Harmonic Oscillator Model Of Aromaticity Index Research Articles

Harmonic oscillator model of aromaticity (HOMA) in conjugated radicals and cations

We propose the extension of Harmonic Oscillator Model of Aromaticity (HOMA), reparametrizing it for radicals and cations. The application is based on a series of experimental structures based on phenalenyl core, found in both radical and cationic forms. Phenalenyl is the first member of the topological spin molecules named triangulenes, looking like triangular graphene flakes, the series being indexed by the number of hexagonal rings at the edge. We discussed the series of triangulenes with n = 2,3 and 4 sizes. In this view, a HOMA parameterization based on computed structures is devised. We found that phenalenyl systems (the n = 2 triangulene) are relatively high aromatic in both radical and cationic forms, with the experimental structures having HOMA indices comprised between 0.89 and 0.99. Along the n-triangulene series, the global aromaticity of radicals decreases, having HOMA = 0.965, 0.932 and 0.925, for computed structures of n = 2, 3 and 4 congeners, respectively, tending to a plateau estimated at HOMA = 0.914 for hypothetical larger systems. The HOMA indices are approximately correlated with the aromatic stabilization estimated in a spin-coupling phenomenology.

Influence of the Solvent on the Stability of Aminopurine Tautomers and Properties of the Amino Group

Amino derivatives of purine (2-, 6-, 8-, and N-NH2) have found many applications in biochemistry. This paper presents the results of a systematic computational study of the substituent and solvent effects in these systems. The issues considered are the electron-donating properties of NH2, its geometry, π-electron delocalization in purine rings and tautomeric stability. Calculations were performed in ten environments, with 1 < ε < 109, using the polarizable continuum model of solvation. Electron-donating properties were quantitatively described by cSAR (charge of the substituent active region) parameter and π-electron delocalization by using the HOMA (harmonic oscillator model of aromaticity) index. In aminopurines, NH2 proximity interactions depend on its position and the tautomer. The results show that they are the main factor determining how solvation affects the electron-donating strength and geometry of NH2. Proximity with the NH∙∙∙HN repulsive interaction between the NH2 and endocyclic NH group results in stronger solvent effects than the proximity with two attractive NH∙∙∙N interactions. The effect of amino and nitro (previously studied) substitution on aromaticity was compared; these two groups have, in most cases, the opposite effect, with the largest being in N1H and N3H purine tautomers. The amino group has a smaller effect on the tautomeric preferences of purine than the nitro group. Only in 8-aminopurine do tautomeric preferences change: N7H is more stable than N9H in H2O.

Assessment of the performance of six indices in predicating the aromaticity of planar porphyrinoids.

Aromaticity is a fundamental chemical concept that has been widely used in explaining the reactivity, stability, structure, and magnetic properties of many molecules such as conjugated macrocycles, metal heterocyclic compounds, and certain metal clusters. Porphyrinoids (including porphyrin) are of particular interest in terms of diverse aromaticity. Various indices therefore have been used to predict the aromaticity of porphyrin-like macrocycles. However, the reliability of these indices for porphyinoids is always questionable. In order to assess the performance of the indices, we have selected six representative indices to predict the aromaticity of 35 porphyrinoids. The calculated values were then compared with the corresponding results obtained from experiments. Our studies suggest that the theoretical prediction by nucleus independent chemical shifts (NICS), topology of the induced magnetic field (TIMF), anisotropy of the induced current density (AICD), and gauge including magnetically induced current method (GIMIC) are essentially consistent with experimental evidence in all 35 cases and thus are preferred indices. Based on density functional theory, the performance of the NICS, TIMF, AICD, GIMIC, harmonic oscillator model of aromaticity (HOMA), and multicenter bond order (MCBO) indices were evaluated theoretically. Molecular geometries were optimized at the M06-2X/6-311G** level. NMR calculations using GIAO or CGST method were performed at the M06-2X/6-311G** level. The above calculations were carried out using Gaussian16 suite. The TIMF, GIMIC, HOMA, and MCBO indices were obtained using the Multiwfn program. The AICD outputs were visualized using the POV-Ray software.

