Predicting variability in the dynamic failure strength of brittle materials considering pre-existing flaws

We perform two-dimensional dynamic fracture simulations of a specimen in biaxial tension, incorporating various distributions of pre-existing microcracks. The simulations consider the spatial distribution of flaws while modeling the discrete failure processes of crack interactions and coalescence, and predict the macroscopic variability in failure strength. The model quantitatively predicts the effect (on the dynamic failure strength) of different shapes of the flaw size distribution function, the random spatial distribution of flaws, and the random local resistance to crack growth (i.e. strength) associated with each flaw. The effect of changing material volumes on the variability in failure strengths is also examined in relation to the flaw size distribution. The effect of loading rate on the variability in failure strengths is presented in a form that will enable improved constitutive modeling using non-local formulations at the continuum scale.     

Monodentate coordination of N-[di(phenyl/ethyl)carbamothioyl]benzamide ligands: synthesis, crystal structure and catalytic oxidation property of Cu(I) complexes

New four-coordinated tetrahedral copper(I) complexes have been synthesized from the reactions between [CuCl(2)(PPh(3))(2)] and N-(diphenylcarbamothioyl)benzamide (HL1) or N-(diethylcarbamothioyl)benzamide (HL2) in benzene. These complexes have been characterized by elemental analyses, IR, UV/Vis, (1)H, (13)C and (31)P NMR spectroscopy. The molecular structure of both the complexes, [CuCl(HL1)(2)(PPh(3))] (1) and [CuCl(HL2)(PPh(3))(2)] (2) were determined by single-crystal X-ray diffraction, which reveals distorted tetrahedral geometry around each Cu(I) ion. The combination of 2 (0.005 mmol) with hydrogen peroxide (2.5 mmol) in acetonitrile is found to be an active catalyst for the oxidation of primary and secondary alcohols (0.5 mmol) to their corresponding acids and ketones, respectively, at room temperature.     

Magneto-structural studies of two new cobalt(II)-N,N-diisobutylisonicotinamide compounds: [CoLCl2]n and [Co(L)2(H2O)4][CoLBr3]2·2H2O

Two similar synthetic pathways using the ligand N,N-diisobutylisonicotinamide (L) with anhydrous CoX(2) salts (being X = Cl(-), Br(-)) led to different species: a one-dimensional system, [CoLCl(2)](n), 1, and an ionic product [Co(L)(2)(H(2)O)(4)][CoLBr(3)](2)·2H(2)O, 2, respectively. Compound 1 is a polymer in which ligand L coordinates to tetrahedral Co(II) ions in a bidentate bridging fashion using the pyridine nitrogen and carbonyl oxygen atoms. Compound 2 consists of one octahedral cationic [Co(L)(2)(H(2)O)(4)](2+) entity and two tetrahedral anionic [CoLBr(3)](-) units. In this system, the ligand molecules coordinate only through the pyridine nitrogen atoms. The magnetic properties of 1 and 2 were investigated in the temperature range of 2.0 to 300.0 K and correlations between both (due to the existence of similar features) examined. The study of the magnetic properties of 1 was carried out by considering each Co(II) ion as a perfectly isolated system, hence, J = 0, but taking into account a significant zero-field splitting contribution due to distortions on the tetrahedral environment of the cobalt atoms. The fit of the magnetic susceptibility data together with reduced magnetization vs H/T measurements provided similar parameters (|D| = 10.8 cm(-1), g(⊥) = 1.92, g(‖) = 2.92 for the former and |D| = 11.04 cm(-1)and g = 2.05 for the latter, respectively). On the other hand, the magnetic response of compound 2 has been analyzed using a model which considers the presence of two tetrahedral and one octahedral Co(ii) ions (Co(Td) and Co(Oh)). The study was carried out in two separated blocks, above and below 80 K, where only the most significant effects at each interval of temperature were considered. As a result, the analysis of the magnetic data shows weak antiferromagnetic interactions between the Co(Oh)and the two Co(Td) ions (J = -0.41 cm(-1)) in 2. The best fit parameters were g(Co(Td)) = 2.89, g(Co(Oh)) = 3.50, |D(Co(Td))| = 10.62 cm(-1), |E(Co(Td))| = 2.95 cm(-1), Δ = 240.9 cm(-1) and J(L-S) = -107.1 cm(-1), from where λ was calculated with a final value of -144.8 cm(-1) (J(L-S) = Aκλ). The approximations performed to obtain these values provide reasonable results in agreement with compound 1 and also to other systems in the literature.     

Alkyltrioctylphosphonium chloride ionic liquids: synthesis and physicochemical properties

A series of alkyltrioctylphosphonium chloride ionic liquids, prepared from trioctylphosphine, and the respective 1-chloroalkane (C(n)H(2n+1)Cl), where n = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12 or 14, is presented. The cynosure of this work is the manner in which the variable chain length impacts the physical properties, such as melting points/glass transitions, thermal stability, density and viscosity. Experimental density and viscosity data were interpreted using QPSR correlations and group contribution methods. We present the first example of an empirical alternation effect for ionic liquids.     

