Influence of the alkyl and alkoxy side chains on the electronic structure and charge-transport properties of polythiophene derivatives

Density Functional Theory has been used to study the structural, electronic and charge-transport properties of two regio-regular head-to-tail polythiophene derivatives, i.e. poly(3-hexyl-thiophene), P3HT, and poly(3-oxyhexyl-thiophene), P3OHT. The effect of substituents on the electronic structure was analyzed by means of bandwidth, bandgap, effective mass, total and partial densities of states and crystal orbital overlap populations. Electronic couplings were estimated from band diagrams as the splitting of the valence band. The neutral and cationic states of isolated oligomers were optimized using the supercell approximation. The hole-transfer rates and mobilities were evaluated according to Marcus's theory. Results provide a compelling illustration of the effect of side chains on the crystal packing, electronic structure and charge-transport properties. Thus, the hole mobility calculated for the alkyl derivative was 0.15 cm(2) V(-1) s(-1) (experimental mobility is 0.10 cm(2) V(-1) s(-1)), while the alkoxy derivative has a theoretical mobility of 0.49 cm(2) V(-1) s(-1). The obtained results hopefully could motivate experimentalists to try out P3OHT for an improved charge carrier mobility.     

Adsorption Mechanism and Structure of the Montmorillonite Complexes with (CH(3))(2)XO (X=C, and S), (CH(3)O)(3)PO, and CH(3)-CN Molecules

The formation of complexes with different ligands in the interlayer space of montmorillonite saturated in Na(+), Mg(2+), Ca(2+), Co(2+), Cu(2+), Ni(2+), Fe(3+), and Cr(3+) was studied. Acetone, acetonitrile, dimethyl sulfoxide, and trimethylphosphate were used as ligands. The nature of the complexes was studied by means of X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, microcalorimetry, and ab initio quantum mechanical methods. In all cases, the organic ligands penetrate into the interlayer space at room temperature, forming complexes stable in vacuum with the interlayer cations. The ligand-cation ratio depends on the valence of the saturating cation. The cation-ligand interaction in these complexes has an ion-dipole electrostatic nature. The complexes are formed by the direct interaction of the oxygen or nitrogen atom of ligand and the interlayer cation. Using the quantum mechanical approach, allow us to determine the disposition of the ligand in these complexes. In all cases, only one layer of ligands is present in the stable complexes. Copyright 2000 Academic Press.     

Hartree-Fock soft Coulomb hole to estimate correlation energies in atoms, molecules and polymers

We have shown that the empirical correction introduced into the Hartree-Fock method to calculate correlation energies for atoms and therefore to remove the error caused by the so-called Coulomb hole can be extended from atoms to molecules and polymers. A reformulation was required of the necessary parameter representation. The reparametrization has been performed staying as close as possible to the original expressions for atoms reported by Chakravorty and Clementi (S.J. Chakravorty and E. Clementi, Phys. Rev. A, 39 (1989) 2290). In addition to their work, where the correlation energy has been calculated with the self-consistent Hartree-Fock wavefunction and the correction integrals, we have performed investigations, including the perturbation operator in the Fock operator, so that the total energy also contains the correlation energy. The applications of this approach to atoms and molecules show that the total electron correlation energies and ionization potentials calculated as differences of total energies can be obtained very satisfactorily. On the basis of the reported calculations it turns out that one obtains better agreement with reference values of more sophisticated calculations when the correction integrals are used to build up the Fock matrix. Furthermore we have found that the magnitude of the correlation energy depends only weakly on the size of the basis sets, which makes this empirical method very attractive for its application to large molecular and polymeric systems.     

DFT research on the dehydroxylation reaction of pyrophyllite 1. First-principle molecular dynamics simulations

The dehydroxylation of pyrophyllite involves the reaction of OH groups and elimination of water molecules through two possible mechanisms, one involving the bridging hydroxyl groups of an octahedral Al (3+) pair and the other two hydroxyl groups reacting across the dioctahedral vacancy. First-principles molecular dynamics simulations at the density functional theory level are used together with the metadynamics algorithm to explore the free-energy surface (FES) of the initial step of the dehydroxylation. We observe that the two possible dehydroxylation mechanisms yield similar activation energies at 0 K, but at high temperatures, the cross mechanism has lower free energy than that of the on-site one. The dehydroxylation process produces different semidehydroxylated intermediates that should be taken into account. The role of the temperature in favoring a dehydroxylation nonconcerted chain mechanism over another is here elucidated, and a novel competitive mechanism, which is assisted by the structural apical oxygens in the high-temperature regime, is proposed.     

Classical and quantum mechanical calculations of conformational probability density distributions. Two-rotor molecules

In order to study the dynamical structure of a two-rotor molecule, such as acetone, as a function of temperature, conformational probability density distributions are computed by using three different approaches: the so-called current approach, the classical approach, and the quantum mechanical oscillator approach. It is found that the three procedures yield comparable results, at least at normal temperature (25°C), although the current and, especially, the classical approaches give rise to too sharp distributions when compared with the quantum mechanical results. Owing to its simplicity, the current approach may be used advantageously, and it is easily extendible to many-rotor systems. Finally, it is verified that deuteration does not affect appreciably the conformation.     

