Itamar Borges

Ab initio modeling of excitonic and charge-transfer states in organic semiconductors: the PTB1/PCBM low band gap system.

A detailed quantum chemical simulation of the excitonic and charge-transfer (CT) states of a bulk heterojunction model containing poly(thieno[3,4-b]thiophene benzodithiophene) (PTB1)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is reported. The largest molecular model contains two stacked PTB1 trimer chains interacting with C60 positioned on top of and lateral to the (PTB1)3 stack. The calculations were performed using the algebraic diagrammatic construction method to second order (ADC(2)). One main result of the calculations is that the CT states are located below the bright inter-chain excitonic state, directly accessible via internal conversion processes. The other important aspects of the calculations are the formation of discrete bands of CT states originating from the lateral C60s and the importance of inter-chain charge delocalization for the stability of the CT states. A simple model for the charge separation step is also given, revealing the energetic feasibility of the overall photovoltaic process.

Topological analysis of the molecular charge density and impact sensitivy models of energetic molecules.

Important explosives of practical use are composed of nitroaromatic molecules. In this work, we optimized geometries and calculated the electron density of 17 nitroaromatic molecules using the Density Functional Theory (DFT) method. From the DFT one-electron density matrix, we computed the molecular charge densities, thus the electron densities, which were then decomposed into electric multipoles located at the atomic sites of the molecules using the distributed multipole analysis (DMA). The multipoles, which have a direct chemical interpretation, were then used to analyze in details the ground state charge structure of the molecules and to seek for correlations between charge properties and sensitivity of the corresponding energetic material. The DMA multipole moments do not present large variations when the size of the Gaussian basis set is changed; the largest variations occurred in the range 10-15% for the dipole and quadrupole moments of oxygen atoms. The charges on the carbon atoms of the aromatic ring of each molecule become more positive when the number of nitro groups increases and saturate when there are five and six nitro groups. The magnitude and the direction of the dipole moments of the carbon atoms, indicators of site polarization, also depend on the nature of adjacent groups, with the largest dipole value being for C-H bonds. The total magnitude of the quadrupole moment of the aromatic ring carbon atoms indicates a decrease in the delocalized electron density due to an electron-withdrawing effect. Three models for sensitivity of the materials based on the DMA multipoles were proposed. Explosives with large delocalized electron densities in the aromatic ring of the component molecule, expressed by large quadrupole values on the ring carbon atoms, correspond to more insensitive materials. Furthermore, the charges on the nitro groups also influence the impact sensitivity.

Theoretical study of the reaction between HF molecules and hydroxyl layers of Mg(OH)2.

The reaction of HF molecules with brucite, Mg(OH)(2), leading to the formation of Mg(OH)(2-x)F(x), was theoretically studied by ab initio density functional theory (DFT) with periodic boundary conditions. We proposed as mechanism for this reaction four elementary steps: adsorption of the HF molecule, OH(-) liberation from brucite as a water molecule, desorption of the newly formed H(2)O, and rearrangement of the F(-) anion into a hydroxyl position. For the Mg(OH)(2-x)F(x) formation, with x = 1/9, the final product, outcome from an initially adsorbed HF molecule, we computed the Helmholtz free energy variation DeltaF = -23 kcal/mol. The calculated frequency for the most intense infrared band, a Mg-F stretching mode, was 342 cm(-1). Two transition states, corresponding to the hydroxyl reacting with a proton forming a water molecule and migration of a fluoride anion into a hydroxyl vacancy, were computed. The calculated reaction barriers indicate that the reaction between Mg(OH)(2) layers and HF molecules is slow and irreversible.

Absorption and Fluorescence Spectra of Poly(p-phenylenevinylene) (PPV) Oligomers: An ab Initio Simulation

The absorption and fluorescence spectra of poly(p-phenylenevinylene) (PPV) oligomers with up to seven repeat units were theoretically investigated using the algebraic diagrammatic construction method to second order, ADC(2), combined with the resolution-of-the-identity (RI) approach. The ground and first excited state geometries of the oligomers were fully optimized. Vertical excitation energies and oscillator strengths of the first four transitions were computed. The vibrational broadening of the absorption and fluorescence spectra was studied using a semiclassical nuclear ensemble method. After correcting for basis set and solvent effects, we achieved a balanced description of the absorption and fluorescence spectra by means of the ADC(2) approach. This fact is documented by the computed Stokes shift along the PPV series, which is in good agreement with the experimental values. The experimentally observed band width of the UV absorption and fluorescence spectra is well reproduced by the present simulations showing that the nuclear ensemble generated should be well suitable for consecutive surface hopping dynamics simulations.

Defesa química: histórico, classificação dos agentes de guerra e ação dos neurotóxicos

Chemical agents are substances used for their toxic effects on humans, animals and plants. The main objective of chemical defense is to develop systems that reduce these effects while minimizing impact on the operational capacity of military troops. In this work, a report on the development of chemical warfare agents since the First World War and their classification is presented. Special attention is given to neurotoxic agents, the most lethal group of chemical agents known to date.