Investigation of intramolecular hydrogen bonding in naphthoquinone derivatives by quantum chemical calculations

AbstractIntramolecular hydrogen‐bonded naphthoquinone derivatives are investigated using both DFT (density functional theory) and MP2 (Møller–Plesset perturbation theory) method. Three different sets of naphthoquinone derivatives, each set consisting of syn and anti conformers with respect to the substitution by (–OH/–NH2) at peri and para position of the ring, are considered for investigation. In all cases, the syn conformer is found to be energetically more stable compared with the anti conformer. However, the hydrogen bond strength of the anti conformer is found to be more as obtained from NBO (natural bond orbital) analysis and QTAIM (quantum theory of atoms in molecules) theory. The O···H–O interactions are associated with covalent character while O···H–N interactions are noncovalent in nature. The NCI (noncovalent interaction) isosurface clearly indicates the formation of O···H–O and O···H–N intramolecular hydrogen bond. The energy partition analysis indicates electrostatic interaction as the dominant force in stabilizing these systems. The aromaticity calculation by HOMA (harmonic oscillator model of aromaticity) index and Bird index indicate the syn conformer with higher aromaticity compared to the anti conformer. The overall stability of the syn conformers is thus dominated by the aromatic stabilization energy instead of hydrogen bonding energy. The global reactivity descriptors further support the higher stability of the syn conformers according to maximum hardness principle (MHP) and minimum electrophilicity principle (MEP).

Density functional estimation of hydride and proton affinities of substituted allenes and heteroallenes

AbstractThe hydride affinity (HA) of 20 allene and heteroallene derivatives containing unsaturated rings such as cyclopentadiene (CP) ring has been computationally investigated using the B3LYP/6‐311++G(3df, 2p) method. The interaction of hydride ion (H−) with the central sp1 carbon of the designed molecules containing electron‐withdrawing groups on the CP ring has been highly desirable, and their HA values were obtained in the range of 369–788 kJ/mol. The aromaticity of CP anion has the key role in the HA increasing and its stability. The harmonic oscillator model of aromaticity (HOMA) and nucleus‐independent chemical shifts (NICS) values associated with the CP ring were calculated and compared before and after the interaction with H−. The calculated data indicated a sharp increase in HOMA and NICS indices for the anions. In the following, the protonation of three designated allenes containing a cyclopropene or cycloheptatriene and a CP ring was also investigated using the density functional theory calculations. Electrostatic potential maps for some key structures were also used for charge distribution analysis.

Substituent Effect in the First Excited Triplet State of Monosubstituted Benzenes.

The structure of 30 monosubstituted benzenes in the first excited triplet T1 state was optimized with both unrestricted (U) and restricted open shell (RO) approximations combined with the ωB97XD/aug-cc-pVTZ basis method. The substituents exhibited diverse σ- and π-electron-donating and/or -withdrawing groups. Two different positions of the substituents are observed in the studied compounds in the T1 state: one distorted from the plane and the other coplanar with a quinoidal ring. The majority of the substituents are π-electron donating in the first group while π-electron withdrawing in the second one. Basically, U- and RO-ωB97XD approximations yield concordant results except for the B-substituents and a few of the planar groups. In the T1 state, the studied molecules are not aromatic, yet aromaticity estimated using the HOMA (harmonic oscillator model of aromaticity) index increases from ca. −0.2 to ca. 0.4 with substituent distortion, while in the S1 state, they are only slightly less aromatic than in the ground state (HOMA ≈0.8 vs ≈1.0, respectively). Unexpectedly, the sEDA(T1) and pEDA(T1) substituent effect descriptors do not correlate with analogous parameters for the ground and first excited singlet states. This is because in the T1 state, the geometry of the ring changes dramatically and the sEDA(T1) and pEDA(T1) descriptors do not characterize only the functional group but the entire molecule. Thus, they cannot provide useful scales for the substituents in the T1 states. We found that the spin density in the T1 states is accumulated at the Cipso and Cp atoms, and with the substituent deformation angle, it nonlinearly increases at the former while decreases at the latter. It appeared that the gap between singly unoccupied molecular orbital and singly occupied molecular orbital (SUMO-SOMO) is determined by the change of the SOMO energy because the former is essentially constant. For the nonplanar structures, SOMO correlates with the torsion angle of the substituent and the ground-state pEDA(S0) descriptor of the π-electron-donating substituents ranging from 0.02 to 0.2 e. Finally, shapes of the SOMO-1 instead of SOMO frontier orbitals in the T1 state somehow resemble the highest occupied molecular orbital ones of the S0 and S1 states. For several planar systems, the shape of the U- and RO-density functional theory-calculated SOMO-1 orbitals differs substantially.