An ab initio comparative structural study of alkenylphosphonic acid derivatives

A comparative study of the electronic structure and conformational properties of alkenylphosphonic acid derivatives with different substituents has been carried out by means of ab initio quantum mechanical methods. The ab initio calculations have been performed using different basis sets. A strongly polarized partial triple bond for the phosphoryl group has been found. A very weak π conjugation has been detected in the C=C/P=O system. Intramolecular hydrogen bonds have been found in 2-cis-carboxyvinylphosphonic acid.     

Gas-phase thermolysis reaction of formaldehyde diperoxide. Kinetic study and theoretical mechanisms

Gas-phase thermolysis reaction of formaldehyde diperoxide (1,2,4,5-tetroxane) was performed in an injection chamber of a gas chromatograph at a range of 463-503 K. The average Arrhenius activation energy and pre-exponential factor were 29.3 ± 0.8 kcal/mol and 5.2 × 10¹³ s⁻¹, respectively. Critical points and reaction paths of the ground singlet and first triplet potential energy surfaces (PES) were calculated, using DFT method at BHANDHLYP/6-311+G^∗∗ level of the theory. Also, G3 calculations were performed on the reactant and products. Reaction by the ground-singlet and first-triplet states turned out to be endothermic and exothermic, respectively. The mechanism in three steps seemed to be the most probable one. An electronically non-adiabatic process appeared, in which a crossing, at an open diradical structure, from the singlet to the triplet state PES occurred, due to a spin-orbit coupling, yielding an exothermic reaction. Theoretical kinetic constant coming from the non- adiabatic transition from the singlet to the triplet state agrees with the experimental values.     

Vibrations of H+(D+) in stoichiometric LiNbO3 single crystal

A first principles quantum mechanical calculation of the vibrational energy levels and transition frequencies associated with protons in stoichiometric LiNbO(3) single crystal has been carried out. The hydrogen contaminated crystal has been approximated by a model one obtains by translating a supercell, i.e., a cluster of LiNbO(3) unit cells containing a single H(+) and a Li(+) vacancy. Based on the supercell model an approximate Hamiltonian operator describing vibrations of the proton sublattice embedded in the host crystal has been derived. It is further simplified to a sum of uncoupled Hamiltonian operators corresponding to different wave vectors (ks) and each describing vibrations of a quasi-particle (quasi-proton). The three dimensional (3D) Hamiltonian operator of k=0 has been employed to calculate vibrational levels and transition frequencies. The potential energy surface (PES) entering this Hamiltonian operator has been calculated point wise on a large set of grid points by using density functional theory, and an analytical approximation to the PES has been constructed by non-parametric approximation. Then, the nuclear motion Schro?dinger equation has been solved by employing the method of discrete variable representation. It has been found that the (quasi-)H(+) vibrates in a strongly anharmonic PES. Its vibrations can be described approximately as a stretching, and two orthogonal bending vibrations. The theoretically calculated transition frequencies agree within 1% with those experimentally determined, and they have allowed the assignment of one of the hitherto unassigned bands as a combination of the stretching and the bending of lower fundamental frequency.     

Exploring the potential energy hypersurface of histamine monocation: Tautomerism in gas phase

The potential energy hypersurface of the histamine monocation is determined by ab initio methods at the STO -4G level using analytical gradient techniques. Three transition states and two minima have been found for the N_τ? H tautomer. One of the transition states connects the trans conformational region with a minimum gauche structure, where the proton of the ammonium group is approximately halfway between the N_π of the imidazole group and the N of the ammonium group, but nearer to the N_π. This minimum connects the potential energy surface of the N_τ? H tautomer with the imidazolium one. In the latter region, three transition states and two minima have been found. Critical points are discussed in relation with experimental data and histamine H₂ receptor models.     

Molecular structure and conformational analysis of chiral alcohols. Application to the enantioselectivity study of lipases

The molecular structures of the chiral compounds 1-phenylethanol, 2-hexanol and 1-phenylethanol acetate have been studied theoretically by ab initio methods. Conformational analysis and electronic structure studies have been carried out with these molecules at STO-3G^* and 6-31G^* basis sets. For the study of the interaction of lipases with substrates, a simplified model of the tetrahedral intermediate has been calculated at the 6-31G^*//4-31G^* level. Molecular mechanics simulations of the interaction of these compounds with the lipases of Candida rugosa, Pseudomonas cepacia and Rhizomucor miehei have been used to study the enantioselectivity of these lipases in the transesterification reaction of the chiral alcohols. The theoretical results have been compared with experimental data and good agreement was observed. It can be concluded that the enantioselectivity of these lipases is controlled by the formation of a tetrahedral intermediate, whereas Michaelis complex formation has a much lower significance.