冠醚构象特生分子力学分析

用分子力学程序ＭＭＰＭ对１２－冠－４、１２－氮冠－４、１２－硫冠－４、１８－冠－６、１８－氮冠－６、１８－硫冠－６的不同构象的能量进行了计算。     

读者评选的最佳产品(前三名)

ＭＰ３ 播放器帝盟Ｒｉｏ　ＰＭＰ　５００ （３６．２％）Ｇｒｕｎｄｉｇ　 ＭＰＡＸＸ ＭＰ１００（１８．５％）富士Ａｘｊａ　 ＦＭＰ－２０００（１６．９％）扫描仪爱克发ＳｎａｐＳｃａｎ １２１２Ｕ（２０．７％）惠普 ＳｃａｎＪｅｔ ５２００Ｃ（１８．１％）爱普生Ｐｅｒｆｅｃｔｉｏｎ １２００Ｕ（１４．９％）数码相机奥林巴斯ＣａｍｅｄｉａＣ３０３０变焦（１７．４％）尼康 Ｃｏｏｌｐｉｘ ９９０（１４．７％）佳能Ｅｕｒｏ－Ｆｏｔｏ　Ｐｏｗｅｒｓｈｏｔ　Ｓ２０（１２．９％）台式机戴尔Ｄｅｌｌ Ｄｉ－ｍｅｎｓｉｏｎ…     

电脑2001全年总目录

信息港１－２ ＩＴ简闻２－２ＩＴ简闻３－２ＩＴ简闻４－２ ＩＴ简闻５－６ 从ＰＣ业的革命到ＩＡ产品的发展５－７ＩＴ简闻５－１３ 大白鲨的奥秘６－８ＩＴ简闻７－８ＩＴ简闻８－８ＩＴ简闻９－８ＩＴ简闻１０－９ ＩＴ简闻１１－１０ＩＴ简闻１２—１３ ＩＴ简闻 硬比拼２－６ 三款 ＧｅＦｏｒｃｅ２ ＭＸ测试 硬件全接触１－６ 初探８１５──钻石ＤＦＩ ＣＳ－６０ＥＣ的测试１－７ Ａｃｅｒ １６速ＤＶＤ－ＲＯＭ区码大破解１－８ 减少硬盘的噪音１－９买只光驱抱回家１－１０ ２５６ＭＢ,你到底需不需要？１－１１是风扇惹的祸─…     

基于CIPH9805的机动车流量与速度检测控制电路

１　芯片特点及功能ＣＩＰＨ９８０５芯片是机动车流量与速度检测芯片,它具有清零功能、计数功能和速度检测功能。可广泛地应用于交通要道车辆流量检测、速度检测,亦可用于大型商场的客流量检测等多种领域。　图１　ＣＩＰＨ９８０５芯片引　　　脚排列功能图２　芯片引脚功能ＣＩＰＨ９８０５芯片引脚排列如图１所示,各引脚功能说明如下：ＲＳＴ：复位输入端;ＣＯＭ０～ＣＯＭ４：ＬＥＤ显示器位选控制输出端,低电平有效;ＸＴＡＬ１,ＸＴＡＬ２：晶体振荡器接入端;接６ＭＨｚ晶振;ＶＤＤ：电源正极（＋５－０Ｖ）;ＧＮＤ：电源地…     

分析与评测

扩展定位音效艺片ＣＭＩ８７３８／ＰＣＩ－６ＣＨＨＲＴＦ Ｃ３Ｄ Ｅｘｔｅｎｓｉｏｎ Ｐｏｓｉｔｉｏｎａｌ Ａｕｄｉｏｃｏｍ ＣｈｉｐＣ－Ｍｅｄａ５．１ＣＨ功能演示系统硬件： ＰＣ（８７３８ ６ＣＨ）－功放（Ａｍｐｌｉｆｉｅｒ－５．１音箱 软件：４／６ＣＨ ＤＥＭＯ Ｖ２． ５ ＨＲＴＦ ３Ｄ 技术 １． ＨＲＴＦ 的来源和标准 ＨＲＴＦ ３Ｄ 来源及标准 ＨＲＴＦ是 Ｈｅａｄ Ｒｅｌａｔｅｄ Ｔｒａｎｓｆｅｒ Ｆｕｎｃｔｉｏｎ的缩写,译为“头部相关传递函数”,最出名的就是 Ｓｅｎｓａｕｒａ的 ＨＲＴＦ ３Ｄ,而 Ｃ－ Ｍｅｄ…     

