A theoretical study of five nitrates: Electronic structure and bond dissociation energies

Three density-functional methods (B3P86, B3PW91, and B3LYP) are employed to investigate the O–NO₂ bond lengths, frontier orbital energies, and O–NO₂ bond dissociation energies (BDEs) of n-propyl nitrate (NPN), isopropyl nitrate (IPN), 2-ethylhexyl nitrate (EHN), triethylene glycol dinitrate (Tri-EGDN), and tetraethylene glycol dinitrate (Tetra-EGDN). It is found that the O–NO₂ bond lengthens (destabilizes) in the order of IPN, NPN, EHN, Tetra-EGDN, and Tri-EGDN. From the data of frontier orbital energies (E_HOMO, E_LUMO), and energy gaps (ΔE), we estimate the relative thermal stability ordering of five nitrates and their corresponding radicals. The predicted BDEs of O–NO₂ bond in NPN, IPN, EHN, Tri-EGDN, and Tetra-EGDN, are 176.6, 174.5, 168.1, 156.1, and 159.3 kJ mol⁻¹, respectively. Based on the finding that the present results of BDEs are well coincident with the experimental results of apparent activation energies from the literature, we can draw a conclusion that the experimental thermolysis of five nitrates is only unimolecular homolytical cleavage of the O–NO₂ bonds.     

Synthesis of non-equilibrium PtxOs1−x Solid solutions. Crystal structure of [Pt(NH3)4][OsCl6]

Several non-equilibrium solid solutions belonging to the platinum-osmium systems Os_0.9Pt_0.1, Os_0.8Pt_0.2, Os_0.5Pt_0.5, Os_0.7Pt_0.3, Os_0.75Pt_0.25 are prepared and studied. The thermal decomposition of [Pt(NH₃)₄][OsCl₆] in the hydrogen and helium atmosphere is investigated. It is found that the Pt_0.5Os_0.5 solid solution develops through the formation of (NH₄)₂[OsCl₆] and a metallic phase based on Pt. The crystal structure of a double complex salt [Pt(NH₃)₄][OsCl₆] is studied (X8-APEX Bruker, 1508 independent reflections, R = 2.04%). Crystal data for PtOsN₄Cl₆H₁₂ are: a = 11.6216(5) Å, b = 11.0016(5) Å, c = 10.3819(5) Å, V = 1327.4(1) ų, space group Cmca, Z = 4, d _x = 3.333 g/cm₃. The coordination polyhedron around Os is octahedral: 〈Os-Cl〉 2.357 Å, ∠Cl-Os-Cl 89.5–90.5°, while around Pt it is square-planar: Pt-N 2.046 Å, ∠ N-Pt-N 89.59° and 90.41°.     

Combining thermal analysis with other techniques to monitor the decomposition of poly(m-phenylene isophthalamide)

The pyrolysis behaviour of Nomex, poly(m-phenylene isophthalamide) fibres under argon has been investigated up to a temperature of 1173 K with different methods to get direct information on the progressive changes taking place in the solid material and its carbon fibre residues. The main stages of the pyrolytic degradation of the fibres were determined by thermal analysis (TG and DTA) and their chemical and morphological evolution through the different steps was subsequently followed by Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM) measurements, respectively, on samples treated to various temperatures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pyrolysis Analysis and Kinetics of Crude Oils

This research presents the results of an experimental study on the determination of pyrolysis behaviour and kinetics of six crude oils by differential scanning calorimetry (DSC) and thermogravimetry (TG/DTG). Crude oil pyrolysis indicated two main temperature ranges where loss of mass was observed. The first region between ambient to 400°C was distillation. The second region between 400 and 600°C was visbreaking and thermal cracking. Arrhenius-type kinetic model is used to determine the kinetic parameters of crude oils studied. It was observed that as crude oils gets heavier (°API decreases) cracking activation energy increases. Activation energy of cracking also show a general trend with asphaltene content. This revised version was published online in July 2006 with corrections to the Cover Date.     

