Stability and Reversibility of Lithium Borohydrides Doped by Metal Halides and Hydrides

In an effort to develop reversible metal borohydrides with high hydrogen storage capacities and low dehydriding temperature, doping LiBH4 with various metal halides and hydrides has been conducted. Several metal halides such as TiCl3, TiF3, and ZnF2 effectively reduced the dehydriding temperature through a cation exchange interaction. Some of the halide doped LiBH4 are partially reversible. The LiBH4 + 0.1TiF3 desorbed 3.5 wt % and 8.5 wt % hydrogen at 150 and 450 °C, respectively, with subsequent reabsorption of 6 wt % hydrogen at 500 °C and 70 bar observed. XRD and NMR analysis of the rehydrided samples confirmed the reformation of LiBH4. The existence of the (B12H12)−2 species in dehydrided and rehydrided samples gives insight into the resultant partial reversibility. A number of other halides, MgF2, MgCl2, CaCl2, SrCl2, and FeCl3, did not reduce the dehydriding temperature of LiBH4 significantly. XRD and TGA-RGA analyses indicated that an increasing proportion of halides such as TiCl3, TiF3, and ZnCl2 from 0.1 to 0.5 mol makes lithium borohydrides less stable and volatile. Although the less stable borohydrides such as LiBH4 + 0.5TiCl3, LiBH4 + 0.5TiF3, and LiBH4 + 0.5ZnCl2 release hydrogen at room temperature, they are not reversible due to unrecoverable boron loss caused by diborane emission. In most cases, doping that produced less stable borohydrides also reduced the reversible hydrogen uptake. It was also observed that halide doping changed the melting points and reduced air sensitivity of lithium borohydrides

Multiple prebiotic metals mediate translation.

Today, Mg2+ is an essential cofactor with diverse structural and functional roles in life's oldest macromolecular machine, the translation system. We tested whether ancient Earth conditions (low O2, high Fe2+, and high Mn2+) can revert the ribosome to a functional ancestral state. First, SHAPE (selective 2'-hydroxyl acylation analyzed by primer extension) was used to compare the effect of Mg2+, Fe2+, and Mn2+ on the tertiary structure of rRNA. Then, we used in vitro translation reactions to test whether Fe2+ or Mn2+ could mediate protein production, and quantified ribosomal metal content. We found that (i) Mg2+, Fe2+, and Mn2+ had strikingly similar effects on rRNA folding; (ii) Fe2+ and Mn2+ can replace Mg2+ as the dominant divalent cation during translation of mRNA to functional protein; and (iii) Fe and Mn associate extensively with the ribosome. Given that the translation system originated and matured when Fe2+ and Mn2+ were abundant, these findings suggest that Fe2+ and Mn2+ played a role in early ribosomal evolution

Transit and Eclipse Analyses of Exoplanet HD 149026b Using BLISS Mapping

The dayside of HD 149026b is near the edge of detectability by the Spitzer Space Telescope. We report on eleven secondary-eclipse events at 3.6, 4.5, 3 x 5.8, 4 x 8.0, and 2 x 16 microns plus three primary-transit events at 8.0 microns. The eclipse depths from jointly-fit models at each wavelength are 0.040 +/- 0.003% at 3.6 microns, 0.034 +/- 0.006% at 4.5 microns, 0.044 +/- 0.010% at 5.8 microns, 0.052 +/- 0.006% at 8.0 microns, and 0.085 +/- 0.032% at 16 microns. Multiple observations at the longer wavelengths improved eclipse-depth signal-to-noise ratios by up to a factor of two and improved estimates of the planet-to-star radius ratio (Rp/Rs = 0.0518 +/- 0.0006). We also identify no significant deviations from a circular orbit and, using this model, report an improved period of 2.8758916 +/- 0.0000014 days. Chemical-equilibrium models find no indication of a temperature inversion in the dayside atmosphere of HD 149026b. Our best-fit model favors large amounts of CO and CO2, moderate heat redistribution (f=0.5), and a strongly enhanced metallicity. These analyses use BiLinearly-Interpolated Subpixel Sensitivity (BLISS) mapping, a new technique to model two position-dependent systematics (intrapixel variability and pixelation) by mapping the pixel surface at high resolution. BLISS mapping outperforms previous methods in both speed and goodness of fit. We also present an orthogonalization technique for linearly-correlated parameters that accelerates the convergence of Markov chains that employ the Metropolis random walk sampler. The electronic supplement contains light-curve files and supplementary figures.Comment: Accepted for publication in Ap

