Aromaticity of monosubstituted derivatives of benzene. The application of out-of-plane ring deformation energy for a quantitative description of aromaticity

Retaining Hückel Aromaticity in the Triplet Excited State of Azobenzene

The implication of the potential concept of aromaticity in the relaxed lowest triplet state of azobenzene, an efficient molecular switch, using elementary aromaticity indices based on magnetic, electronic, and geometric criteria has been discussed. Azobenzene exhibits a major Hückel aromatic character retained in the diradical lowest relaxed triplet state (T₁ ) by virtue of a twisted geometry with partial delocalization of unpaired electrons in the perpendicular p-orbitals of two nitrogen atoms to the corresponding phenyl rings. The computational analysis has been expanded further to stilbene and N-diphenylmethanimine for an extensive understanding of the effect of closed-shell Hückel aromaticity in double-bond-linked phenyl rings. Our analysis concluded that stilbene has Hückel aromatic character in the relaxed T₁ state and N-diphenylmethanimine has a considerable Hückel aromaticity in the phenyl ring near the carbon atom while a paramount Baird aromaticity in the phenyl ring near the nitrogen atom of the C=N double bond. The results reveal the application of excited-state aromaticity as a general tool for the design of molecular switches.

On the aromaticity of uracil and its 5-halogeno derivatives as revealed by theoretically derived geometric and magnetic indexes

The problem of aromaticity in heterocyclic rings of uracil and its 5-halogenoderivatives (5XU) was analyzed theoretically by calculating modified harmonic oscillator model of aromaticity (HOMA) for Heterocycle Electron Delocalization (HOMHED), nucleus-independent chemical shift parameters (NICS) and the so-called scan experiments, using helium-3 atom as a magnetic probe. The impact of halogen electronegativity on C5 atom’s NBO charges was also investigated. Water, as a polar environment, has a negligible impact on 5XU aromaticity. The most stable diketo tautomer shows a very low aromaticity while the “rare” dihydroxy form (tautomer No 6) is aromatic and resembles benzene. This is in agreement with traditional drawing of chemical formula of uracil’s six-membered ring, directly showing three alternating single and double bonds in its tautomer No 6. No good correlation between magnetic and geometric indexes of aromaticity for the studied 5XU tautomers was found. Linear correlation between the magnitude of NICS minimum, as well as the distance of the minimum above uracil ring plane center from 3He NMR chemical shift scan plot with respect to halogen electronegativity were observed. A strong linear dependence of magnetic index of aromaticity and the electronegativity of 5X substituent was observed.

Aromaticity of benzene derivatives: an exploration of the Cambridge Structural Database

The harmonic oscillator model of aromaticity (HOMA) index, one of the most popular aromaticity indices for solid-state benzene rings in the Cambridge Structural Database (CSD), has been analyzed. The histograms of HOMA for benzene, for benzene derivatives with one formyl, nitro, amino or hydroxy group as well as the histograms for the derivatives with two formyl, nitro, amino or hydroxy groups inortho,metaandparapositions were investigated. The majority of the substituted benzene derivatives in the CSD are characterized by a high value of HOMA, indicating fully aromatic character; however, the distribution of the HOMA value from 1 to about 0 indicates decreasing aromaticity down to non-aromatic character. Among the benzene derivatives investigated, a significant decrease in aromaticity can be related to compounds with diamino and dinitro groups in themetaposition.

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.

Spectroscopic characterization of C-4 substituted 3,5-dichloro-4H-1,2,6-thiadiazines

The effect of C-4 substitution with electron withdrawing groups on the structure and electronic properties of three 3,5-dichloro-4H-1,2,6-thiadiazines is characterized using resonance Raman, absorption and photoluminescence spectroscopies.

Substituents and environment influences on aromaticity of peri- and para-substituted naphthalenes

The influence of substituents and environment on the aromaticity of the naphthalene ring is shown for a series of peri- and para-substituted naphthalenes. Crystal structure geometries are compared with the single molecule structures in vacuum (optimized at the B3LYP/6-311++G** level) and with structures determined in media of different polarity. The harmonic oscillator model of aromaticity (HOMA) index of the naphthalene rings has been used to characterize the aromaticity of the investigated molecules. It has been shown that the ellipticity of the C2-C3 (C6-C7) bonds can be applied as a measure of participation of the quinoid resonance structure and through-resonance effect between the para-substituents.

