Overview of edge electrostatic turbulence experiments on TCV

Experiments on edge turbulence in the TCV tokamak have been performed for the first time at the beginning of 2003. This paper presents an overview of some of the results obtained, concentrating in particular on two areas: universality of density fluctuations and the dynamical coupling between radial turbulent-driven fluxes and parallel flows.Plasma fluctuations in the edge of the TCV tokamak have been found to exhibit statistical properties which are universal across a broad range of discharge conditions. Analysis of the time series of density fluctuations in the entire scrape-off layer (SOL) region from just inside the magnetic separatrix to the plasma-wall interface, yields a probability distribution function (PDF) of density which conforms closely to a Gamma distribution. In the wall vicinity, the density fluctuations exhibit clear evidence of self-similarity and are characterised by a PDF with universal shape and with a standard deviation proportional to the mean density. It is also found that radial particle-flux fluctuations scale solely with the mean density. Such findings indicate that it may be possible to improve the prediction of transport in the critical plasma-wall interaction region of future large-scale tokamaks.Recent experiments on JET [C. Hidalgo et al.: Phys. Rev. Lett. 91 (2003) 065001] have investigated a possible link between turbulent transport and the parallel flows. Similar experiments have been performed on TCV for a variety of plasma conditions and flow magnitudes. Although correlations have been found as seen on JET, especially in the wall vicinity, it appears that the magnitude of the coupling is insufficient to drive any significant flow.Presented at the Workshop Electric Fields Structures and Relaxation in Edge Plasmas, Nice, France, October 26–27, 2004.     

Spectroscopic Characterization of a [C−F−C]+ Fluoronium Ion in Solution

We report the first spectroscopic evidence for a [C?F?C]⁺ fluoronium ion in solution. Extensive NMR studies (¹⁹F, ¹H, ¹³C) characterize a symmetric cage‐like species in which fluorine exhibits substantial covalent bonding to each of the two carbon atoms involved in the three‐center interaction. Experimental NMR data comport well with calculated values to lend credence to the structural assignment. As the culminating experiment, a Saunders isotopic perturbation test confirmed the symmetric structure. Congruent with the trend in other types of onium ions, the calculated charge at fluorine moves in a more positive (less negative) direction from the neutral. It is this important trend that explains in part the extraordinary historical difficulty in making theoretical predictions of fluoronium ions come true in solution, and why it takes fluorine captured in a cage to produce, finally, a stable ion and complete the historical arc of the organic halonium ion story.     

On Using the Volatile Mem-Capacitive Effect of TiO<Subscript>2</Subscript> Resistive Random Access Memory to Mimic the Synaptic Forgetting Process

In this work, we report on mimicking the synaptic forgetting process using the volatile mem-capacitive effect of a resistive random access memory (RRAM). TiO₂ dielectric, which is known to show volatile memory operations due to migration of inherent oxygen vacancies, was used to achieve the volatile mem-capacitive effect. By placing the volatile RRAM candidate along with SiO₂ at the gate of a MOS capacitor, a volatile capacitance change resembling the forgetting nature of a human brain is demonstrated. Furthermore, the memory operation in the MOS capacitor does not require a current flow through the gate dielectric indicating the feasibility of obtaining low power memory operations. Thus, the mem-capacitive effect of volatile RRAM candidates can be attractive to the future neuromorphic systems for implementing the forgetting process of a human brain.     

Temperature dependence of the unimolecular decomposition of pernitric acid and its atmospheric implications

The decay of pernitric acid (HO₂NO₂) in the presence of excess nitric oxide has been studied in a 5800-liter, Teflon-lined chamber over the temperature range 254 to 283 K at 1 atm pressure of N₂ by Fourier transform infrared spectroscopy. A heterogeneous reaction of NO₂ and H₂O₂ was used to generate HO₂NO₂ with less than 20% HNO₃ and less than 5% NO₂ present as impurities. The HO₂NO₂ had lifetimes of 5 to 20 h in our chamber, presumably determined by heterogeneous loss to the walls. Two paths have been proposed for the reaction of NO₂ with HO₂:

(1), NO₂ + HO₂ → HONO + O₂ (2). In this study the ratio k₁/k₂ was calculated to be greater than 10³ throughout the temperature range studied. The homogeneous unimolecular decay of the HO₂NO₂, reaction (3), was investigated by adding excess NO in order to remove HO₂ by reaction (4).

(3), HO₂ + NO → NO₂ + OH (4). The rate constant k₃ was determined to be 1.4 × 10¹⁴ exp(?20700 ± 500/RT)s^?1. The thermal decomposition lifetimes of HO₂O₂ at 1 atm total pressure calculated from k₃ are 12 s at 298 K, 5 min at 273 K and 1 month at 220 K. Implications of these results for the role of pernitric acid in the lower and upper atmosphere are discussed.     

Examining Trichloroisocyanuric Acid and Oxalyl Chloride in Complementary Approaches to Fluorination of Group 15 Heteroatoms

A mild, oxidative fluorination approach to a variety of fluorinated phosphorus compounds using trichloroisocyanuric acid (TCICA) and KF was developed as a complement to a recent study on deoxygenative fluorination using oxalyl chloride. Herein, the syntheses of several fluorinated organophosphorus compounds are reported, and both TCICA/KF and oxalyl chloride/KF conditions are compared and contrasted throughout. Initial investigations of the method on other group 15 heteroatoms (i. e., As, Sb, and Bi) are also reported, with varied success. This work notably extends the known TCICA/KF reactivity series to another group of elements beyond previously studied chalcogens (S, Se, and Te) and halogens (iodine) and expands the utility of the previously reported oxalyl chloride/KF method.     

