全文获取类型
收费全文 | 29153篇 |
免费 | 6509篇 |
国内免费 | 4318篇 |
专业分类
化学 | 19153篇 |
晶体学 | 320篇 |
力学 | 3105篇 |
综合类 | 261篇 |
数学 | 2587篇 |
物理学 | 14554篇 |
出版年
2024年 | 68篇 |
2023年 | 380篇 |
2022年 | 626篇 |
2021年 | 947篇 |
2020年 | 1325篇 |
2019年 | 1088篇 |
2018年 | 970篇 |
2017年 | 1125篇 |
2016年 | 1573篇 |
2015年 | 1486篇 |
2014年 | 1864篇 |
2013年 | 2817篇 |
2012年 | 2009篇 |
2011年 | 2085篇 |
2010年 | 1789篇 |
2009年 | 2073篇 |
2008年 | 2122篇 |
2007年 | 2172篇 |
2006年 | 1980篇 |
2005年 | 1687篇 |
2004年 | 1458篇 |
2003年 | 1325篇 |
2002年 | 970篇 |
2001年 | 799篇 |
2000年 | 737篇 |
1999年 | 644篇 |
1998年 | 545篇 |
1997年 | 443篇 |
1996年 | 379篇 |
1995年 | 363篇 |
1994年 | 321篇 |
1993年 | 247篇 |
1992年 | 205篇 |
1991年 | 192篇 |
1990年 | 143篇 |
1989年 | 133篇 |
1988年 | 136篇 |
1987年 | 107篇 |
1986年 | 96篇 |
1985年 | 101篇 |
1984年 | 71篇 |
1983年 | 32篇 |
1982年 | 62篇 |
1981年 | 59篇 |
1980年 | 43篇 |
1979年 | 42篇 |
1978年 | 24篇 |
1977年 | 33篇 |
1976年 | 17篇 |
1974年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Jiayong Tang Dr. Xia Huang Dr. Tengfei Qiu Xiyue Peng Dr. Tingting Wu Prof. Lei Wang Dr. Bin Luo Prof. Lianzhou Wang 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(6):1921-1940
The increasing demand for high-performance rechargeable energy storage systems has stimulated the exploration of advanced electrode materials. MXenes are a class of two-dimensional (2D) inorganic transition metal carbides/nitrides, which are promising candidates in electrodes. The layered structure facilitates ion insertion/extraction, which offers promising electrochemical characteristics for electrochemical energy storage. However, the low capacity accompanied by sluggish electrochemical kinetics of electrodes as well as interlayer restacking and collapse significantly impede their practical applications. Recently, interlayer space engineering of MXenes by different chemical strategies have been widely investigated in designing functional materials for various applications. In this review, an overview of the most recent progress of 2D MXenes engineering by intercalation, surface modification as well as heterostructures design is provided. Moreover, some critical challenges in future research on MXene-based electrodes have been also proposed. 相似文献
993.
Dr. Sara Sabater Dr. David Schmidt Dr. Heidi Schmidt Dr. Maximilian W. Kuntze-Fechner Dr. Thomas Zell Connie J. Isaac Dr. Nasir A. Rajabi Harry Grieve Dr. William J. M. Blackaby Dr. John P. Lowe Prof. Stuart A. Macgregor Dr. Mary F. Mahon Prof. Dr. Udo Radius Prof. Michael K. Whittlesey 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(52):13221-13234
The addition of PPh2H, PPhMeH, PPhH2, P(para-Tol)H2, PMesH2 and PH3 to the two-coordinate Ni0 N-heterocyclic carbene species [Ni(NHC)2] (NHC=IiPr2, IMe4, IEt2Me2) affords a series of mononuclear, terminal phosphido nickel complexes. Structural characterisation of nine of these compounds shows that they have unusual trans [H−Ni−PR2] or novel trans [R2P−Ni−PR2] geometries. The bis-phosphido complexes are more accessible when smaller NHCs (IMe4>IEt2Me2>IiPr2) and phosphines are employed. P−P activation of the diphosphines R2P−PR2 (R2=Ph2, PhMe) provides an alternative route to some of the [Ni(NHC)2(PR2)2] complexes. DFT calculations capture these trends with P−H bond activation proceeding from unconventional phosphine adducts in which the H substituent bridges the Ni−P bond. P−P bond activation from [Ni(NHC)2(Ph2P−PPh2)] adducts proceeds with computed barriers below 10 kcal mol−1. The ability of the [Ni(NHC)2] moiety to afford isolable terminal phosphido products reflects the stability of the Ni−NHC bond that prevents ligand dissociation and onward reaction. 相似文献
994.
