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71.
Complexes in which a sigma-H--E bond (E=H, B, Si, C) acts as a two-electron donor to the metal center are called sigma complexes. Clues that it is possible to interconvert sigma ligands without a change in oxidation state derive from C--H activation reactions effecting isotope exchange and from dynamic rearrangements of sigma complexes (see Frontispiece). Through these pathways, metathesis of M--E bonds can occur at late transition metals. We call this process sigma-complex-assisted metathesis, or sigma-CAM, which is distinct from the familiar sigma-bond metathesis (typical for d(0) metals and requiring no intermediate) and from oxidative-reductive elimination mechanisms (inherently requiring intermediates with changed oxidation states and sometimes involving sigma complexes). There are examples of sigma-CAM mechanisms in catalysis, especially for alkane borylation and isotope exchange of alkanes. It may also occur in silylation and alkene hydrogenation. 相似文献
72.
73.
Schäfer A Reissmann M Schäfer A Saak W Haase D Müller T 《Angewandte Chemie (International ed. in English)》2011,50(52):12636-12638
Well-shuffled: An unexpected substituent distribution reaction via alkyldiarylsilylium ions leads to a distribution of substituents. Starting from alkyldiaryl silanes, this reaction provides a facile synthetic approach to sterically highly hindered triarylsilylium ions. These silylium ions can be applied in dihydrogen activation reactions. 相似文献
74.
为了探寻OsO+与H2气相反应的机理,用密度泛函理论方法 UB3LYP,全优化了该反应的加成(氧化加成和[2+2]环加成)-消除、氢抽提-反弹,以及氧端插入等四种可能路径中所有可能的反应物、中间体、过渡态和产物在六重态、四重态和二重态等三个自旋态下的几何结构,计算了各种机理反应的势能面.结果表明,标题反应为自旋禁阻反应,反应起始自四重态,最终产物为六重态基态,整个反应放热21.0 kJ mol-1.因反应络合物相对于入口通道有太正Gibbs函数,氧端插入机理是高能的过程.其他三种机理都具有多(或二)态反应性(MSR或TSR).其中,两种加成-消除机理的最低能量路径都可能经由四重态-二重态-四重态-六重态的三次自旋翻转,抽提-反弹机理的最低能量路径可能经历由四重态-六重态的自旋翻转.抽提-反弹机理由势能面一路攀升的吸热氢抽提过程和几乎无能垒的强放热的反弹过程组成,所以按该机理反应在常温常压下难以发生.两种加成-消去机理的决速步(第二个H的迁移步)相同,虽然其位垒稍高,为156.9 kJ mol-1,但与其进程中前面的强放热步骤耦合,常温常压下该反应是可以发生的.其中,协同环加成步的位垒仅28.7 kJ mol-1,比第一个H的还原消去步的位垒低113.7 kJ mol-1,所以竞争的结果是,常温常压下[2+2]环加成-消去机理比氧化加成-消去机理在动力学上更有利. 相似文献
75.
Jianjun Liu Dr. You Han Qingfeng Ge Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(7):1685-1695
Storage facility : A dihydrogen complex formed in transition‐metal‐doped NaAlH4 was found to play important roles in hydrogen release/uptake (see figure). Electronic structure analysis revealed that the electron transfer between hydrogen and Al groups was mediated by the d orbitals of transition metals. Hydrogen release/uptake from the transition‐metal‐doped NaAlH4 was accompanied by an exchange of Al? H and H? H bond ligands through σ‐bond metathesis.
76.
CCSD(T)/aug-cc-pVTZ//ωB97XD/aug-cc-pVTZ calculations were performed for halogen-bonded complexes. Here, the molecular hydrogen, cyclopropane, cyclobutane and cyclopentane act as Lewis base units that interact through the electrons of the H–H or C–C σ-bond. The FCCH, ClCCH, BrCCH and ICCH species, as well as the F2, Cl2, Br2 and I2 molecular halogens, act as Lewis acid units in these complexes, interacting through the σ-hole localised at the halogen centre. The Quantum Theory of Atoms in Molecules (QTAIM), the Natural Bond Orbital (NBO) and the Energy Decomposition Analysis (EDA) approaches were applied to analyse these aforementioned complexes. These complexes may be classified as linked by A–X···σ halogen bonds, where A = C, X (halogen). However, distinct properties of these halogen bonds are observed that depend partly on the kind of electron donor: dihydrogen, cyclopropane, or another cycloalkane. Examples of similar interactions that occur in crystals are presented; Cambridge Structural Database (CSD) searches were carried out to find species linked by the A–X···σ halogen bonds. 相似文献
77.
用分子动力学模拟方法研究了不同温度下磷酸二氢钾(KDP)水溶液的内能和微结构. 水分子被看作简单点电荷模型,磷酸二氢根被看作七节点模型. 另外系统地研究了溶液的内能和径向分布函数. 在饱和温度附近,局域粒子数密度有很大的波动;饱和溶液的比热要比不饱和溶液的比热高表明溶液在低于饱和温度的过程中经历了结晶过程;水中的氧原子跟磷酸二氢根中的氢原子的径向分布函数表明二者之间形成很强的氢键结构;在KDP水溶液中,钾阳离子跟磷酸二氢根中的氧原子有很强的相互作用,并且在饱和溶液中形成的离子对数量更多. 相似文献
78.
Antonio Peluso Nicola Gargiulo Paolo Aprea Francesco Pepe 《Separation & Purification Reviews》2019,48(1):78-89
Biogas is one of the most promising renewable sources of energy. However, it is also a gas mixture containing acidic gases, such as H2S, useless for energetic purposes, environmentally harmful and damaging for energy conversion devices. This review focuses on nanoporous materials as adsorbents of H2S for biogas purification processes. Cation-exchanged zeolites and impregnated activated carbons have been thoroughly studied since many years for this application, providing good results, in particular for what concerns activated carbons, despite having a limited regenerability. Amino-functionalized ordered mesoporous silicas produced very interesting results, both in terms of adsorption performances and regeneration capacity, but they are largely untested in large-scale “real-life” applications, and deserve further investigations, in particular for H2S and CO2 discrimination. On the contrary, despite reporting very good results, there are only few papers dealing with H2S adsorption on nanoporous metal organic frameworks. 相似文献
79.
80.
A. Jayarama M. R. Suresh Kumar S. M. Dharmaprakash R. Chitra R. R. Choudhury 《Pramana》2008,71(5):905-910
Thiourea-doped ammonium dihydrogen phosphate (TADP) exhibits nonlinear optical property and the second harmonic generation efficiency of these crystals is three times that of pure ammonium dihydrogen phosphate (ADP) crystal. In this context, the study of structural distortion in the thiourea-doped ADP crystal is significant, hence single neutron diffraction investigations were undertaken. The final R-factors are: R[F 2 > 2σ(F 2)] = 0.11, Goodness of fit(S) = 1.15. Though the dopant could not be located from the difference Fourier map, the cell parameters (a = b = 7.531(3) A, c = 7.544(5) Å) were found to be significantly greater than that of pure ADP at RT (a = b = 7.502(1) Å, c = 7.546(1) Å). This indicates that the dopant concentration in the crystals is small but enough to bring changes in the overall average structure. 相似文献