399 research outputs found
Replacing C6F5 groups with Cl and H atoms in frustrated Lewis pairs : H-2 additions and catalytic hydrogenations
2-(Dialkylamino) phenylboranes containing the BXZ group, where X, Z = C6F5, Cl, and H, were prepared in a few synthetic steps and demonstrated the cleavage of H-2 under mild conditions. Depending on the nature of the dialkylamino group, X, and Z, the stability of the produced zwitterionic H-2 adducts varies from isolated solids indefinitely stable in an inert atmosphere to those quickly equilibrating with the initial aminoborane and H-2. Using a combined experimental/computational approach on a series of isostructural aminoboranes (dialkylamino = 2,2,6,6-tetramethylpiperid-1-yl), it was demonstrated that the electro-negativity and the steric effect of the substituents generally follow the trend C6F5 similar to Cl >> H. This observation is useful for designing new FLPs for practical applications. As an example, we demonstrated the hydrogenation of alkynes to cis-alkenes under mild conditions that was catalyzed by a chloro-analogue of the C6F5-substituted aminoborane developed previously. The presence of a BHCl group in the amino-chloroboranes or in their H-2 adducts features facile redistribution of the H and Cl atoms and the formation of polychloro and polyhydrido species.Peer reviewe
Spontaneous N-15 Nuclear Spin Hyperpolarization in Metal-Free Activation of Parahydrogen by Molecular Tweezers
The ability of frustrated Lewis pairs (FLPs) to activate H-2 is of significant interest for metal-free catalysis. The activation of H-2 is also the key element of parahydrogen-induced polarization (PHIP), one of the nuclear spin hyper polarization techniques. It is demonstrated that o-phenylene-based ansa-aminoboranes (AABs) can produce H-1 nuclear spin hyperpolarization through a reversible interaction with parahydrogen at ambient temperatures. Heteronuclei are useful in NMR and MRI as well because they have a broad chemical shift range and long relaxation times and may act as background-free labels. We report spontaneous formation of N-15 hyperpolarization of the N-H site for a family of AABs. The process is efficient at the high magnetic field of an NMR magnet (7 T), and it provides up to 350-fold N-15 signal enhancements. Different hyperpolarization effects are observed with various AAB structures and in a broad temperature range. Spontaneous hyperpolarization, albeit an order of magnitude weaker than that for N-15, was also observed for B-11 nuclei.Peer reviewe
Event-by-event fluctuations in Mean and Mean in sqrt(s_NN) = 130 GeV Au+Au Collisions
Distributions of event-by-event fluctuations of the mean transverse momentum
and mean transverse energy near mid-rapidity have been measured in Au+Au
collisions at sqrt(s_NN) = 130 GeV at RHIC. By comparing the distributions to
what is expected for statistically independent particle emission, the magnitude
of non-statistical fluctuations in mean transverse momentum is determined to be
consistent with zero. Also, no significant non-random fluctuations in mean
transverse energy are observed. By constructing a fluctuation model with two
event classes that preserve the mean and variance of the semi-inclusive p_T or
e_T spectra, we exclude a region of fluctuations in sqrt(s_NN) = 130 GeV Au+Au
collisions.Comment: 10 pages, RevTeX 3, 7 figures, 4 tables, 307 authors, submitted to
Phys. Rev. C on 22 March 2002. Plain text data tables for the points plotted
in figures for this and previous PHENIX publications are (will be made)
publicly available at
http://www.phenix.bnl.gov/phenix/WWW/run/phenix/papers.htm
Net Charge Fluctuations in Au + Au Interactions at sqrt(s_NN) = 130 GeV
Data from Au + Au interactions at sqrt(s_NN) = 130 GeV, obtained with the
PHENIX detector at RHIC, are used to investigate local net charge fluctuations
among particles produced near mid-rapidity. According to recent suggestions,
such fluctuations may carry information from the Quark Gluon Plasma. This
analysis shows that the fluctuations are dominated by a stochastic distribution
of particles, but are also sensitive to other effects, like global charge
conservation and resonance decays.Comment: 6 pages, RevTeX 3, 3 figures, 307 authors, submitted to Phys. Rev.
Lett. on 21 March, 2002. Plain text data tables for the points plotted in
figures for this and previous PHENIX publications are (will be made) publicly
available at http://www.phenix.bnl.gov/phenix/WWW/run/phenix/papers.htm
Flow Measurements via Two-particle Azimuthal Correlations in Au + Au Collisions at sqrt(s_NN) = 130 GeV
Two particle azimuthal correlation functions are presented for charged
hadrons produced in Au + Au collisions at RHIC sqrt(s_NN) = 130 GeV. The
measurements permit determination of elliptic flow without event-by-event
estimation of the reaction plane. The extracted elliptic flow values v_2 show
significant sensitivity to both the collision centrality and the transverse
momenta of emitted hadrons, suggesting rapid thermalization and relatively
strong velocity fields. When scaled by the eccentricity of the collision zone,
epsilon, the scaled elliptic flow shows little or no dependence on centrality
for charged hadrons with relatively low p_T. A breakdown of this epsilon
scaling is observed for charged hadrons with p_T > 1.0 GeV/c for the most
central collisions.Comment: 6 pages, RevTeX 3, 4 figures, 307 authors, submitted to Phys. Rev.