Theoretical investigations on valence vibronic transitions

This article reviews previously employed methods to study several valence electronic transitions, optically forbidden or not, enhancing intensity through vibronic coupling. Electronic transition dipole moments were calculated using several ab initio methods including electron correlation. In this method the square of the electronic transition dipole moments are directly calculated along the normal coordinates of vibration and then expanded with a polynomial function. Afterwards, analytical vibrational integration using harmonic wave functions, of the square of the transition moments function, allows us to obtain partial (i.e. for each vibrational mode) and total optical oscillator strengths (OOS), for the vibronic transition of interest. We illustrate the accuracy of the method through valence transitions of benzene (C6H6), formaldehyde (H2CO), acetone (C3H6O) and formic acid (HCOOH).

Influences on the calculation of accurate and basis set extrapolated oscillator strengths: the transition of H2O

The vertical optical oscillator strength (OOS) for the transition was calculated using complete active space self-consistent field followed by multireference configuration interaction (CAS/MR-CI) wavefunctions and Gaussian basis sets of the aug-cc-pVXZ and d-aug-cc-pVXZ types (X = D, T, Q, 5). We discuss the dependence of the results on the size of the CAS space, the MR-CI calculation and the Gaussian basis set. The results were extrapolated using the F(X) = A + Bexp(−CX) function. The oscillator strengths were obtained at the length (fL) and velocity (fV) forms. The most extensive treatment gave fL and fV values, with extrapolated values between brackets, 0.048 74 (0.048 71) and 0.054 15 (0.054 12), respectively.

How to find an optimum cluster size through topological site properties: MoSxmodel clusters

Computational investigations in catalysis frequently use model clusters to represent realistically the catalyst and its reaction sites. Detailed knowledge of the molecular charge, thus electronic density, of a cluster would then allow physical and chemical insights of properties and can provide a procedure to establish their optimum size for catalyst studies. For this purpose, an approach is suggested to study model clusters based on the distributed multipole analysis (DMA) of molecular charge properties. After full density functional theory (DFT) geometry optimization of each cluster, DMA computed from the converged DFT one‐electron density matrix allowed the partition of the corresponding cluster charge distribution into monopole, dipole, and quadrupole moments on the atomic sites. The procedure was applied to MoS2 model clusters Mo10S18, Mo12S26, Mo16S32, Mo23S48, and Mo27S54. This analysis provided detailed features of the charge distribution of each cluster, focused on the 1010 (Mo or metallic edge) and 1010 (sulfur edge) active planes. Properties of the Mo27S54 cluster, including the formation of HDS active surfaces, were extensively discussed. The effect of cluster size on the site charge distribution properties of both planes was evaluated. The results showed that the Mo16S32 cluster can adequately model both active planes of real size Mo27S54. These results can guide future computational studies of MoS2 catalytic processes. Furthermore, this approach is of general applicability.

A multireference configuration interaction study of the photodynamics of nitroethylene.

Extended multireference configuration interaction with singles and doubles (MR-CISD) calculations of nitroethylene (H2C=CHNO2) were carried out to investigate the photodynamical deactivation paths to the ground state. The ground (S0) and the first five valence excited electronic states (S1–S5) were investigated. In the first step, vertical excitations and potential energy curves for CH2 and NO2 torsions and CH2 out-of-plane bending starting from the ground state geometry were computed. Afterward, five conical intersections, one between each pair of adjacent states, were located. The vertical calculations mostly confirm the previous assignment of experimental spectrum and theoretical results using lower-level calculations. The conical intersections have as main features the torsion of the CH2 moiety, different distortions of the NO2 group and CC, CN, and NO bond stretchings. In these conical intersections, the NO2 group plays an important role, also seen in excited state investigations of other nitro molecules. Based on the conical intersections found, a photochemical nonradiative deactivation process after a π–π* excitation to the bright S5 state is proposed. In particular, the possibility of NO2 release in the ground state, an important property in nitro explosives, was found to be possible.

Probing topological electronic effects in catalysis: thiophene adsorption on NiMoS and CoMoS clusters

A general two-step theoretical approach to study electronic redistributions in catalytic processes is presented. In the first step, density functional theory (DFT) is used to fully optimize two geometries: the cluster representing the catalyst and the cluster plus adsorbed molecule system. In the second step, the converged electron density is divided into multipoles centered on atomic sites according to a distributed multipole analysis which provides detailed topological information on the charge redistribution of catalyst and molecule before and after adsorption. This approach is applied to thiophene adsorption on the 10-10 metal edge of Ni(Co)MoS catalysts and compared to the same reaction on bare MoS2. Calculated adsorption energies, geometries and multipole analysis indicate weak thiophene chemisorption on both cases. A Coulombic bond model showed that surface metal-sulfur bond strengths in Ni(Co)MoS promoted catalysts are considerably smaller than in bare MoS2, thus confirming the origin of the enhancement of hydrodesulfurization (HDS) activity in these catalysts.