Theoretical study of beryllium (II) complexes using CATIVIC: New parametric method

Calculations of several beryllium complexes {[Be(H₂O)_n]²⁺ (n = 1–4), [BeOH(H₂O)_n]⁺ (n = 1–3), and [Be(OH)₂(H₂O)_n] (n = 1, 2)} were carried out to compare different ab initio (density functional theory, MP2) and parametric (PM3(tm), CATIVIC) methods. Results show that the parametric method CATIVIC gives geometries and energies closer to the ab initio geometries than the PM3(tm) method due to the inclusion of the atomic excitation energies of the neutral atoms as well as the ions and to the dependence of the molecular parameters on the system charge. The molecular electronic density analysis of the Be? O bonds shows that the Be–water interaction in the [Be(H₂O)_n]²⁺ complexes can be considered as a closed‐shell interaction with a σ character in the bond while in the [Be(OH)₂(H₂O)_n] complexes the Be? water bond have π character. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Theoretical and experimental investigation of the second hyperpolarizabilities of methyl orange

We describe experimental and theoretical studies of the third-order nonlinear optical coefficients of methyl orange solutions under different pH conditions. A combination of semiempirical and ab initio methods was adopted to investigate the most stable geometrical structures possible for this molecule. The experimental data obtained using the Z-scan technique for the third-order nonlinear optical properties of this compound has allowed the determination of the nonlinear refractive index and nonlinear absorption coefficient under picosecond excitation in the visible (532 nm) spectral region. From those experimental results, the second hyperpolarizability of methyl orange was inferred both for acidic and alkaline solutions. Comparison of these values to the results predicted by semiempirical methods suggests that even at low pH, when the probability of cis-trans isomerization is increased, the trans conformation of the methyl orange molecule should dominate the nonlinear spectra of this compound. The theoretical results were used as an auxiliary tool to identify possible trends on the nonlinear properties changes as a function of the distinct molecular conformations adopted by the methyl orange molecule under different pH conditions.     

Compartmentalization Effects on Miniemulsion Polymerization with Oil‐Soluble Initiator

A dynamic mathematical model is developed to represent miniemulsion polymerization reactions using oil‐soluble initiator. Both simulation and experimental results show that the evolution of conversion varies with particle diameter and that reactions become slower as the particle size is increased. For each experimental condition, a limiting diameter is obtained, above which the reaction kinetics does not change anymore with the diameter. These results are explained in terms of the compartmentalization of growing radicals during the miniemulsion polymerizations. The evolution of conversion in bulk polymerizations shows that the kinetic behavior above the limiting diameter is similar to the kinetic behavior of bulk reactions.

     

Effect of the AL-MCM-41 catalyst on the catalytic pyrolysis of atmospheric petroleum residue (ATR)

Thermogravimetry (TG) was used in this study to evaluate thermal and catalytic pyrolysis of Atmospheric Petroleum Residue (ATR) which can be found in the state of Rio Grande do Norte/Brazil, after a process of atmospheric distillation of petroleum. The utilized sample in the process of catalytic pyrolysis was Al-MCM-41, a mesoporous material. The procedures for obtaining the thermogravimetric curves were performed in a thermobalance with heating rates of 5, 10, and 20 °C min⁻¹. From TG, the activation energy was determined using the Flynn–Wall kinetic method, which decreased from 161 kJ mol⁻¹, for the pure ATR, to 71 kJ mol⁻¹, in the presence of the Al-MCM-41, showing the efficiency of the catalyst in the pyrolysis of Atmospheric Petroleum Residue.