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991.
镇元素簇合物是人们最感兴趣的簇合物之一l‘-’].已有的。作主要是研究它的稳定性和电子结构,对其成键性质还研究得不多·为止匕我们对Lkin为元素簇合物中所含原子个数,e为该化合物的电荷)进行了定域化研究,并得出一些有意义的结果·1计算我4rl采用GAMESS90程序m,在4-31G基组下进行了正则分子轨道SCFi十算(闭壳层采用RHF,开壳层采用UHF).然后,以QSU90程序问,用B0yS方法进行了定域化计算,得到定域分子轨道(LM),再根据LMO的系数和集居数分析,判断各分子中原子的成键情况问.共计算了18个簇合物.它们的原子数… 相似文献
992.
993.
以甲基丙烯酸缩水甘油酯为单体,二甲基丙烯酸乙二醇酯为交联剂,甲苯与正庚烷为有机致孔剂,20%的甘油水溶液为内水相(超孔致孔剂),利用二次乳化法制备了(W/O)/W乳液,通过紫外光引发悬浮聚合生成两类孔型高分子微球(BiPB).BiPB孔径为双峰分布,范围分别在10~100nm和1000~7300nm之间;而其体积平均粒径、比表面积、湿密度、静态吸附容量与不含超孔的微孔介质(MiPB)接近.修饰相同二乙胺密度的BiPB和MiPB介质虽在较低流速(5cm/min)下有相近的动态吸附容量,但在高流速(40cm/min)下BiPB的动态吸附容量约为MiPB的动态吸附容量的3倍,表明BiPB介质的超孔结构对孔内传质的强化作用,因而其更适合于高速的蛋白质色谱分离. 相似文献
994.
995.
Two new triterpene lactones, polysperlactones A ( 2 ) and B ( 3 ), were isolated from the stems of Kadsura polysperma, together with the known compounds heteroclitalactone D ( 1 ) and schisanlactone E ( 4 ). Their structures were elucidated by spectroscopic methods, including 2D‐NMR and HR‐MS techniques. The configuration of 1 was confirmed by X‐ray analysis. Compounds 2 and 3 are members of a rare class of 3,4‐secolanostane metabolites with ring‐expanded or cyclized structures, respectively. 相似文献
996.
Understanding effect of wall structure on the hydrothermal stability of mesostructured silica SBA-15
Zhang F Yan Y Yang H Meng Y Yu C Tu B Zhao D 《The journal of physical chemistry. B》2005,109(18):8723-8732
Mesostructured silica SBA-15 materials with different structural parameters, such as pore size, pore volume, and wall thickness, etc., were prepared by varying the postsynthesis hydrothermal treatment temperature and adding inorganic salts. The hydrothermal stabilities of these materials in steam (100% water vapor) were systematically investigated using a variety of techniques including powder X-ray diffraction, transmission electron microscopy, nitrogen sorption, and (29)Si solid-state NMR. The effect of the pore size, microporosity or mesoporosity, and wall thickness on the stability was discussed. The results show that all of the SBA-15 materials have a good hydrothermal stability under steam of 600 degrees C for at least 24 h. N(2) sorption measurements show that the Brumauer-Emmett-Teller surface area of SBA-15 materials is decreased by about 62% after treatment under steam at 600 degrees C for 24 h. The materials with thicker walls and more micropores show relatively better hydrothermal stability in steam of 600 degrees C. Interestingly, we found that the microporosity of the mesostructured silica SBA-15 is a very important factor for the hydrothermal stability. To the materials with more micropores, the recombination of Si-O-Si bonds during the high-temperature steam treatment may not cause direct destruction to the wall structure. As a result, SBA-15 materials with more micropores show better stability in pure steam of 600 degrees C. Nevertheless, these materials are easily destroyed in steam of 800 degrees C for 6 h. Two methods to effectively improve the hydrothermal stability are introduced here: one is a high-temperature treatment, and another is a carbon-propping thermal treatment. Thermal treatment at 900 degrees C can enhance the polymerization degree of Si-O-Si bonds and effectively improve the hydrothermal stability of these SBA-15 materials in 800 degrees C steam for 12 h. But, this approach will cause very serious shrinkage of the mesopores, resulting in smaller pore diameter and low surface area. A carbon-propping thermal treating method was employed to enhance the polymerization of Si-O-Si bonds and minimize the serious shrinkage of mesopores at the same time. It was demonstrated to be an effective method that can greatly improve the hydrothermal stability of SBA-15 materials in 800 degrees C steam for 12 h. Furthermore, the SBA-15 materials obtained by using the carbon-propping method possess larger pores and higher surface area after the steam treatment at 800 degrees C compared to the materials from the direct thermal treatment method after the steam treatment. 相似文献
997.
Zhao L Zhu G Zhang D Di Y Chen Y Terasaki O Qiu S 《The journal of physical chemistry. B》2005,109(2):764-768
Using 1,2-bis(trimethoxysilyl)ethane as organosiloxane precursor and a triblock copolymer surfactant, Pluronic F127, as template, a highly ordered mesoporous organosilica with large cagelike pores has been successfully synthesized. Its structure was resolved to be 3-D cubic Fmm by high-resolution transmission electron microscopy. The intergrowth of hexagonal close-packed and cubic close-packed phases was observed in this material. The effect of different siliceous precursors on the mesostructure was also investigated. 相似文献
998.
The first hyperpolarizabilities and origin of nonlinear optical (NLO) properties of arylimido molybdate derivatives have been investigated by density functional theory (DFT). The molecular orbital character analysis reveals that organoimido-to-polyanion charge transfer may be responsible for the NLO properties of this kind of molybdate derivatives. The NLO study shows intra-ion charge transfer is helpful to increase the first hyperpolarizability of arylimido molybdate derivatives. The lengthening of organoimido pi-conjugation enhances the betavec value. System 4 has the largest betavec value at the static electronic field, 1.238 x 10(-27)esu. Orbital analysis shows that the degree of charge transfer between polyanion cluster and organic segment was increased when the second organoimido polyanion was introduced. The present investigation provides important insight into NLO origin and properties of polyanion arylimido molybdate derivatives. 相似文献
999.
HUN-7293 (1), a naturally occurring cyclic heptadepsipeptide, is a potent inhibitor of cell adhesion molecule expression (VCAM-1, ICAM-1, E-selectin), the overexpression of which is characteristic of chronic inflammatory diseases. Representative of a general approach to defining structure-function relationships of such cyclic (depsi)peptides, the parallel synthesis and evaluation of a complete library of key HUN-7293 analogues are detailed enlisting solution-phase techniques and simple acid-base liquid-liquid extractions for isolation and purification of intermediates and final products. Significant to the design of the studies and unique to solution-phase techniques, the library was assembled superimposing a divergent synthetic strategy onto a convergent total synthesis. An alanine scan and N-methyl deletion of each residue of the cyclic heptadepsipeptide identified key sites responsible for or contributing to the biological properties. The simultaneous preparation of a complete set of individual residue analogues further simplifying the structure allowed an assessment of each structural feature of 1, providing a detailed account of the structure-function relationships in a single study. Within this pharmacophore library prepared by systematic chemical mutagenesis of the natural product structure, simplified analogues possessing comparable potency and, in some instances, improved selectivity were identified. One potent member of this library proved to be an additional natural product in its own right, which we have come to refer to as HUN-7293B (8), being isolated from the microbial strain F/94-499709. 相似文献
1000.