Similarities and differences in the electron ionization-induced fragmentation of structurally related N-alkoxymethyl lactams and sultams

Unimolecular fragmentation patterns of the molecular ions of selected lactams and sultams bearing alkoxymethyl group at the nitrogen atom were studied. The main common fragmentation reaction observed for all compounds studied in this work is the elimination of an aldehyde molecule. This reaction is considered to proceed via two different mechanisms. For lactams, hydrogen rearrangement within an alkoxymethyl group is observed, which leads to the appropriate N-methyl derivatives. For sultams, transfer of the methyl group to the nitrogen and oxygen atoms, proceeding through an ion-neutral complex, dominates. Another important fragmentation channel characteristic exclusively for lactams is the loss of an alkyl radical. This process takes place within the N-alkoxymethyl moiety, yielding the appropriate protonated ion of N-formyllactams. This process is accompanied by relatively high kinetic energy release.     

Diffusion of Cesium Ions in Agar Gel Containing Alkali Metal Bromides

The present paper gives an account of different aspects of the tracer diffusion of Cs⁺ ions in alkali metal bromides. We have measured the diffusion coefficients, D, of cesium ions in 1% agar gel medium at 25 ^∘C using a zone-diffusion technique over a concentration range of 5 × 10⁻⁵ to 0.1 mol,dm⁻³. The values of the diffusion coefficients were found to deviate from theory, which are explained on the basis of different types of interactions occurring in the ion-gel-water system. The study is also focused on the effect of alkali metal bromides on the obstruction effect and activation energy for the tracer-diffusion of cesium ions in agar gel medium. It is observed that both parameters, extent of obstruction, ∝, and activation energy, E, decrease with increasing charge density of the cation of the supporting electrolyte. The influence of these trends is explained on the basis of competitive hydration between the ions and agar molecules, and the relative distortion in the water structure that is brought about by these different ions and agar molecules.     

Heavy metal phosphate nanophases in silica: influence of radiolysis probed via f-electron state properties

We have assessed the feasibility of carrying out time- and wavelength-resolved laser-induced fluorescence measurements of radiation damage in glassy silica. The consequences of alpha decay of Es-253 in LaPO₄ nanophases embedded in silica were probed based on excitation of 5f states of Cm³⁺, Bk³⁺, and Es³⁺ ions. The recorded emission spectra and luminescence decays showed that alpha decay of Es-253 ejected Bk-249 decay daughter ions into the surrounding silica and created radiation damage within the LaPO₄ nanophases. This conclusion is consistent with predictions of an ion transport code commonly used to model ion implantation. Luminescence from the ⁶D_7/2 state of Cm³⁺was used as an internal standard. Ion-ion energy transfer dominated the dynamics of the observed emitting 5f states and strongly influenced the intensity of observed spectra. In appropriate sample materials, laser-induced fluorescence provides a powerful method for fundamental investigation of alpha-induced radiation damage in silica.     

Thermodynamics of liquid mixtures containing n-alkanes and strongly polar components: VE and C P E of mixtures with either pyridine or piperidine

Excess molar volumes V _E and excess molar heat capacities C _P ^/E at constant pressure have been obtained, as a function of mole fraction x₁, for several binary liquid mixtures belonging either to series I: pyridine+n-alkane (C_lH_2l+2), with l=7, 10, 14, 16, or series II: piperidine+n-alkane, with l=7, 8, 10, 12, 14. The instruments used were a vibrating-tube densimeter and a Picker flow microcalorimeter, respectively. V ^E of pyridine+n-heptane shows a S-shaped composition dependence with a small negative part in the region rich in pyridine (x₁>0.90). All the other systems show positive V ^E only. The excess volumes increase with increasing chain length l of the n-alkane. The excess molar heat capacities of the mixtures belonging to series II are all negative, except for a small positive part for piperidine+n-heptane in the region rich in piperidine (x₁>0.87). The C _P ^/E at the respective minima, C _P ^/E (x_1,min ), become more negative with increasing l, and the x_1,min values range from about 0.26 (l=7) to 0.39 (l=14). Most interestingly, mixtures of series I exhibit curves of C _P ^/E against x₁ with two minima and one maximum, the so-called W-shape curves.Dedicated to Professor A. Néckel on the occasion of his 65^th birthday. Communicated in part at the XVII^èmes Journées de Calorimétrie, d'Analyse Thermique et de Thermodynamique Chimique, Ferrara, Italy, 27–30 October, 1986.     