Lett. on 11 April 2002. Plain text data tables for the points plotted in
figures for this and previous PHENIX publications are (will be made) publicly
available at http://www.phenix.bnl.gov/phenix/WWW/run/phenix/papers.htm
Centrality Dependence of Charged Particle Multiplicity in Au-Au Collisions at sqrt(s_NN)=130 GeV
We present results for the charged-particle multiplicity distribution at
mid-rapidity in Au - Au collisions at sqrt(s_NN)=130 GeV measured with the
PHENIX detector at RHIC. For the 5% most central collisions we find
. The results,
analyzed as a function of centrality, show a steady rise of the particle
density per participating nucleon with centrality.Comment: 307 authors, 43 institutions, 6 pages, 4 figures, 1 table Minor
changes to figure labels and text to meet PRL requirements. One author added:
M. Hibino of Waseda Universit
Measurement of the mid-rapidity transverse energy distribution from GeV Au+Au collisions at RHIC
The first measurement of energy produced transverse to the beam direction at
RHIC is presented. The mid-rapidity transverse energy density per participating
nucleon rises steadily with the number of participants, closely paralleling the
rise in charged-particle density, such that E_T / N_ch remains relatively
constant as a function of centrality. The energy density calculated via
Bjorken's prescription for the 2% most central Au+Au collisions at
sqrt(s_NN)=130 GeV is at least epsilon_Bj = 4.6 GeV/fm^3 which is a factor of
1.6 larger than found at sqrt(s_NN)=17.2 GeV (Pb+Pb at CERN).Comment: 307 authors, 6 pages, 4 figures, 1 table, submitted to PRL 4/18/2001;
revised version submitted to PRL 5/24/200
Deuteron and antideuteron production in Au+Au collisions at sqrt(s_NN)=200 GeV
The production of deuterons and antideuterons in the transverse momentum
range 1.1 < p_T < 4.3 GeV/c at mid-rapidity in Au + Au collisions at
sqrt(s_NN)=200 GeV has been studied by the PHENIX experiment at RHIC. A
coalescence analysis comparing the deuteron and antideuteron spectra with those
of protons and antiprotons, has been performed. The coalescence probability is
equal for both deuterons and antideuterons and increases as a function of p_T,
which is consistent with an expanding collision zone. Comparing (anti)proton
yields p_bar/p = 0.73 +/- 0.01, with (anti)deuteron yields: d_bar/d = 0.47 +/-
0.03, we estimate that n_bar/n = 0.64 +/- 0.04.Comment: 326 authors, 6 pages text, 5 figures, 1 Table. Submitted to PRL.
Plain text data tables for the points plotted in figures for this and
previous PHENIX publications are (or will be) publicly available at
http://www.phenix.bnl.gov/papers.htm
Mid-Rapidity Direct-Photon Production in p+p Collisions at sqrt(s) = 200 GeV
A measurement of direct photons in p+p collisions at sqrt(s)=200 GeV is
presented. A photon excess above background from pi^0 --> gamma+gamma, eta -->
gamma+gamma, and other decays is observed in the transverse momentum range 5.5
< p_T < 7 GeV/c. The result is compared to a next-to-leading-order perturbative
QCD calculation. Within errors, good agreement is found between the QCD
calculation and the measured result.Comment: 330 authors, 7 pages text, RevTeX, 2 figures, 2 tables. Submitted to
Physical Review D. Plain text data tables for the points plotted in figures
for this and previous PHENIX publications are (or will be) publicly available
at http://www.phenix.bnl.gov/papers.htm
Suppressed pi^0 Production at Large Transverse Momentum in Central Au+Au Collisions at sqrt(s_NN) = 200 GeV
Transverse momentum spectra of neutral pions in the range 1 < p_T < 10 GeV/c
have been measured at mid-rapidity by the PHENIX experiment at RHIC in Au+Au
collisions at sqrt(s_NN) = 200 GeV. The pi^0 multiplicity in central reactions
is significantly below the yields measured at the same sqrt(s_NN) in peripheral
Au+Au and p+p reactions scaled by the number of nucleon-nucleon collisions. For
the most central bin, the suppression factor is ~2.5 at p_T = 2 GeV/c and
increases to ~4-5 at p_T ~= 4 GeV/c. At larger p_T, the suppression remains
constant within errors. The deficit is already apparent in semi-peripheral
reactions and increases smoothly with centrality.Comment: 326 authors, 6 pages text, RevTeX, 3 figures, 2 tables. Submitted to
PRL. Plain text data tables for the points plotted in figures for this and
previous PHENIX publications are (or will be) publicly available at
http://www.phenix.bnl.gov/papers.htm
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