Investigation of the Abstraction and Dissociation Mechanism in the Nitrogen Trifluoride Channels: Combined Post-Hartree-Fock and Transition State Theory Approaches.

The present paper concludes our series of kinetics studies on the reactions involved in the complex mechanism of nitrogen trifluoride decomposition. Two other related reactions that, along with this mechanism, take part in an efficient boron nitride growth process are also investigated. We report results concerning two abstraction reactions, namely NF2 + N ⇄ 2NF and NF3 + NF ⇄ 2NF2, and two dissociations, N2F4 ⇄ 2NF2 and N2F3 ⇄ NF2 + NF. State-of-the-art electronic structure calculations at the CCSD(T)/cc-pVTZ level of theory were considered to determine geometries and frequencies of reactants, products, and transition states. Extrapolation of the energies to the complete basis set limit was used to obtain energies of all the species. We applied transition state theory to compute thermal rate constants including Wigner, Eckart, Bell, and deformed theory corrections in order to take tunneling effects into account. The obtained results are in good agreement with the experimental data available in the literature and are expected to provide a better phenomenological understanding of the NF3 decomposition role in the boron nitride growth for a wide range of temperature values.

Polaron Properties in Armchair Graphene Nanoribbons.

By means of a 2-D tight-binding model with lattice relaxation in a first-order expansion, we report different polaron properties depending on the armchair graphene nanoribbons width family as well as on its size. We find that representatives of the 3p+2 family do not present a polaronic-mediated charge transport. As for 3p and 3p+1 families, the polaron behavior was completely dependent on the system's width. In particular, we observed a greater degree of delocalization for broader nanoribbons; narrower nanoribbons of both families, on the contrary, typically presented a more localized polaronic-type transport. Energy levels and occupation numbers analysis are performed to rigorously assess the nature of the charge carrier. Time evolution in the scope of the Ehrenfest molecular dynamics was also carried out to confirm the collective behavior and stability of the carrier as a function of time. We were able to determine that polarons in nanoribbons of 3p family present higher mobility than those in 3p+1 nanoribbons. These results identify the transport process that takes place for each system, and they allow the prediction of the mobility of the charge carriers as a function of the structural properties of the system, thus providing guidance on how to improve the efficiency of graphene nanoribbon-based devices.

Vibrational and electronic structure analysis of a carbon dioxide interaction with functionalized single-walled carbon nanotubes.

Electronic and vibrational properties of different single-walled carbon nanotubes (SWNTs) interacting with a CO2 molecule are investigated through the use of density functional theory (DFT) calculations and the discrete variable representation (DVR) method, respectively. We observed a considerable geometry difference between pristine and doped nanotubes. Consequently, a greater binding energy between the former type of nanotubes and the adsorbing molecule is achieved, a fact that finds experimental support and leads us to consider cobalt-doped nanotubes as promising candidates for chemical sensors. From the vibrational point of view, we note that the zigzag chirality tends to present higher values of vibrational frequencies for most of the states considered regardless of the nanotubes being doped or not. The potential energy curves (PECs) for the interactions between CO2 and all of the considered nanotubes together with spectroscopic constants are provided, and the reliability of the performed calculations makes the data a useful source of comparison for future works.

Exciton dissociation and charge carrier recombination processes in organic semiconductors.

Exciton dissociation and charge recombination processes in organic semiconductors, with thermal effects taken into account, are described in this paper. Here, we analyzed the mechanisms of polaron-excitons dissociation into free charge carriers and the consequent recombination of those carriers under thermal effects on two parallel π-conjugated polymers chains electronically coupled. Our results suggest that exciton dissociation in a single molecule give rise to localized, polaron-like charge carrier. Besides, we concluded that in the case of interchain processes, the bimolecular polaron recombination does not lead to an usual exciton state. Rather, this type of recombination leads to an oscillating dipole between the two chains. The recombination time obtained here for these processes are in agreement with the experimental results. Finally, our results show that temperature effects are essential to the relaxation process leading to polaron formation in a single chain, as in the absence of temperature, this process was not observed. In the case of two chains, we conclude that temperature effects also help the bimolecular recombination process, as observed experimentally.

Bosonic Charge Carriers in Necklace-like Graphene Nanoribbons.