Ab initio Computational Modeling of Glyphosate Analogs: Conformational Perspective

A historical perspective on the application of conformational analysis to structure-based ligand design approach is presented. The application of isodensity molecular electrostatic potential surfaces with the conformational energy surfaces (CES) have allowed us to reach pertinent conclusions for aiding synthetic and biochemical studies. Here we illustrate such an application on the modeling of the potent analogs of an important, environmentally stringent herbicidal compound glyphosate by constructing conformational energy surfaces. The systems were modeled by substituting F, Cl, and NH— OH moiety to the position of pharmacophoric nitrogen center in glyphosate structure. All the calculations were thoroughly performed with ab initio MO theory at Hartree–Fock method using 3-21G(d) basis functions. On the basis of the results, we identified the bioactive conformations for N-fluoro-glyphosate, N-chloro-glyphosate, and N-hydroxyamino-glyphosate as (−38^∘, 77^∘), (−61^∘, 111^∘), and (−167^∘, −169^∘), respectively. Geometry optimization of certain selected conformations of these compounds using hybrid DFT method with 6–31+G(d) basis functions provides nearly equal values of φ and ψ. Moreover, the results indicate that the global minimum structures of N-fluoro and N-chloro analogs of glyphosate show cyclic conformation whereas the N-hydroxyamino-glyphosate global minimum structure shows spyrocyclic and zig-zag conformation. Also, the predicted bioactive conformation of N-hydroxyamino analog optimally overlaps with glyphosate backbone in EPSPS complex with 0.1 Å RMSD value. However, the other two compounds slightly deviate from the backbone of glyphosate with RMSD of 0.92 Å for N-fluoro-glyphosate and 0.83 Å for N-chloro-glyphosate. The linear N-hydroxyamino-glyphosate exhibits relatively more number of intermolecular hydrogen bond interactions as compared to the other two analogs. Further, comparison of CES of previously studied glyphosate analogs such as N-hydroxy-glyphosate (2.2 μM) and N-amino-glyphosate (0.61 μM) with the present systems reveals the order of activity as: N-hydroxyamino-glyphosate > N-fluoro-glyphosate > N-chloro-glyphosate based on CES flexibility. Also, the calculated heats of formation of N-fluoro-glyphosate, N-chloro-glyphosate, and N-hydroxyamino-glyphosate are −288, −209, and −288 kcal/mol, respectively, which clearly indicate that the N-hydroxyamino and N-fluoro analogs of glyphosate are thermodynamically more stable than N-amino-glyphosate (−278 kcal/mol).     

Cathodoluminescence Depth Profiling in SiO2:Ge Layers

 For investigation of the luminescent center profile cathodoluminescence measurements are used under variation of the primary electron energy E ₀ = 2…30 keV. Applying a constant incident power regime (E ₀·I ₀ = const), the depth profiles of luminescent centers are deduced from the range of the electron energy transfer profiles dE/dx. Thermally grown SiO₂ layers of thickness d = 500 nm have been implanted by Ge⁺-ions of energy 350 keV and doses (0.5–5)10¹⁶ ions/cm². Thus Ge profiles with a concentration maximum of (0.4 – 4) at% at the depth of d_m≅240 nm are expected. Afterwards the layers have been partially annealed up to T _a = 1100 °C for one hour in dry nitrogen. After thermal annealing, not only the typical violet luminescence (λ = 400 nm) of the Ge centers is strongly increased but also the luminescent center profiles are shifted from about 250 nm to 170 nm depth towards the surface. This process should be described by Ge diffusion processes, precipitation and finally Ge nanocluster formation. Additionally, a Ge surface layer is piled-up extending to a depth of roughly 25 nm.