Quasi-classical trajectory study of the dynamics of the Cl + CH4→ HCl + CH3 reaction

We present an on-the-fly classical trajectory study of the Cl + CH(4)→ HCl + CH(3) reaction using a specific reaction parameter (SRP) AM1 Hamiltonian that was previously optimized for the Cl + ethane reaction [S. J. Greaves et al., J. Phys Chem A, 2008, 112, 9387]. The SRP-AM1 Hamiltonian is shown to be a good model for the potential energy surface of the title reaction. Calculated differential cross sections, obtained from trajectories propagated with the SRP-AM1 Hamiltonian compare favourably with experimental results for this system. Analysis of the vibrational modes of the methyl radical shows different scattering distributions for ground and vibrationally excited products.     

Determination of arsenic in water samples by Total Reflection X-Ray Fluorescence using pre-concentration with alumina

The determination of arsenic in water samples requires techniques of high sensitivity. Total Reflection X-Ray Fluorescence (TXRF) allows the determination but a prior separation and pre-concentration procedure is necessary. Alumina is a suitable substrate for the selective separation of the analytes. A method for separation and pre-concentration in alumina, followed by direct analysis of the alumina is evaluated. Quantification was performed using the Al–Kα and Co–Kα lines as internal standard in samples prepared on an alumina matrix, and compared to a calibration with aqueous standards. Artificial water samples of As (III) and As (V) were analyzed after the treatment. Fifty milliliters of the sample at ppb concentration levels were mixed with 10 mg of alumina. The pH, time and temperature were controlled. The alumina was separated from the slurry by centrifugation, washed with de-ionized water and analyzed directly on the sample holder. A pre-concentration factor of 100 was found, with detection limit of 0.7 μgL⁻¹. The percentage of recovery was 98% for As (III) and 95% for As (V) demonstrating the suitability of the procedure.     

On the representation of a number as a sum of two fourth powers

Published in Matematicheskie Zametki, Vol. 55, No. 2, pp. 47–58, February, 1994.     

Analysis of uranium azide and nitride complexes by atmospheric pressure chemical ionization mass spectrometry

Atmospheric pressure chemical ionization mass spectrometry (APCI-MS) has been used to characterize the air-sensitive paramagnetic organouranium azide and nitride complexes [(C5Me5)2UN3(mu-N3)]3 and [(C5Me5)U(mu-I)2]3N, respectively. The trimetallic complex [(C5Me5)U(mu-I)2]3E had been identified by X-ray crystallography, but the data did not definitively identify E as N3- versus O2- or (OH)-, a common problem in heavy-element nitride complexes involving metals with variable oxidation states. A comparison of the 250 degrees C APCI-MS spectra of products made from NaN3 and Na15NNN showed mixed [M]+ and [M + H]+ envelopes at expected ion intensities for the 14N and 15N isotopomers. A compilation of U-C(C5Me5) and U-I bond distance data for U3+ and U4+ is also reported that shows that the ranges for the two oxidation states have significant overlap.     

Imaging the nonadiabatic dynamics of the CH3 + HCl reaction

LAB-frame velocity distributions of Cl-atoms produced in the photoinitiated reaction of CH(3) radicals with HCl have been measured for both the ground Cl ((2)P(3/2)) and excited Cl* ((2)P(1/2)) spin-orbit states using a DC slice velocity-map ion imaging technique. The similarity of these distributions, as well as the average internal excitation of methane co-products for both Cl and Cl* pathways, suggest that all the reactive flux proceeds through the same transition state on the ground potential energy surface (PES) and that the couplings which promote nonadiabatic transitions to the excited PES correlating to Cl* occur later in the exit channel, beyond the TS region. The nature of these couplings is discussed in light of initial vibrational excitation of CH(3) radicals as well as previously reported nonadiabatic reactivity in other polyatomic molecule reactions. Furthermore, the scattering of the reaction products, derived using the photoloc method, suggests that at the high collision energy of our experiment (E(coll) = 22.3 kcal mol(-1)), large impact parameter collisions are favoured with a reduced kinematic constraint on the internal excitation of the methane co-product.     

Evaluation of field desorption mass spectrometry for the analysis of polyethylene

Field desorption mass spectrometry (FD-MS) has been evaluated for the analysis of low molecular weight polyethylene by using samples in the molecular weight range 600–2000 u as determined by gel permeation chromatography. The repeat units and end groups were characterized by FD-MS, but it was demonstrated that accurate molecular weight distribution data cannot be obtained for polyethylene by FD-MS because there is mass discrimination against the higher molecular weight polymers.     

On the use of adaptive hierarchical meshes for numerical simulation of separated flows

This paper describes the use of adaptive hierarchical grids to predict incompressible separated flow at low Reynolds number. The grids consist of a quadtree system of hierarchical Cartesian meshes which are generated by recursive subdivision about seeding points. The governing equations are discretized in collocated primitive variable form using finite volumes and solved using a pressure correction scheme. The mesh is locally adapted at each time step, with panel division or removal dependent on the vorticity magnitude. The resulting grids have fine local resolution and are economical in array size. Results are presented for unidirectional, impulsively started flow past a circular and a square cylinder at various Reynolds numbers up to 5000 and 250 respectively. It is clear that hierarchical meshes may offer gains in efficiency when applied to complex flow domains or strongly sheared flows. However, as expected, the stepped approximation to curved boundaries resulting from the Cartesian quadtree representation adversely affects the accuracy of the results for flow past a circular cylinder. © 1998 John Wiley & Sons, Ltd.     

Correlation between electron capture negative chemical ionization mass spectrometric fragmentation and calculated internal energies for polychlorinated biphenyls

Correlationbse tween molecular structure and fragmentation observed in electron capture negative chemical ionization mass spectra (moderator gas = methane) of 49 selected tetrachlorinated, pentachlorinated, and hexachlorinated biphenyls have been investigated by using molecular modeling. The semiempirical general molecular orbital program MOPAC was used to calculate molecular properties for biphenyl and the 209 polychlorinated biphenyls. The mass spectrometric ionization and fragmentation processes were found to be linked to the number of chlorine atoms present on the biphenyl, and to the number of those chlorine atoms in the ortho (2, 2′, 6, and 6′) positions. The intensity of molecular ions increased with the number of chlorine atoms present, but this was counteracted by enhanced fragmentation as the number of ortho position chlorine atoms increased. The molecular parameters that were most closely linked with the number of ortho chlorine atoms were the twist angle between the phenyl rings and the energy of the lowest unoccupied molecular orbital (LUMO). It is suggested that fragmentation occurs when the energy of the ionizing electron exceeds the energy difference between the LUMO and LlJMO + 1 orbitals.