全文获取类型
收费全文 | 1615篇 |
免费 | 71篇 |
国内免费 | 2篇 |
专业分类
化学 | 1108篇 |
晶体学 | 6篇 |
力学 | 19篇 |
数学 | 215篇 |
物理学 | 340篇 |
出版年
2024年 | 3篇 |
2023年 | 13篇 |
2022年 | 55篇 |
2021年 | 52篇 |
2020年 | 44篇 |
2019年 | 48篇 |
2018年 | 29篇 |
2017年 | 28篇 |
2016年 | 80篇 |
2015年 | 53篇 |
2014年 | 63篇 |
2013年 | 66篇 |
2012年 | 127篇 |
2011年 | 146篇 |
2010年 | 111篇 |
2009年 | 67篇 |
2008年 | 101篇 |
2007年 | 135篇 |
2006年 | 76篇 |
2005年 | 67篇 |
2004年 | 58篇 |
2003年 | 40篇 |
2002年 | 29篇 |
2001年 | 12篇 |
2000年 | 12篇 |
1999年 | 16篇 |
1998年 | 10篇 |
1997年 | 6篇 |
1996年 | 13篇 |
1995年 | 18篇 |
1994年 | 16篇 |
1993年 | 10篇 |
1992年 | 9篇 |
1991年 | 6篇 |
1990年 | 4篇 |
1989年 | 6篇 |
1987年 | 3篇 |
1985年 | 3篇 |
1983年 | 4篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1969年 | 2篇 |
1968年 | 2篇 |
1962年 | 2篇 |
1905年 | 3篇 |
1904年 | 2篇 |
排序方式: 共有1688条查询结果,搜索用时 15 毫秒
41.
Ultraviolet (UV) spectrophotometric and nonaqueous volumetric methods are described for the determination of carvedilol in pharmaceutical formulations. Linearity, precision, and accuracy were evaluated according to the validation guidelines of the International Conference on Harmonization and the United States Pharmacopeia for both methods. The UV spectrophotometric procedure was performed in ethanol at 244 nm. Good linearity was obtained between 2 and 7 microg/mL with a correlation coefficient of 0.9999. The intra- and interday precision values were <2% for all samples analyzed. The accuracy, determined from recovery studies, was between 97.5 and 102.2%. The other procedure was based on the volumetric quantitation of carvedilol in a nonaqueous medium with 0.01 M perchloric acid and 1% violet crystal as the indicator. The validation of the volumetric method yielded good results that included linearity (r of > 0.999), precision (relative standard deviations of <2% for intra- and interday precision), and accuracy (96.4-102.4%). The methods were applied to tablets and compounded capsules. Statistical analysis by analysis of variance showed no significant difference between the results obtained by the proposed methods. 相似文献
42.
43.
Michela Iezzi Simone 《Tetrahedron letters》2005,46(34):5761-5765
Efficient SN2 ring closure of open chain trihydroxytriflates—in which the leaving group is on a primary carbon adjacent to a quaternary centre—provides access to tetrahydrofurans with branched carbon chains from branched carbohydrate lactones; the first examples of a new class of branched chain tetrahydrofuran α,α-disubstituted-δ-sugar amino acid scaffolds are described. 相似文献
44.
Frank Seela Simone Budow Henning Eickmeier Hans Reuter 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(1):o54-o57
The title compound, 4‐amino‐1‐(2‐deoxy‐β‐d ‐erythropentofuranosyl)‐5‐(prop‐1‐ynyl)pyrimidin‐2(1H)‐one, C12H15N3O4, shows two conformations in the crystalline state which differ mainly in the glycosylic bond torsion angle and the sugar pucker. Both molecules exhibit an anti glycosylic bond conformation, with torsion angles χ = −135.0 (2) and −156.4 (2)° for molecules 1 and 2, respectively. The sugar moieties show a twisted C2′‐endo sugar pucker (S‐type), with P = 173.3 and 192.5° for molecules 1 and 2, respectively. The crystal structure is characterized by a three‐dimensional network that is stabilized by several intermolecular hydrogen bonds between the two conformers. 相似文献
45.
