A comparative ab initio and DFT study of neutral aniline oligomers

The relationship between structure and excited-state properties in polyanilines from geminal-based methods

We employ state-of-the-art quantum chemistry methods to study the structure-to-property relationship in polyanilines (PANIs) of different lengths and oxidation states. Specifically, we focus on leucoemeraldine, emeraldine, and pernigraniline in their tetramer and octamer forms. We scrutinize their structural properties, HOMO and LUMO energies, HOMO-LUMO gaps, and vibrational and electronic spectroscopy using various Density Functional Approximations (DFAs). Furthermore, the accuracy of DFAs is assessed by comparing them to experimental and wavefunction-based reference data. We perform large-scale orbital-optimized pair-Coupled Cluster Doubles (oo-pCCD) calculations for ground and electronically excited states and conventional Configuration Interaction Singles (CIS) calculations for electronically excited states in all investigated systems. The EOM-pCCD+S approach with pCCD-optimized orbitals allows us to unambiguously identify charge transfer and local transitions across the investigated PANI systems-an analysis not possible within a delocalized canonical molecular orbital basis obtained, for instance, by DFAs. We show that the low-lying part of the emeraldine and pernigraniline spectrum is dominated by charge transfer excitations and that polymer elongation changes the character of the leading transitions. Furthermore, we augment our study with a quantum informational analysis of orbital correlations in various forms of PANIs.

Density functional theory and phytochemical study of Pistagremic acid

We report here for the first time a comparative theoretical and experimental study of Pistagremic acid (P.A). We have developed a theoretical model for obtaining the electronic and spectroscopic properties of P.A. The simulated data showed nice correlation with the experimental data. The geometric and electronic properties were simulated at B3LYP/6-31 G (d, p) level of density functional theory (DFT). The optimized geometric parameters of P.A were found consistent with those from X-ray crystal structure. Differences of about 0.01 and 0.15 Å in bond length and 0.19-1.30° degree in the angles, respectively; were observed between the experimental and theoretical data. The theoretical vibrational bands of P.A were found to correlate with the experimental IR spectrum after a common scaling factor of 0.963. The experimental and predicted UV-Vis spectra (at B3LYP/6-31+G (d, p)) have 36 nm differences. This difference from experimental results is because of the condensed phase nature of P.A. Electronic properties such as Ionization Potential (I.P), Electron Affinities (E.A), co-efficient of highest occupied molecular orbital (HOMO), co-efficient of lowest unoccupied molecular orbital (LUMO) of P.A were estimated for the first time however, no correlation can be made with experiment. Inter-molecular interaction and its effect on vibrational (IR), electronic and geometric parameters were simulated by using Formic acid as model for hydrogen bonding in P.A.

Lithium Intercalation of Phenyl-Capped Aniline Dimers: A Study by Photoelectron Spectroscopy and Quantum Chemical Calculations

Structural and electronic properties of pristine and lithium-intercalated, phenyl-capped aniline dimers as a model for the lithium-polyaniline system have been studied by photoelectron spectroscopy and quantum chemical calculations. It was found that the electronic structure of reduced and oxidized forms of oligoanilines is only weakly affected by isomerism. Upon intercalation, charge transfer from the Li-atoms is remarkable and highly localized at N-atomic sites, where configurations are energetically favored in which both N atoms of the dimers are bound to Li atoms. Conversion of nitrogen sites is different for the two forms of aniline dimers and incomplete up to high intercalation levels, indicating a pronounced role of solid-state effects in the formation of such compounds.

MNDO-PM3 Study of the Early Stages of the Chemical Oxidative Polymerization of Aniline

The theoretical approach to the study of aniline polymerization mechanism has been based on the MNDO-PM3 semi-empirical quantum chemical computations of the heat of formation of aniline dimer and trimer intermediates. The oxidation in aqueous medium without added acid is analyzed. The aniline nitrenium cation is proposed to be the reactive electrophilic species generated by the oxidation of aniline with a two-electron oxidant, ammonium peroxydisulfate, in the initiation phase. 4-Aminodiphenylamine and its fully oxidized form, N-phenyl-1,4-benzoquinonediimine, are the main dimeric products. 2-Aminodiphenylamine and its fully oxidized form, N-phenyl-1,2-benzoquinonediimine, are the most important side products of aniline dimerization. The influence of protonation on the oxidizability of aniline and reaction intermediates was studied. The dominant aniline oligomers have been shown to be linear as well as branched. The importance of reactivity difference between fully oxidized aniline oligomers composed of odd or even numbers of constitutional aniline units is pointed out. The species with odd numbers of aniline units, having nitrenium cationic nature, are much more reactive. The oxidation combined with intramolecular cyclization reaction pathways leads to substituted phenazines, characteristic of two-dimensional PANI chain-growth.

Doping of Polyaniline by Acid−Base Chemistry: Density Functional Calculations with Periodic Boundary Conditions

Calculations with Gaussian orbitals and periodic boundary conditions using several density functionals are carried out to study the proton-doping of polyaniline. We explore previously proposed mechanisms to explain the spectacular increase of the electrical conductivity of polyaniline upon protonation. The structural and spectroscopic theoretical predictions for both polaron and bipolaron lattices agree quite well with the experimental data, and we find that the bipolaron structure is lower in energy.