Analogy between real irreducible tensor operator and molecular two-particle operator

. Molecular matrix elements of a physical operator are expanded in terms of polycentric matrix elements in the atomic basis by multiplying each by a geometrical factor. The number of terms in the expansion can be minimized by using molecular symmetry. We have shown that irreducible tensor operators can be used to imitate the actual physical operators. The matrix elements of irreducible tensor operators are easily computed by choosing rational irreducible tensor operators and irreducible bases. A set of geometrical factors generated from the expansion of the matrix elements of irreducible tensor operator can be transferred to the expansion of the matrix elements of the physical operator to compute the molecular matrix elements of the physical operator. Two scalar product operators are employed to simulate molecular two-particle operators. Thus two equivalent approaches to generating the geometrical factors are provided, where real irreducible tensor sets with real bases are used. Received: 3 September 1996 / Accepted: 19 December 1996     

A new method for the derivation of exact vibration-rotational kinetic energy operator in internal coordinates

An efficient angular momentum method is presented and used to derive analytic expressions for the vibration-rotational kinetic energy operator of polyatomic molecules.The vibration-rotational kinetic energy operator is expressed in terms of the total angular momentum operator J,the angular momentum operator J and the momentum operator p conjugate to Z in the molecule-fixed frame Not only the method of derivation is simpler than that in the previous work,but also the expressions ot the kinetic energy operators arc more compact.Particularly,the operator is easily applied to different vibrational or rovibrational problems of the polyatomic molecules by variations of matrix elements Gn of a mass-dependent constant symmetric matrix     

基于毛细管电泳的天然产物中酶抑制剂筛选研究进展

人类疾病的发生往往与体内各种酶的功能失调密切相关,因此酶一直是目前新药研发的重要靶标。天然产物是发现新药的宝贵资源,但是由于成分复杂,活性筛选一直受制于耗时费力的分离纯化过程。毛细管电泳(CE)技术由于具有样品和试剂消耗少、灵活多样的分离模式且不受样品基质干扰的特点,可以直接从粗提物开始筛选活性成分,在复杂样品活性筛选中显示出独特的优势。该文综述了近十年来CE在天然产物中酶抑制剂筛选的研究进展。其中重点介绍了CE应用于重要药物靶标,包括转移酶(激酶)、水解酶以及氧化还原酶等方面的应用,总结了用于酶抑制剂筛选的电泳分离模式和酶动力学研究,并展望了CE用于天然产物中活性成分筛选的应用前景。     

火焰原子吸收光谱法测定天然气转化催化剂中的氧化钾

建立火焰原子吸收光谱法测定天然气转化催化剂中氧化钾的分析方法.该方法将催化剂样品和助溶剂四硼酸锂熔融后再用盐酸溶解定容,采用火焰原子吸收光谱仪对样品溶液进行测定.在优化的实验条件下,钾离子的质量浓度在0.05～0.50 mg/L范围内与吸光度呈良好线性关系,相关系数为0.9995.钾的方法检出限为0.001mg/L,定...     

3-(噁唑-5-基)吲哚类天然产物生物活性及其衍生物的合成研究进展

3-(噁唑-5-基)吲哚类天然产物如Pimprinine,Streptochlorin等,广泛存在于海洋微生物中,因其具有多样的生物活性,在医药和农药领域中很有研究潜力.3-(噁唑-5-基)吲哚类天然产物的合成方法有很多研究报道,在吲哚结构上构建噁唑环是合成此类天然产物的关键.总结了已报道的3-(噁唑-5-基)吲哚类天然产物的生物活性,并对构建3-(噁唑-5-基)吲哚骨架的合成方法及部分主要反应机理进行了综述,探讨了3-(噁唑-5-基)吲哚类骨架作为一种优势活性结构在未来的应用前景.     

制备型高效液相色谱在天然产物分离中的应用

制备型高效液相色谱法在天然产物的分离纯化中得到广泛应用。从制备型高效液相色谱法的技术分类及其在天然产物研究中的实际应用方面对目前国内外制备型高效液相色谱技术研究现状进行了综述,对其发展动态进行了展望。     

硫化天然橡胶的本征自修复与固相回收加工

通过小分子模拟试验和应力松弛,发现甲基丙烯酸铜(MA-Cu)比氯化铜(CuCl_2)更适合用作硫化天然橡胶中二硫键交换反应催化剂,并且硫化天然橡胶中的二硫键交换反应需要在高于120°C的温度下才能高效进行,这保障了相关制品在较低温工作时的结构与性能稳定.进一步的实验结果表明,硫化天然橡胶在MA-Cu辅助作用下获得了多次自修复与固相回收加工性能,损伤自修复后的硫化天然橡胶与固相回收加工的再生硫化天然橡胶,其拉伸强度均可恢复到原始材料强度的80%左右.     

Application of Cyclopentadiene and its Derivatives in Natural Product Synthesis

Approaches toward the synthesis of iridoids and 9-isocyanopupukeanane starting from cy-clopentadiene are discussed.     

β-Silylacrylate as a Multipurpose Reagent in Organic Synthesis

Acrylates are well known electrophilic alkenes having multitude of applications in organic synthesis. They are very good acceptors in Michael addition reactions and are good enophile/dienophile/dipolarophile partners in cycloaddition reactions. Replacing the β-alkyl/aryl groups in acrylates by a silicon group would be interesting. In addition to the conventional reactions displayed by acrylates, β-silylacrylates (β-SAs) can show reactivity specifically related to the silicon group. Many conventional organic reactions such as hydrodimerization, organocatalytic asymmetric Michael additions, inter- and intra-molecular Diels–Alder reactions, and asymmetric 1,3-dipolar cycloadditions have been used to generate the complex chemical entities from β-SAs. Some of the reaction outcomes were vastly influenced by the silicon substituent. This review describes the practical synthesis β-SAs and their use as starting point in complex molecule generation including total synthesis of some natural products/bioactive molecules.     

Effects of the type of crosslink on viscoelastic properties of natural rubber

The viscoelastic properties of various crosslinked natural rubbers, NR, were investigated by mechanical spectroscopy. The glass transition temperature, T_g, was found to be dependent on both the crosslink density and the crosslink type. Higher values of T_g were obtained for sulfur-crosslinked NR than for peroxide-crosslinked NR at the same crosslink density. The greater influence of the sulfur content on T_g may be attributed to polysulfidic crosslinks and cyclic sulfide structures favored at high sulfur contents. Sulfur-vulcanized NRs with monosulfidic crosslinks, favored at relatively high accelerator/sulfur ratios, have properties more similar to the peroxide-cured NR with simple carbon(SINGLE BOND)carbon crosslinks covalent bonds, resulting in only small shifts in T_g. A qualitative analysis of monosulfidic crosslinks and polysulfidic structures was performed with ¹³C solid-state NMR spectroscopy. The storage modulus, E′, in the rubbery plateau region increased with increasing crosslink density. However, the crosslink type did not influence the moduli values as much as it influenced the T_g values. Different methods of detecting the crosslink density were also discussed. © 1996 John Wiley & Sons, Inc.