Graph theoretical models describe perfectly the aromatic features of these hydrocarbon fragments, showing how they can be understood as a superposition of specific neutral PBHs. The large aromatic character of these systems suggests they may be relatively stable upon formation at combustion conditions, like those given in the interstellar medium. It has been postulated that closed-shell fragments of PBHs may play an important role in the photoluminescent phenomenon known as extended red emission.

Face-to-Face Stacks of Trinuclear Gold(I) Trihalides with Benzene, Hexafluorobenzene, and Borazine: Impact of Aromaticity on Stacking Interactions

The interplay of electrostatics, charge transfer, and dispersion forces contributing to the interaction energies in 1:1, 1:2, and 2:1 binary stacks of the c-Au(3)(μ(2)-X)(3) (X = F, Cl, Br, I) clusters with benzene, hexafluorobenzene, or borazine were investigated by employing a multitude of electronic structure computational techniques. The molecular and electronic structures, stabilities, bonding features, and magnetotropicity of [c-Au(3)(μ(2)-X)(3)](n)(L)(m) (X = halide; L = C(6)H(6), C(6)F(6), B(3)N(3)H(6); n, m ≤ 2) columnar binary stacks have been investigated by DFT calculations employing the M05-2X functional. The novel binary stacks could be considered as the building blocks of extended columnar supramolecular assemblies formulated as {[c-Au(3)(μ(2)-X)(3)](C(6)H(6))}(∞), {[c-Au(3)(μ(2)-X)(3)](2)(C(6)F(6))}(∞), and {[c-Au(3)(μ(2)-X)(3)](B(3)N(3)H(6))(2)}(∞). In all binary stacks, with a few exceptions, the plane of the alternating c-Au(3)(μ(2)-X)(3) and L (C(6)H(6), C(6)F(6), B(3)N(3)H(6)) stacking participants adopt an almost parallel face-to-face (pff) orientation. The observed trends in the intermolecular distances R in the [c-Au(3)(μ(2)-X)(3)](n)(L)(m) (X = halide; L = C(6)H(6), C(6)F(6), B(3)N(3)H(6); n, m ≤ 2) columnar binary stacks are explained by the diverse intermolecular interactions characterizing the stacks, since the three ligands L and the c-Au(3)(μ(2)-X)(3) cyclic trinuclear clusters (CTCs) exhibit diverse physical properties being important determinants of the intermolecular interactions (consisting of covalent, electrostatic, and dispersion forces). The properties considered are the zz tensor components of quadrupole moment, Q(zz), polarizability, α(zz), nucleus-independent chemical shift, NICS(zz)(1), along with the molecular electrostatic potential, MEP(0), and surface area (S). Energy decomposition analysis (EDA) at the revPBE-D3/TZ2P level revealed that the dominant term in the stacking interactions arises mainly from dispersion and electrostatic forces, while the contribution of covalent interactions are predicted to be small. On the other hand, charge decomposition analysis (CDA) illustrated very small charge transfer from the L stacking participants toward the c-Au(3)(μ(2)-X)(3) clusters. Excellent linear correlations of the interaction energy, ΔE(int), and its components (ΔE(disp), ΔE(elstat), ΔE(orb), and ΔE(Pauli)) with calculated physical properties related to dispersion, covalent, and electrostatic forces have been established. The most important finding is the excellent linear relationship between ΔE(int) and the NICS(zz)(1) magnetic criterion of aromaticity, indicating that ΔE(int) is also affected by the coupling of the induced magnetic fields of the interacting stacking participants. The magnetotropicity of the binary stacks evaluated by the NICS(zz)-scan curves indicated an enhancement of the diatropicity in the space between the interacting inorganic and organic rings, probably due to the superposition of the diamagnetic ring currents of the interacting ring systems. The energy splitting in dimer (ESID) model was employed to estimate the charge transport of electrons and holes between the ligands L and the [c-Au(3)(μ(2)-X)(3)] clusters in [c-Au(3)(μ(2)-X)(3)](L) 1:1 binary stacks.

Capturing the Antiaromatic #6094C68 Carbon Cage in the Radio‐Frequency Furnace

Although all fullerenes do not satisfy the classical aromaticity condition, as a result of their nonplanar nature, they experience effective stabilization due to extensive cyclic π-electron delocalization and exhibit pronounced "spherical aromaticity". This feature has raised the question of the opposite phenomenon, that is, the existence of antiaromatic carbon cages. Here the first experimental evidence of the existence of antiaromatic fullerenes is reported. The elusive (#6094)C(68) was effectively captured as C(68)Cl(8) by in situ chlorination in the gas phase during radio-frequency synthesis. The chlorinated cage was separated by means of multistage HPLC, and its connectivity unambiguously determined by single-crystal X-ray analysis. Halogen-stripped pristine (#6094)C(68) was monitored by mass spectrometry of the chlorinated C(68)Cl(8) cage. Quantum chemical calculations reveal the highly antiaromatic character of (#6094)C(68), in accordance with all geometric, energetic, and magnetic criteria of aromaticity. Chlorine addition leads to substantial stabilization of the cage owing to aromatization in the resulting C(68)Cl(8), which explains its high abundance in the primary fullerene soot. This work provides new insights into the process of fullerene formation and better understanding of aromaticity phenomena in general.

