Validation of a Monte Carlo simulation of the plane Couette flow of a rarefied gas

We report and discuss the results of a direct Monte Carlo simulation of the flow of a rarefied gas flowing between two parallel plates when one of them moves in its own plane. The boundary conditions are assumed to be of the bounceback type and the molecules to be Maxwell's. Under this condition the moments can be computed exactly, following a method used by Ikenberry and Truesdell in the unbounded case. This allows a comparison of the Monte Carlo methods with the exact solution and an evaluation of its accuracy.     

Kinetic analysis of the carboxypeptidase A hydrolysis of oligopeptides by reversed-phase high-performance liquid chromatography

A reversed-phase high-performance liquid chromatographic (HPLC)-based method was developed to follow the time course of the hydrolytic action of pancreatic carboxypeptidase A on oligopeptide substrates of the general formula benzoyl(glycyl)n-L-Phe, n being in the range 1-5. The proposed procedure is sensitive at the nanomolar level of substrate, but also allows the kinetics of substrate hydrolysis to be investigated over a wide range of concentrations. The prior quenching of the carboxypeptidase activity and the use of isocratic conditions for the rapid separation of the substrates and their products (in less than 8 min for the slowest one), allow the automation of the procedure, which could become an easy and versatile alternative to the commonly used spectrophotometric methods. A comparative evaluation of the use of a spectrophotometric method to measure carboxypeptidase A activity on the same substrates indicated that the latter is not valid for long oligopeptides (n greater than or equal to 2) at concentrations higher than 5 mM owing to an interference of a physico-chemical nature. For benzoylglycyl-L-Phe (n = 1), the apparent kinetic parameters were calculated by means of the HPLC method and by a well established spectrophotometric procedure, and both yielded similar values.     

Indium-mediated reduction of hydroxylamines to amines

[reaction: see text] A novel and simple procedure for reduction of hydroxylamines to the corresponding amines by means of indium powder in aqueous media is reported. Applicability to one-pot reactions and isoxazolidine N-O bond reduction is also demonstrated. A catalytic version of the process using 2-5% In in the presence of other metals (Zn, Al) has been successfully developed.     

The Role ofChirocephalus diaphanusin the Early Recognition of Environmental Pollution by Trace Elements

A study was undertaken to evaluate whether the fairy shrimpChirocephalus diaphanus(Crustacea: Anostraca) could be considered as a species capable of biomonitoring environmental contamination by accumulating trace elements. This preliminary investigation aimed at assessing the levels of As, Cd, Cu, Hg, Pb, Se, and Zn in natural waters as well as in adults and cysts of fairy shrimps sampled from five different sites in central Italy. Trace elements were determined by inductively coupled plasma (ICP)-based spectrometry. In particular, ICP–mass spectrometry (ICP-MS) was resorted to in most cases due to its high detection power. Pretreatment of samples (adults and cysts) consisted basically in the HNO₃-assisted microwave digestion of the material. The highest levels of trace elements were found in the anostracan cysts, with concentration ranges (μg/g) of 0.464–3.11 for As, up to 0.820 for Cd, 2.62–13.0 for Cu, 0.011–0.213 for Hg, 0.966–8.46 for Pb, 0.295–2.45 for Se, and 16.4–50.4 for Zn. On the other hand, the lowest concentrations were found in natural waters, with values close to the ICP-MS detection power for some elements. These data are probative of a bioconcentration process, the extent of which is proportional to the environmental level of each element. This may turn out to be useful in planning biological monitoring and assessing ecotoxicological consequences which might ensue from the diffusion of the exploitation of such organisms in aquaculture.     

An Experimental and Mechanistic Investigation of the Complexities Arising During Inhibition of the Briggs?Rauscher Reaction by Antioxidants

