1,590 research outputs found
Benzene CâH Bond Activation in Carboxylic Acids Catalyzed by O-Donor Iridium(III) Complexes: An Experimental and Density Functional Study
The mechanism of benzene CâH bond activation by [Ir(ÎŒ-acac-O,O,C^3)(acac-O,O)(OAc)]_2 (4) and [Ir(ÎŒ-acac-O,O,C^3)(acac-O,O)(TFA)]_2 (5) complexes (acac = acetylacetonato, OAc = acetate, and TFA = trifluoroacetate) was studied experimentally and theoretically. Hydrogenâdeuterium (H/D) exchange between benzene and CD_(3)COOD solvent catalyzed by 4 (ÎH^⥠= 28.3 ± 1.1 kcal/mol, ÎS^⥠= 3.9 ± 3.0 cal K^(â1) mol^(â1)) results in a monotonic increase of all benzene isotopologues, suggesting that once benzene coordinates to the iridium center, there are multiple H/D exchange events prior to benzene dissociation. B3LYP density functional theory (DFT) calculations reveal that this benzene isotopologue pattern is due to a rate-determining step that involves acetate ligand dissociation and benzene coordination, which is then followed by heterolytic CâH bond cleavage to generate an iridium-phenyl intermediate. A synthesized iridium-phenyl intermediate was also shown to be competent for H/D exchange, giving similar rates to the proposed catalytic systems. This mechanism nicely explains why hydroarylation between benzene and alkenes is suppressed in the presence of acetic acid when catalyzed by [Ir(ÎŒ-acac-O,O,C^3)(acac-O,O)(acac-C^3)]_2 (3) (Matsumoto et al. J. Am. Chem. Soc. 2000, 122, 7414). Benzene H/D exchange in CF_(3)COOD solvent catalyzed by 5 (ÎH^⥠= 15.3 ± 3.5 kcal/mol, ÎS^⥠= â30.0 ± 5.1 cal K^(â1) mol^(â1)) results in significantly elevated H/D exchange rates and the formation of only a single benzene isotopologue, (C_(6)H_(5)D). DFT calculations show that this is due to a change in the rate-determining step. Now equilibrium between coordinated and uncoordinated benzene precedes a single rate-determining heterolytic CâH bond cleavage step
Quantification of the Impact of Photon Distinguishability on Measurement-Device- Independent Quantum Key Distribution
Measurement-Device-Independent Quantum Key Distribution (MDI-QKD) is a two-photon protocol devised to eliminate eavesdropping attacks that interrogate or control the detector in realized quantum key distribution systems. In MDI-QKD, the measurements are carried out by an untrusted third party, and the measurement results are announced openly. Knowledge or control of the measurement results gives the third party no information about the secret key. Error-free implementation of the MDI-QKD protocol requires the crypto-communicating parties, Alice and Bob, to independently prepare and transmit single photons that are physically indistinguishable, with the possible exception of their polarization states. In this paper, we apply the formalism of quantum optics and Monte Carlo simulations to quantify the impact of small errors in wavelength, bandwidth, polarization and timing between Aliceâs photons and Bobâs photons on the MDI-QKD quantum bit error rate (QBER). Using published single-photon source characteristics from two-photon interference experiments as a test case, our simulations predict that the finite tolerances of these sources contribute (4.04±20/âNsifted )% to the QBER in an MDI-QKD implementation generating an Nsifted-bit sifted key
Heterolytic CH Activation with a Cyclometalated Platinum(II) 6-Phenyl-4,4â-di-tert-butyl-2,2-Bipyridine Complex
The more electron-rich, thermally, air, and protic stable, cyclometalated Pt(II)(NNC) trifluoroacetate complex (3) (NNC = Îș^3-6-phenyl-4,4â-di-tert-butyl-2,2â-bipyridine) was synthesized with the expectation that it would be less susceptible to H_2O inhibition than the Pt(bpym)(TFA)_2 system (bpym = Îș^2-2,2â-bipyrimidine) for the catalytic oxidation of hydrocarbons. Complex 3 was found to catalyze the H/D exchange between benzene and trifluoroacetic acid via CH activation but at a rate slower than the Pt(bpym) complex. Experimental and theoretical studies show that while the use of the more electron-rich NNC ligand motif lowered the ÎH for substrate coordination relative to the Pt(bpym) system, a larger increase in the barrier for CH cleavage led to an increase in the overall barrier for CH activation
Mechanical and hydrologic basis for the rapid motion of a large tidewater glacier: 1. Observations
Measurements of glacier flow velocity and basal water pressure at two sites on Columbia Glacier, Alaska, are combined with meteorological and hydrologic data to provide an observational basis for assessing the role of water storage and basal water pressure in the rapid movement of this large glacier. During the period from July 5 to August 31, 1987, coordinated observations were made of glacier surface motion and of water level in five boreholes drilled to (or in one case near to) the glacier bed at two sites, 5 and 12 km from the terminus. Glacier velocities increased downglacier in this reach from about 4 m d^â1 to about 7 m d^â1. Three types of time variation in velocity and other variables were revealed: (1) Diurnal fluctuation in water input/output, borehole water level, and ice velocity (fluctuation amplitude 5 to 8%); (2) Speed-up events in glacier motion (15â30% speed up), lasting about 3 days, and occurring at times of enhanced input of water, in some cases from rain and in others from ice ablation enhanced by strong, warm winds; (3) âExtra-slowdownâ events, in which, after a speed-up event, the ice velocity decreased in about 3 days to a level consistently lower than that prior to the speed-up event. All of the time variations in velocity were due, directly or indirectly, to variations in water input to the glacier. The role of basal water in causing the observed glacier motions is interpreted by Kamb et al. (this issue)