Entropy versus aromaticity in the conformational dynamics of aromatic rings

Comparison of the results of Car-Parrinello molecular dynamics simulations of isolated benzene, pyrimidine and 1,2,4-triazine molecules reveals that the unusually low population of planar geometry of the benzene ring is caused by entropy effects despite its high aromaticity. The decrease in symmetry of the molecule results in smaller changes in entropy and Gibbs free energy due to out-of-plane deformations of the ring, leading to an increase in the population of planar geometry of the ring. This leads to differences in the topology of potential energy and Gibbs free energy surfaces.

A new approach for aromaticity criterion based on electrostatic field gradient

In this research, electric field gradient (EFG), which is the first derivative of electric field, is applied for evaluation of aromaticity of 89 cyclic organic compounds. In our calculations, DFT procedure (b3lyp) with basis set 6-311++G has been employed, and the obtained electronic structures and frequency test has been done for optimized geometries. The aromaticity evaluated for these compounds by EFG procedure is successfully compared with other well-known indices in literature, especially with nuclear independent chemical shift (NICS). These comparisons show that EFG method of assessment of aromaticity can be used as a rather valid procedure for this purpose. Flexibility and simplicity of EFG make this method a rather easy procedure for assessment of aromaticity. Since EFG method of aromaticity evaluation does not need specific programming and it can be done by known software such as Gaussian, therefore, the availability for everyone to calculate desired aromaticity by this method is one of attractive feature of it similar to NICS.

Growth format, electronic architecture, magnetic, and optical properties of aromaticcyclo-Cu3Au3homotops

Bimetallic Cu(3)Au(3) clusters have been investigated using electronic structure calculation techniques (DFT) to understand their electronic, magnetic, and optical properties as well as the geometrical structures. The most stable homotop is the planar cyclo-[Cu(3)(micro-Au)(3)] form consisting of a triangular positively charged Cu(3) structural core with negatively charged Au atoms occupying exposed positions. This structure is characterized by the maximum number of heterobonds and peripheral positions of Au atoms. Possible growth formats of the cyclo-[Cu(3)(micro-Au)(3)] homotops have been explored following both the edge-capping and the stepwise metal atom substitution mechanism. The bonding pattern along with the density of states (DOS) plots of the cyclo-[Cu(3)(micro-Au)(3)] homotop are thoroughly analyzed and compared with those of the pure cyclo-[Cu(3)(micro-Cu)(3)] and cyclo-[Au(3)(micro-Au)(3)] clusters. Particular attention was paid on the stability of these bimetallic clusters in relation with the ring-shaped electron density distribution (aromaticity). It was found that all 3-membered metal rings exhibit significant aromatic character, which was verified by a number of established criteria of aromaticity, such as structural, energetic, magnetic (NICS profiles), and out-of-plane ring deformability criteria. The NICS (1) values correlate well with the out-of-plane ring deformation energy. Finally, a comprehensive analysis of the optical spectra of the CuAu, Cu(2), and Au(2) diatomics and the cyclo-[Cu(3)(micro-Au)(3)], cyclo-[Cu(3)(micro-Cu)(3)], and cyclo-[Au(3)(micro-Au)(3)] clusters placed the electronic assignments of the optical transitions on a firm footing.

Aromaticity of Distorted Benzene Rings: Exploring the Validity of Different Indicators of Aromaticity

The effect of three in-plane (bond length alternation, bond length elongation, and clamping) and three out-of-plane deformations (boatlike, chairlike, and pyramidalization) on the aromaticity of the benzene molecule has been analyzed employing seven widely used indicators of aromaticity. It is shown that only the aromatic fluctuation index (FLU) is able to indicate the expected loss of aromaticity because of distortion from the equilibrium geometry in all deformations analyzed. As FLU has been shown previously to fail in other particular situations, we conclude that there is not yet a single indicator of aromaticity that works properly for all cases. Therefore, to reach safer conclusions, aromaticity analyses should be carried out employing a set of aromaticity descriptors on the basis of different physical manifestations of aromaticity.