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排序方式: 共有71条查询结果,搜索用时 15 毫秒
1.
We report the crystal and molecular structures of the complex of 18-C-6 with H3O+BF4− (I) and the complex of 18-C-6 with BF3OH2·H2O (II). The different modes of appearance of the “BF3” species as BF3, BF3OH2, BF3OH2·H2O and BF4−, as well as their structurally significant intermolecular and intramolecular interactions, are discussed. In complex I the oxonium ion is bound at the centre of the 18-C-6 macrocycle. The oxonium oxygen is located practically equidistant (2.68–2.73 Å) from the six macrocyclic ethereal oxygens. The BF4− counter-ion is positioned 7.3 Å away from the oxonium ion in the same general plane of the crown ether. This anion is not involved in any direct intermolecular contacts, a fact that may explain why it is spherically disordered. In complex II there is no guest molecule (or ion) present in the “cavity” of the macrocycle, but there are two hydrogen-bonded systems of BF3OH2·H2O that are interacting with the crown ether on either side of the general macrocyclic plane. Complex II features three types of hydrogen bonds—the O(water)-HO(crown) bonds (2.83 and 2.85 Å), the O(water)H-O(BF3) bond (2.49 Å) and the O(BF3)-HO(crown) bond (2.65 Å). The strong intermolecular O(crown)O(water)O(BF3) and O(crown)O(BF3) interactions stabilize the normally unstable BF3OH2·H2O species. 相似文献
2.
Dvora Barnea Ovadia Shoham Yehuda Taitel 《International Journal of Multiphase Flow》1980,6(5):387-397
An improved system of conductance probes is used to identify the flow patterns in two phase horizontal, near horizontal and upward flows. The results show that this system is very well suited to distinguish among flow patterns consistent with visual observations. 相似文献
3.
Lamed R Kenig R Morag E Yaron S Shoham Y Bayer EA 《Applied biochemistry and biotechnology》2001,90(1):67-73
Previous work from our group [Morag (Morgenstern), E., Bayer, E. A., and Lamed, R. (1991), Appl. Biochem. Biotechnol.
30, 129–136] has demonstrated an anomalous electrophoretic mobility pattern for scaffoldin, the 210-kDa cellulosome-integrating
subunit of Clostridium thermocellum. Subsequent evidence [Morag, E., Bayer, E. A., and Lamed, R. (1992), Appl. Biochem. Biotechnol.
33, 205–217] indicated that the effect could be attributed to a nonproteolytic fragmentation of the subunit into a defined series
of lowermolecular-weight bands. In the present work, a recombinant segment of the scaffoldin subunit was employed to determine
the site(s) of bond breakage. An Asp-Pro sequence within the cohesin domain was identified to be the sensitive peptide bond.
This sequence appears quite frequently in the large cellulosomal proteins, and the labile bond may be related to an as yet
undescribed physiological role in the hydrolysis of cellulose by cellulosomes. 相似文献
4.
Edward S. Letzter 《代数通讯》2013,41(4):1889-1893
5.
Severe slugging can occur in a pipeline-riser system operating at low liquid and gas rates. The flow of gas into the riser can be blocked by liquid accumulation at the base of the riser. This can cause formation of liquid slugs of a length equal to or longer than the height of the riser. A cyclic process results in which a period of no liquid production into the separator occurs, followed by a period of very high liquid production. This study is an experimental and theoretical investigation of two methods for eliminating this undesirable phenomenon, using choking and gas lift. Choking was found to effectively eliminate or reduce the severity of the slugging. However, the system pressure might increase to some extent. Gas lift can also eliminate severe slugging. While choking reduces the velocities in the riser, gas lift increases the velocities, approaching annular flow. It was found that a relatively large amount of gas was needed before gas injection would completely stabilize the flow through the riser. However, gas injection reduces the slug length and cycle time, causing a more continuous production and a lower system pressure. Theoretical models for the elimination of severe slugging by gas lift and choking have been developed. The models enable the prediction of the flow behavior in the riser. One model is capable of predicting the unstable flow conditions for severe slugging based on a static force balance. The second method is a simplified transient model based on the assumption of a quasi-equilibrium force balance. This model can be used to estimate the characteristics of the flow, such as slug length and cycle time. The models were tested against new severe slugging data acquired in this study. An excellent agreement between the experimental data and the theoretical models was found. 相似文献
6.
Edward S. Letzter 《Archiv der Mathematik》2017,109(6):499-503
We observe that a finitely generated algebraic algebra R (over a field) is finite dimensional if and only if the associated graded ring \({{\text {gr}}}{R}\) is right noetherian, if and only if \({{\text {gr}}}{R}\) has right Krull dimension, if and only if \({{\text {gr}}}{R}\) satisfies a polynomial identity. 相似文献
7.
Synthesis of the title compound () is presented herein as part of an approach to the synthesis of substituted dodecahedranes. 相似文献
8.
We introduce the notion of predicted decrease approximation (PDA) for constrained convex optimization, a flexible framework which includes as special cases known algorithms such as generalized conditional gradient, proximal gradient, greedy coordinate descent for separable constraints and working set methods for linear equality constraints with bounds. The new scheme allows the development of a unified convergence analysis for these methods. We further consider a partially strongly convex nonsmooth model and show that dual application of PDA-based methods yields new sublinear convergence rate estimates in terms of both primal and dual objectives. As an example of an application, we provide an explicit working set selection rule for SMO-type methods for training the support vector machine with an improved primal convergence analysis. 相似文献
9.
Two strong convergence theorems for Bregman strongly nonexpansive operators in reflexive Banach spaces 总被引:1,自引:0,他引:1
We study the convergence of two iterative algorithms for finding common fixed points of finitely many Bregman strongly nonexpansive operators in reflexive Banach spaces. Both algorithms take into account possible computational errors. We establish two strong convergence theorems and then apply them to the solution of convex feasibility, variational inequality and equilibrium problems. 相似文献
10.
Two strong convergence theorems for a proximal method for finding common zeroes of maximal monotone operators in reflexive Banach spaces are established. Both theorems take into account possible computational errors. 相似文献