Electron transfer and ligand addition to atomic mercury cations in the gas phase: kinetic and equilibrium studies at 295 k

Results are reported for experimental measurements of the room-temperature chemical reactions between ground-state Hg*+ ions and 16 important environmental and biological gases: SF6, CO, CO2, N2O, D2O, CH4, CH3F, O2, CH3Cl, OCS, CS2, NH3, C6F6, NO2, NO*, and C6H6. The inductively coupled plasma/selected-ion flow tube tandem mass spectrometer used for these measurements has provided both rate and equilibrium constants. Efficient electron transfer (>19%) is observed with CS2, NH3, C6F6, NO2, NO*, and C6H6, molecular addition occurs with D2O, CH4, CH3F, CH3Cl, and OCS, and SF6, CO, CO2, N2O, and O2 showed no measurable reactivity with Hg*+. Theory is used to explore the stabilities and structures of both the observed and unobserved molecular adducts of Hg*+, and reasonable agreement is obtained with experimental observations, given the uncertainties of the theory and experiments. A correlation is reported between the Hg*+ and proton affinities of the ligands investigated. Solvation of Hg*+ with formic acid was observed to increase the rate of electron transfer from NO* by more than 20%.     

Fierz bilinear formulation of the Maxwell–Dirac equations and symmetry reductions

We study the Maxwell–Dirac equations in a manifestly gauge invariant presentation using only the spinor bilinear scalar and pseudoscalar densities, and the vector and pseudovector currents, together with their quadratic Fierz relations. The internally produced vector potential is expressed via algebraic manipulation of the Dirac equation, as a rational function of the Fierz bilinears and first derivatives (valid on the support of the scalar density), which allows a gauge invariant vector potential to be defined. This leads to a Fierz bilinear formulation of the Maxwell tensor and of the Maxwell–Dirac equations, without any reference to gauge dependent quantities. We show how demanding invariance of tensor fields under the action of a fixed (but arbitrary) Lie subgroup of the Poincaré group leads to symmetry reduced equations. The procedure is illustrated, and the reduced equations worked out explicitly for standard spherical and cylindrical cases, which are coupled third order nonlinear PDEs. Spherical symmetry necessitates the existence of magnetic monopoles, which do not affect the coupled Maxwell–Dirac system due to magnetic terms cancelling. In this paper we do not take up numerical computations. As a demonstration of the power of our approach, we also work out the symmetry reduced equations for two distinct classes of dimension 4 one-parameter families of Poincaré subgroups, one splitting and one non-splitting. The splitting class yields no solutions, whereas for the non-splitting class we find a family of formal exact solutions in closed form.     

Large Eddy Simulation and PIV Measurements of Unsteady Premixed Flames Accelerated by Obstacles

In gas explosions, the unsteady coupling of the propagating flame and the flow field induced by the presence of blockages along the flame path produces vortices of different scales ahead of the flame front. The resulting flame–vortex interaction intensifies the rate of flame propagation and the pressure rise. In this paper, a joint numerical and experimental study of unsteady premixed flame propagation around three sequential obstacles in a small-scale vented explosion chamber is presented. The modeling work is carried out utilizing large eddy simulation (LES). In the experimental work, previous results (Patel et al., Proc Combust Inst 29:1849–1854, 2002) are extended to include simultaneous flame and particle image velocimetry (PIV) measurements of the flow field within the wake of each obstacle. Comparisons between LES predictions and experimental data show a satisfactory agreement in terms of shape of the propagating flame, flame arrival times, spatial profile of the flame speed, pressure time history, and velocity vector fields. Computations through the validated model are also performed to evaluate the effects of both large-scale and sub-grid scale (SGS) vortices on the flame propagation. The results obtained demonstrate that the large vortical structures dictate the evolution of the flame in qualitative terms (shape and structure of the flame, succession of the combustion regimes along the path, acceleration-deceleration step around each obstacle, and pressure time trend). Conversely, the SGS vortices do not affect the qualitative trends. However, it is essential to model their effects on the combustion rate to achieve quantitative predictions for the flame speed and the pressure peak.     

Myrotoxins: a new class of macrocyclic trichothecenes

An isolate of $\text{Myrothecium roridum}$ which is a pathogen on tomatoes in Texas produces a new class of C27 macrocyclic trichothecenes which contains a nonconjugated diene system that includes a vinyl ether group.     

A thermodynamic approach, using speciation studies, towards the evaluation and design of bone-seeking radiopharmaceuticals as illustrated for 117mSn(II)-PEI-MP