PC散件价格传真(参考价)

（２００１年３月,上海）主板华硕 ＣＵＢＸ－Ｌ／ＣＵＳＬ２／ＣＵＳＬ２－Ｃ ８８０／１２５０／１０８０元微星６１２０／６１５３／６３０９ＮＬ１００ １１００／６８０／８２０元升技 ＳＥ６／ＫＴ７／ＫＴ７Ａ １０３０／９８０／１１３０元技嘉 ６８６ＢＸＣ／６０ＸＭ７Ｅ／６０ＸＭ７Ｅ－１７８０／１１８０／１０９０元艾威ＢＤ１００／ＶＤ１３３／ＶＤ１３３ ＰＲＯ／Ｗ０２ ８８０／７００／９３０／１２８０元Ｉｎｔｅｌ ８１５ＥＰ／８１５Ｅ／８２０ １０９０／１１７０／１１３０元承启 ６ＡＴＡ２／６ＡＴＡ４／６ＢＴ…     

PC散件价格传真(参考价)

（２００１年５月,上海）主板华硕ＣＵＢＸ－Ｌ／ＣＵＳＬ２／ＣＵＳＬ２－Ｃ ８７０／１２２０／１０５０元华硕 ＣＵＶ４Ｘ／Ｐ３Ｃ－Ｅ／Ｐ３Ｖ ８８０／１３６０／７３０元微星 ６１２０／６１５３／６３０９ＮＬ１００ ８５０／６８０／８００元微星 ６９４Ｄ ＰＲＯ／８１５Ｅ ＰＲＯ／８１５ＥＰ １１８０／１０５０／９４０元升技 ＳＥ６／ＫＴ７／ＫＴ７Ａ １０２０／９５０／９９５元技嘉 ６ＶＸ７－４Ｘ－Ａ／６ＶＸ７Ｂ－４Ｘ／６ＶＸＥ７＋ ８５０／９８０／６７０元技嘉 ＢＸ－Ｄ／ＢＸ－ＤＳ／ＢＸ２０００／ＢＸ２０００＋…     

硬件新看台

本期新品 １．ＧＡ－ＳＶ２０００显示卡 ２．显示器三胞胎Ｓｏｎｙ Ｐ２６０、Ｐ２６、Ｐ７６ ３．微星最新力作 Ｋ７Ｘ Ｐｒｏ主板 ４．改头换面的ＢＸ芯片主板ＣＵＢＸ ５．先锋１６倍速ＤＶＤ－ＲＯＭ ６．ＤＴＴＰ－２５００数码音箱 ７．最酷的ＭＰ３播放器ＤＭＰ－１００ ８．惠普新型 ＵＰＳ ＨＪＵ８８５０１系列 ９．家庭网络产品ＤＨＮ－９１０ １０．笔记本电脑富贵－族ＰＣ－ＭＪ１４０Ｍ １１． ３２０转换卡 ＳｌｏｃｋｅｔⅢＧＡ－ＳＶ２０００显示卡 自从Ｓ３去年推出Ｓａｖａｇｅ２０００芯片以来,市面上支持Ｓａｖａｇ…     

多晶SiC薄膜的生长及其压阻特性

在Ａｒ、ＣＨ４的混合气体中用反应性磁控溅射法生长了多品ＳｉＣ薄膜。溅射靶使用了单晶硅片。ＳｉＣ薄膜的生长速率１０～４０ｎｍ／ｍｉｎ。Ａｒ的分压为６０ｍｔｏｒｒ（１ｔｏｒｒ＝１３３．３２２Ｐａ，以下同），ＣＨ４的分压为４０ｍｔｏｒｒ，衬底温度为７５０℃时，薄膜具有显著的多晶特性。杂质浓度、Ｈａｌｌ迁移率、电阻率分别为１０（１８）ｃｍ（－３）、１０ｃｍ（２）／Ｖ·ｓ、１０（－１）１０（－２）Ωｃｍ。压阻系数约为１５。     

