2,334 research outputs found
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, ÎS(^+) , 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 ÎS(^+) 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 ÎC(^+)/ÎS(^+), where ÎC(^+) 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
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
The bar PANDA focussing-lightguide disc DIRC
bar PANDA will be a fixed target experiment internal to the HESR antiproton storage ring at the future FAIR complex. The ANDA detector requires excellent particle-identification capabilities in order to achieve its scientific potential. Cherenkov counters employing the DIRC principle were chosen as PID detectors for the Target Spectrometer. The proposed Focussing-Lightguide Disc DIRC will cover the forward part of the Target Spectrometer acceptance in the angular range between 5° and 22°. Its design includes a novel approach to mitigate dispersion effects in the solid radiator of a DIRC counter using optical elements. The dispersion correction will enable the Focussing-Lightguide Disc DIRC to provide pion-kaon identification for momenta well above 3.5 GeV/c
Galaxy Halo Masses from Galaxy-Galaxy Lensing
We present measurements of the extended dark halo profiles of bright early
type galaxies at redshifts 0.1 to 0.9 obtained via galaxy-galaxy lensing
analysis of images taken at the CFHT using the UH8K CCD mosaic camera. Six half
degree fields were observed for a total of 2 hours each in I and V, resulting
in catalogs containing ~20 000 galaxies per field. We used V-I color and I
magnitude to select bright early type galaxies as the lens galaxies, yielding a
sample of massive lenses with fairly well determined redshifts and absolute
magnitudes M ~ M_* \pm 1. We paired these with faint galaxies lying at angular
distances 20" to 60", corresponding to physical radii of 26 to 77 kpc (z = 0.1)
and 105 to 315 kpc (z = 0.9), and computed the mean tangential shear of the
faint galaxies. The shear falls off with radius roughly as expected for flat
rotation curve halos. The shear values were weighted in proportion to the
square root of the luminosity of the lens galaxy. Our results give a value for
the average mean rotation velocity of an L_* galaxy halo at r~50-200 kpc of v_*
= 238^{+27}_{-30} km per sec for a flat lambda (Omega_m0 = 0.3, Omega_l0 = 0.7)
cosmology (v_* = 269^{+34}_{-39} km per sec for Einstein-de Sitter), and with
little evidence for evolution with redshift. We compare to halo masses measured
by other groups/techniques. We find a mass-to-light ratio of ~121\pm28h(r/100
kpc) and these halos constitute Omega ~0.04 \pm 0.01(r/100 kpc) of closure
density. (abridged)Comment: Accepted for publication in ApJ (minor modifications) - 32 pages, 11
figs, 5 table
Faint, Evolving Radio AGN in SDSS Luminous Red Galaxies
We detect and study the properties of faint radio AGN in Luminous Red
Galaxies (LRGs). The LRG sample comprises 760,000 objects from a catalog of LRG
photometric redshifts constructed from the Sloan Digital Sky Survey (SDSS)
imaging data, and 65,000 LRGs from the SDSS spectroscopic sample. These
galaxies have typical 1.4 GHz flux densities in the 10s-100s of microJy, with
the contribution from a low-luminosity AGN dominating any contribution from
star formation. To probe the radio properties of such faint objects, we employ
a stacking technique whereby FIRST survey image cutouts at each optical LRG
position are sorted by the parameter of interest and median-combined within
bins. We find that median radio luminosity scales with optical luminosity
(L_opt) as L_1.4 GHz ~ L_opt^(beta), where beta appears to decrease from beta ~
1 at z = 0.4 to beta ~ 0 at z = 0.7, a result which could be indicative of AGN
cosmic downsizing. We also find that the overall LRG population, which is
dominated by low-luminosity AGN, experiences significant cosmic evolution
between z = 0.2 and z = 0.7. This implies a considerable increase in total AGN
heating for these massive ellipticals with redshift. By matching against the
FIRST catalog, we investigate the incidence and properties of LRGs associated
with double-lobed (FR I/II) radio galaxies. (Abridged)Comment: 12 pages, 9 figures, Accepted by A
Optically Faint Microjansky Radio Sources
We report on the identifications of radio sources from our survey of the
Hubble Deep Field and the SSA13 fields, both of which comprise the deepest
radio surveys to date at 1.4 GHz and 8.5 GHz respectively. About 80% of the
microjansky radio sources are associated with moderate redshift starburst
galaxies or AGNs within the I magnitude range of 17 to 24 with a median of I =
22 mag. Thirty-one (20%) of the radio sources are: 1) fainter than 25 mag,
with two objects in the HDF 28.5, 2) often identified with very red
objects 4, and 3) not significantly different in radio properties than
the brighter objects. We suggest that most of these objects are associated with
heavily obscured starburst galaxies with redshifts between 1 and 3. However,
other mechanisms are discussed and cannot be ruled out with the present
observations.Comment: to appear in Astrophysical Journal Letters, 3 figures, 1 tabl
Resolving the Submillimeter Background: the 850-micron Galaxy Counts
Recent deep blank field submillimeter surveys have revealed a population of
luminous high redshift galaxies that emit most of their energy in the
submillimeter. The results suggest that much of the star formation at high
redshift may be hidden to optical observations. In this paper we present
wide-area 850-micron SCUBA data on the Hawaii Survey Fields SSA13, SSA17, and
SSA22. Combining these new data with our previous deep field data, we establish
the 850-micron galaxy counts from 2 mJy to 10 mJy with a >3-sigma detection
limit. The area coverage is 104 square arcmin to 8 mJy and 7.7 square arcmin to
2.3 mJy. The differential 850-micron counts are well described by the function
n(S)=N_0/(a+S^3.2), where S is the flux in mJy, N_0=3.0 x 10^4 per square
degree per mJy, and a=0.4-1.0 is chosen to match the 850-micron extragalactic
background light. Between 20 to 30 per cent of the 850-micron background
resides in sources brighter than 2 mJy. Using an empirical fit to our >2 mJy
data constrained by the EBL at lower fluxes, we argue that the bulk of the
850-micron extragalactic background light resides in sources with fluxes near 1
mJy. The submillimeter sources are plausible progenitors of the present-day
spheroidal population.Comment: 5 pages, accepted by The Astrophysical Journal Letter
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