Blood plasma modeling has proved effective in the evaluation of clinical observations recorded for baboon and rat tests with ¹⁵³Sm- ethylenediaminetetraphosphonic acid (EDTMP) as well as for ¹⁶⁶Ho-EDTMP. In the search for a cure for metastatic bone cancer, ^117mSn with its conversion electrons of discrete energies shows low bone marrow toxicity, providing the opportunity to increase the administered dose. Selective accumulation in lesions would capitalize on this advantage. The 10-30 kDa fraction of the water-soluble polymer polyethyleneimine, functionalized with methylene phosphonate groups (PEI-MP) and labeled with ^99mTc, has shown selective uptake into bone tumours. This paper relates the speciation of Sn(II)-PEI-MP and other known ^117mSn(II) containing bone-seeking radiopharmaceuticals in blood plasma. Apparent formation constants for the complexation of Sn^II with PEI-MP, DTPA, HEDP and other important blood plasma ligands were measured potentiometrically or estimated by linear free energy relationships (LFER). These data were added to the ECCLES database in order to construct a blood plasma model for Sn^II. From this model it is predicted that Sn^II will remain bound to the polymer (PEI-MP) in blood plasma and therefore, have only slight reticuloendothelial uptake. Preliminary primate studies indeed proved that the complex between Sn^II and PEI-MP remains intact in blood plasma, which is consistent with the observation for PEI-MP labeled with ^99mTc. From these data, it was also possible to explain in retrospect the lower bone uptake, the slow blood clearance and the liver uptake of the agents ^117mSn(II) DTPA and ^117mSn(II) HEDP agents as reported in the literature.     

Controlled and optimally controlled multiplexing systems: A numerical exploration

Large controlled multiplexing systems are approximated by diffusion type processes yielding a very efficient way of approximation and good numerical methods. The limit equations are an efficient aggregation of the original system, and provide the basis of the actual numerical approximation to the control problem. The numerical approximations have the structure of the original problem, but are generally much simpler. The control can occur in a variety of places; e.g., leaky bucket controllers, control of marked cells at the transmitter buffer, or control of the transmitter speed. From the point of view of the limit equations, those are equivalent. Various forms of the optimal control problem are explored, where the main aim is to control or balance the losses at the control with those due to buffer overflow. It is shown that much can be saved via the use of optimal controls or reasonable approximations to them. We discuss systems with one to three classes of sources, various aggregation methods and control approximation schemes. There are qualitative comparisons of various systems with and without control and a discussion of the variations of control and performance as the systems data and control bounds vary. The approach is a very useful tool for providing both qualitative and quantitative information which would be hard to get otherwise. The results have applications to various forms of the ATM and broadband integrated data networks.The work was partially supported by AFOSR-91-0375 and (AFOSR) F49620-92-J-088-1DEF.The work was partially supported by grants (AFOSR) F49620-92-J-008-1DEF, AFOSR-91-03750.This work was partially supported by DAAH04-93-0070 (ARO) and AFOSR-91-0375.     

Nitric oxide as an electron donor, an atom donor, an atom acceptor, and a ligand in reactions with atomic transition-metal and main-group cations in the gas phase

The room-temperature reactions of nitric oxide with 46 atomic cations have been surveyed systematically across and down the periodic table using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Rate coefficients and product distributions were measured for the reactions of first-row cations from K+ to Se+, of second-row cations from Rb+ to Te+ (excluding Tc+), and of third-row cations from Cs+ to Bi+. Reactions both first and second order in NO were identified. The observed bimolecular reactions were thermodynamically controlled. Efficient exothermic electron transfer was observed with Zn+, As+, Se+, Au+, and Hg+. Bimolecular O-atom transfer was observed with Sc+, Ti+, Y+, Zr+, Nb+, La+, Hf+, Ta+, and W+. Of the remaining 32 atomic ions, all but 8 react in novel termolecular reactions second order in NO to produce NO+ and the metal-nitrosyl molecule, the metal-monoxide cation and nitrous oxide, and/or the metal-nitrosyl cation. K+, Rb+, Cs+, Ga+, In+, Tl+, Pb+, and Bi+ are totally unreactive. Further reactions with NO produce the dioxide cations CaO2+, TiO2+, VO2+, CrO2+, SrO2+, ZrO2+, NbO2+, RuO2+, BaO2+, HfO2+, TaO2+, WO2+, ReO2+, and OsO2+ and the still higher order oxides WO3+, ReO3+, and ReO4+. NO ligation was observed in the formation of CaO+(NO), ScO+(NO), TiO+(NO), VO+(NO)(1-3), VO2+(NO)(1-3), SrO+(NO), SrO2+(NO)1,2, RuO+(NO)(1-3), RuO2+(NO)1,2, OsO+(NO)(1-3), and IrO+(NO). The reported reactivities for bare atomic ions provide a benchmark for reactivities of ligated atomic ions and point to possible second-order NO chemistry in biometallic and metal-surface environments leading to the conversion of NO to N2O and the production of metal-nitrosyl molecules.     

Improved precision and accuracy for high-performance liquid chromatography/Fourier transform ion cyclotron resonance mass spectrometric exact mass measurement of small molecules from the simultaneous and controlled introduction of internal calibrants via a second electrospray nebuliser

The use of a second electrospray nebuliser has proved to be highly successful for exact mass measurement during high-performance liquid chromatography/Fourier transform ion cyclotron resonance mass spectrometry (HPLC/FTICRMS). Much improved accuracy and precision of mass measurement were afforded by the introduction of the internal calibration solution, thus overcoming space charge issues due to the lack of control over relative ion abundances of the species eluting from the HPLC column. Further, issues of suppression of ionisation, observed when using a T-piece method, are addressed and this simple system has significant benefits over other more elaborate approaches providing data that compares very favourably with these other approaches. The technique is robust, flexible and transferable and can be used in conjunction with HPLC, infusion or flow injection analysis (FIA) to provide constant internal calibration signals to allow routine, accurate and precise mass measurements to be recorded.