Motivated by the success of graphene in flat optoelectronics, several carbon allotropes have recently been proposed. One of these allotropes, graphene nanoribbons (GNRs) with a singular "necklace-like" atomic structure, was recently synthesized through a bottom-up chemical approach. The absorption spectrum exhibited a band gap of 1.4 eV for this novel GNR geometry. Guided by its exciting electronic and structural properties, investigations should be performed to outline the major features of this material focused on expanding organic-based energy conversion and storage applications. In particular, the formation and dynamics of charge carriers are crucial in defining the material's performance. Here we describe the formation and transport of charge carriers in necklace-like graphene nanoribbons (NGNRs). A 2D tight-binding Hamiltonian endowed with lattice relaxation effects constitutes the basis of the theoretical approach employed to examine the carrier formation and dynamics in these lattices. Results demonstrate that polarons and effective boson species are spontaneously generated in NGNRs by the addition of holes or electrons to the system. Both types of generated quasiparticles are dynamically stable and can move at surprisingly low electric-field regimes. Remarkably, the formation of effective bosons is a process triggered by a higher density of added charges. The understanding of the carriers' formation and transport in NGNRs can pave the way for their broad usage in producing novel optoelectronic applications, added to the possibility of Bose-Einstein condensate phenomena.

Rovibrational energies and spectroscopic constants for H2O-Ng complexes.

In this work, rovibrational energies and spectroscopic constants for the water -Ng complexes (Ng = He, Ne, Ar, Kr and Xe) were calculated through two different approaches (by solving the Nuclear Schrödinger equation and by applying the Dunham's method) and using two different potential energy curves (PEC). These PEC were determined using potential parameters obtained through molecular beam scattering experiments and accurate theoretical calculation, respectively. It was found that the theoretical rovibrational energies are in a good agreement (only for the lowest numbers of vibrational states) with those obtained through experimental PEC. Another important conclusions was regarding the calculated first two rovibrational energies for the H 2 O-Ar system, that are in a good agreement with the experimental data.

A detailed reactive cross section study of X + Li2 → Li + LiX, with X = H, D, T, and Mu.

In this work we apply quasiclassical trajectory theory to the X + Li2 → Li + LiX reactions, with X standing for H, D, T, and Mu, in order to determine dynamical properties such as state-to-state reactive cross-section, rotational, vibrational, and translational product distributions. By using the literature benchmark potential energy surface, we were able to predict the aforementioned dynamical property in remarkable qualitative agreement with data in the literature for the H + Li2 → Li + LiH channel. Particularly, our results points toward the well known cross section independence with ro-vibrational excitations for high excitation regimes. Since the methodology is known to be well suited for the other species, as we considered the same PES, our results are expected to be similarly accurate for D, T, and Mu. The present work consists on a significant progress in this area of research, since previous theoretical calculations-based on known potential energy surface-deviated from the experimental results.

Níveis séricos de teofilina e velocidade de administraçäo por vía endovenosa / Serum theophylline levels and intravenous administration velocity

Com o objetivo de avaliar a influência da velocidade de administraçäo de aminofilina por via endovenosa sobre a concentraçäo sanguínea inicial de teofilina, foram avaliados 21 voluntários normais divididos em dois grupos. Grupo I - Dez indivíduos quais foi administrada aminofilina (5,6 mg/kg) diluída em 100ml de soro fisiológico em 20 minutos. Grupo II - Onze voluntários em que a mesma dose foi injetada diluída em 10ml de soro, em cinco minutos. Amostras sanguíneas para dosagem da teofilina sérica foram colhidas imediatamente após a injeçäo (tempo 0) e após 3 e 5 minutos. Os valores médios obtidos nos grupos I e II, foram respectivamente (microng/ml): 0' - 9,70 ñ 2,07 e 33,87 ñ 13,09; 3' - 9,41 ñ 2,45 e 22,39 ñ 8,03; 5' - 8,82 ñ 2,21 e 18.01 ñ 6,02. O nível máximo de teofilinemia no grupo I foi de 13,7 microng/ml e no grupo II 57,1 microng/ml. Somente em dois indivíduos deste grupo os níveis foram inferiores a 20microng/ml no tempo zero. Como a distribuiçäo da teofilina do compartimento vascular para o extravascular é rápida, pode-se assumir que a concentraçäo sanguínea reflita os valores teciduais. Assim a administraçäo endovenosa rápida coloca os pacientes por um curto período sob risco de efeitos colaterais potencialmente graves. Concluímos que a dose de ataque de aminofilina deve ser administrada por via endovenosa em 20 minutos, eliminando-se o perigoso hábito das injeçöes rápidas