全文获取类型
收费全文 | 5110篇 |
免费 | 704篇 |
国内免费 | 333篇 |
专业分类
化学 | 4050篇 |
晶体学 | 15篇 |
力学 | 482篇 |
综合类 | 54篇 |
数学 | 929篇 |
物理学 | 617篇 |
出版年
2024年 | 7篇 |
2023年 | 46篇 |
2022年 | 209篇 |
2021年 | 289篇 |
2020年 | 230篇 |
2019年 | 155篇 |
2018年 | 182篇 |
2017年 | 154篇 |
2016年 | 270篇 |
2015年 | 256篇 |
2014年 | 288篇 |
2013年 | 504篇 |
2012年 | 321篇 |
2011年 | 289篇 |
2010年 | 227篇 |
2009年 | 291篇 |
2008年 | 281篇 |
2007年 | 285篇 |
2006年 | 271篇 |
2005年 | 256篇 |
2004年 | 226篇 |
2003年 | 222篇 |
2002年 | 152篇 |
2001年 | 98篇 |
2000年 | 94篇 |
1999年 | 67篇 |
1998年 | 76篇 |
1997年 | 71篇 |
1996年 | 71篇 |
1995年 | 34篇 |
1994年 | 29篇 |
1993年 | 30篇 |
1992年 | 35篇 |
1991年 | 18篇 |
1990年 | 19篇 |
1989年 | 14篇 |
1988年 | 19篇 |
1987年 | 11篇 |
1986年 | 10篇 |
1985年 | 7篇 |
1984年 | 5篇 |
1982年 | 11篇 |
1981年 | 2篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1978年 | 1篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有6147条查询结果,搜索用时 15 毫秒
91.
M. M. Kabachnic E. V. Zobnina V. Yu. Pavlov I. O. Konstantinov G. V. Ponomarev I. P. Beletskaya 《Russian Chemical Bulletin》2005,54(1):262-265
A preparative procedure is proposed for the synthesis of α-aminophosphonates of natural porphyrins by the microwave-assisted Kabachnik—Fields reaction.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 256–259, January, 2005. 相似文献
92.
Boris S. Krumgalz Rita Pogorelsky Kenneth S. Pitzer 《Journal of solution chemistry》1995,24(10):1025-1038
The ion interaction approach developed by Pitzer was used for the prediction of volumetric properties of mixed electrolyte solutions at 25°C based on parameters calculated from experimental data for single-solute electrolyte solutions. Such an approach was shown to be especially effective for application to the calculation of volumetric properties of natural hypersaline brines and of industrial electrolyte solutions of large complexity. The use of the latest recommended sets of volumetric ion interaction parameters for single electrolyte solutions and symmetrical mixing parameters for Na–K–Cl ion combinations considerably improved the precision of the density calculations of highly concentrated mixed electrolyte solutions and of various natural waters. 相似文献
93.
Tonino CarusoAldo Spinella 《Tetrahedron》2003,59(39):7787-7790
An improved methodology for the preparation of copper(I) alkynides has been developed using cesium carbonate as base. In the presence of cesium carbonate and CuI various 1-alkynes were converted smoothly to the corresponding copper alkynides which in situ reacted with propargylic halides in order to synthesize skipped diynes. 相似文献
94.
Marimganti S Yasmeen S Fischer D Maier ME 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(22):6687-6700
The synthesis of the omega-amino acid 4 is described utilizing a two-dimensional synthesis strategy combined with an enzymatic differentiation of homotopic ester groups. The amino acid 4 features two non-bonded interactions that result in conformational constraints on a cyclic construct. This amino acid was incorporated into the four macrolactams 17, 22, 31, and 37. The ring in 17 and 22 is 18-membered, whereas 31 and 37 have a 19-membered ring. The pairs with the same ring size differ in a N-methyl group. For the larger macrolactams (31 and 37) conformational analysis showed that the macrocyclic rings are somewhat more rigid than in the natural lead, the depsipeptide jasplakinolide. Nevertheless, their conformations are comparable to the natural product. There are no intramolecular hydrogen bonds, neither is the cis-rotamer populated in the N-methyl compound 37. Due to the increased flexibility of the smaller macrolactams 17 and 22 and signal overlap, a distinct solution structure could not be obtained for these compounds. The amino acid 4 should be useful for restricting the conformation of other small peptides. 相似文献
95.
