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991.
Monnell JD Stapleton JJ Dirk SM Reinerth WA Tour JM Allara DL Weiss PS 《The journal of physical chemistry. B》2005,109(43):20343-20349
The electronic properties of alkanethiolate [CH3(CH2)nS-, n = 9 and 11] and alkaneselenolate [CH3(CH2)nSe-, n = 9 and 11] self-assembled monolayers on Au{111} have been quantitatively compared. Simultaneously acquired apparent tunneling barrier height (ATBH) and scanning tunneling microscopy (STM) images reveal that alkanethiolate molecules have a lower barrier to tunneling, and therefore a higher conductance than alkaneselenolates of the same alkyl chain length. Molecular and contact conductance differences were elucidated by using observed STM topographic tunneling height differences between the analogous species. This apparent topographic difference combined with comparative ATBH data indicate that the observed decrease in conductance for alkaneselenolates compared to alkanethiolates originates exclusively from the Au-chalcogenide physical, chemical, and electronic contact. 相似文献
992.
Drevogenin A was converted in several steps (acetylation, hydrogenation, dehydration, hydrogenation, the haloform reaction and energetic alkaline hydrolysis) into 3β, 11α, 12β-trihydroxy-5α-etianic acid, which could be characterised by its crystalline methyl ester ( 15 ) and its tri-O-acetyl methyl ester ( 16 ). The same acid was obtained by partial synthesis starting from hecogenin. Taking into consideration earlier results [1], the structure of drevogenin P is proved to be 3β, 11α, 12β, 14β-tetrahydroxy-20-oxo-Δ5-pregnene ( 7 ). Energetic hydrolysis of dihydro-3-O-acetyldrevogenin A gave a mixture of 17αH- and 17βH-desacyl-kondurangogenin A, which were obtained in crystalline form after separation by chromatography. The only difference between the basic structures of the drevogenins and kondurangogenin A is the presence of a double bond in the 5-position in the former. 相似文献
993.
George H. Weiss Edmund A. DiMarzio Richard J. Gaylord 《Journal of statistical physics》1986,42(3-4):567-572
A general expression is derived for the Laplace transform of the probability density of the first passage time for the span of a symmetric continuous-time random walk to reach levelS. We show that when the mean time between steps is finite, the mean first passage time toS is proportional toS
2. When the pausing time density is asymptotic to a stable density we show that the first passage density is also asymptotically stable. Finally when the jump distribution of the random walk has the asymptotic formp(j)A/|j|
+1, 0 < < 2 it is shown that the mean first passage time toS goes likeS
. 相似文献
994.
995.
We present analytical and numerical results for the probability distributions of the number of sitesS as a function of the number of shellsl for several ensembles of percolation clusters generated on a Cayley tree at criticality. We find that for the incipient infinite percolation cluster the probability distribution isP(S¦l)~(S/l
4)exp(- aS/l
2) for Sl1. 相似文献
996.
997.
998.
We investigate the homotopy type of
(M)/TOP(M), where M is a compact manifold, TOP(M) is the simplicial group of homeomorphisms of M which restrict to the identity on ∂M, and
is the simplicial group of block homeomorphisms of M which restrict to the identity on ∂M. In the so-called topological concordance stable range of M, we obtain an expression in terms of the topological Whitehead spectrum of M. If M is smooth, we also investigate the homotopy type of
(M)/DIFF(M); in the smooth concordance stable range of M, it has an expression in terms of the smooth Whitehead spectrum of M. 相似文献
999.
1000.
Katja Lindenberg V. Seshadri K. E. Shuler George H. Weiss 《Journal of statistical physics》1980,23(1):11-25
We have studied the mean first passage time for the first of aset of random walkers to reach a given lattice point on infinite lattices ofD dimensions. In contrast to the well-known result ofinfinite mean first passage times for one random walker in all dimensionsD, we findfinite mean first passage times for certain well-specified sets of random walkers in all dimensions, exceptD = 2. The number of walkers required to achieve a finite mean time for the first walker to reach the given lattice point is a function of the lattice dimensionD. ForD > 4, we find that only one random walker is required to yield a finite first passage time, provided that this random walker reaches the given lattice point with unit probability. We have thus found a simple random walk property which sticks atD > 4.Supported in part by a grant from Charles and Renée Taubman and by the National Science Foundation, Grant CHE78-21460. 相似文献