Dr. Michael Jäger Dr. Helmar Görls Dr. Wolfgang Günther Prof. Dr. Ulrich S. Schubert 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(6):2150-2157
A new modular approach based on Pd‐catalyzed C? C bond formation is presented for the assembly of a benzannulated azocine scaffold, the key intermediate in the synthesis of functionalized azadibenzocyclooctynes (aza‐DIBOs). The intramolecular ring‐closing Heck coupling was investigated by variation of the C? X bond. The reaction rate is limited by the initial oxidative addition step and the regiochemistry strongly depends on the auxiliary phosphine. Under optimized conditions, the 8‐endo regioisomer was obtained in 71 % yield over two steps (with no protecting group chemistry) or in one pot, inclusive of C? N bond formation. The practical generation of the octyne triple bond of a prototypical N‐benzoyl aza‐DIBO, without the need for chromatographic purification, is also described. The structural features, including those of the ring‐strained cyclic octyne, were elucidated by NMR spectroscopy and X‐ray crystallographic analysis. The high reactivity of the N‐benzoyl aza‐DIBO synthesized is demonstrated in a strain‐promoted azide–alkyne cycloaddition reaction with an alkyl azide (k=0.38 M ?1 s?1). 相似文献
995.
Dr. Randa K. Gabr Prof. Dr. Takuji Hatakeyama Prof. Dr. Kazuhiro Takenaka Prof. Dr. Shinobu Takizawa Yoshihiro Okada Prof. Dr. Masaharu Nakamura Prof. Dr. Hiroaki Sasai 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(29):9518-9525
The reaction pathway of an enantioselective 5‐endo‐trig‐type cyclization of 3‐alkenoic acids catalyzed by a chiral palladium–spiro‐bis(isoxazoline) complex, Pd–SPRIX, has been studied by density functional theory calculations. The most plausible pathway involves intramolecular nucleophilic attack of the carboxylate moiety on the C?C double bond activated by Pd–SPRIX and β‐H elimination from the resulting organopalladium intermediate. The enantioselectivity was determined in the cyclization step through the formation of a π‐olefin complex, in which one of the two enantiofaces of the olefin moiety was selected. The β‐H elimination occurs via a seven‐membered cyclic structure in which the acetate ligand plays a key role in lowering the activation barrier of the transition state. In the elimination step, the SPRIX ligand was found to behave as a monodentate ligand due to the hemilability of one of the isoxazoline units thereby facilitating the elimination. Natural population analysis of this pathway showed that the more weakly electron‐donating SPRIX ligand, compared with the bis(oxazoline) ligand, BOX, facilitated the formation of the π‐olefin complex intermediate, leading to a smaller overall activation energy and a higher reactivity of the Pd–SPRIX catalyst. 相似文献
996.
997.
998.
999.
Dario Romano Efren Alberto Andablo‐Reyes Sara Ronca Sanjay Rastogi 《Journal of polymer science. Part A, Polymer chemistry》2013,51(7):1630-1635
The use of a hindered phenol to trap free trimethylaluminum (TMA) in methylaluminoxane (MAO) solutions has been reported to improve the performance of single‐site, homogeneous catalysts for olefin polymerization. In the present study, with the help of rheological analyses, we have investigated and compared the molecular weight, molecular weight distribution and entanglement density of ultrahigh molecular weight polyethylene synthesized with a single‐site catalyst activated by MAO and phenol‐modified MAO. While the number average molecular weight (Mn) of the obtained polymers remains the same for both activations, a higher yield and a higher entanglement density are found in the initial stages of polymerization on using phenol‐modified MAO as the cocatalyst. These results suggest that on using the phenol‐modified MAO as activator, a higher number of active sites are obtained. Surprisingly in the presence of untreated MAO, a tail in the higher molecular mass region is produced. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 相似文献
1000.
Markus Bursch Lukas Kunze Dr. Amol M. Vibhute Dr. Andreas Hansen Prof. Dr. Kana M. Sureshan Dr. Peter G. Jones Prof. Dr. Stefan Grimme Prof. Dr. Daniel B. Werz 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(14):4627-4639
The noncovalent interactions between azides and oxygen-containing moieties are investigated through a computational study based on experimental findings. The targeted synthesis of organic compounds with close intramolecular azide–oxygen contacts yielded six new representatives, for which X-ray structures were determined. Two of those compounds were investigated with respect to their potential conformations in the gas phase and a possible significantly shorter azide–oxygen contact. Furthermore, a set of 44 high-quality, gas-phase computational model systems with intermolecular azide–pnictogen (N, P, As, Sb), –chalcogen (O, S, Se, Te), and –halogen (F, Cl, Br, I) contacts are compiled and investigated through semiempirical quantum mechanical methods, density functional approximations, and wave function theory. A local energy decomposition (LED) analysis is applied to study the nature of the noncovalent interaction. The special role of electrostatic and London dispersion interactions is discussed in detail. London dispersion is identified as a dominant factor of the azide–donor interaction with mean London dispersion energy-interaction energy ratios of 1.3. Electrostatic contributions enhance the azide–donor coordination motif. The association energies range from −1.00 to −5.5 kcal mol−1. 相似文献