全文获取类型
收费全文 | 15282篇 |
免费 | 1318篇 |
国内免费 | 633篇 |
专业分类
化学 | 15224篇 |
晶体学 | 323篇 |
力学 | 17篇 |
综合类 | 9篇 |
数学 | 4篇 |
物理学 | 1656篇 |
出版年
2024年 | 8篇 |
2023年 | 142篇 |
2022年 | 412篇 |
2021年 | 470篇 |
2020年 | 474篇 |
2019年 | 354篇 |
2018年 | 292篇 |
2017年 | 268篇 |
2016年 | 542篇 |
2015年 | 546篇 |
2014年 | 581篇 |
2013年 | 1136篇 |
2012年 | 711篇 |
2011年 | 657篇 |
2010年 | 745篇 |
2009年 | 744篇 |
2008年 | 764篇 |
2007年 | 783篇 |
2006年 | 756篇 |
2005年 | 702篇 |
2004年 | 752篇 |
2003年 | 668篇 |
2002年 | 1397篇 |
2001年 | 366篇 |
2000年 | 310篇 |
1999年 | 311篇 |
1998年 | 288篇 |
1997年 | 211篇 |
1996年 | 418篇 |
1995年 | 395篇 |
1994年 | 142篇 |
1993年 | 111篇 |
1992年 | 128篇 |
1991年 | 79篇 |
1990年 | 51篇 |
1989年 | 67篇 |
1988年 | 66篇 |
1987年 | 36篇 |
1986年 | 40篇 |
1985年 | 55篇 |
1984年 | 39篇 |
1983年 | 24篇 |
1982年 | 22篇 |
1981年 | 19篇 |
1980年 | 14篇 |
1979年 | 10篇 |
1978年 | 13篇 |
1975年 | 7篇 |
1971年 | 48篇 |
1970年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
951.
Naiem Ahmad Wani Vivek Kumar Gupta Rajni Kant Subrayashastry Aravinda Rajkishor Rai 《Acta Crystallographica. Section C, Structural Chemistry》2014,70(8):776-779
The crystal structure of N‐[(1‐{2‐oxo‐2‐[2‐(pyrazin‐2‐ylcarbonyl)hydrazin‐1‐yl]ethyl}cyclohexyl)methyl]pyrazine‐2‐carboxamide monohydrate (Pyr‐Gpn‐NN‐NH‐Pyr·H2O), C19H23N7O3·H2O, reveals an unusual trans–gauche (tg−) conformation for the gabapentin (Gpn) residue around the Cγ—Cβ (θ1) and Cβ—Cα (θ2) bonds. The molecular conformation is stabilized by intramolecular N—H...N hydrogen bonds and weak C—H...O interactions. The packing of the molecules in the crystal lattice shows a network of strong N—H...O and O—H...O hydrogen bonds together with weak C—H...O and π–π inteactions. 相似文献
952.
Viktor Vrbel Július Sivý Peter af tefan Marchalín 《Acta Crystallographica. Section C, Structural Chemistry》2014,70(8):817-822
Molecules of (S)‐6‐oxo‐1‐(thiophen‐2‐ylmethyl)piperidine‐2‐carboxylic acid, C11H13NO3S, crystallize as single enantiomers in the space group P21 and the thiophene ring is disordered over two positions, while (S)‐6‐oxo‐1‐(thiophen‐3‐ylmethyl)piperidine‐2‐carboxylic acid, C11H13NO3S, crystallizes as a single enantiomer in the space group P212121. Their absolute configurations were confirmed by anomalous dispersion effects in diffraction measurements on the crystals. The molecules of each compound are linked by a combination of strong O—H...O hydrogen bonds and weak C—H...O interactions, resulting in two‐ and three‐dimensional networks, respectively, in the crystal structures. 相似文献
953.
