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91.
Heteroatom polysilylenes with Cl, OR and NR2 substituents attached to the silicon chain through the heteroatom are described. The relationship between the single crystal X-ray structures of octachlorocyclotetrasilane Si4Cl8 and perchloropolysilylene, (SiCl2)n, is discussed with reference to possible mechanisms for its phototopochemical synthesis from Si4Cl8. Substitution with alcohols yields the bis(alkoxy)polysilylenes, and the origin of the red-shifted UV spectra are discussed in terms of the electronic interaction between oxygen n and silicon orbitals. Substitution with secondary amines yields soluble, yellow polymers with even longer wavelength UV absorption maxima. Effects of the alkyl chain length on hydrolytic stability are considered. 相似文献
92.
We present a detailed de Haas-van Alphen effect study of the perovskite CaVO3, offering an unprecedented test of electronic structure calculations in a 3d transition metal oxide. Our experimental and calculated Fermi surfaces are in good agreement, but only if we ignore large orthorhombic distortions of the cubic perovskite structure. Subtle discrepancies may shed light on an apparent conflict between the low energy properties of CaVO3, which are those of a simple metal, and high energy probes which reveal strong correlations that place CaVO3 on the verge of a metal-insulator transition. 相似文献
93.
Julian Schwinger 《Letters in Mathematical Physics》1992,24(3):227-230
A proper-energy method is used to find the energy of attraction between two dielectric slabs. Independent derivations of the attractive force are also presented. 相似文献
94.
Mixtures of N2O, H2, O2, and trace amounts of NO and NO2 were photolyzed at 213.9 nm, at 245°–328°K, and at about 1 atm total pressure (mostly H2). HO2 radicals are produced from the photolysis and they react as follows: Reaction (1b) is unimportant under all of our reaction conditions. Reaction (1a) was studied in competition with reaction (3) from which it was found that k1a/k31/2 = 6.4 × 10?6 exp { z?(1400 ± 500)/RT} cm3/2/sec1/2. If k3 is taken to be 3.3 × 10?12 cm3/sec independent of temperature, k1a = 1.2 × 10?11 exp {?(1400 ± 500)/RT} cm3/sec. Reaction (2a) is negligible compared to reaction (2b) under all of our reaction conditions. The ratio k2b/k1 = 0.61 ± 0.15 at 245°K. Using the Arrhenius expression for k1a given above leads to k2b = 4.2 × 10?13 cm3/sec, which is assumed to be independent of temperature. The intermediate HO2NO2 is unstable and induces the dark oxidation of NO through reaction (?2b), which was found to have a rate coefficient k?2b = 6 × 1017 exp {?26,000/RT} sec?1 based on the value of k1a given above. The intermediate can also decompose via Reaction (10b) is at least partially heterogeneous. 相似文献
95.
The competitive reactions between 2-trifluoromethylpropene (TMP) and OCS for O(3P) atoms were studied between 300° and 523°K, using the mercury-senstitized photolysis of N2O as a source of O(3P). From the known value for the rate constant of the O(3P) + TMP reaction, k3 was found to be 1.6 × 10?11 exp (?4500/RT) cm3/particle-sec, where reaction (3) is Mixtures of O3 and OCS were photolyzed at 197°, 228°, 273°, and 299°K with radiation above 4300 Å to produce O(3P) from the photolysis of O3, and thus study the competition between reaction (3) and From the above value of k3, k1 could be computed. When combined with all the previous data, the best espression for k1 is k1 = 1.2 × 10?11 exp (?4300/RT) cm3/particle-sec. 相似文献
96.
The purpose of this note is to generalize a theorem of Tamura’s [3] providing a self-contained and, we think, more elementary
proof than Tamura’s in that it avoids using the theory of contents. Tamura’s result states that a semigroup S satisfies an
identify xy=f(x,y) with f(x,y) a word of length greater than 2 which starts with y and ends in x if and only if S is an inflation
of a semilattice of groups satisfying the same identity. We investigate semigroups as in Tamura’s Theorem, except that we
permit f(x,y) to vary with x and y. 相似文献
97.
Our earlier work on the formation of particulate NH4NO3 in the NH3? O3 reaction at 25°C is extended to include air as a diluent and H2O vapor as an additive. More extensive data at different values of [NH3]/[O3]0 were obtained also, where [O3]0 is the initial O3 concentration. In our earlier work we concluded that NH4NO3 vapor was dissociated to NH3 + HNO3 and that the HNO3 was removed by diffusion to the walls with a rate coefficient kdiff = 0.4 min?1 or by condensation on the suspended particles. Particles were nucleated by 8 NH3? HNO3 pairs when their concentration product reached 5.8 × 1027 molec2/cm6 with a rate coefficient knucl of 6.2 × 10?224 cm45/min and removed by coagulation with a rate coefficient kcoag of 1.3 × 10?7 cm3/min. A corrected calculation modifies the number of pairs required to 6–7 with a correspondingly changed value of knucl. With the more extensive data of the present study the indications are that the vapor-phase NH4NO3 monomer is not dissociated and that its diffusion constant for loss to the walls varies between 0.3 and 0.9 min?1 for different reaction conditions. Nucleation occurs when the NH4NO3 vapor concentration reaches 1.0 × 1012 molec/cm3 via. where r is 9 and the nucleation rate coefficient knucl is 3 × 10?108 cm24/min. With 5.0 or 9.5 torr of H2O vapor present, there is an excess of particles produced over that expected from this rate coefficient, indicating an additional nucleation step in which H2O vapor participates directly to produce a hydrated salt. The coagulation coefficient of (1.87 ± 0.14) × 10?7 cm3/min found here is in good agreement with that found previously. 相似文献
98.
99.
100.
The overall Kaplan-Meier estimate of the survival curve underlying a study population does not individually account for the ages at which patients were first seen and thence followed up in the study. Since the number of patients in these age-specific subgroups varies quite arbitrarily for a given study, the overall Kaplan-Meier curve which assumes that all patients were seen from the beginning of the study usually either overestimates or underestimates the survival probability, depending on the varying sample sizes. In this paper, we therefore propose an estimate of the overall survival curve underlying a study population based on the individual age-specific Kaplan-Meier estimates and the probability distribution of the age at which patients were first seen in the study. In the particular case when there is no censoring, it is shown that the usual overall Kaplan-Meier survival curve which assumes that all patients irrespective of their age at entry into the study were followed from the beginning of the study is identical with the proposed estimate. This result does not seem to necessarily hold good in the presence of arbitrary censoring. Further, the estimate is unbiased and consistent, a property which the Kaplan-Meier estimate also enjoys. The efficiency of the proposed estimate relative to the Kaplan-Meier estimate is being investigated and will be reported along with results of its application in a separate communication. 相似文献