Theoretical investigation of tube-like supramolecular structures formed through bifurcated lithium bonds.

The stability of three supramolecular naostructures, which are formed through the aggregation of identical belts of [12] arene containing p-nitrophenyllithium, 1,4-dilithiatedbenzene and 1,4-dinitrobenzene units, is investigated by density functional theory. The electrostatic potential calculations indicate the ability of these belts in forming bifurcated lithium bonds (BLBs) between the Li atoms of one belt and the oxygen atoms of the NO2 groups in the other belt, which is also confirmed by deformation density maps and quantum theory of atoms in molecules (QTAIM) analysis. Topological analysis and natural bond analysis (NBO) imply to ionic character for these BLBs with binding energies up to approximately - 60 kcal mol-1. The many-body interaction energy analysis shows the strong cooperativity belongs to the configuration with the highest symmetry (C4v) containing p-nitrophenyllithium fragments as the building unit. Therefore, it seems that this configuration could be a good candidate for designing a BLB-based supramolecular nanotube with infinite size in this study.

Achieved negative differential resistance behavior of Si/B-substituted into a C6 chain sandwiched between capped carbon nanotube junctions.

Electronic transport properties of a pristine C6 chain and Si/B-substituted into the C6 chain sandwiched between two (5, 5) capped carbon nanotube electrodes were investigated through first-principles calculations based on non-equilibrium Green's functions (NEGF) conjugated with density functional theory (DFT). Si and B substitutions will affect the I-V curve of a pristine C6 chain. In the I-V characteristics, multi negative differential resistance (NDR) with large peak to valley ratio (PVR) and rectifying actions were observed. The NDR behavior originates from the joining and moving of conduction orbitals inside and outside of the bias window at a certain bias voltage. Furthermore, the assessment of transmission coefficient and distribution of molecular orbitals reveals that the rectifying performance is the result of the asymmetric distribution of the frontier molecular orbitals in the central region and their coupling with the electrodes. Multi NDR behavior of B substitution under very low bias voltage is a unique property of our proposed devices. Moreover, the CNT|C-(B-C)2-C|CNT molecular device shows a high PVR up to 31.8, which demonstrates that the proposed devices can be useful for molecular switching in nanoelectronic devices.

N-Derivatives of Shannon entropy density as response functions.

The exact first and second order partial derivatives of Shannon entropy density with respect to the number of electrons at constant external potential are introduced as new descriptors for prediction of the active sites of a molecule. The derivatives, which are a measure of the inhomogeneity of electron density, are calculated both exactly (from analytical forms) and approximately (using the finite difference method) for some molecular systems. According to the maximum entropy principle, the extreme value of the first order derivative on the surface of a given molecule should determine the active sites of the molecule in electrophilic and nucleophilic attack. The second order derivative indicates where the Shannon entropy is more concentrated or depleted during the electron exchange. Although these derivatives on the surfaces of helium and neon atoms are uniform, the corresponding values for argon, krypton and xenon atoms are not. This could explain the greater tendency of heavy noble gas atoms to form stable compounds. A dual descriptor is also defined as the difference between the left and right first order derivatives of Shannon entropy density, which allows one to simultaneously predict the preferable sites for electrophilic and nucleophilic attack over the system at point r. Therefore, the reactivity of an atom in a molecule requires the non-uniformity of the first and second order derivatives of Shannon entropy density on the surface of that atom.

Atomic Zero Steric Potential and the Regioselectivity of Reactions.

A new function, called zero steric potential (ZSP(r) = |∇ρ(r)|(2) - 2ρ(r)∇(2)ρ(r)), is proposed from the framework of density functional theory (DFT) to describe the regioselectivity of some selected reactions, including Diels-Alder reactions, addition of acids to alkenes, and Paternò-Büchi reactions. The ability of atomic zero steric potential (AZSP), which is obtained by integration of ZSP over an atomic basin, in predicting the major products of these reactions is checked. It is shown that for each reaction the least AZSP difference is observed for those atoms for which their binding leads to the main product. Therefore, it seems that the AZSP index could be used for predicting the reactive sites of reactants in a given chemical reaction.

Evaluation of absolute hardness: a new approach.

By taking the energy to be a Morse-like function of the number of electrons, E(N) = α{1 - e(-ß(N-δ))}(2) - κ, the electronic chemical potential and global hardness values for a set of atoms and some molecules are calculated from the accurate definitions of these two concepts and using the hybrid B3LYP functional and 6-311++G** basis set. By a comparison between the obtained hardnesses and the corresponding experimental values, it is found that the proposed model yields better values for hardnesses with respect to those that are obtained from the other frequently used methods. It is claimed that the difference between the calculated and experimental hardness values may arise from the approximate equation used for the evaluation of experimental hardnesses. Both of the calculated and experimental molecular hardnesses are used to investigate the change of hardness during the course of some exothermic reactions according to the maximum hardness principle (MHP). It is shown that the obtained hardnesses of reactions from the calculated hardnesses (Δη(calc)) are more successful in predicting the directions of these reactions than those that are evaluated from the experimental hardnesses (Δη(exp)).

Shannon entropy as a new measure of aromaticity, Shannon aromaticity.

Based on the local Shannon entropy concept in information theory, a new measure of aromaticity is introduced. This index, which describes the probability of electronic charge distribution between atoms in a given ring, is called Shannon aromaticity (SA). Using B3LYP method and different basis sets (6-31G**, 6-31+G** and 6-311++G**), the SA values of some five-membered heterocycles, C(4)H(4)X, are calculated. Significant linear correlations are observed between the evaluated SAs and some other criteria of aromaticity such as ASE, Lambda and NICS indices. According to the obtained relationships, the range of 0.003 < SA < 0.005 is chosen as the boundary of aromaticity/antiaromaticity. Using B3LYP/6-31+G** level of theory, the Shannon aromaticities for a series of mono-substituted benzene derivatives are calculated and analyzed. It is found that the least standard deviation between the aromaticities and the best linear correlation with the Hammett substituent constants are observed for the new index in comparison with the other indices. Also the values of the new index are evaluated for some substituted penta- and heptafulvenes, which successfully predict the order of aromaticity in these compounds. Applying this index to some non-benzonoids, linear and angular polyacenes also give satisfactory results and prove to be quite suitable for determining the local aromaticity of different rings in polyaromatic hydrocarbons.

A new scale of electronegativity based on electrophilicity index.

By calculating the energies of neutral and different ionic forms (M2+, M+, M, M-, and M2-) of 32 elements (using B3LYP/6-311++G** level of theory) and taking energy (E) to be a Morse-like function of the number of electrons (N), the electrophilicity values (omega) are calculated for these atoms. The obtained electrophilicities show a good linearity with some commonly used electronegativity scales such as Pauling and Allred-Rochow. Using these electrophilicities, the ionicities of some diatomic molecules are calculated, which are in good agreement with the experimental data. Therefore, these electrophilicities are introduced as a new scale for atomic electronegativity, chi(omega)0. The same procedure is also performed for some simple polyatomic molecules. It is shown that the new scale successfully obeys Sanderson's electronegativity equalization principle and for those molecules which have the same number of atoms, the ratio of the change in electronegativity during the formation of a molecule from its elements to the molecular electronegativity (Delta chi/chi omega) is the same.