Luiz F. A. Ferrão

A product branching ratio controlled by vibrational adiabaticity and variational effects: Kinetics of the H + trans-N2H2 reactions

The abstraction and addition reactions of H with trans-N(2)H(2) are studied by high-level ab initio methods and density functional theory. Rate constants were calculated for these two reactions by multistructural variational transition state theory with multidimensional tunneling and including torsional anharmonicity by the multistructural torsion method. Rate constants of the abstraction reaction show large variational effects, that is, the variational transition state yields a smaller rate constant than the conventional transition state; this results from the fact that the variational transition state has a higher zero-point vibrational energy than the conventional transition state. The addition reaction has a classical barrier height that is about 1 kcal∕mol lower than that of the abstraction reaction, but the addition rates are lower than the abstraction rates due to vibrational adiabaticity. The calculated branching ratio of abstraction to addition is 3.5 at 200 K and decreases to 1.2 at 1000 K and 1.06 at 1500 K.

Hydrogen Abstraction from the Hydrazine Molecule by an Oxygen Atom

Thermochemical and kinetics properties of the hydrogen abstraction from the hydrazine molecule (N2H4) by an oxygen atom were computed using high-level ab initio methods and the M06-2X DFT functional with aug-cc-pVXZ (X = T, Q) and maug-cc-pVTZ basis sets, respectively. The properties along the reaction path were obtained using the dual-level methodology to build the minimum energy path with the potential energy surface obtained with the M06-2X method and thermochemical properties corrected with the CCSD(T)/CBS//M06-2X/maug-cc-pVTZ results. The thermal rate constants were calculated in the framework of variational transition-state theory. Wells on both sides of the reaction (reactants and products) were found and considered in the chemical kinetics calculations. Additionally, the product yields were investigated by means of a study of the triplet and singlet surfaces of the N2H4 + O → N2H2 + H2O reaction.

Dehydrogenation of N2HX (X = 2 − 4) by nitrogen atoms: Thermochemical and kinetics

Thermochemical and kinetics of sequential hydrogen abstraction reactions from hydrazine by nitrogen atoms were studied. The dehydrogenation was divided in three steps, N2H4 + N, N2H3 + N, and N2H2 + N. The thermal rate constants were calculated within the framework of canonical variational theory, with zero and small curvature multidimensional tunnelling corrections. The reaction paths were computed with the BB1K/aug-cc-pVTZ method and the thermochemical properties were improved with the CCSD(T)/CBS//BB1K/aug-cc-pVTZ approach. The first dehydrogenation step presents the lowest rate constants, equal to 1.22 × 10(-20) cm(3) molecule(-1) s(-1) at 298 K.

A theoretical study of the inversion and rotation barriers in Methyl-substituted Amines

Barrier heights of the internal rotation and inversion motions of methylamine, dimethylamine and trimethylamine molecules were calculated with the CCSD(T)//B3LYP methodology in combination with the cc-pVTZ, cc-pVQZ, and cc-pCVTZ basis sets of Dunning. The complete basis set (CBS) extrapolation procedure and core-valence (CV) correlation effects are also examined to the barrier heights. Our best estimate results (CCSD(T)/CBS+CV//B3LYP/cc-pVQZ) are in very good agreement with the experimental data, indicating the use of this methodology to provide accurate predictions of structures and barrier heights for other systems

Theoretical and experimental study for the biomimetic recognition of levothyroxine hormone on magnetic molecularly imprinted polymer

This study addresses the rational design of a magnetic molecularly imprinted polymer (magnetic-MIP) for the selective recognition of the hormone levothyroxine. The theoretical study was carried out by the density functional theory (DFT) computations considering dispersion interaction energies, and using the D2 Grimmes correction. The B97-D/def2-SV(P)/PCM method is used not only for studying the structure of the template the and monomer-monomer interactions, but also to assess the stoichiometry, noncovalent binding energies, solvation effects and thermodynamics properties such as binding energy. Among the 13 monomers studied in silico, itaconic acid is the most suitable according to the thermodynamic values. In order to assess the efficiency of the computational study, three different magnetic-MIPs based on itaconic acid, acrylic acid and acrylamide were synthesized and experimentally compared. The theoretical results are in agreement with experimental binding studies based on laser confocal microscopy, magneto-actuated immunoassay and electrochemical sensing. Furthermore, and for the first time, the direct electrochemical sensing of L-thyroxine preconcentrated on magnetic-MIP was successfully performed on magneto-actuated electrodes within 30 min with a limit of detection of as low as 0.0356 ng mL-1 which cover the clinical range of total L-thyroxine. Finally, the main analytical features were compared with the gold standard method based on commercial competitive immunoassays. This work provides a thoughtful strategy for magnetic molecularly imprinted polymer design, synthesis and application, opening new perspectives in the integration of these materials in magneto-actuated approaches for replacing specific antibodies in biosensors and microfluidic devices.

