Arrhenius parameters in the solvolysis of alkyl chlorides and bromides

The reactivities of alkyl halides, BX, in nucleophilic substitution reactions increase in the order RF C-C1 > C-Br > C-I. On the other hand some authors have concluded that a change in the entropy of activation, ΔS(^+) , plays the most important part in controlling reaction rate in this series. In many cases, however, the activation parameters of the different halides referred to different temperatures. Such comparisons may be misleading since recent work has clearly shown that E and ΔS(^+) can vary with temperature; any valid comparison of these parameters must, therefore, involve quantities which all refer to the same temperature.' A study of the reactions of several pairs of alkyl chlorides and bromides with aqueous acetone is now reported. Reaction rates, activation parameters and the temperature coefficients of these parameters have been determined and the results show that, for hydrolysis at the same temperature, the change in rate caused by replacing an alkyl chloride by the corresponding bromide arises almost entirely from a change in the activation energy; this applies to both S(_N)1 and S(_N)2 reactions. It has recently been suggested that the value of ΔC(^+)/ΔS(^+), where ΔC(^+) is the heat capacity of activation, should be independent of the nature of the substrate in SNl solvolysis and that this ratio will have a lower value for solvolysis by mechanism S(_N)2 under the same experimental conditions. This suggestion was based on results observed with alkyl chlorides. All the alkyl chlorides and bromides now studied behave in accordance with the requirements of this hypothesis. During this work the solvolysis of benzyl bromide was studied and the results indicated that this substance reacted by mechanism S(_N)2. This is of interest, for although the hydrolysis of benzyl chloride occurs near the point which marks the transition from reaction by mechanism S(_N)2 to reaction by mechanism S(_N)1, the replacement of the chlorine atom by a bromine atom does not appear to cause a major mechanistic change

An Extremely Luminous Galaxy at z=5.74

We report the discovery of an extremely luminous galaxy lying at a redshift of z=5.74, SSA22-HCM1. The object was found in narrowband imaging of the SSA22 field using a 105 Angstrom bandpass filter centered at 8185 Angstroms during the course of the Hawaii narrowband survey using LRIS on the 10 m Keck II Telescope, and was identified by the equivalent width of the emission W_lambda(observed)=175 Angstroms, flux = 1.7 x 10^{-17} erg cm^{-2} s^{-1}). Comparison with broadband colors shows the presence of an extremely strong break (> 4.2 at the 2 sigma level) between the Z band above the line, where the AB magnitude is 25.5, and the R band below, where the object is no longer visible at a 2 sigma upper limit of 27.1 (AB mags). These properties are only consistent with this object's being a high-z Ly alpha emitter. A 10,800 s spectrum obtained with LRIS yields a redshift of 5.74. The object is similar in its continuum shape, line properties, and observed equivalent width to the z=5.60 galaxy, HDF 4-473.0, as recently described by Weymann et al. (1998), but is 2-3 times more luminous in the line and in the red continuum. For H_0 = 65 km s^{-1} Mpc^{-1} and q_0 = (0.02, 0.5) we would require star formation rates of around (40, 7) solar masses per year to produce the UV continuum in the absence of extinction.Comment: 5 pages, 4 figures, Latex with emulateapj style file; to appear in the Astrophysical Journal (Letters

Associations between environmental exposures and asthma control and exacerbations in young children : a systematic review

Peer reviewedPublisher PD

A Flux-Limited Sample of z~1 Ly-alpha Emitting Galaxies in the CDFS

We describe a method for obtaining a flux-limited sample of Ly-alpha emitters from GALEX grism data. We show that the multiple GALEX grism images can be converted into a three-dimensional (two spatial axes and one wavelength axis) data cube. The wavelength slices may then be treated as narrowband images and searched for emission-line galaxies. For the GALEX NUV grism data, the method provides a Ly-alpha flux-limited sample over the redshift range z=0.67-1.16. We test the method on the Chandra Deep Field South field, where we find 28 Ly-alpha emitters with faint continuum magnitudes (NUV>22) that are not present in the GALEX pipeline sample. We measure the completeness by adding artificial emitters and measuring the fraction recovered. We find that we have an 80% completeness above a Ly-alpha flux of 10^-15 erg/cm^2/s. We use the UV spectra and the available X-ray data and optical spectra to estimate the fraction of active galactic nuclei in the selection. We report the first detection of a giant Ly-alpha blob at z<1, though we find that these objects are much less common at z=1 than at z=3. Finally, we compute limits on the z~1 Ly-alpha luminosity function and confirm that there is a dramatic evolution in the luminosity function over the redshift range z=0-1.Comment: 18 pages, in press at The Astrophysical Journa

A new multi-modal dataset for human affect analysis

In this paper we present a new multi-modal dataset of spontaneous three way human interactions. Participants were recorded in an unconstrained environment at various locations during a sequence of debates in a video conference, Skype style arrangement. An additional depth modality was introduced, which permitted the capture of 3D information in addition to the video and audio signals. The dataset consists of 16 participants and is subdivided into 6 unique sections. The dataset was manually annotated on a continuously scale across 5 different affective dimensions including arousal, valence, agreement, content and interest. The annotation was performed by three human annotators with the ensemble average calculated for use in the dataset. The corpus enables the analysis of human affect during conversations in a real life scenario. We first briefly reviewed the existing affect dataset and the methodologies related to affect dataset construction, then we detailed how our unique dataset was constructed

An Integrated Picture of Star Formation, Metallicity Evolution, and Galactic Stellar Mass Assembly

We present an integrated study of star formation and galactic stellar mass assembly from z=0.05-1.5 and galactic metallicity evolution from z=0.05-0.9 using a very large and highly spectroscopically complete sample selected by rest-frame NIR bolometric flux in the GOODS-N. We assume a Salpeter IMF and fit Bruzual & Charlot (2003) models to compute the galactic stellar masses and extinctions. We determine the expected formed stellar mass density growth rates produced by star formation and compare them with the growth rates measured from the formed stellar mass functions by mass interval. We show that the growth rates match if the IMF is slightly increased from the Salpeter IMF at intermediate masses (~10 solar masses). We investigate the evolution of galaxy color, spectral type, and morphology with mass and redshift and the evolution of mass with environment. We find that applying extinction corrections is critical when analyzing galaxy colors; e.g., nearly all of the galaxies in the green valley are 24um sources, but after correcting for extinction, the bulk of the 24um sources lie in the blue cloud. We find an evolution of the metallicity-mass relation corresponding to a decrease of 0.21+/-0.03 dex between the local value and the value at z=0.77 in the 1e10-1e11 solar mass range. We use the metallicity evolution to estimate the gas mass of the galaxies, which we compare with the galactic stellar mass assembly and star formation histories. Overall, our measurements are consistent with a galaxy evolution process dominated by episodic bursts of star formation and where star formation in the most massive galaxies (>1e11 solar masses) ceases at z<1.5 because of gas starvation. (Abstract abridged)Comment: 48 pages, Accepted by the Astrophysical Journa