Magnetic criteria of aromaticity in a benzene cation and anion: how does the Jahn–Teller effect influence the aromaticity?

The aromatic/antiaromatic behavior of the Jahn–Teller (JT) active benzene cation and anion has been investigated using Density Functional Theory (DFT) calculations of Nuclear Independent Chemical Shifts (NICS) and magnetic susceptibility. NICS parameters have been scanned along the Intrinsic Distortion Path (IDP) for the benzene cation showing antiaromaticity which decreases with increasing deviation from D6h to D2h symmetry. Changes in NICS values along the IDP from D6h to C2v in the benzene anion revealed non-aromatic character.

ChemInform Abstract: The Large closo-Borane Dianions, BnHn2- (n = 13-17) Are Aromatic, Why Are They Unknown?

The relative stabilities of the unknown larger closo-borane dianions B(n)()H(n)()(2)(-) (n = 13-17), were evaluated at the B3LYP/6-31G level of density functional theory by comparing the average energies, E/n, and also by the energies using the model equation: B(n)()(-)(1)H(n)()(-)(1)(2)(-) + B(6)H(10) --> B(n)()H(n)()(2)(-) + B(5)H(9) (n = 6-17). Starting with the small closo-borane, B(5)H(5)(2)(-), the sequential addition of BH groups is represented by formal transfer from B(6)H(10) to build up larger and larger clusters. Most of the energies for these sequential steps are exothermic, but not for the B(12)H(12)(2)(-) to B(13)H(13)(2)(-) and the B(14)H(14)(2)(-) to B(15)H(15)(2)(-) stages. The cumulative total energies (DeltaH(add)) of these BH group additions, based on B(5)H(5)(2)(-) as the reference zero, tend to increase with increasing cluster size. DeltaH(add) indicates that the larger unknown closo-boranes B(13)H(13)(2)(-) to B(17)H(17)(2)(-) are more stable than B(9)H(9)(2)(-), B(10)H(10)(2)(-), and B(11)H(11)(2)(-); this agrees with E/n and with Lipscomb's earlier conclusion based on the PRDDO average energies. B(13)H(13)(2)(-), B(14)H(14)(2)(-), and B(15)H(15)(2)(-) are less stable than B(12)H(12)(2)(-), which has the lowest average energy on a per vertex basis among the closo-borane dianions. However, the total DeltaH(add) treatment indicates the larger B(16)H(16)(2)(-) and B(17)H(17)(2)(-) to be favorable relative to B(12)H(12)(2)(-), because of the larger number of vertexes. The formation of B(13)H(13)(2)(-) from B(12)H(12)(2)(-) is especially unfavorable. The further formation of B(14)H(14)(2)(-) and B(15)H(15)(2)(-) via BH transfer also is endothermic. These are not the only thermodynamic difficulties in building up large closo-borane dianions beyond B(12)H(12)(2)(-). The highly exothermic disproportionation of larger and smaller closo-borane dianions, e.g., B(12+)(n)()H(12+)(n)()(2)(-) + B(12)(-)(n)()H(12)(-)(n)()(2)(-) --> 2B(12)H(12)(2)(-) (n = 1-5), also indicate possible synthetic problems in preparing larger closo-boranes with more than 12 vertexes under condition where smaller boranes are present. All the larger closo-B(n)()H(n)()(2)(-) (n = 13-17) cluster exhibit three-dimensional aromaticity, judging from the computed Nucleus Independent Chemical Shifts (NICS), which range from -30.9 to -36.5 ppm. The trends in NICS values are similar to the variations in the bond length alternations, Deltar. Thus, the qualitative relationships between geometric and magnetic criteria of aromaticity found earlier for the smaller clusters extends to the larger closo-borane dianions, B(n)()H(n)()(2)(-) (n = 13-17).

Topological resonance energy, bond resonance energy, and circuit resonance energy

AbstractCircuit resonance energy (CRE) is a key quantity that links energetic and magnetic criteria of aromaticity for a polycyclic π‐system. Topological resonance energy (TRE) correlates well with the sum of CREs for an entire π‐system. We found that bond resonance energy (BRE), so far defined to estimate the degree of kinetic stability for a polycyclic π‐system, also correlates very well with the corresponding quantity defined with CREs. These findings justify the view that graph‐theoretical quantities, such as TRE and BRE, can be used safely even though they rely on unrealistic reference structures. Copyright © 2007 John Wiley & Sons, Ltd.