Singlet La and Lb Bands for N-Acenes (N = 2–7): A CASSCF/CASPT2 Study

In this work CASPT2 calculations of polyacenes (from naphthalene to heptacene) were performed to find a methodology suitable for calculations of the absorption spectra, in particular of the La (B2u state) and Lb (B3u state) bands, of more extended systems. The effect of the extension of the active space and of freezing σ orbitals was investigated. The MCSCF excitation energy of the B2u state is not sensitive to the size of the active space used. However, the CASPT2 results depend strongly on the amount of σ orbitals frozen reflecting the ionic character of the B2u state. On the other hand, the excitation energies of the B3u state are much more sensitive to the size of the active space used in the calculations reflecting its multiconfigurational character. We found a good agreement with experimental data for both bands by including 14 electrons in 14 π orbitals in the active space followed by the CASPT2(14,14) perturbation scheme in which both σ and π orbitals are included.

Thermochemical and Kinetics of Hydrazine Dehydrogenation by an Oxygen Atom in Hydrazine-Rich Systems: A Dimer Model.

The kinetics of the reaction of N2H4 with oxygen depends sensitively on the initial conditions used. In oxygen-rich systems, the rate constant shows a conventional positive temperature dependence, while in hydrazine-rich setups the dependence is negative in certain temperature ranges. In this study, a theoretical model is presented that adequately reproduces the experimental results trend and values for hydrazine-rich environment, consisting of the hydrogen abstraction from the hydrazine (N2H4) dimer by an oxygen atom. The thermochemical properties of the reaction were computed using two quantum chemical approaches, the coupled cluster theory with single, double, and noniterative triple excitations (CCSD(T)) and the M06-2X DFT approach with the aug-cc-pVTZ and the maug-cc-pVTZ basis sets, respectively. The kinetic data were calculated with the improved canonical variational theory (ICVT) using a dual-level methodology to build the reaction path. The tunneling effects were considered by means of the small curvature tunneling (SCT) approximation. Potential wells on both sides of the reaction ((N2H4)2 + O → N2H4···N2H3 + OH) were determined. A reaction path with a negative activation energy was found leading, in the temperature range of 250-423 K, to a negative dependence of the rate constant on the temperature, which is in good agreement with the experimental measurements. Therefore, the consideration of the hydrazine dimer model provides significantly improved agreement with the experimental data and should be included in the mechanism of the global N2H4 combustion process, as it can be particularly important in hydrazine-rich systems.

Preparation of Paraffin-Based Solid Combustible for Hybrid Propulsion Rocket Motor

Paraffin stands out as a promising solid combustible grain for the classical hybrid propulsion rocket motor. However, its low mechanical properties increase the risk of grain cracking and rupturing. The purpose of this experimental study is to address the existing drawbacks by formulating paraffin particles within a hydroxyl-terminated polybutadiene binder. Paraffin particles are prepared by crystallization in emulsion, which allows the control of particle size distribution, based on the droplets breakup and coalescence equilibrium. The paraffin particles are suspended in hydroxyl-terminated polybutadiene, and the maximum paraffin loading (62% volume fraction) is attained with the use of bimodal systems. A combustible grain (paraffin particles/hydroxyl-terminated polybutadiene) is cast with the maximum loading and submitted to thermal and mechanical tests. The results are compared to hydroxyl-terminated polybutadiene and paraffin wax counterparts, and they meet the criteria of performance and safety requi...

Thermochemical and Kinetics of CH3SH + H Reactions: The Sensitivity of Coupling the Low and High-Level Methodologies

The reaction system formed by the methanethiol molecule (CH3SH) and a hydrogen atom was studied via three elementary reactions, two hydrogen abstractions and the C-S bond cleavage (CH3SH + H → CH3S + H2 (R1); → CH2SH + H2 (R2); → CH3 + H2S (R3)). The stable structures were optimized with various methodologies of the density functional theory and the MP2 method. Two minimum energy paths for each elementary reaction were built using the BB1K and MP2 methodologies, and the electronic properties on the reactants, products, and saddle points were improved with coupled cluster theory with single, double, and connected triple excitations (CCSD(T)) calculations. The sensitivity of coupling the low and high-level methods to calculate the thermochemical and rate constants were analyzed. The thermal rate constants were obtained by means of the improved canonical variational theory (ICVT) and the tunneling corrections were included with the small curvature tunneling (SCT) approach. Our results are in agreement with the previous experimental measurements and the calculated branching ratio for R1:R2:R3 is equal to 0.96:0:0.04, with kR1 = 9.64 × 10-13 cm3 molecule-1 s-1 at 298 K.

Hydrazine decomposition on a small platinum cluster: the role of N2H5 intermediate

This work presents a comprehensive DFT study on the interaction between hydrazine derivatives with a platinum catalyst surface, which is represented by a tetrahedral Pt4 cluster model. Three separate reaction pathways were investigated; two of which are related to possible pathways of NH3 formation. The first pathway describes the intramolecular transfer of one hydrogen atom in the hydrazine molecule forming the NHNH3 intermediate, then dissociating into NH and NH3. The second describes the addition of one external hydrogen atom to hydrazine forming N2H5, followed by its dissociation to NH2 and NH3. The third reaction pathway involves the formation of N2H3 by means of hydrogen abstraction by an external hydrogen. The reactions were studied in both the absence and the presence of a Pt4 cluster. We find that the assistance of the Pt4 cluster lacks a systematic effect on the reactions barrier heights. It is also shown that the ammonia formation can possibly proceed through the formation of the N2H5 intermediate, leading to more exothermic intermediate steps in the presence of the Pt4 cluster.