三款815芯片主板

Ｉｎｔｅｌ８１５芯片（Ｓｏｌａｎｏ）已 经发布,而其性能也普遍 获得好评。依照不同的南桥 芯片以及使用者的不同需要, 升技为Ｓｏｌａｎｏ芯片设计了两 种主机板－ＳＬ６和ＳＥ６。ＳＬ６ 是采用８１５（ＩＣＨ１）芯片组的 主机板。ＳＬ６是专为ＦＣ－ＰＧＡ ＣＰＵ设计的ＡＴＸ规格主机 板,支持 Ｕｌｔｒａ ＤＭＡ６６硬盘传 输,ＳＤＲＡＭ和升技专利Ｓｏｆｔ－ ＭｅｎｕⅡ。ＳＬ６也支持 １３３ ＦＳＢ, ＡＣＰＩ（Ａｄｖａｎｃｅｄ Ｃｏｎｆｉｇｕｒａｔｉｏｎ ａｎｄ Ｐｏｗｅｒ Ｍａｎａｇｅｍｅｎｔ）和 ４Ｘ ＡＧＰ,或是你…     

Conformational Study of the Structure of dibenzo-18-crown-6. Comparison with 18-crown-6

We report for the first time the conformational analysis of dibenzo-18-crown-6, db18c6. The conformational search was carried out using the CONFLEX conformational search method of cyclic molecules. Energies were calculated for the low-lying predicted conformations at different levels of theory up to the G3MP2 level. At the G3MP2 level, the predicted ground state (GS) conformation was more stable than the experimental conformation by only 1.60 kcal/mol. Strong similarity was found between the GS structure and experimental conformations of db18c6 and 18-crown-6, 18c6. The GS and experimental conformations of db18c6 are non-planar. This allows db18c6 to exist in optically active enantiomers. Similar to 18c6, it was concluded that the db18c6 structure is stabilized by intramolecular hydrogen bond. We also performed the computations for the water and chloroform solution phase, where the same conformation was predicted as the GS conformation.     

Conformational study of the structure of 18-thiacrown-6

Conformational analysis was performed for 18-thiacrown-6 (18t6) using the CONFLEX method and the MMFF94s force field. Computations were performed for some of the low energy conformations at the HF, B3LYP, CAM-B3LYP, M06, M06L, M062x, M06HF and MP2 levels. The computations were also performed using the DFT-D3 method along with the TPSS and PBE functionals. The study predicted a new C₂ conformation as the ground state conformation of 18t6. This new C₂ conformation is more stable than the experimentally known solid state conformation by 4.7 kcal/mol at the MP2/6-311G** level. This conformation has all of the SCCS dihedral angles adopt exodentate structure. However, the experimentally known conformation of the solid phase has two of the SCCS dihedral angles violating this exodentate rule. It was concluded that for 18t6 stability a linear dihedral SCCS angle requirement is more important than a gauche CSCC dihedral angle requirement.     

三角网格模型的快速树搜索算法及可设计性分析

在蛋白质折叠格子模型的可设计性特征研究中,为了克服以往方格模型具有奇偶问题这一缺点,本文利用三角网格模型来进行穷举搜索。在简化的网格模型中,序列折叠为某一结构的能量值为在结构心部疏水氨基酸的个数取负值。在蛋白质折叠模型的二维4 5 6 5 4三角网格中穷举了所有的序列和致密结构。其中序列由两类氨基酸(疏水氨基酸和亲水氨基酸)组成,排除正反对称序列共2~(12) 2~(23)=8392704种不同序列。在由24个格点组成的三角网格模型中共得到219093种简化结构串。在穷尽搜索算法中,为实现快速搜索,通过树结构将相似的结构串尽量聚类,通过计算各树结点的目标能量值以减少搜索算法中所需的计算量。经并行实验验证,利用该树结构可使快速搜索算法达到指数级加速比。最后对计算所得结果进行了统计分析。     

The hydration structure of 18-crown-6/K+ complex as studied by Monte Carlo simulation using ab initio fitted potential

The intermolecular potential between a 18-crown-6/K+ complex and a water molecule is derived from 1200 energy points obtained from quantum chemical calculations using the 6-31G** basis set. The ab initio fitted potential was then applied to study the structural properties of the complex in an aqueous solution using the Monte Carlo simulation method. The radial distribution function (RDF) centered at K+ to the oxygen atom of water shows a sharp first peak at 2.88 A. The corresponding coordination number, integrated up to the first minimum at 3.76 A, is 2 water molecules. The results indicate clearly that the 18-crown-6/K+ complex was solvated by the two nearest neighbors, one above and other below the ligand's plane. Evaluation was focused on the precise position and orientation of the two water molecules. It was found that the oxygen atoms of the two nearest neighbors bind to the K+ while their hydrogen atoms rotate freely around the vector perpendicular to the ligand's molecular plane.     