本文综述了利用分子对称性简化天然有机产物合成设计的策略,讨论了具有下述四种对称性的分子的合成:一、对称中心和旋转对称轴;二、对称平面;三、潜在的对称性;四、局部的对称性。 相似文献
96.
The total synthesis of the polyenoyltetramic acid polycephalin C is described. Key steps of the synthesis include a double Swern oxidation, double Takai reaction and a double Stille reaction. In addition, the absolute stereochemistry of the ring junction has been determined by synthesis of both isomers and comparison of their CD spectra with natural polycephalin C. 相似文献
97.
DTA/TG and TG/DTG thermal studies and XRD investigations were carried out on pastes of lime-pozzolan binders to examine the
relative hydration process. The binders were prepared with two natural pozzolans and hydrated lime, mixed together in different
proportions. The main hydrated phases formed in the pastes are calcium silicate hydrate (CSH) and mono-carboaluminate. The
growth of CSH was greater for the paste of the pozzolan richer in reactive constituents. For pastes with a higher lime/pozzolan
ratio, a slowing of the formation of CSH was observed, while the formation of monocarboaluminate was more intense. The calcium
hydroxide contents of the pastes obtained by thermogravimetric analysis made it possible to determine the pozzolanic activities
of the two pozzolans under examination. The mortars for the various lime-pozzolan binders displayed characteristic hardening.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
98.
Quantum chemical calculations at mPW1PW91 level, with full geometry optimization, using the 6-31g(d) basis set, and GIAO (gauge including atomic orbitals) 13C NMR chemical shifts using the 6-31g(d,p) basis set, are here utilized as a support to define the configurational features of the natural product passifloricin A, whose previously proposed relative configuration has been recently shown, by synthetic studies, to be incorrect. This study suggests that the relative stereostructure for passifloricin A corresponds to the δ-lactone of the (5R,7R,9S,11R)-tetrahydroxyhexacos-2-enoic acid. 相似文献
99.
Francisco Sarabia 《Tetrahedron letters》2003,44(41):7671-7675
A synthetic approach to the [15]-membered stevastelins, a novel class of immunosuppressant agents, is reported based on a macrolactamization route to the 2,3-epoxy derivative 6. The synthesis of this compound was achieved via a stereoselective epoxidation of the allylic alcohol 13, followed by a coupling reaction with a variety of peptide derivatives to give the epoxy peptides 7-10. After an extensive study of cyclizations with these precursors, the best result was achieved with the macrolactamization of 8 in the presence of DEPC, to obtain the epoxy cyclic depsipeptide 6 in 42% yield. From this product, an epoxy analogue of stevastelin B, compound 27, was prepared. Finally, the synthesis of the natural product was attempted through the opening of the oxirane ring contained in 6 and 26, with a variety of methyl cuprate reagents, but without success. 相似文献
100.
The paper represents a further development of our hydrate hypothesis of simplest living matter origination (SLMO) from inorganic and simplest organic mineral substances. It is supposed that the sources of SLMO are simplest aliphatic hydrocarbons, niters, and phosphates. Thermodynamic calculations demonstrating a principal possibility for simultaneous syntheses of different nitrogen bases, d-ribose, and desoxy-d-ribose from these source substances are presented. The most probable principal mechanism of SLMO and the principal conditions necessary for realization of the proposed chemism of SLMO in the framework of the proposed mechanism are considered. A principal means for testing the hypothesis is proposed. 相似文献