Rim Benali‐Cherif Radhwane Takouachet El‐Eulmi Bendeif Nourredine Benali‐Cherif 《Acta Crystallographica. Section C, Structural Chemistry》2014,70(7):689-692
Two polymorphs of L‐aspartic acid hydrochloride, C4H8NO4+·Cl−, were obtained from the same aqueous solution. Their crystal structures have been determined from single‐crystal data collected at 100 K. The crystal structures revealed three‐ and two‐dimensional hydrogen‐bonding networks for the triclinic and orthorhombic polymorphs, respectively. The cations and anions are connected to one another via N—H...Cl and O—H...Cl interactions and form alternating cation–anion layer‐like structures. The two polymorphs share common structural features; however, the conformations of the L‐aspartate cations and the crystal packings are different. Furthermore, the molecular packing of the orthorhombic polymorph contains more interesting interactions which seems to be a favourable factor for more efficient charge transfer within the crystal. 相似文献
954.
A combined computational and 13C NMR study was used to investigate the formation of mixed aggregates of 1-methoxyallenyllithium and lithium chloride in tetrahydrofuran (THF) solution. The observed and calculated chemical shifts, as well as the calculated free energies of mixed aggregate formation (MP2/6-31+G(d)), are consistent with the formation of a mixed dimer as the major species in solution. Free energies of mixed dimer, trimer, and tetramer formation were calculated by using the B3LYP and MP2 methods and the 6-31+G(d) basis set. The two methods generated different predictions of which mixed aggregates will be formed, with B3LYP/6-31+G(d) favoring mixed trimers and tetramers in THF solution, and MP2/6-31+G(d) favoring mixed dimers. Formation of the sterically unhindered mixed dimers is also consistent with the enhanced reactivity of these compounds in the presence of lithium chloride. The spectra are also consistent with some residual 1-methoxyallenyllithium tetramer, as well as small amounts of higher mixed aggregates. Although neither computational method is perfect, for this particular system, the calculated free energies derived using the MP2 method are in better agreement with experimental data than those derived using the B3LYP method. 相似文献
955.
Mohamed Naji Jean‐Yves Colle Ondrej Bene Mark Sierig Jouni Rautio Patrick Lajarge Dario Manara 《Journal of Raman spectroscopy : JRS》2015,46(9):750-756
An approach for Raman measurements of highly radioactive samples is presented here. The innovative part of this approach lies in the fact that no single part of the Raman equipment is in direct contact with the radioactive sample, as the sample is sealed in an alpha‐tight capsule. Raman analysis is effectively performed through the optical‐grade quartz window closing the capsule. This allows performing micro‐Raman measurements on radioactive samples with no limitations on the laser source wavelength, polarisation mode, spectrometer mode and microscope mode (provided the focal length of the microscope objective is greater than the thickness of the quartz window and with sub mg samples). Some example results are shown and discussed. In particular, some spectral features of americium‐containing oxide nuclear fuel specimens are presented. Raman spectra clearly reveal in these specimens the presence of abundant oxygen defects induced in the fcc fluorite lattice by trivalent americium. In order to complete the analysis the Raman spectrum of pure americium dioxide was also measured with a lower energy excitation source compared with previous research. The current results seem to be consistent with the possible occurrence of a photolysis process induced by the Raman laser, resulting in the formation of hyperstoichiometric americium sesquioxide Am2O3 + z. Such a photolytic process is deemed to be unavoidable when visible lasers are used as excitation sources for the Raman analysis of americium dioxide. © 2015 The Authors Journal of Raman Spectroscopy Published by John Wiley & Sons, Ltd. 相似文献
956.
957.
Tobias Dunaj Prof. Dr. Carsten von Hänisch 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(69):e202202932
In this work, we report the preparation of multiple interpnictogen chain compounds with three consecutive pnictogen atoms and terminal Ar2Bi fragments (Ar=Ph, Mes). Symmetrical compounds of the form Ar2Bi−E(tBu)−Bi2Ar ( 1 : Ar=Ph, E=P; 2 : Ar=Ph, Mes, E=As) as well as ternary interpnictogen compounds of the form Ar2Bi−E1(tBu)−E2tBu2 (Ar=Ph, Mes; 4 : E1=P, E2=As; 5 : E1=P, E2=Sb; 6 : E1=As, E2=P) were prepared. The decomposition in solution at room temperature and under the influence of light was studied for compounds 1 – 6 . The reactivity of 1Ph and 2Ph with the small N-heterocyclic carbene 1,3,4,5-tetramethylimidazol-2-ylidene (Me2IMe) was also studied. In the case of 1Ph , the formation and consecutive decomposition of Me2IMe=PtBu ( 8 ) was observed in solution. Hence, it was shown that 1Ph can react as a “masked phosphinidene”. In the case of 2Ph , no reaction with Me2IMe was observed. All isolated compounds were analysed by NMR and IR spectroscopy, mass spectrometry, elemental analysis and single-crystal X-ray diffraction. 相似文献
958.