Three Queries about the HOMA Index.

HOMA (Harmonic Oscillator Model of Aromaticity) is a simple, successful, and widely used geometrical aromaticity index. However, HOMA can also be used as a general molecular descriptor appropriate for any type of molecule. It reaches the global maximum for benzene, whereas the potent magnetic aromaticity NICS index has no lower or upper limits. Hence, questions arise and go beyond mere differences between the geometric and magnetic aspects of aromaticity: (1) Does a molecule of aromaticity greater than that of benzene, but undisclosed by the HOMA definition, exist? (2) Can the Kekuléne cyclohexatriene moiety with HOMA = 0 exist as a part of a larger system? (3) Can the geometrical aromaticity index be defined better? Our answer to the first query is “It is not likely enough”, to the second, “Why not define HOMA using a less mysterious molecule than cyclohexatriene?”, and to the third, “It is possible to construct another fair geometrical index, but is it better for evaluating aromaticity?” To find these answers, we have studied: (1) the HOMA and NICS indices of over 50 hexahomosubstituted benzenes, (2) the HOMA, as well as EN and GEO, indices of over 100 triply fused hexasubstituted benzenes, and (3) the HOMA and new Geometrical Auxiliary Index (GAI) , of different unsaturated and saturated, aromatic and aliphatic hydrocarbons including all alkane constitutional isomers composed of up to nine carbon atoms.

Investigation of molecular structure and solvent/temperature effect on tautomerism in (E)-4,6-dibromo-3-methoxy-2-[(p-tolylimino)methyl]phenol, a new thermochromic Schiff base, by using XRD, FT-IR, UV-vis, NMR and DFT methods.

The molecular structure and the solvent/temperature effect on the tautomerism in a new Schiff base, (E)-4,6-dibromo-3-methoxy-2-[(p-tolylimino)methyl]phenol, were investigated using spectroscopic (NMR, UV-vis, FT-IR), crystallographic (XRD), computational (DFT and TD-DFT) methods and harmonic oscillator model of aromaticity (HOMA). The XRD, DFT and FT-IR results show that the compound exists in the phenol-imine form in the solid state. HOMA indices support the aromatic structure of the compound. DFT calculations were performed to understand proton transfer process and relatively close values were obtained for the energies of tautomers. UV-vis studies prove the solvent dependence of the tautomerism in the compound by revealing the existence of both phenol-imine and keto-amine forms in polar solvents and only the phenol-imine form in apolar solvent. The TD-DFT results for the electronic transitions lead to the same conclusion as the absorption spectra. 1H NMR and 13C NMR studies at room and low (-60 °C) temperatures indicate that the tautomeric equilibrium occurs rapidly in the compound. Therefore, it is difficult to observe two tautomers. However, the presence of tautomeric structures is clearly seen in acetone‑d6, alternatively underlying the solvent and temperature dependence of tautomerism in the title compound.

Supramolecular structures of rhenium(I) complexes mediated by ligand planarity via the interplay of substituents.

