Application of AIM parameters at ring critical points for estimation of pi-electron delocalization in six-membered aromatic and quasi-aromatic rings

The AIM parameters at the ring critical point (the electron density and its Laplacian, the total electron energy density and both its components, potential and kinetic electron energy densities), have been intercorrelated with aromaticity indices: the geometry-based HOMA and the magnetism-based NICS, NICS(1), and NICS(1)(zz). A set of 33 phenylic rings having possibly a diversified aromatic character, and a set of 20 quasi-rings formed by intramolecular hydrogen and lithium bonds, have been taken into consideration. It has been found that the density of total electron energy, H, may serve as a new quantitative characteristic of pi-electron delocalization. The dependences between H values and aromaticity indices are correlated (cc(H/HOMA)=0.99, cc(H/NICS(1)zz)=0.95).     

Relation between pi-electron localization/delocalization and H-bond strength in derivatives of omicron-hydroxy-schiff bases

Detailed investigations of electronic effects taking place within the molecular system of o-hydroxy Schiff bases have been performed. The analysis of geometry, local and global aromaticity, selected AIM-based parameters, and finally, pi-electron currents induced in the systems under consideration have been performed on the basis of quantum chemical calculations at the B3LYP/6-311+G** level of theory. The relation between localization/delocalization of pi-electrons within the whole system has been described. It has been shown that the character of the bond which is common to the phenylic ring and the quasi-ring formed as a result of H-bond formation has a crucial impact on the strength of H-bonding. The strongest H-bonds can be observed for the systems in which the sequence of formally single and double bonds within the H-bridged quasi-ring enable a pi-electronic coupling. These observations indicate that pi-electron effects play a fundamental role in the stabilization of the hydrogen bridge within omicron-hydroxy Schiff bases. Analysis of pi-ring currents induced by a magnetic field perpendicular to the molecular plane of selected analyzed systems confirms these conclusions.     

Why are the kinked polyacenes more stable than the straight ones? A topological study and introduction of a new topological index of aromaticity

A topological model for estimating the stability of benzenoid hydrocarbons (BHs) is presented showing an acceptable linear dependence on Hess-Schaad resonance energy per pi-electron values. The topological measure of stability is accessible by use of pencil calculation and is based on counting cis-type fragments of double bonds in all canonical structures of a given BH. Evidence is given that infinite chains of straight linear polyacenes are always less stable than the kinked ones.     

How the shape of the NH2 group depends on the substituent effect and H-bond formation in derivatives of aniline

The geometry and electronic structure of the amino group in aniline and its derivatives are very sensitive to both intramolecular interactions such as substituent effects and intermolecular ones such as H-bonding. An analysis of experimental geometries retrieved from the CSD base and computational modeling of aniline and its derivatives and their H-bonded complexes by use of B3LYP/6-311+G** and MP2/aug-cc-pVDZ showed that the degree of pyramidalization of the amino group depends on H-bonding, which exists in two forms, (i) NH...B (base) and (ii) N...HB (Br?nsted acid), both of which affect the shape of the NH2 group. The effect may be significantly enhanced by a substituent through resonance interaction from electron-attracting substituents. The NH...B interactions lead to a substantial planarization of the group, whereas N...HB interactions do not. The natural bond orbital analysis allowed the authors to show that the changes in occupancy of the "lone pair" orbital and in geometry parameters describing pyramidalization of the group depend on the substituent constants.     

Electrochemical investigations of intermediates in electroreduction of aromatic nitro- and nitroso-compounds in DMF: Part III. Electrochemical characteristics of additional (red-ox) systems formed during the electroreduction of nitroso-, azoxy- and azobenzene

In cyclic voltammetric measurements of DMF solutions of nitroso-, azoxy- and azobenzene using HMDE it was found that besides the usual peaks of red-ox processes an additional, new system of peaks was observed. Its properties were nearly identical for all three depolarizers. Taking into account various possible interpretations, based on experimental facts observed, the most probable is to attribute this additional system of peaks to electrode reaction of the material of electrode (i.e. Hg) in the presence of adsorbed azobenzene dianion.     

(1Z,3Z)-1,4-Di(pyridin-2-yl)buta-1,3-diene-2,3-diol: the planar highly conjugated symmetrical enediol with multiple intramolecular hydrogen bonds

1H, (13)C, and (15)N NMR spectral data show that in chloroform solution (1Z,3Z)-1,4-di(pyridin-2-yl)buta-1,3-diene-2,3-diol, OO, is in ca. 9:1 equilibrium with (3Z)-3-hydroxy-1,4-di(pyridin-2-yl)but-3-en-2-one, OK, while no 1,4-di(pyridin-2-yl)-2,3-butanedione, KK, was detected. The species present in the tautomeric mixture were identified by comparing their experimental chemical shifts with those known for similar compounds as well as with the theoretically calculated (GIAO-HF/DFT) values. Ab initio calculations show that OK and especially the highly conjugated OO forms are preferred in the tautomeric mixtures both in vacuo and in chloroform solution. Comparison of experimental (Arrhenius) and calculated (ab initio) energies of OK shows that the MP2/6-31G//RHF/6-31G method gives the most precise results. There are one and two strong O-H.N hydrogen bonds present in the molecules of the former and latter compound, respectively. Other tautomeric forms, e.g., dienaminedione [(1Z,4Z)-1,4-di[pyridin-2(1H)-ylidene]butane-2,3-dione], and their rotamers were found to have higher energies. The single-crystal X-ray diffraction studies show that dienediol OO is the only tautomeric form present in the crystal at 173 K. Its almost perfectly planar molecule is stabilized by two strong intramolecular O-H.N hydrogen bonds.