Combustion chemistry of enols: possible ethenol precursors in flames

Before the recent discovery that enols are intermediates in many flames, they appeared in no combustion models. Furthermore, little is known about enols' flame chemistry. Enol formation in low-pressure flames takes place in the preheat zone, and its precursors are most likely fuel species or the early products of fuel decomposition. The OH + ethene reaction has been shown to dominate ethenol production in ethene flames although this reaction has appeared insufficient to describe ethenol formation in all hydrocarbon oxidation systems. In this work, the mole fraction profiles of ethenol in several representative low-pressure flames are correlated with those of possible precursor species as a means for judging likely formation pathways in flames. These correlations and modeling suggest that the reaction of OH with ethene is in fact the dominant source of ethenol in many hydrocarbon flames, and that addition-elimination reactions of OH with other alkenes are also likely to be responsible for enol formation in flames. On this basis, enols are predicted to be minor intermediates in most flames and should be most prevalent in olefinic flames where reactions of the fuel with OH can produce enols directly.     

Imaging CIN(3) photodissociation from 234 to 280 nm

We report Cl((2)P(3/2)) and Cl*((2)P(1/2)) fragment images following ClN(3) photolysis in the 234-280 nm region measured by velocity map imaging. Kinetic energy distributions change shape with photolysis wavelength from bimodal at 234 and 240 nm to single peak at 266 and 280 nm. Where two peaks exist, their ratio is significantly different for Cl and Cl* fragments. The single peak of 266 and 280 nm and the faster peak at 234 and 240 nm are assigned to a Cl + linear-N(3) dissociation channel, in agreement with previous work. The slow peak in the bimodal distributions is assigned to the formation of a high energy form (HEF) of N(3). Candidates for the identity of HEF-N(3) are discussed. Combining our data with photofragmentation translational spectroscopy results, we determined the threshold for the appearance of HEF-N(3) at 4.83 +/- 0.17 eV photolysis energy. This threshold behavior is similar to recently reported results on the wavelength dependence of HN(3) photolysis, where the threshold was associated with a ring closed isomer of HN(3) on the S(1) potential energy surface. We also note that the HEF-N(3) formation threshold observed for ClN(3) occurs where the energy available to the products equals the isomerization barrier from linear to cyclic-N(3).     

Site-selective X-ray spectroscopy on an asymmetric model complex of the [FeFe] hydrogenase active site

The active site for hydrogen production in [FeFe] hydrogenase comprises a diiron unit. Bioinorganic chemistry has modeled important features of this center, aiming at mechanistic understanding and the development of novel catalysts. However, new assays are required for analyzing the effects of ligand variations at the metal ions. By high-resolution X-ray absorption spectroscopy with narrow-band X-ray emission detection (XAS/XES = XAES) and density functional theory (DFT), we studied an asymmetrically coordinated [FeFe] model complex, [(CO)(3)Fe(I)1-(bdtCl(2))-Fe(I)2(CO)(Ph(2)P-CH(2)-NCH(3)-CH(2)-PPh(2))] (1, bdt = benzene-1,2-dithiolate), in comparison to iron-carbonyl references. Kβ emission spectra (Kβ(1,3), Kβ') revealed the absence of unpaired spins and the low-spin character for both Fe ions in 1. In a series of low-spin iron compounds, the Kβ(1,3) energy did not reflect the formal iron oxidation state, but it decreases with increasing ligand field strength due to shorter iron-ligand bonds, following the spectrochemical series. The intensity of the valence-to-core transitions (Kβ(2,5)) decreases for increasing Fe-ligand bond length, certain emission peaks allow counting of Fe-CO bonds, and even molecular orbitals (MOs) located on the metal-bridging bdt group of 1 contribute to the spectra. As deduced from 3d → 1s emission and 1s → 3d absorption spectra and supported by DFT, the HOMO-LUMO gap of 1 is about 2.8 eV. Kβ-detected XANES spectra in agreement with DFT revealed considerable electronic asymmetry in 1; the energies and occupancies of Fe-d dominated MOs resemble a square-pyramidal Fe(0) for Fe1 and an octahedral Fe(II) for Fe2. EXAFS spectra for various Kβ emission energies showed considerable site-selectivity; approximate structural parameters similar to the crystal structure could be determined for the two individual iron atoms of 1 in powder samples. These results suggest that metal site- and spin-selective XAES on [FeFe] hydrogenase protein and active site models may provide a powerful tool to study intermediates under reaction conditions.     

Non-stationary forward flux sampling

We present a method, Non-Stationary Forward Flux Sampling, that allows efficient simulation of rare events in both stationary and non-stationary stochastic systems. The method uses stochastic branching and pruning to achieve uniform sampling of trajectories in phase space and time, leading to accurate estimates for time-dependent switching propensities and time-dependent phase space probability densities. It is suitable for equilibrium or non-equilibrium systems, in or out of stationary state, including non-Markovian or externally driven systems. We demonstrate the validity of the technique by applying it to a one-dimensional barrier crossing problem that can be solved exactly, and show its usefulness by applying it to the time-dependent switching of a genetic toggle switch.     

Nonaqueous capillary electrophoresis-mass spectrometry: a versatile, straightforward tool for the analysis of alkaloids from psychoactive plant extracts

In this study we show that a nonaqueous capillary electrophoresis mass spectrometry (NACE-MS) method carefully optimized by a design of experiment can be applied to a very large number of alkaloids in different plant extracts. It is possible to characterize the pattern of the psychoactive alkaloids in several plant samples and preparations thereof, each presenting different challenges in their analysis. The method is shown to be able to separate structurally closely related substances, diastereomers and further isobaric compounds, to separate members of different alkaloid classes within one run and to tolerate significant matrix load. A comparison with methods presented in the literature reveals that a near-generic NACE-MS method for the fast profiling of alkaloids in forensically relevant plant samples has been developed.     

A note on bounds for non-linear multivalued homogenized operators

In this paper we study the behaviour of maximal monotone multivalued highly oscillatory operators. We construct Reuss-Voigt-Wiener and Hashin-Shtrikmann type bounds for the minimal sections of G-limits of multivalued operators by using variational convergence and convex analysis.     

On the propagation of polarization sets for systems of real principal type

We define and study the polarization sets for vector-valued distributions. Systems of real principal type are defined, and we prove that for such systems of pseudodifferential operators we have a unique propagation of the polarization sets for the solutions.