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1.
Alistair J Usher Nigel T Lucas Simon Petrie Mark G Humphrey Anthony C Willis 《Journal of organometallic chemistry》2004,689(1):50-57
Reaction of WH(CO)3(η-C5Me5) with IrCl(CO)2(4-H2NC6H4Me) affords WIr3(μ-CO)3(CO)8(η-C5Me5) in low yield. A structural study reveals a WIr2-centred plane of bridging carbonyls, in contrast to the crystal structure of WIr3(CO)11(η-C5H5) (all-terminal carbonyl distribution). DFT calculations reveal an increasing proclivity to adopt an all-terminal CO disposition for clusters MIr3(CO)11(η-C5H5) in the gas phase on proceeding from M=Cr to Mo and then W, consistent with structural studies in the solid state for which the tungsten-containing cluster is the only all-terminal example. Increasing electron donation from the ligands in the tungsten system (either from phosphine substitution or cyclopentadienyl permethylation) suffices to impose a plane of bridging carbonyls in the ground state structure. 13C NMR fluxionality studies reveal that CO exchange mechanism(s) for WIr3(CO)11(η-C5H5) and the related tetrahedral cluster W2Ir2(CO)10(η-C5H5)2 are very fast and involve all carbonyls on the clusters. DFT calculations on MIr3(CO)11(η-C5H5) (M=Cr, Mo) substantiate a ‘merry-go-round’ mechanism for carbonyl scrambling in these systems, a result which is consistent with the scrambling behaviour seen in the NMR fluxionality studies on the W-containing congener. 相似文献
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The use of a nitrogen electron cyclotron resonance (ECR) plasma source has allowed the growth of GaAsN at GaAs substrate temperatures as high as 600 °C, unlike the case for growth using radio frequency (RF) plasma sources, for which there is significant loss of nitrogen at substrate temperatures as low as 480-520 °C. Photoluminescence (PL) intensities are significantly improved at a substrate temperature of 600 °C and are further improved slightly by using an ion trap to extract charged species from the beam. As the trap voltage is increased there is a reduction in the total nitrogen concentration, as measured by secondary ion mass spectrometry (SIMS), and a slight increase in the active nitrogen concentration, as measured by PL. These observations are consistent, for example, with charged and active nitrogen species together being involved in the formation of point defects, however more work is needed to clarify what may well prove to be a complex situation. 相似文献
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Michael Usher 《Israel Journal of Mathematics》2011,184(1):1-57
We assign to each nondegenerate Hamiltonian on a closed symplectic manifold a Floer-theoretic quantity called its “boundary
depth,” and establish basic results about how the boundary depths of different Hamiltonians are related. As applications,
we prove that certain Hamiltonian symplectomorphisms supported in displaceable subsets have infinitely many nontrivial geometrically
distinct periodic points, and we also significantly expand the class of coisotropic submanifolds which are known to have positive
displacement energy. For instance, any coisotropic submanifold of contact type (in the sense of Bolle) in any closed symplectic
manifold has positive displacement energy, as does any stable coisotropic submanifold of a Stein manifold. We also show that
any stable coisotropic submanifold admits a Riemannian metric that makes its characteristic foliation totally geodesic, and
that this latter, weaker, condition is enough to imply positive displacement energy under certain topological hypotheses. 相似文献
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E. Eisenhandler W.R. Gibson C. Hojvat P.I.P. Kalmus L.C.Y.Lee Chi Kwong T.W. Pritchard E.C. Usher D.T. Williams M. Harrison W.H. Range M.A.R. Kemp A.D. Rush J.N. Woulds G.T.J. Arnison A. Astbury D.P. Jones A.S.L. Parsons 《Physics letters. [Part B]》1973,47(6):531-535
We present measurements from a spark chamber experiment of the differential cross-sections for p → π?π+, K?K+ at 20 momenta in the range 0.8–2.4 GeV/c (c.m. energy 2.02 to 2.57 GeV). The c.m. angular range was . There are about 2000π?π+ events and about 300 K?K+ events at each momentum. 相似文献
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