Robust s+/- pairing in CaK[Fe(1-x)Ni(x)]4As4$ (x = 0 and 0.05) from the response to electron irradiation
Controlled point-like disorder introduced by 2.5 MeV electron irradiation was
used to probe the superconducting state of single crystals of \CaKx\
superconductor at and 0.05 doping levels. Both compositions show an
increase of the residual resistivity and a decrease of the superconducting
transition temperature, at the rate of 0.19
K(\textmucm) for and 0.38 K(\textmucm) for
0.05, respectively. In Ni - doped, , compound the coexisting
spin-vortex crystal (SVC) magnetic phase is suppressed at the rate of
0.16 K(\textmucm). Low - temperature
variation of London penetration depth is well approximated by the power law,
with 2.5 for and 1.9
for in the pristine state. Electron irradiation leads to the exponent
increase above 2 in suggesting superconducting gap with
significant anisotropy that is smeared by the disorder scattering. Detailed
analysis of and evolution with disorder is consistent
with two effective nodeless superconducting energy gaps due to robust s
pairing. Overall the behavior of \CaKx\ at is similar to a slightly
overdoped \BaK\ at 0.5 and at to an underdoped
composition at 0.2
The Nuclear Spectroscopic Telescope Array (NuSTAR)
The Nuclear Spectroscopic Telescope Array (NuSTAR) is a NASA Small Explorer
mission that will carry the first focusing hard X-ray (5 -- 80 keV) telescope
to orbit. NuSTAR will offer a factor 50 -- 100 sensitivity improvement compared
to previous collimated or coded mask imagers that have operated in this energy
band. In addition, NuSTAR provides sub-arcminute imaging with good spectral
resolution over a 12-arcminute field of view. After launch, NuSTAR will carry
out a two-year primary science mission that focuses on four key programs:
studying the evolution of massive black holes through surveys carried out in
fields with excellent multiwavelength coverage, understanding the population of
compact objects and the nature of the massive black hole in the center of the
Milky Way, constraining explosion dynamics and nucleosynthesis in supernovae,
and probing the nature of particle acceleration in relativistic jets in active
galactic nuclei. A number of additional observations will be included in the
primary mission, and a guest observer program will be proposed for an extended
mission to expand the range of scientific targets. The payload consists of two
co-aligned depth-graded multilayer coated grazing incidence optics focused onto
solid state CdZnTe pixel detectors. To be launched in early 2012 on a Pegasus
rocket into a low-inclination Earth orbit. Data will be publicly available at
GSFC's High Energy Astrophysics Science Archive Research Center (HEASARC)
following validation at the science operations center located at Caltech.Comment: 9 pages, 5 figures, to appear in Proceedings of the SPIE, Space
Telescopes and Instrumentation 2010: Ultraviolet to Gamma Ra
The Redshift Search Receiver 3 mm Wavelength Spectra of 10 Galaxies
The 3 mm wavelength spectra of 10 galaxies have been obtained at the Five
College Radio Astronomy Observatory using a new, very broadband receiver and
spectrometer, called the Redshift Search Receiver (RSR). The RSR has an
instantaneous bandwidth of 37 GHz covering frequencies from 74 to 111 GHz, and
has a spectral resolution of 31 MHz (~100 km/s). During tests of the RSR on the
FCRAO 14 m telescope the complete 3 mm spectra of the central regions of NGC
253, Maffei 2, NGC1068, IC 342, M82, NGC 3079, NGC 3690, NGC 4258, Arp 220 and
NGC 6240 were obtained. Within the wavelength band covered by the RSR, 20
spectral lines from 14 different atomic and molecular species were detected.
Based on simultaneous fits to the spectrum of each galaxy, a number of key
molecular line ratios are derived. A simple model which assumes the emission
arises from an ensemble of Milky Way-like Giant Nolecular Cloud cores can
adequately fit the observed line ratios using molecular abundances based on
Galactic molecular cloud cores. Variations seen in some line ratios, such as
13CO/HCN and HCO+/HCN, can be explained if the mean density of the molecular
gas varies from galaxy to galaxy. However, NGC 3690, NGC 4258 and NGC 6240 show
very large HCO+/HCN ratios and require significant abundance enhancement of
HCO+ over HCN, possible due to the proximity to active galactic nucleus
activity. Finally, the mass of dense molecular gas is estimated and we infer
that 25-85 % of the total molecular gas in the central regions of these
galaxies must have densities greater than 10^4 cm^-3.Comment: accepted for publication in A.
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