The crystal and molecular structures of two ReI tricarbonyl complexes, namely fac-tricarbonylchlorido[1-(4-fluorocinnamoyl)-3-(pyridin-2-yl-κN)pyrazole-κN2]rhenium(I), [ReCl(C17H12FN3O)(CO)3], (I), and fac-tricarbonylchlorido[1-(4-nitrocinnamoyl)-3-(pyridin-2-yl-κN)pyrazole-κN2]rhenium(I) acetone monosolvate, [ReCl(C17H12ClN4O3)(CO)3]·C3H6O, (II), are reported. The complexes form centrosymmetric dimers that are linked into one-dimensional columns by C-H...Cl and N-O...H interactions in (I) and (II), respectively. C-H...Cl interactions in (II) generate two R21(7) loops that merge into a single R21(10) loop. These interactions involve the alkene, pyrazole and benzene rings, hence restricting the ligand rotation and giving rise to a planar conformation. Unlike (II), complex (I) exhibits a twisted conformation of the ligand and a pair of molecules forms a centrosymmetric dimer with an R22(10) loop via C-H...O interactions. The unique supramolecular structures of (I) and (II) are determined by their planarity and weak interactions. The planar conformation of (II) provides a base for appreciable π-π stacking interactions compared to (I). In addition, an N-O...π interaction stabilizes the supramolecular structure of (II). We report herein the first n→π* interactions of ReI tricarbonyl complexes, which account for 0.33 kJ mol-1. Intermolecular C-H...Cl and C-H...O interactions are present in both complexes, with (II) showing a greater preference for these interactions compared to (I), with cumulative contributions of 48.7 and 41.5%, respectively. The influence of inductive (fluoro) and/or resonance (nitro) effects on the π-stacking ability was further supported by LOLIPOP (localized orbital locator-integrated π over plane) analysis. The benzene ring of (II) demonstrated a higher π-stacking ability compared to that of (I), which is supported by the intrinsic planar geometry. The HOMA (harmonic oscillator model of aromaticity) index of (I) revealed more aromaticity with respect to (II), suggesting that NO2 greatly perturbed the aromaticity. The Hirshfeld fingerprint (FP) plots revealed the preference of (II) over (I) for π-π contacts, with contributions of 6.8 and 4.4%, respectively.

Aromaticity of peri- and para-Substituted Naphthalene-1-carbaldehyde. Comparison with 1-Nitronaphthalene.

Aromaticity of the naphthalene ring substituted by formyl or nitro and secondary amine groups in peri and para positions has been investigated. HOMA (harmonic oscillator model of aromaticity) index of the naphthalene rings has been used to characterize the aromaticity of the investigated molecules. Ellipticity of C2-C3 (C6-C7) bonds, obtained by means of topological analysis of the electron density, has been used as a measure of the through-resonance effect between the para substituents. Dependence of the HOMA values on the rotation angle of formyl and nitro groups to the aromatic plane for naphthalene-1-carbaldehyde and their derivatives with dimethylamino group in para position has been analyzed. It has been shown that the nitro group has a stronger resonance effect and causes greater decrease of aromaticity than the formyl group only for planar or close to planar conformation. The attractive and repulsive peri interactions have been analyzed by using the QTAIM and NCI methods. Interrelation between the peri and para interactions has been discussed.

Toward Molecular Magnets of Organic Origin via Anion-π Interaction Involving m-Aminyl Diradical: A Theoretical Study.

Here we study a set of novel magnetic organic molecular species with different halide ions (fluoride, chloride, bromide) absorbed ∼2 Å above or below the center of an aromatic π-ring in an m-aminyl diradical. Focus is on the nature of anion-π interaction and its impact on magnetic properties, specifically on magnetic anisotropy and on intramolecular magnetic exchange coupling. In the development of single molecule magnets, magnetic anisotropy is considered to be the most influential factor. A new insight regarding the magnetic anisotropy that determines the barrier height for relaxation of magnetization of m-aminyl diradical-derived anionic complexes is obtained from calculations of the axial zero-field-splitting (ZFS) parameter D. The noncovalent anion-π interaction strongly influences magnetic anisotropy in m-aminyl-halide diradical complexes. In particular, the change of D values from positive (for the m-aminyl diradical, m-aminyl diradical/fluoride, and m-aminyl diradical/chloride complexes) to negative D-values in m-aminyl diradical complexes containing bromide signals a change from oblate to prolate type of spin-density distribution. Furthermore, the noncovalent halide-π interactions lead to large values of intramolecular magnetic exchange coupling coefficients J exhibiting a ferromagnetic sign. The magnitude of J steadily increases going from anionic complexes containing fluoride to chloride and then to bromide. Relations are sought between the magnetic exchange coupling coefficients J and aromaticity, namely structural HOMA (harmonic oscillator model of aromaticity) and magnetic NICS (nucleus independent chemical shift) aromaticity indices, in particular, the NICSzz(+1) component. Finally, possible numerical checks on the conditions relating to validity of the well-known Yamaguchi's formula for calculating the exchange coupling coefficient J in diradical systems are discussed.

