Harmonic Oscillator Model Of Aromaticity Values Research Articles

Stable Diarylamine-Substituted Tris(2,4,6-trichlorophenyl)methyl Radicals: One-Step Synthesis, Near-Infrared Emission, and Redox Chemistry

Stable Diarylamine-Substituted Tris(2,4,6-trichlorophenyl)methyl Radicals: One-Step Synthesis, Near-Infrared Emission, and Redox Chemistry

Heteroatom effects on aromaticity of five-membered rings in acenaphthylene analogs.

The pattern of cyclic conjugation was thoroughly studied in the series of N- and P-acenaphthylene derivatives using several different aromaticity indices: the energy effect (ef), multicenter delocalization index (MCI), harmonic oscillator model of aromaticity (HOMA) index, and nucleus independent chemical shifts (NICS). The Kekulé-structure-based reasoning predicts that there would be no cyclic conjugation in the "empty" five-membered heteroatom-containing rings in the studied molecules. It was found that according to the ef, MCI, and HOMA values, the extent of cyclic conjugation in the pentagonal rings is strongly influenced by the number and mutual arrangement of the hexagonal rings. In addition, it was revealed that in some of the examined molecules, the intensity of cyclic conjugation in the "empty" pentagons is even stronger than that of some hexagonal rings within the same molecule. The obtained results refute what one would expect based on "chemical intuition," which is usually strongly rooted to the Kekulé structures.Graphical abstract.

Harnessing aromaticity and intramolecular hydrogen bonding to tailor organosuperbases by using 2,4,6-cycloheptatriene-1-imine scaffold

The density functional theory (DFT) calculations at B3LYP/6-31 + G(d,p) level were performed for a series of new 2,4,6-cycloheptatriene-1-imine scaffolds which have potential to introduce as organosuperbases. In the present study, we have exploited the 2,4,6-cycloheptatriene-1-imines which strategically substituted at 2 and 7 positions by two potential anchoring arms having lone pair possessing atoms. The designed organic superbases were found to have the higher basicity than the benchmarked molecule, 1,8-bis(dimethylamino)-naphthalene. The proposed superbase B14 bearing guanidine and aniline groups possesses the highest proton affinity value as 1143.8 kJ mol−1. The protonation enthalpy, Gibbs free energy, harmonic oscillator model of aromaticity (HOMA), and nucleus-independent chemical shift (NICS) were used for the basicity comparison. After the protonation of the designed molecules, the positive charge on the conjugate acid is expected to be delocalized in the cycloheptatriene ring and led to its aromatization. The results indicate that the HOMA and NICS values of the ring in the conjugate acid are strongly elevated. On the other hand, very strong intramolecular hydrogen bonding is formed after protonation. These two factors enhance the stability of the conjugate acids and thereby increase the basicity of the designed molecules.

Anion˗π and Intramolecular Hydrogen Bond Interactions in the Various Complexes of 1,3,5-Triamino-2,4,6-trinitrobenzene with H-, F-, Cl- and Br- Anions

The quantum chemical calculations were performed to investigate the interplay between the anion˗π and intramolecular hydrogen bond (IMHB) interactions in the various complexes of 1,3,5-triamino-2,4,6-trinitrobenzene (ANB) with Hˉ, Fˉ, Clˉ and Brˉ anions. For better understanding the cooperative effects, the parent molecules (ANB) and the corresponding complexes of 1,3,5˗trinitrobenzene with the mentioned anions are also considered, as a set of the reference points. In this regard, the IMHB and anion˗π interactions are comprehensively analyzed by energetic, geometrical, spectroscopic and topological descriptors. It was found that the coexistence of the anion˗π and IMHB increases the strength of both interactions. Furthermore, the influences of the anion-π and IMHB interactions on the significance of the π˗electron delocalization (π˗ED) of the resonance assisted hydrogen bond (RAHB) units and the aromaticity of the benzene ring are evaluated by the harmonic oscillator model of aromaticity (HOMA), as a geometry based index. According to the HOMA values, we concluded that the coexistence of the mentioned interactions decreases the aromaticity of benzene ring and increases the π˗ED of RAHB units. Finally, the strength of the non-covalent interactions and the significance of π˗ED and aromaticity strongly depend on type of the anion.

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.

Aromatic changes in isoelectronic derivatives of phenalenyl radicals by central carbon replacement

A computational study of the aromatic characteristics of phenalenyl (PLY) upon replacement of the central carbon atom by Si, Ge, N, P, As, B, Al and Ga atoms comprising isoelectronic series with different charges (0, +1, −1) has been performed at the B3LYP/6-311++G(d,p) level. Two different geometries have been obtained, one planar and another one bowl-shaped. PLY derivatives exhibit a loss of the aromatic character in all cases indicated by NICS (Nucleus-Independent Chemical Shifts) values. These aromatic features are also in agreement with the pyramidalisation undergone by the central atom. The current density maps for those planar systems corroborate NICS findings. These variations in the aromaticity have been rationalised in terms of charge localisation. Harmonic oscillator model of aromaticity (HOMA) and para-delocalisation indices (PDI) have been used to evaluate the electron density delocalisation. HOMA values do not correlate with NICS but PDI indices follow the same trend than NICS.

Local aromaticity in naphtho-annelated fluoranthenes: can the five-membered rings be more aromatic than the six-membered rings?