Ab initio calculations of NO2 and SO2 chemisorption onto non-polar ZnO surfaces

The authors present an ab initio study of NO₂ and SO₂ chemisorption onto non-polar ZnO and ZnO surfaces with the aim of providing theoretical hints for further developments in gas sensors. From first principles calculations (DFT-GGA approximation), the most relevant surface reduction scenarios are analyzed and, subsequently, considered in the chemisorption study. First, calculations indicate that NO₂ adsorbs avidly onto Zn surface atoms. This is compatible with the oxidizing character of NO₂. Second, results also explain the sensor poisoning by SO₂ adsorption (since this molecule competes with NO₂ for the same adsorption sites) and indicate that poisoning can only be reverted at typical operation temperatures (T ≤ 700 °C) in the case of stoichiometric ZnO surfaces.     

Past is for free: on the complexity of verifying linear temporal properties with past

We study the complexity of satisfiability and model checking problems for fragments of linear-time temporal logic with past (PLTL). We consider many fragments of PLTL, obtained by restricting the set of allowed temporal modalities, the use of negations or the nesting of future formulas into past formulas. Our results strengthen the widely accepted fact that past is for free, in the sense that allowing symmetric past-time modalities does not bring additional theoretical complexity. This result holds even for small fragments and even when nesting future formulas into past formulas.Received: 4 September 2002, Published online: 17 March 2004     

Least squares estimation of nonlinear spatial trends

Results on asymptotic and finite sample properties of an estimator of a nonlinear regression function when errors are spatially correlated, and when the spatial dependence structure is unknown are derived. The proposed method is based on a generalized nonlinear least squares approach, taking into account the spatial covariance. Weak consistency of the regression parameters estimator is derived, along with its asymptotic Gaussian limit. The behavior of the proposed estimator is illustrated with a simulation study, considering different correlation structures in and a more general case including a spatial covariate. The method is also applied to two real data cases.     

Theoretical study of stereoselective reduction controlled by NADH analogs

The potential energy surfaces (PES) of 2-methyl-4-(R)-methyl-1,4-dihydropyridine-3-carboxamide (4R-DM, 1), 2-methyl-4-(S)-methyl-1,4-dihydropyridine-3-carboxamide (4S-DM, 2) and 2-methyl-1,4-dihydropyridine-3-carboxamide (MM, 3) have been explored with ab initio calculations at the RHF/6-311G(**) and MP2/6-311G(**) levels of theory. In agreement with previous experimental and computational results, the PES provides three minima for each of the above molecules. The calculations reported herein indicate that the cisoid conformation is most favorable in gas phase and hydrophobic environments. Nevertheless, the preference of the cis conformation can be controlled by different solvents. The most favorable conformation in methanol, water, and probably in the polar (or water medicated) enzyme active sites, however, would be the one in which the carbonyl group is in a transoid position and is syn to Hsyn. In addition, our calculations suggest that the carbonyl group in the syn, rather than anti, position relative to Hsyn is preferred. These observations are in very good agreement with previous computational and experimental results. Our computational studies have provided an explanation as to why the transoid conformation is preferred in enzyme active sites as well as in many other NADH mimics. Furthermore, these new data imply that the stereoselectivity of NADH analogs can be controlled by means of changing solvents in which the reaction is carried out.     

Incremental View Maintenance By Base Relation Tagging in Distributed Databases

The incremental view maintenance problem deals with the efficient updating of materialized views in response to updates to base relations. This paper considers the problem in a distributed database environment, with communication cost minimization as the primary objective. The views considered are defined based on the relational join operation. The approach is to use yes/no tags as auxiliary data on tuples in the base relations to indicate whether the tuples participate in joins. These tags will help avoid sending irrelevant data over the network and thus reduce the communication cost. Two basic view maintenance algorithms are proposed using the tags. In addition to reducing communication costs, an important feature of these two basic algorithms is that they derive the exact change to views without looking at the old views. This feature allows us to maintain certain aggregates on views without actually materializing the views themselves; this feature is useful in applications such as active databases where many conditions or constraints must be tested whenever updates occur, since a condition is true exactly when some corresponding view has nonzero number of tuples. The paper then combines the use of tags with the counting algorithm to derive a tagged counting algorithm that further reduces the communication cost. The paper illustrates the algorithms by examples and studies their performance via a statistical analysis. The illustrating examples and the performance analysis show that, under uniform distribution with reasonable join participation rates, the use of tags significantly improves the efficiency of view maintenance over similar algorithms without tags. The performance analysis also identifies the situations where a particular algorithm is superior to others. The use of tags for memoing values of subexpressions in a view definition is also explored in the paper.