Quantitative Assessment of Aromaticity and Antiaromaticity Utilizing Vibrational Spectroscopy.

Vibrational frequencies can be measured and calculated with high precision. Therefore, they are excellent tools for analyzing the electronic structure of a molecule. In this connection, the properties of the local vibrational modes of a molecule are best suited. A new procedure is described, which utilizes local CC stretching force constants to derive an aromaticity index (AI) that quantitatively determines the degree of π-delocalization in a cyclic conjugated system. Using Kekulé benzene as a suitable reference, the AIs of 30 mono- and polycyclic conjugated hydrocarbons are calculated. The AI turns out to describe π-delocalization in a balanced way by correctly describing local aromatic units, peripheral, and all-bond delocalization. When comparing the AI with the harmonic oscillator model of AI, the latter is found to exaggerate the antiaromaticity of true and potential 4n π-systems or to wrongly describe local aromaticity. This is a result of a failure of the Badger relationship (the shorter bond is always the stronger bond), which is only a rule and therefore cannot be expected to lead to an accurate description of the bond strength via the bond length. The AI confirms Clar's rule of disjoint benzene units in many cases, but corrects it in those cases where peripheral π-delocalization leads to higher stability. [5]-, [6]-, [7]-Circulene and Kekulene are found to be aromatic systems with varying degree of delocalization. Properties of the local vibrational modes provide an accurate description of π-delocalization and an accurate AI.

New bio-active azo-azomethine based Cu(II) complexes

In this study, new azo-azomethine ligands L1H–L3H and their Cu(II) complexes were synthesized and characterized by the analytical and spectroscopic methods such as elemental analyses, infrared, mass, 1H and 13C NMR spectra. Molecular structures of the azo-azomethine ligands were determined by single crystal X-ray diffraction studies. X-ray data revealed that the compound L1H adopts azo-enamine tautomer form in the solid state while the compounds L2H and L3H favor azo-imine form. The harmonic oscillator model of aromaticity (HOMA) indexes for rings of synthesized azo-azomethine ligands were calculated in order to investigate of azo-enamine tautomer in the solid state. In L2H, the central phenol ring shows non-aromatic character with HOMA index value of 0.4872 suggesting the keto-amine tautomer in the solid state. All three azo-azomethine ligands involve in an intramolecular hydrogen bonding (NH⋯O for L1H and OH⋯N resulting in a S(6) graph set motif. The effect of solvents with different polarities on keto-enamine/phenol-imine tautomers was studied by UV-Vis spectroscopy. In the structure of the Cu(II) complexes, the central metal atom is coordinated to two phenolate oxygen atoms and two imine nitrogen atoms of two azo-azomethine molecules in a distorted square-planar geometry. All the synthesized azo-azomethine ligands and their Cu(II) complexes were screened for their cytotoxicity against HeLa (human uterus carcinoma) cell lines. Finally, antioxidant capacities of the compounds were determined by different antioxidant tests such as, free radical scavenging activity, metal chelating activity, and reduction power activity.

Substituent effects in nitro derivatives of carbazoles investigated by comparison of low-temperature crystallographic studies with density functional theory (DFT) calculations.