X-ray Crystallographic Structure of the Potent Antiplasmodial Compound 2,7-Dibromocryptolepine Acetic Acid Solvate

Abstract  The structure of 2,7-dibromocryptolepine acetic acid solvate, C₁₆H₁₁N₂Br₂ [1.5(C₂H₄O₂)][C₂H₃O₂⁻][0.5H₂O], M_r = 460.17, has been determined from X-ray diffraction data. The crystals are monoclinic, space group P2₁/c with Z = 4 molecules per unit cell and a = 7.3243(3), b = 18.7804(6), c = 15.8306(7) ?, β = 94.279(1)°, V_c = 2171.5(2) ?, crystal density D_c = 1.667 g/cm³. The structure was determined using direct methods and refined by full-matrix least-squares to a conventional R-index of 0.0496 for 4,908 reflections and 258 parameters. The cryptolepine nucleus of the 2,7-dibromocryptolepine molecule is highly planar and the two Br atoms are in this plane within 0.06 and 0.01 ?, respectively. The crystal structure is maintained via hydrogen bonding between N(10) in the cryptolepine nucleus and the oxygen of one of the three solvated acetic acid molecules. The acetic acid molecules also form hydrogen bonded chains. Acetic acid B is deprotonated and its two C–O bond lengths are equivalent, unlike those in A and C. Acetic acid C lies very close to a crystallographic centre of symmetry. To avoid overlap the two repeats cannot exist together and are subject to 50% statistical disorder. O(1C) of this methanol is furthest from the two-fold axis and its occupancy refines to a value of 1.0 and is assumed to exist alternately as a water oxygen hydrogen bonding to methanol O(1C) across the two-fold axis at a distance of 2.775 ?. Index Abstract  The antiplasmodial activity of the analogue 2,7-dibromocryptolepine is nine times greater than that of cryptolepine itself against chloroquine-resistant Plasmodium falciparum (multi-drug resistant strain K1) with IC₅₀ values = 0.44 and 0.049 μM, respectively. This analogue also showed promising in vivo activity against P. berghei in mice. Parasitaemia was suppressed by 91.4% compared to untreated infected control animals at a dose of 25 mg/kg given daily by i.p. injection with no apparent toxicity to the mice, in contrast to cryptolepine which was toxic to mice when given i.p. at 20 mg/kg. Further experiments showed a dose-dependent relationship with ED₅₀ and ED₉₀ values of 6.92 and 21.46 mg/kg/day, respectively. Although 2,7-dibromocryptolepine was not toxic to the mice its cytotoxic activity is similar to that of cryptolepine, but unlike cryptolepine it does not appear to intercalate into DNA as assessed using thermodenaturation techniques (ΔTm values = 3 and 9 °C, respectively). Like cryptolepine, 2,7-dibromocryptolepine inhibits the formation of haemozoin, but its increased antiplasmodial potency does not appear to be due to more potent inhibition of haemozoin formation, nor to increased accumulation into the acidic parasite food vacuole suggesting that 2,7-dibromocryptolepine has additional modes of action compared to cryptolepine. The X-ray structure of this compound will help to determine whether or not 2,7-dibromocryptolepine is able to intercalate into DNA and facilitate the design of new cryptolepine analogues with DNA binding properties appropriate for antimalarial (with no DNA intercalation) or anticancer (sequence-specific binding) applications.      

Making the SF5 Group More Accessible: A Gas‐Reagent‐Free Approach to Aryl Tetrafluoro‐λ6‐sulfanyl Chlorides

Modern pentafluorosulfanyl (SF₅) chemistry has an Achilles heel: synthetic accessibility. Herein, we present the first approach to aryl‐SF₄Cl compounds (key intermediates in state‐of‐the‐art aryl‐SF₅ synthesis) that overcomes the reliance on hazardous fluorinating reagents and/or gas reagents (e.g. Cl₂) by employing easy‐to‐handle trichloroisocyanuric acid, potassium fluoride, and catalytic amounts of acid. These simple, mild conditions allow direct access to aryl‐SF₄Cl intermediates that either have not been or cannot be demonstrated using previous methods. Furthermore, the same approach provides access to aryl‐SF₃ and aryl‐SeF₃ compounds, which extend the applications of this chemistry beyond arene SF₅‐functionalization, and demonstrate its ability to address a more general oxidative fluorination problem.     

A re-examination of the circular dichroism of the calicheamicin enediyne/dienone chromophoric interaction

A series of calicheamicin derivatives have been made in an effort to delineate the origin of the strong circular dichroism (CD) of calicheamicin reported previously. The CD spectrum of calicheamicin (I) was compared with that of fragments II and III, which contain either the enediyne/dienone or a thiobenzoate chromophore alone. NaBH(4) reduction of calicheamicin produced two analogues (IV and V) that have no dienone. This allowed the assessment of possible exciton coupling between the enediyne on the warhead and the thiobenzoate on the tail. It was found that the strong negative 312/272 nm exciton split in the CD of calicheamicin is due largely to the enediyne/dienone interaction. Contributions from the thiobenzoate or its interaction with the enediyne have been ruled out. [structure: see text]