9,009 research outputs found

    Isolation of a Chiral Anthracene Cation Radical: X-Ray Crystallography and Computational Interrogation of its Racemization

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    Chiral cation-radical salts hold significant promise as charge-transfer materials, chiroptical switches, and electron-transfer catalysts for enantioselective synthesis. Herein we demonstrate that the readily-available chiral 9,10-diphenyleanthracene derivative (i.e.SANT) forms a robust cation radical, whose structure was elucidated by X-ray crystallography and DFT calculations. While SANT was observed to racemize on a timescale (t1/2) of 1.1 hours, a computational conformational search and kinetic analysis of the racemization pathway led us to identify a simple methyl substituted SANT derivative, which does not racemize (racemization t1/2 1013–1017 years)

    The late time radio emission from SN 1993J at meter wavelengths

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    We present the investigations of SN 1993J using low frequency observations with the Giant Meterwave Radio Telescope. We analyze the light curves of SN 1993J at 1420, 610, 325 and 243 MHz during 7.5107.5-10 years since explosion.The supernova has become optically thin early on in the 1420 MHz and 610 MHz bands while it has only recently entered the optically thin phase in the 325 MHz band. The radio light curve in the 235 MHz band is more or less flat. This indicates that the supernova is undergoing a transition from an optically thick to optically thin limit in this frequency band. In addition, we analyze the SN radio spectra at five epochs on day 3000, 3200, 3266, 3460 and 3730 since explosion. Day 3200 spectrum shows a synchrotron cooling break. SN 1993J is the only young supernova for which the magnetic field and the size of the radio emitting region are determined through unrelated methods. Thus the mechanism that controls the evolution of the radio spectra can be identified. We suggest that at all epochs, the synchrotron self absorption mechanism is primarily responsible for the turn-over in the spectra. Light curve models based on free free absorption in homogeneous or inhomogeneous media at high frequencies overpredict the flux densities at low frequencies. The discrepancy is increasingly larger at lower and lower frequencies. We suggest that an extra opacity, sensitively dependent on frequency, is likely to account for the difference at lower frequencies. The evolution of the magnetic field (determined from synchrotron self absorption turn-over) is roughly consistent with Bt1B \propto t^{-1}. Radio spectral index in the optically thin part evolves from α0.81.0\alpha \sim 0.8-1.0 at few tens of days to 0.6\sim 0.6 in about 10 years.Comment: 37 pages, 9 figures in LaTex; scheduled for ApJ 10 September 2004, v612 issue; send comments to: [email protected]

    Synchrotron aging and the radio spectrum of SN 1993J

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    We combine the GMRT low frequency radio observations of SN 1993J with the VLA high frequency radio data to get a near simultaneous spectrum around day 3200 since explosion. The low frequency measurements of the supernova determine the turnover frequency and flux scale of the composite spectrum and help reveal a steepening in the spectral index, Δα0.6\Delta \alpha \sim 0.6, in the optically thin part of the spectrum. This is the first observational evidence of a break in the radio spectrum of a young supernova. We associate this break with the phenomenon of synchrotron aging of radiating electrons. From the break in the spectrum we calculate the magnetic field in the shocked region independent of the equipartition assumption between energy density of relativistic particles and magnetic energy density. We determine the ratio of these two energy densities and find that this ratio is in the range: 8×1065×1048\times 10^{-6}-5\times 10^{-4}. We also predict the nature of the evolution of the synchrotron break frequency with time, with competing effects due to diffusive Fermi acceleration and adiabatic expansion of the radiative electron plasma.Comment: 12 pages, 2 figures. Accepted for publication in ApJ

    A Simultaneous Deterministic Perturbation Actor-Critic Algorithm with an Application to Optimal Mortgage Refinancing

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    Wedevelopasimulation-based,two-timescale actorcritic algorithm for infinite horizon Markov decision processes with finite state and action spaces, with a discounted reward criterion. The algorithm is of the gradient ascent type and performs a search in the space of stationary randomized policies. The algorithm uses certain simultaneous deterministic perturbation stochastic approximation (SDPSA) gradient estimates for enhanced performance. We show an application of our algorithm on a problem of mortgage refinancing. Our algorithm obtains the optimal refinancing strategies in a computationally efficient manner

    Analysis of Toxicants by Gas Chromatography

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    Characteristics of gravity waves generated in a convective and a non-convective environment revealed from hourly radiosonde observation under CPEA-II campaign

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    Analyses of hourly radiosonde data of temperature, wind, and relative humidity during four days (two with convection and two with no convection) as a part of an intensive observation period in CPEA-2 campaign over Koto Tabang (100.32° E, 0.20° S), Indonesia, are presented. Characteristics of gravity waves in terms of dominant wave frequencies at different heights and their vertical wavelengths are shown in the lower stratosphere during a convective and non-convective period. Gravity waves with periods ~10 h and ~4–5 h were found dominant near tropopause (a region of high stability) on all days of observation. Vertical propagation of gravity waves were seen modified near heights of the three identified strong wind shears (at ~16, 20, and 25 km heights) due to wave-mean flow interaction. Between 17 and 21 km heights, meridional wind fluctuations dominated over zonal wind, whereas from 22 to 30 km heights, wave fluctuations with periods ~3–5 h and ~8–10 h in zonal wind and temperature were highly associated, suggesting zonal orientation of wave propagation. Gravity waves from tropopause region to 30 km heights were analyzed. In general, vertical wavelength of 2–5 km dominated in all the mean-removed (~ weekly mean) wind and temperature hourly profiles. Computed vertical wavelength spectra are similar, in most of the cases, to the source spectra (1–16 km height) except that of zonal wind spectra, which is broad during active convection. Interestingly, during and after convection, gravity waves with short vertical wavelength (~2 km) and short period (~2–3 h) emerged, which were confined in the close vicinity of tropopause, and were not identified on non-convective days, suggesting convection to be the source for them. Some wave features near strong wind shear (at 25 km height) were also observed with short vertical wavelengths in both convective and non-convective days, suggesting wind shear to be the sole cause of generation and seemingly not associated with deep convection below. A drop in the temperature up to ~4–5 K (after removal of diurnal component) was observed at ~16 km height near a strong wind shear (~45–55 m s<sup>−1</sup> km<sup>−1</sup>) during active period of convection

    Recovery of Non-ferrous Metallic Values from Metallurgical Wastes

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    With the increased industrial expansion and ever incre-asing consumption of non-ferrous metals in India, utili-sation of low grade and complex ores, recovery of metals from waste products like slag, ashes and dross, apart from conservation of the non-ferrous metals not available in the country, by their judicious uses and by substitution, wherever possible, is a matter of great importance
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