全文获取类型
收费全文 | 79153篇 |
免费 | 7233篇 |
国内免费 | 6050篇 |
专业分类
化学 | 52528篇 |
晶体学 | 877篇 |
力学 | 1357篇 |
综合类 | 748篇 |
数学 | 7284篇 |
物理学 | 29642篇 |
出版年
2024年 | 94篇 |
2023年 | 743篇 |
2022年 | 1246篇 |
2021年 | 2385篇 |
2020年 | 2267篇 |
2019年 | 2316篇 |
2018年 | 1959篇 |
2017年 | 2332篇 |
2016年 | 3156篇 |
2015年 | 3042篇 |
2014年 | 3552篇 |
2013年 | 6588篇 |
2012年 | 4740篇 |
2011年 | 5137篇 |
2010年 | 4274篇 |
2009年 | 5339篇 |
2008年 | 5205篇 |
2007年 | 5541篇 |
2006年 | 4587篇 |
2005年 | 3418篇 |
2004年 | 2972篇 |
2003年 | 2598篇 |
2002年 | 2285篇 |
2001年 | 1851篇 |
2000年 | 1424篇 |
1999年 | 1241篇 |
1998年 | 1089篇 |
1997年 | 928篇 |
1996年 | 873篇 |
1995年 | 829篇 |
1994年 | 784篇 |
1993年 | 754篇 |
1992年 | 729篇 |
1991年 | 490篇 |
1990年 | 378篇 |
1989年 | 305篇 |
1988年 | 396篇 |
1987年 | 298篇 |
1986年 | 276篇 |
1985年 | 371篇 |
1984年 | 296篇 |
1983年 | 162篇 |
1982年 | 342篇 |
1981年 | 518篇 |
1980年 | 461篇 |
1979年 | 488篇 |
1978年 | 390篇 |
1977年 | 287篇 |
1976年 | 249篇 |
1973年 | 170篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
Combining Organometallic Reagents,the Sulfur Dioxide Surrogate DABSO,and Amines: A One‐Pot Preparation of Sulfonamides,Amenable to Array Synthesis 下载免费PDF全文
Alex S. Deeming Dr. Claire J. Russell Prof. Michael C. Willis 《Angewandte Chemie (International ed. in English)》2015,54(4):1168-1171
We describe a method for the synthesis of sulfonamides through the combination of an organometallic reagent, a sulfur dioxide equivalent, and an aqueous solution of an amine under oxidative conditions (bleach). This simple reaction protocol avoids the need to employ sulfonyl chloride substrates, thus removing the limitation imposed by the commercial availability of these reagents. The resultant method allows access to new chemical space, and is also tolerant of the polar functional groups needed to impart favorable physiochemical properties required for medicinal chemistry and agrochemistry. The developed chemistry is employed in the synthesis of a targeted 70 compound array, prepared using automated methods. The array achieved a 93 % success rate for compounds prepared. Calculated molecular weights, lipophilicities, and polar surface areas are presented, demonstrating the utility of the method for delivering sulfonamides with drug‐like properties. 相似文献
102.
103.
Extraordinary transport behavior of gases in isothermally annealed poly(4‐methyl‐1‐pentene) membranes 下载免费PDF全文
Ywu‐Jang Fu Cheng‐Lee Lai Chien‐Chieh Hu Yi‐Ming Sun Shuan‐Ying Wu Jung‐Tsai Chen Shu‐Hsien Huang Wei‐Song Hung Kueir‐Rarn Lee 《Journal of Polymer Science.Polymer Physics》2016,54(22):2368-2376
Poly(4‐methyl‐1‐pentene) (PMP) membranes were modified through isothermal annealing to investigate the change of their crystalline structure and rigid and mobile amorphous fractions (RAF and MAF), assuming a three‐phase model, affected the gas transport behavior. The crystalline structure was characterized by wide‐angle X‐ray diffraction (WAXD) and small‐angle X‐ray scattering (SAXS) techniques, and the free volume properties were analyzed by positron annihilation lifetime spectroscopy. Compared with the pristine membrane, the annealed membranes show higher crystallinity; the crystals undergo partial structural change from form III to form I. The lamellar crystal thickness, rigid amorphous fraction thickness, and long period in the lamellar stacks increase with crystallinity. The annealed PMP membranes exhibit higher permeability due to the increase in larger size free volumes in MAF and higher selectivity due to the increase in smaller size free volumes in RAF, respectively. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2368–2376 相似文献
104.
Romain Bénéteau Anne Boussonnière Jean‐Christophe Rouaud Prof. Dr. Jacques Lebreton Dr. Jérôme Graton Prof. Dr. Denis Jacquemin Dr. Muriel Sebban Prof. Dr. Hassan Oulyadi Ghanem Hamdoun Dr. Amber N. Hancock Prof. Dr. Carl H. Schiesser Dr. Fabrice Dénès 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(14):4809-4824
α‐Bromo aluminium acetals are suitable substrates for Ueno–Stork‐like radical cyclisations affording γ‐lactols and acid‐sensitive methylene‐γ‐lactols in high yields. The mechanistic study herein sets the scope and limitation of this reaction. The influence of the halide (or chalcogenide) atom X (X=Cl, Br, I, SPh, SePh) in the precursors α‐haloesters, as well as influence of the solvent and temperature was studied. The structure of the aluminium acetal intermediates resulting from the reduction of the corresponding α‐haloesters has been investigated by low‐temperature 13C‐INEPT diffusion‐ordered NMR spectroscopy (DOSY) experiments and quantum calculations, providing new insights into the structures of these thermally labile intermediates. Oxygen‐bridged dimeric structures with a planar Al2O2 ring are proposed for the least hindered aluminium acetals, while monomeric structures seem to prevail for the most hindered species. A comparison against the radical cyclisation of aluminium acetals derived from allyl and propargyl alcohols with the parent Ueno–Stork has been made at the BHandHLYP/6‐311++G(d,p) level of theory, highlighting mechanistic similarities and differences. 相似文献
105.