Jochen C. Lauer Dr. Avinash S. Bhat Chantal Barwig Nathalie Fritz Tobias Kirschbaum Dr. Frank Rominger Prof. Dr. Michael Mastalerz 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(51):e202201527
The pollution of groundwater with nitrate is a serious issue because nitrate can cause several diseases such as methemoglobinemia or cancer. Therefore, selective removal of nitrate by efficient binding to supramolecular hosts is highly desired. Here we describe how to make [2+3] amide cages in very high to quantitative yields by applying an optimized Pinnick oxidation protocol for the conversion of corresponding imine cages. By NMR titration experiments of the eight different [2+3] amide cages with nitrate, chloride and hydrogen sulfate we identified one cage with an unprecedented high selectivity towards nitrate binding vs. chloride (S=705) or hydrogensulfate (S>13500) in CD2Cl2/CD3CN (1 : 3). NMR experiments as well as single-crystal structure comparison of host-guest complexes give insight into structure-property-relationships. 相似文献
959.
Or Dishi Yuval Rahav Dr. Raanan Carmieli Prof. Ori Gidron 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(62):e202202082
Macrocyclic furans are predicted to switch between global aromaticity and antiaromaticity, depending on their oxidation states. However, the macrocyclic furans reported to date are stabilized by electron withdrawing groups, which result in inaccessible oxidation states. To circumvent this problem, a post-macrocyclization approach was applied to introduce methylene-substituted macrocyclic furans, which display an extremely low oxidation potential of −0.23 vs. Fc/Fc+, and are partially oxidized in ambient conditions. Additional oxidation to the dication results in aromaticity switching to a global 30πe− aromatic state, as indicated by the formation of a strong diatropic current observed in the 1H NMR spectrum. NICS and ACID calculations support this trend and provide evidence for a different pathway for the global current in the neutral and dicationic states. According to these findings, macrocyclic furans can be rendered as promising p-type materials with stable oxidation states. 相似文献
960.
采用密度泛函理论(DFT)计算研究了苯酚、邻甲酚、愈创木酚在不同结构Ru-Fe(211)表面上吸附活化性能和加氢脱氧反应路径.结果表明,Ru掺杂能促进H2分子在Fe(211)表面上解离,提高加氢脱氧反应速率.酚类在1Ru_(ads)-Fe(211)表面上吸附比在1Ru_(sub)-Fe(211)表面上更稳定,苯酚和邻甲酚脱羟基步骤能垒分别降低0.13和0.28 eV,有利于生成芳烃.愈创木酚在1Ru_(sub)-Fe(211)表面上加氢脱氧优势路径是先脱甲氧基生成苯酚,苯酚再加氢脱氧生成产物苯(速控步骤能垒1.16 eV);而在1Ru_(ads)-Fe(211)表面上愈创木酚先脱羟基再脱甲基生成苯酚的路径更具有动力学优势(速控步骤能垒1.21 eV).计算结果表明Ru掺杂方式影响Fe催化剂对酚反应分子的吸附稳定性以及加氢脱氧反应路径和性能.与1Ru掺杂Fe(211)催化剂相比,增加Ru原子数形成4Ru_(ads)-Fe(211),能够进一步提高酚类反应物的吸附强度,但导致加氢脱氧反应能垒升高.因此,在Fe催化剂上以表面吸附的形式掺杂少量贵金属Ru更利于酚类加氢脱氧生成芳烃. 相似文献