Galia Madjarova

Fully Doped Oligomers of Emeraldine Salt: Polaronic versus Bipolaronic Configuration

Calculations for model oligomers of the emeraldine salt with UBLYP/6-31G*/PCM are performed. The models differ in number of monomers, in the position of the counterions (Cl(-)), and in multiplicity. The molecular features affected most prominently by the protonation, namely, structure, energetics, and electron and spin density partitioning are analyzed. The results show unequivocally that the studied molecular characteristics are essentially size independent. The octamer profiles of all parameters are repeated in the dodecamer and the hexadecamer. The bipolaronic forms are energetically more favorable than the polaronic ones within the chosen protocol. The electronic structure in the intermediate multiplicities differs from the bipolaronic and polaronic periodicity. The geometrical changes and electron density redistribution upon increase of multiplicity illustrate the pathway of intramolecular bipolaron-polaron conversion. The orbital analysis rationalizes the observed behavior of the oligomers.

On the nature of the spin exchange interaction in poly (m-aniline)

Abstract The energy spectra of two infinite 1D models of the poly(meta-aniline) (PMA) namely the neutral and cationic radical forms are theoretically investigated. The band structure is characterized by a wide energy gap with a half-filled band of nearly degenerate molecular orbitals. Different contributions (potential, kinetic, and indirect exchange) to the effective spin exchange between the unpaired electrons in the half-filled band are calculated. The PMA exhibits a net spin exchange of ferromagnetic nature, i.e. the ground state of the PMA is characterized by maximum spin multiplicity. The ferromagnetic interaction is due to the predominant contribution of the direct (potential) Coulomb exchange.

Calculated and experimental spectra of some 1,8-naphthalimide derivatives

The influence of different electron donating substituens on the absorption of 1,8-naphthalimide derivatives was studied and compared with the theoretically (AM1,PPP-SCF-CI) calculated predictions. It was shown that the experimental results and the calculated electronic transitions are in good agreement, assuming that in relation to the solvent polarity the possible mesomeric forms contribute to a different extent. In cases where a labile hydrogen atom is present, tautomerism takes place.

Molecular dynamics approach to water structure of HII mesophase of monoolein

The goal of the present work is to study theoretically the structure of water inside the water cylinder of the inverse hexagonal mesophase (H(II)) of glyceryl monooleate (monoolein, GMO), using the method of molecular dynamics. To simplify the computational model, a fixed structure of the GMO tube is maintained. The non-standard cylindrical geometry of the system required the development and application of a novel method for obtaining the starting distribution of water molecules. A predictor-corrector schema is employed for generation of the initial density of water. Molecular dynamics calculations are performed at constant volume and temperature (NVT ensemble) with 1D periodic boundary conditions applied. During the simulations the lipid structure is kept fixed, while the dynamics of water is unrestrained. Distribution of hydrogen bonds and density as well as radial distribution of water molecules across the water cylinder show the presence of water structure deep in the cylinder (about 6 Å below the GMO heads). The obtained results may help understanding the role of water structure in the processes of insertion of external molecules inside the GMO∕water system. The present work has a semi-quantitative character and it should be considered as the initial stage of more comprehensive future theoretical studies.

Unit Cell Structure of Water-Filled Monoolein into Inverted Hexagonal (HII) Mesophase Modeled by Molecular Dynamics

The study investigates the unit cell structure of inverted hexagonal (H(II)) mesophase composed of monoolein (1-monoolein, GMO) and water using atomistic molecular dynamics methods without imposing any restraints on lipid and water molecules. Statistically meaningful and very contrast images of the radial mass density distribution, scrutinizing also the separate components water, monoolein, the polar headgroups of the lipids, the double bond, and the termini of the hydrocarbon chain (the tail), are obtained. The lipid/water interface structure is analyzed based on the obtained water density distribution, on the estimated number of hydrogen bonds per monoolein headgroup, and on the headgroup-water radial distribution functions. The headgroup mass density distribution demonstrates hexagonal shape of the monoolein/water interface that is well-defined at higher water/monoolein ratios. Water interacts with the headgroups by forming a three-layer diffusive mass density distribution, and each layers shape is close to hexagonal, which is an indication of long-range structural interactions. It is found that the monoolein headgroups form a constant number of hydrogen bonds leaving an excessive amount of water molecules outside the first lipid coordination sphere. Furthermore, the quantity of water at the monoolein/water interface increases steadily upon extension of the unit cell, so the interface should have a very dynamic structure. Investigation of the hydrocarbon residues reveals high compression and well-expressed structuring of the tails. The tails form a very compressed and constrained structure of defined layers across the unit cell with properties corresponding to a more densely packed nonpolar liquid (oil). Due to the hexagonal shape the 2D packing frustration is constant and does not depend on the water content. All reported structural features are based on averaging of the atomic coordinates over the time-length of the simulation trajectories. That kind of processing allows the observation of the water/GMO interface shape and its stability and mobility at a time scale close to the ones of the intermolecular interactions.

Nature of the magnetic interaction in organic radical crystals. I. Alternant systems

Abstract A theoretical approach is applied for estimation of the nature (ferro- or antiferromagnetic) of the spin exchange interaction between the unpaired electrons within the half-filled band of 1-D organic radical crystals, consisting of weakly interacting alternant radicals. The various contributions to the Heisenberg effective exchange integral, Jeff - direct, kinetic, and indirect spin exchange - are evaluated quantitatively for several model crystals in order to establish the nature and magnitude of the exchange coupling between the unpaired spins. General rules are formulated showing the dependence of Jeff on the crystal topology and geometry.

Influence of the Level of Protonation on the Geometry and the Electronic Structure of Emeraldine Oligomers

A number of studies prove the existence of magnetically active states in polyaniline and claim polaronic nature of conductivity, but the molecular structure of polarons and bipolarons with account of the solvent effect has not been exhausted. Alongside with conductivity, the optical and magnetic properties of the polymer related to its practical application could be rationalized by the elucidation of this problem. The purpose of this chapter is the assessment of the degree of protonation on the spatial and electronic structure of hydrated polyaniline oligomers. Neutral and protonated emeraldine octamers are modeled to this end. UHF, UBLYP, and UB3LYP with 6-31G* basis set were employed for optimization of the geometry in aqueous medium (PCM). Various structural parameters: bond lengths, valence, and torsion angles, were analyzed and compared. The distribution of Mulliken and NBO charge density and Mulliken atomic spin density was discussed.

Structure and energy spectra of non-alternant analogues of the di- and triphenylmethine radicals and their ions

Abstract The results of a theoretical study of the structure and the energy spectra of iso-π-electronic non-alternant analogues ( NA ) of the di-and triphenylmethine radicals and their ions are presented. Based on the Sachs theorem, it is shown that the NA have also a non-bonding MO which is a characteristic feature of odd-alternant π-systems. The transition energies of the NA are similar trathose of the corresponding di- and triphenylmethine species. The electron transitions of NA radicals are connected with a significant π-charge density redistribution and a change of the value and direction of the dipole moment.