This paper presents the application of the eMergy evaluation, introduced by H.T. Odum as an efficient methodology of environmental accounting. The results of this analysis are a part of a wider sustainability assessment for the Province of Venice realized in 2003. Within a territorial sustainability analysis, it is necessary to implement an environmental accounting method, which is able: to precisely consider the boundaries of the investigated system (spatial scale); to arbitrarily separate the life of the investigated system into periods (temporal scale); to account for both economic and natural support to a given territorial system in order to implement a sustainable management of resources; to assess all the items supporting the system on the basis of a physical unit that is objective (while the traditional economic evaluation method is based on individual preferences, hence it is subjective). The Province of Venice is a very complex system due, on one hand, to the heavy and consolidated presence of industrial activity since the beginning of the 20th century and, on the other hand, to the principal natural ecosystem which characterizes it: the lagoon, the largest wetland in the Mediterranean Sea. The role of the lagoon, as a provider of resources and services is represented by the analysis of two aspects: the lagoon as an ecosystem on the basis of the evaluation of all flows that cyclically support its biotic activity and the lagoon as a productive system on the basis of the contribution of both natural and "economic" inputs that support human activity. In particular, the case of clam fishing will be presented to put on evidence the importance of natural inputs that are systematically neglected by the traditional accounting methodologies. 相似文献
46.
H. -G. Rubahn E. Konz S. Schiemann K. Bergmann 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1991,22(1):401-406
The recently developed STIRAP-technique (J. Chem. Phys. 92, 5363 (1990)) is applied to efficient population transfer and alignment of metastable Ne* atoms. Alignment parameters in the range ?0.3≧A 0 2 ≧?1.0 are observed. The degree of alignment can be controlled by changing the angle between the field vectors of the linearly polarized pump- and Stokes-lasers. The transfer efficiency depends also on this angle. The results of numerical calculations agree satisfactorily with the experimental data. 相似文献
47.
Mian Chin Gregory L. Durst Simone R. Head Paul L. BockJohn A. Mosbo 《Journal of organometallic chemistry》1994,470(1-2):73-85
Molecular mechanics (MM2) calculations were performed on 54 conformations of 18 phosphines (PH3; PH3−nRn, where n = 1,…3, and R = Me and Et, n = 1 or 2 and R =iPr, and n = 1 and R =tBu, PMe2Et, PMeEt2, and PPhMe2, and PPh2R where R = Me, Et, iPr, tBu and Ph). The results are compared to those previously obtained from MINDO/3 and MNDO calculations, and to experimental data. Single conformer cone angles and weighted average cone angles were calculated from MM2 optimized geometries employing Tolman's general definition, and they are compared to Tolman's values, MINDO/3 results, and T.L. Brown's ER values. Of the cone angle definitions used, the weighted average values are suggested as the best single representation of phosphine ligand sizes. The steric parameters (cone angle and ER values) alone, and in conjunction with electronic parameters, are correlated with experimental data. 相似文献
48.
On the 31st of May and the 1st of June, 2007, a workshop to celebrate 10 years of atomic and molecular science was held at Elettra. The occasion marked the 10th anniversary since the first beamtime was officially allocated to a user at the Gas Phase beamline, namely George King of Manchester University. As a satisfied user, King was not only present, but had beamtime during the workshop. 相似文献
49.
Synthesis and Rearrangement of P‐Nitroxyl‐Substituted PIII and PV Phosphanes: A Combined Experimental and Theoretical Case Study
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Tobias Heurich Dr. Zheng‐Wang Qu Dr. Senada Nožinović Dr. Gregor Schnakenburg Dr. Hideto Matsuoka Prof. Dr. Stefan Grimme Prof. Dr. Olav Schiemann Prof. Dr. Rainer Streubel 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(29):10102-10110
Low‐temperature generation of P‐nitroxyl phosphane 2 (Ph2POTEMP), which was obtained by the reaction of Ph2PH ( 1 ) with two equivalents of TEMPO, is presented. Upon warming, phosphane 2 decomposed to give P‐nitroxyl phosphane P‐oxide 3 (Ph2P(O)OTEMP) as one of the final products. This facile synthetic protocol also enabled access to P‐sulfide and P‐borane derivatives 7 and 13 , respectively, by using Ph2P(S)H ( 6 ) or Ph2P(BH3)H ( 11 ) and TEMPO. Phosphane sulfide 7 revealed a rearrangement to phosphane oxide 8 (Ph2P(O)STEMP) in CDCl3 at ambient temperature, whereas in THF, thermal decomposition of sulfide 7 yielded salt 10 ([TEMP‐H2][Ph2P(S)O]). As well as EPR and detailed NMR kinetic studies, indepth theoretical studies provided an insight into the reaction pathways and spin‐density distributions of the reactive intermediates. 相似文献
50.