Separation behavior of U(VI) and Th(IV) on a cation exchange column using 2,6-pyridine dicarboxylic acid as a complexing agent and its application for the rapid separation and determination of U and Th by ion chromatography

The retention behavior of U and Th as their 2,6-pyridine dicarboxylic acid (PDCA) complexes on a cation exchange column was investigated under low pH conditions. Based on the observed retention characteristics, an ion chromatographic method for the rapid separation of uranium and thorium in isocratic elution mode using 0.08 mM PDCA and 0.24 M KNO(3) in 0.22 M HNO(3) as the eluent was developed. Both uranium and thorium were eluted as their PDCA complexes within 2 min, whereas the transition and lanthanide metal cations were eluted as an unresolved broad peak after thorium. Under the optimized conditions both U and Th have no interference either from alkali and alkaline earth elements up to a concentration ratio of 1:500 or from other elements up to 1:100. The detection limits (LOD) of U and Th were calculated as 0.04 and 0.06 ppm, respectively (S/N=3). The precision in the measurement of peak area of 0.5 ppm of both U and Th was better than 5% and a linear calibration in the concentration range of 0.25-25 ppm of U and Th was obtained. The method was successfully applied to determine U and Th in effluent water samples.     

Deprotonated N-(2,4-Dinitrophenyl)amino Acids Undergo Cyclization in Solution and the Gas Phase

The collisionally activated mass spectral fragmentations of N-(2,4-dinitrophenyl)alanine and phenylalanine [M - H](-) may be gas-phase analogs of the base-catalyzed cyclization of N-(2,4-dinitrophenyl)amino acids in aqueous dioxane. This latter reaction is one source of the 2-substituted 5-nitro-1H-benzimidazole-3-oxides, which are antibacterial agents. The fragmentation of both compounds, established by tandem mass spectrometric experiments and supported by molecular modeling using DFT methods, indicate that the [M - H](-) ions dissociate via sequential eliminations of CO(2) and H(2)O to produce deprotonated benzimidazole-N-oxide derivatives. The gas-phase cyclization reactions are analogous to the base-catalyzed cyclization in solution, except that in the latter case, the reactant must be a dianion for the reaction to occur on a reasonable time scale. The cyclization of N-(2-nitrophenyl)phenylalanine, which has one less nitro group, requires a stronger base for the cyclization than the compound with a second nitro group at the 4-position. Following losses of CO(2) and H(2)O are expulsions of both neutral molecules and free radicals, the latter being examples of violations of the even-electron ion rule.     

Structural, spectral, optical and dielectric properties of copper and glycine doped LAHCl single crystals

Cu2+ and glycine doped L-arginine monohydrochloride monohydrate (LAHCl) single crystals were grown by slow solvent evaporation technique. The grown single crystals were confirmed by X-ray diffraction study and the interaction of dopants with LAHCl molecule was identified in Fourier transform infrared spectra. The crystalline perfection of pure and doped crystals was analyzed by high resolution X-ray diffraction studies. Vickers microhardness and UV-visible spectroscopy were carried out respectively to study the mechanical stability and optical transmittance of pure and doped LAHCl single crystals. He-Ne laser of wavelength 632.8 nm was used to measure refractive index and birefringence of grown crystals. The second harmonic generation efficiency was also measured for pure and doped LAHCl single crystals using Nd:YAG laser.     

A mild and convenient synthesis of quinoxalines via cyclization-oxidation process using DABCO as catalyst

An efficient and general method has been described for the synthesis of quinoxalines by the reaction of 1,2-diamines with phenacyl bromides in the presence of DABCO. The method is suitable for the preparation of functionalized quinoxalines.     

Characterization of Nα-Fmoc-protected ureidopeptides by electrospray ionization tandem mass spectrometry (ESI-MS/MS): differentiation of positional isomers

Four pairs of positional isomers of ureidopeptides, FmocNH‐CH(R₁)‐φ(NH‐CO‐NH)‐CH(R₂)‐OY and FmocNH‐CH(R₂)‐φ(NH‐CO‐NH)‐CH(R₁)‐OY (Fmoc = [(9‐fluorenyl methyl)oxy]carbonyl; R₁ = H, alkyl; R₂ = alkyl, H and Y = CH₃/H), have been characterized and differentiated by both positive and negative ion electrospray ionization (ESI) ion‐trap tandem mass spectrometry (MS/MS). The major fragmentation noticed in MS/MS of all these compounds is due to ? N? CH(R)? N? bond cleavage to form the characteristic N‐ and C‐terminus fragment ions. The protonated ureidopeptide acids derived from glycine at the N‐terminus form protonated (9H‐fluoren‐9‐yl)methyl carbamate ion at m/z 240 which is absent for the corresponding esters. Another interesting fragmentation noticed in ureidopeptides derived from glycine at the N‐terminus is an unusual loss of 61 units from an intermediate fragment ion FmocNH = CH₂⁺ (m/z 252). A mechanism involving an ion‐neutral complex and a direct loss of NH₃ and CO₂ is proposed for this process. Whereas ureidopeptides derived from alanine, leucine and phenylalanine at the N‐terminus eliminate CO₂ followed by corresponding imine to form (9H‐fluoren‐9‐yl)methyl cation (C₁₄H₁₁⁺) from FmocNH = CHR⁺. In addition, characteristic immonium ions are also observed. The deprotonated ureidopeptide acids dissociate differently from the protonated ureidopeptides. The [M ? H]^? ions of ureidopeptide acids undergo a McLafferty‐type rearrangement followed by the loss of CO₂ to form an abundant [M ? H ? Fmoc + H]^? which is absent for protonated ureidopeptides. Thus, the present study provides information on mass spectral characterization of ureidopeptides and distinguishes the positional isomers. Copyright © 2010 John Wiley & Sons, Ltd.