The crystal structure of 9H-carbazole, C12H9N, (I), has been redetermined at low temperature for use as a reference structure in a comparative study with the structures of 1-nitro-9H-carbazole, C12H8N2O2, (II), and 9-nitrocarbazole, C12H8N2O2, (III). The molecule of (I) has crystallographically imposed mirror symmetry (Z' = 0.5). All three solid-state structures are slightly nonplanar, the dihedral angles between the planes of the arene and pyrrole rings ranging from 0.40 (7)° in (III) to 1.82 (18)° in (II). Nevertheless, a density functional theory (DFT) study predicts completely planar conformations for the isolated molecules. To estimate the influence of nitro-group substitution on aromaticity, the HOMA (harmonic oscillator model of aromaticity) descriptor of π-electron delocalization has been calculated in each case. The HOMA indices for the isolated and solid-state molecules are relatively consistent and decrease in value for aromatic rings that are substituted with a π-electron-withdrawing nitro group. Substitution of the arene ring influences the π-electron delocalization in the ring only weakly, showing strong resistance to a perturbation of its geometry, contrary to what is observed for nitro substitution of the five-membered heterocyclic pyrrole ring. In (II), the molecules are arranged in near-planar dimers connected to each other by strong N-H···O hydrogen bonds that stack parallel to the crystallographic b axis. A similar stacking arrangement is observed in (III), although here the stacked structure is formed by stand-alone molecules.

Substituent effects in trans-p,p'-disubstituted azobenzenes: X-ray structures at 100 K and DFT-calculated structures.

The crystal and molecular structures of two para-substituted azobenzenes with π-electron-donating -NEt2 and π-electron-withdrawing -COOEt groups are reported, along with the effects of the substituents on the aromaticity of the benzene ring. The deformation of the aromatic ring around the -NEt2 group in N,N,N',N'-tetraethyl-4,4'-(diazenediyl)dianiline, C20H28N4, (I), may be caused by steric hindrance and the π-electron-donating effects of the amine group. In this structure, one of the amine N atoms demonstrates clear sp(2)-hybridization and the other is slightly shifted from the plane of the surrounding atoms. The molecule of the second azobenzene, diethyl 4,4'-(diazenediyl)dibenzoate, C18H18N2O4, (II), lies on a crystallographic inversion centre. Its geometry is normal and comparable with homologous compounds. Density functional theory (DFT) calculations were performed to analyse the changes in the geometry of the studied compounds in the crystalline state and for the isolated molecules. The most significant changes are observed in the values of the N=N-C-C torsion angles, which for the isolated molecules are close to 0.0°. The HOMA (harmonic oscillator model of aromaticity) index, calculated for the benzene ring, demonstrates a slight decrease of the aromaticity in (I) and no substantial changes in (II).

1-[(1-Ethoxypropylidene)amino]-2-ethyl-4-(4-hydroxybenzyl)imidazol-5(4H)-one

The racemic title compound, C(17)H(23)N(3)O(3), isolated from the reaction of L-(-)-tyrosine hydrazide with triethyl orthopropionate in the presence of a catalytic quantity of p-toluenesulfonic acid (p-TsOH), crystallizes with Z' = 1 in a centrosymmetric monoclinic unit cell. The molecule contains two planar fragments, viz. the benzene and imidazole rings, linked by two C-C single bonds. The dihedral angle between the two planes is 59.54 (5)° and the molecule adopts a synclinal conformation. The HOMA (harmonic oscillator model of aromaticity) index, calculated for the benzene ring, demonstrates no substantial interaction between the two π-electron delocalization regions in the molecule. In the crystal structure, there is an O-H...N hydrogen bond that links the molecules along the c axis.

Coordinating Properties of Pyrone and Pyridinone Derivatives, Tropolone and Catechol toward the VO2+ Ion: An Experimental and Computational Approach