The objective of this work is to study electron-vibrational interaction (EVI) and concentration quenching and their manifestation in experimental photoluminescence spectra of Cu+ ion in various lithium based phosphors namely, Li2SO4, LiNaSO4 and LiKSO4. The main parameters of EVI, such as the Stokes shift, Huang-Rhys factor and zero-phonon line positions, were estimated. The studied systems shows strong electron lattice coupling. The validity of results was established by modeling the shape of the emission spectra, which was found to be in good agreement with experimental photoluminescence spectra. The concentration quenching study is also carried out for these compounds. The studied systems correspond to the nearest neighbor energy transfer mechanism. 相似文献
106.
Inside Cover: Photoinduced Formation of an Azobenzene‐Based CD‐Active Supramolecular Cyclic Dimer (Chem. Eur. J. 18/2015) 下载免费PDF全文
107.
Elsye Agustina Jeungchoon Goak Suntae Lee Youngho Seo Jun-Young Park Naesung Lee 《ChemistryOpen》2015,4(5):613-619
Iron catalysts have been used widely for the mass production of carbon nanotubes (CNTs) with high yield. In this study, UV/visible spectroscopy was used to determine the Fe catalyst content in CNTs using a colorimetric technique. Fe ions in solution form red–orange complexes with 1,10-phenanthroline, producing an absorption peak at λ=510 nm, the intensity of which is proportional to the solution Fe concentration. A series of standard Fe solutions were formulated to establish the relationship between optical absorbance and Fe concentration. Many Fe catalysts were microscopically observed to be encased by graphitic layers, thus preventing their extraction. Fe catalyst dissolution from CNTs was investigated with various single and mixed acids, and Fe concentration was found to be highest with CNTs being held at reflux in HClO4/HNO3 and H2SO4/HNO3 mixtures. This novel colorimetric method to measure Fe concentrations by UV/Vis spectroscopy was validated by inductively coupled plasma optical emission spectroscopy, indicating its reliability and applicability to asses Fe content in CNTs. 相似文献
108.
价廉易得的L-亮氨酸先以苄基同时保护氨基及羧基得(S)-2-(二苄胺基)-4-甲基戊酸苄酯,进而在碱性条件下与乙腈发生亲核取代反应得(S)-4-(二苄胺基)-6-甲基-3-氧代庚腈,再经硼氢化钠选择性还原羰基得(3S,4S)-4-(二苄胺基)-3-羟基-6-甲基庚腈,用双氧水氧化得(3S,4S)-4-(二苄胺基)-3-羟基-6-甲基庚酸,最后在Pd(OH)2/C-H2作用下脱掉苄基得到(3S,4S)-4-氨基-3-羟基-6-甲基庚酸,即(3S,4S)-statine。整个合成路线总产率为33.6%。 相似文献
109.
Back Cover: Understanding the Origins of Nucleophilic Hydride Reactivity of a Sodium Hydride–Iodide Composite (Chem. Eur. J. 21/2016) 下载免费PDF全文
110.
Synthesis of thermally stable aromatic poly(spiroorthocarbonate)s having a Cardo or bent structure 下载免费PDF全文
Masaki Moritsugu Ryota Seto Kozo Matsumoto Takeshi Endo 《Journal of polymer science. Part A, Polymer chemistry》2016,54(10):1409-1416
Novel poly(spiroorthocarbonate)s [poly(SOC)]s having a Cardo or bent structure were synthesized by polycondensation of several bis‐catechols having fluorene (BCFL), spirobisindane (BCSPI), or spirobischromane (BCSPC) in the structure with 2,2,6,6‐tetrachlorobenzo[1,2‐d:4,5‐d’]bis[1,3]dioxole (4ClBD). Synthesis of poly(SOC)s was confirmed by NMR and IR spectrometry. The poly(SOC)s obtained from BCFL or BCSPC were soluble in common organic solvents. The glass transition temperature of the poly(SOC)s was not detected by differential scanning calorimetry (DSC) in the range of 50–300 °C. The 10 wt % decomposition temperature of the poly(SOC)s was found to be above 400 °C. These results indicated the high thermal stability of the poly(SOC)s. Soluble poly(SOC)s could be possessed to form a film on a glass plate by the spin coat method. The obtained polymer films were 0.2 μm in thickness with 95% light transmission in the optical wavelength range. These results suggested that the Cardo or bent structure may block the packing of the main‐chain of the structure, which improves the solubility of the polymers, increases transparency, and enhances the thermal stability of SOCs. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1409‐1416 相似文献