All Kekulé-structure-based theories predict that the central five-membered ring in fluoranthene and naphtho-annelated fluoranthenes is nonaromatic. In the present work, a detailed study of the local aromaticity in a series of naphtho-annelated fluoranthene derivatives was performed by means of the following aromaticity indices: the energy effect (ef), bond resonance energy (BRE), multicenter delocalization indices (MCI), harmonic oscillator model of aromaticity (HOMA) index, nucleus-independent chemical shifts (NICS), and ring current maps. It was found that, according to the ef, BRE, MCI, and HOMA values, the pentagonal rings in some naphtho-annelated fluoranthenes can be even more aromatic than some hexagonal rings in the respective molecules. The magnetic indices do not support the results obtained by the energetic, electron delocalization, and geometrical aromaticity indices.

3,5-Diphenyl-1,2,4-triazin-6(1H)-one: synthesis, and X-ray and DFT-calculated structures

The title compound, C(15)H(11)N(3)O, (I), was obtained by the air oxidation of 3,5-diphenyl-4,5-dihydro-1,2,4-triazin-6(1H)-one. In the crystal structure, (I) forms centrosymmetric hydrogen-bonded dimers through pairs of N-H...N hydrogen bonds. The molecular structure of (I) deviates somewhat from planarity in the crystalline state, whereas a density functional theory (DFT) study predicts a completely planar conformation (C(s) point-group symmetry) for the isolated molecule. The solid-state conformation of (I) is stabilized by intramolecular hydrogen bonds, viz. one C-H...O interaction, which forms a six-membered ring, and three C-H...N interactions that each form five-membered rings. To estimate the influence of the intramolecular hydrogen-bonded rings on the aromaticity of the phenyl rings, the HOMA (harmonic oscillator model of aromaticity) descriptor of π-electron delocalization has been calculated for conformations of (I) with and without intramolecular hydrogen bonds. In the planar conformation of (I), the HOMA values for both benzene rings are lower than in hypothetical conformations without intramolecular hydrogen bonds.

A DFT Study on the Magnetostructural Property of Ferromagnetic Heteroverdazyl Diradicals with Phenylene Coupler

We have theoretically designed five different m-phenylene coupled high-spin bis-heteroverdazyl diradicals and their analogous p-phenylene coupled low-spin positional isomers. The geometry-based aromaticity index, harmonic oscillator model of aromaticity (HOMA) values for both the couplers (local HOMA), and the whole diradicals (global HOMA) have been calculated for all the diradicals. We also qualitatively relate these HOMA values with the intramolecular magnetic exchange coupling constants (J), calculated using a broken symmetry approach within unrestricted density functional theory framework. Structural aromaticity index HOMA of linkage specific benzene rings in our designed diradical systems shows that the aromatic character depends on the planarity of the molecule and it controls the sign and magnitude of J. The predicted J values are explained on the basis of spin polarization maps, average dihedral angles, and magnetic orbitals. The effect of the spin leakage phenomenon on magnetic exchange coupling constant and that on HOMA values of certain phosphaverdazyl systems has been explicitly discussed. In addition, a similar comparison is made between the calculated exchange coupling constants and corresponding HOMA values. The main novelty of this work stands on the consideration of the aromatic behavior by means of the geometrical index HOMA. We also estimate another aromaticity index, nucleus independent chemical shift (NICS) values for the phenylene coupler in each diradical to measure aromaticity and compare its value with that of HOMA. The ground state stabilities of these diradicals have also been compared.

An explicit solvent quantum chemistry study on the water environment influence on the interactions of fluoride with phenol

The phenol complexes with fluoride in water solution were analyzed for different F⋯O separation distances ranging from 2.6 A up to 4.0 A with 0.2 A intervals. The quenching molecular dynamics runs were followed by hybrid quantum mechanics–molecular mechanics (QM–MM) geometry optimization at B3LYP/6–311+G** level. The analysis of MD trajectories allowed for detailed characteristics of hydration patterns and mobility of fluoride and phenol hydroxyl group. The explicit solvent molecules taken into account in QM–MM studies impose solute geometry changes according to fluctuations of the structure of its nearest neighborhood. Hence, fluctuations of HOMA (harmonic oscillator model of aromaticity index) values were observed corresponding to different conformations of water molecules around the solute. However, the screening role of the water molecules was noticed, which reduces the influence of the fluoride compared with the gas phase. Similarly as in vapor state, also in water solution the formation of hydrogen bond between the fluoride and the hydroxyl group of phenol is energetically favorable.

Local Aromaticities in Large Polyacene Molecules

There has been controversy on the relative aromaticities of individual rings in a large polyacene molecule. Nucleus-independent chemical shift (NICS) values suggest that the highly reactive inner rings might be more aromatic than the outer ones and even more aromatic than benzene. We evaluated the bond resonance energies (BREs) and hypothetical geometry-independent pi-electron currents for a series of linear polyacenes and noticed that for large polyacene molecules the inner rings are never more aromatic than the outer ones. Global HOMA (harmonic oscillator model of aromaticity) values are highly correlative with percentage topological resonance energies (% TREs) but not with average NICS values. Magnetic properties, such as NICS and ring-current intensity, are highly dependent on molecular geometry and so must be carefully related to aromaticity.

An ab initio study on nucleic acid bases aromaticities

Aromatic character of nucleic acid bases was characterized by means of magnetic and geometric properties. Optimized geometries at DFT/6-311+G** level were used for calculation of nucleus independent chemical shift (NICS) and harmonic oscillator model of aromaticity (HOMA) values. Aromaticities of both pyrimidine and imidazole rings were analyzed. Solvent impact on HOMA values was estimated. Predictions of both methods are in qualitative agreement and suggest more aromatic character of purines rather than pyrimidines. Aromaticity decreases in the following sequence: adenine>guanine>cytosine>thymine≈uracil. Significant impact of protonation on the aromatic character was noticed. Protonation of adenine reduced its aromatic character of both pyrimidine and imidazole rings. However, protonation of guanine and all pyrimidines leads to increase of aromaticity compared to neutral species.