AbstractThe interaction of the VO2+ ion with pyrone derivatives and tropolone, which form very effective antidiabetic compounds, is critically re‐examined. The binary systems with ethylmaltol (Hema) and tropolone (Htrop) were studied in aqueous solution and in the solid state through the combined application of spectroscopic (EPR, UV/Vis and IR) and pH‐potentiometric techniques. The results were compared with those of the systems with maltol (Hma) and kojic acid (Hkoj) and rationalized on the basis of DFT simulations. All the ligands L– form [VOL]+, cis‐[VOL2(H2O)] and cis‐[VOL2(OH)]– species in aqueous solutions and a square‐pyramidal [VOL2] complex in the solid state, which transforms into cis‐[VOL2(solvent)] when it is dissolved in water or in a coordinating solvent. The coordinating properties of the ligands studied were compared with those of pyridinone [3‐hydroxy‐1,2‐dimethyl‐4(1H)pyridinone (Hdhp), and 1,2‐diethyl‐3‐hydroxy‐4(1H)pyridinone (Hdepp)] derivatives and catechol (H2cat), and were explained by postulating that from pyrones to pyridinones and to catechol the donor set changes progressively from (CO, O–) to (O–, O–). DFT calculations allowed us to determine the relative stability of the four possible structures (square pyramidal, trans‐ and two cis‐octahedral) of the bis‐chelated species and the aromaticity of the protonated, neutral and deprotonated form of the ligands through the calculation of the HOMA (harmonic oscillator model of aromaticity) index. The relationship between the electric charge on the oxygen donors, the mean distances and the difference between the lengths of C–Oket and C–Ophen bonds with (i) the pKa of the ligands, (ii) the pK of deprotonation of the equatorially coordinated water molecule in cis‐[VOL2(H2O)], and (iii) the 51V hyperfine coupling constant (Az) of [VOL2], cis‐[VOL2(H2O)] and cis‐[VOL2(OH)]– was also found and discussed.

The π-Electron Delocalization Imposed by Thermal Vibrations of Substituted Benzene Analogues in Mediums of Varying Polarities

The unique set of aromaticity indices was identified for thermally induced changes of π-electron delocalization by means of PCA (Principal Component Analysis). It was demonstrated that solvents polarity can influence not only the values of aromaticity indices but also their contribution to Principal Components. Therefore, in different phases one should select different indices for a proper description of the aromaticity. The found aromaticity diversities indices were provided for aniline as well as p-nitrosoaniline and it was found that the geometry of the latter one is highly sensitive to solvents polarity changes. Thus, all of the aromaticity indices experienced a reduction of their values, with the HOMA (Harmonic Oscillator Model of Aromaticity) index being the most sensitive. It was also found that there were such vibrations which could alter this trend and lead to apparent increase of aromaticity.

Aromaticity of topological isomers of cyclazines and their [14] to [18] annulenes

The structures and magnetic properties of the parent cyclazines [2.2.2], [3.2.2], [3.3.2], [4.2.2], [3.3.3], [4.3.2], their topological isomers and the doubly bridged [14] to [18] annulenes were studied computationally at the B3LYP/6-311+G ** level of density functional theory (DFT). The magnetic criteria Λ (magnetic susceptibility exaltations, MSE), ∑NICS (the summation of nucleus-independent chemical shifts), and 1H NMR chemical shifts, geometry-based HOMA (harmonic oscillator model of aromaticity) index and HOMO–LUMO gaps were used to elucidate their aromatic and anti-aromatic properties. The excellent correlation between Λ and the summation of NICS(0) and NICS(1) demonstrates that these two originally proposed indices are still good indicators of the whole aromaticity of these polycyclic ring systems. The computed NMR chemical shifts are also in good agreement with available experimental data. In general, there is a good relationship between different criteria of aromaticity.

On the Harmonic Oscillator Model of Electron Delocalization (HOMED) Index and its Application to Heteroatomic π-Electron Systems

The HOMA (Harmonic Oscillator Model of Aromaticity) index, reformulated in 1993, has been very often applied to describe π-electron delocalization for mono- and polycyclic π-electron systems. However, different measures of π-electron delocalization were employed for the CC, CX, and XY bonds, and this index seems to be inappropriate for compounds containing heteroatoms. In order to describe properly various resonance effects (σ-π hyperconjugation, n-π conjugation, π-π conjugation, and aromaticity) possible for heteroatomic π-electron systems, some modifications, based on the original HOMA idea, were proposed and tested for simple DFT structures containing C, N, and O atoms. An abbreviation HOMED was used for the modified index.