A method to determine the relative antioxidant capacity of radical scavengers based on the inhibition of the oscillations of the Briggs? Rauscher (BR) oscillating reaction was previously reported. A semiquantitative mechanistic interpretation of the inhibitory effects required two steps to obtain simulated inhibition times in very good agreement with the experimental ones. The first step is inhibitory, involving H‐atom transfer from antioxidant to the HOO^. radical; the second step is a first‐order degradation of the antioxidant to unspecified products. Since the degradation may be due to oxidation and/or iodination of the antioxidant, we studied the kinetics of the subsystems IO (H⁺)+antioxidant and I₂(H⁺)+antioxidant. We used 2,5‐ and 2,6‐dihydroxybenzoic acids, caffeic acid (=3‐(3,4‐dihydroxyphenyl)prop‐2‐enoic acid), ferulic acid (=3‐(4‐hydroxy‐3‐methoxyphenyl)prop‐2‐enoic acid), pyrocatechol (=benzene‐1,2‐diol), and hydroquinone (=benzene‐1,4‐diol) as antioxidants. Spectra in the wavelength range 500–250 nm were repeated at given time intervals to follow the peaks of the iodine and oxidation products, which were mainly quinones. For the iodination of the above diphenols (=benzenediol derivatives) the substitution and/or addition reactions with I₂ or HOI were found to be relatively slow compared to oxidation by IO . Approximate rate constants for oxidation were obtained on the basis of a reasonable kinetic model by using a suitable numerical integration program. Although these complexities can arise also in the completely inhibited BR oscillator, we believe that the inhibitory effects are due to the HOO^. scavenging action by diphenols or by quinones since HOO^. radicals are also potential reducing agents. We propose two steps that could maintain a small reservoir of diphenol, while both quinone and diphenol deplete HOO^. radicals. In short, the complexities do not affect the method for monitoring the relative activity of antioxidants based on the BR oscillating reaction. The effects of temperature on the inverse of the oscillatory time in the BR‐uninhibited system, on the inverse of inhibition times, and on the time length of the resumed oscillations for four antioxidants were also investigated. Apparent average activation energies were obtained.     

Carbonyl derivatives of phthalocyaninatoiron(II), especially those containing group VI axial donor atoms. Crystal and molecular structure of carbonyl(N,N-dimethylformamide)phthalocyaninatoiron(II) and mössbauer studies of some of the products

The carbonyl adduct of phthalocyaninatoiron(II), FePc, with N,N-dimethylformamide (DMF) as axial ligand, FePc(CO)DMF, was prepared by the reaction of iron carbonyls, Fe(CO)₅ or Fe₂(CO)₉, with o-phthaalonitrile in DMF as solvent. Several carbonyl adducts of FePc of general formula FePc (CO)L are reported, with L being a ligand with oxygen, sulphur and nitrogen donor atoms (L = tetrahydrofuran, H₂O, CH₃OH, dimethylsulphoxide, tetrahydrothiophene, ammonia, n-propylamine, diethylamine, triethylamine). The crystal and molecular structure of FePc(CO)DMF·DMF was investigaed by X-ray diffraction methods. The compound has a monoclinic unit cell and space group P2₁/n, a 9.86(1), b 17.35(3), c 19.79(4) », β 87.9(2)°, Z = 4, U 3383 »³, D₃ 1.458 g cm^?3. The iron atom is hexacoordinated to the four inner nitrogen atoms of the macrocyle, to carbon monoxide (Fe—C distance 1.72(2) ») and to DMF (Fe—O distance 2.07(1) »). The extra DMF occupies lattice sites. All of the compounds reported in this paper are substantially diamagnetic. Mössbauer spectra show typical isomer shift parameters for the bis-adducts and for the carbonyl adduct, substantially independent of the nature of the axial ligand. The quadrupole splitting parameter of the carbonyl adducts is strongly affected by the nature of the axial ligand.     

Penning ionization of N2O molecules by He*(2(3,1)S) and Ne*(3P2,0) metastable atoms: a crossed beam study

The energetics of [Rg... N2O]* autoionizing collision complexes (where Rg=He or Ne) and their dynamical evolution have been studied in a crossed beam apparatus, respectively, by Penning ionization electron spectroscopy (PIES) and by mass spectrometry (MS) techniques in the thermal energy range. The PIES spectra, detected by an electron energy analyzer, were recorded for both complexes at four different collision energies. Such spectra allowed the determination of the energy shifts for Penning electron energy distributions, and the branching ratios for the population of different electronic states and for the vibrational population in the molecular nascent ions. For the [Ne...N2O]* collision complex it was found, by MS, that the autoionization leads to the formation of N2O+, NO+, O+, and NeN2O+ product ions whose total and partial cross sections were measured in the collision energy range between 0.03 and 0.2 eV. The results are analyzed exploiting current models for the Penning ionization process: the observed collision energy dependence in the PIES spectra as well as in the cross sections are correlated with the nature of the N2O molecule orbitals involved in the ionization and are discussed in term of the Rg-N2O interaction potentials, which are estimated by using a semiempirical method developed in our laboratory.     

1,4-thiazepines, 1,4-benzothiazepin-5-ones,and 1,4-benzothioxepin orthoamides via multicomponent reactions of isocyanides

The intramolecular Ugi four-component condensation between 6-oxo-4-thiacarboxylic acids, benzylamines, and cyclohexyl isocyanide gave hexahydro-1,4-thiazepin-5-ones and 1,4-benzothiazepin-5-ones, in some cases with high stereoselectivity, and the intramolecular Passerini three-component reaction, in the presence of catalytic amine, gave tetracyclic 1,4-benzothioxepin orthoamides.