Spitzer IRAC Secondary Eclipse Photometry of the Transiting Extrasolar Planet HAT-P-1b

We report Spitzer/IRAC photometry of the transiting giant exoplanet HAT-P-1b during its secondary eclipse. This planet lies near the postulated boundary between the pM and pL-class of hot Jupiters, and is important as a test of models for temperature inversions in hot Jupiter atmospheres. We derive eclipse depths for HAT-P-1b, in units of the stellar flux, that are: 0.080% +/- 0.008%,[3.6um], 0.135% +/- 0.022%,[4.5um],0.203% +/- 0.031%,[5.8um], and $0.238% +/- 0.040%,[8.0um]. These values are best fit using an atmosphere with a modest temperature inversion, intermediate between the archetype inverted atmosphere (HD209458b) and a model without an inversion. The observations also suggest that this planet is radiating a large fraction of the available stellar irradiance on its dayside, with little available for redistribution by circulation. This planet has sometimes been speculated to be inflated by tidal dissipation, based on its large radius in discovery observations, and on a non-zero orbital eccentricity allowed by the radial velocity data. The timing of the secondary eclipse is very sensitive to orbital eccentricity, and we find that the central phase of the eclipse is 0.4999 +/- 0.0005. The difference between the expected and observed phase indicates that the orbit is close to circular, with a 3-sigma limit of |e cosw| < 0.002.Comment: 5 pages, 6 figures, 1 table. Accepted by The Astrophysical Journal, 10 Nov 200

The Structure, Anharmonic Vibrational Frequencies, and Intensities of NNHNN+

A semi-global potential energy surface (PES) and quartic force field (QFF) based on fitting high-level electronic structure energies are presented to describe the structures and spectroscopic properties of NNHNN+. The equilibrium structure of NNHNN+ is linear with the proton equidistant between the two nitrogen groups and thus of D(sub h) symmetry. Vibrational second-order perturbation theory (VPT2) calculations based on the QFF fails to describe the proton rattle motion, i.e., the antisymmetric proton stretch, due to the very flat nature of PES around the global minimum, but performs properly for other modes with sharper potential wells. Vibrational self-consistent field/virtual state configuration interaction (VSCF/VCI) calculations using a version of MULTIMODE without angular momentum terms successfully describe this motion and predict the fundamental to be at 759 cm(exp -1). This is in good agreement with the value of 746 cm(exp -1) from a fixed-node diffusion Monte Carlo calculation and the experimental Ar-tagged result of 743 cm(exp -1). Other VSCF/VCI energies are in good agreement with other experimentally reported ones. Both double-harmonic intensity and rigorous MULTIMODE intensity calculations show the proton transfer fundamental has a very strong intensity

Spitzer transit and secondary eclipse photometry of GJ 436b

We report the results of infrared (8 mu m) transit and secondary eclipse photometry of the hot Neptune exoplanet, GJ 436b using Spitzer. The nearly photon-limited precision of these data allows us to measure an improved radius for the planet and to detect the secondary eclipse. The transit (centered at HJD = 2454280.78149 +/- 0.00016) shows the flat-bottomed shape typical of infrared transits, and it precisely defines the planet-to-star 0.00016 radius ratio (), independent of the stellar properties. However, we obtain the planetary radius, 0.0839 +/- 0.0005 as well as the stellar mass and radius, by fitting to the transit curve simultaneously with an empirical mass-radius relation for M dwarfs (M = R). We find R* = M* 0.47 +/- 0.02 in solar units, and R-p = 27,600 +/- 1170 km 4.33 +/- 0.18 R-circle plus). This radius significantly exceeds the radius of a naked ocean planet and requires a gaseous hydrogen-helium envelope. The secondary eclipse occurs at phase, proving a significant orbital 0.587 +/- 0.005 eccentricity (e = 0.150 +/- 0.012). The amplitude of the eclipse [(5.7 +/- 0.8) x 10(-4)] indicates a brightness tem- perature for the planet of T = 712 +/- 36 K. If this is indicative of the planet\u27s physical temperature, it suggests T = 712 +/- 36 the occurrence of tidal heating in the planet. An uncharacterized second planet likely provides ongoing gravitational perturbations that maintain GJ 436b\u27s orbit eccentricity over long timescales