全文获取类型
收费全文 | 108551篇 |
免费 | 26688篇 |
国内免费 | 6800篇 |
专业分类
化学 | 110241篇 |
晶体学 | 993篇 |
力学 | 4443篇 |
综合类 | 465篇 |
数学 | 9223篇 |
物理学 | 16674篇 |
出版年
2024年 | 464篇 |
2023年 | 4507篇 |
2022年 | 2634篇 |
2021年 | 3825篇 |
2020年 | 6898篇 |
2019年 | 5961篇 |
2018年 | 3718篇 |
2017年 | 2379篇 |
2016年 | 8588篇 |
2015年 | 8827篇 |
2014年 | 8637篇 |
2013年 | 10100篇 |
2012年 | 8143篇 |
2011年 | 6214篇 |
2010年 | 7581篇 |
2009年 | 7461篇 |
2008年 | 5655篇 |
2007年 | 4678篇 |
2006年 | 3622篇 |
2005年 | 3489篇 |
2004年 | 2862篇 |
2003年 | 2466篇 |
2002年 | 3188篇 |
2001年 | 2411篇 |
2000年 | 2103篇 |
1999年 | 1149篇 |
1998年 | 698篇 |
1997年 | 640篇 |
1996年 | 673篇 |
1995年 | 614篇 |
1994年 | 541篇 |
1993年 | 598篇 |
1992年 | 464篇 |
1991年 | 386篇 |
1990年 | 314篇 |
1989年 | 292篇 |
1988年 | 298篇 |
1987年 | 253篇 |
1985年 | 271篇 |
1980年 | 252篇 |
1977年 | 345篇 |
1976年 | 406篇 |
1975年 | 491篇 |
1974年 | 513篇 |
1973年 | 332篇 |
1972年 | 390篇 |
1971年 | 371篇 |
1970年 | 556篇 |
1969年 | 427篇 |
1968年 | 458篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
Highly Selective Copper‐Catalyzed Asymmetric [3+2] Cycloaddition of Azomethine Ylides with Acyclic 1,3‐Dienes 下载免费PDF全文
María González‐Esguevillas Ana Pascual‐Escudero Dr. Javier Adrio Prof. Dr. Juan C. Carretero 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(12):4561-4565
The first examples of the catalytic asymmetric 1,3‐dipolar cycloaddition of azomethine ylides with acyclic activated 1,3‐dienes (and 1,3‐enynes) are described. Under copper catalysis, a selective cycloaddition at the terminal γ,δ‐C?C bond is observed. In addition, depending on the ligand used, either the exo or the endo adduct can be obtained with high selectivity. Under appropriate reaction conditions, the acyclic 1,6‐addition product is detected, suggesting a stepwise mechanism. The resulting C4‐alkenyl‐substituted pyrrolidines are suitable substrates for further access to polycyclic systems, as highlighted by the preparation of hexahydrochromeno[4,3‐b]pyrrole and the tetracyclic core of the alkaloid gracilamine. 相似文献
62.
Beyond Stereoselectivity,Switchable Catalysis: Some of the Last Frontier Challenges in Ring‐Opening Polymerization of Cyclic Esters 下载免费PDF全文
Dr. Sophie M. Guillaume Dr. Evgueni Kirillov Dr. Yann Sarazin Prof. Dr. Jean‐François Carpentier 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(22):7988-8003
Metal‐based catalysts and initiators have played a pivotal role in the ring‐opening polymerization (ROP) of cyclic esters, thanks to their high activity and remarkable ability to control precisely the architectures of the resulting polyesters in terms of molar mass, dispersity, microstructure, or tacticity. Today, after two decades of extensive research, the field is slowly reaching maturity. However, several challenges remain, while original concepts have emerged around new types or new applications of catalysis. This Review is not intended to comprehensively cover all of these aspects. Rather, it provides a personal overview of the very recent progress achieved in some selected, important aspects of ROP catalysis—stereocontrol and switchable catalysis. Hence, the first part addresses the development of new metal‐based catalysts for the isoselective ROP of racemic lactide towards stereoblock copolymers, and the use of syndioselective ROP metal catalysts to control the monomer sequence in copolymers. A second part covers the development of ROP catalysts—primarily metal‐based catalysts, but also organocatalysts—that can be externally regulated by the use of chemical or photo stimuli to switch them between two states with different catalytic abilities. Current challenges and opportunities are highlighted. 相似文献
63.
64.
65.
66.
Yoshiaki Murata Cheng-Huang Lin Totaro Imasaka 《Fresenius' Journal of Analytical Chemistry》1993,346(6-9):543-544
Summary A supersonic jet spectrum of 9,10-dichloroanthracene is measured by stimulated-emission-pumping fluorescence dip spectrometry and conventional fluorescence spectrometry. The performance obtained is compared for these spectrometric methods, providing same information concerned with the energy level of the ground state. The former is more preferential for measurement of a high-resolution spectrum, since the spectral resolution is determined by the linewidth of the dumping laser. On the other hand, the latter is more preferential for measurement with better sensitivity at the expense of the spectral resolution, since the fluorescence throughput can be improved by increasing the slit width of the monochromator.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday 相似文献
67.
68.
The theory of tree-growing (RECPAM approach) is developed for outcome variables which are distributed as the canonical exponential family. The general RECPAM approach (consisting of three steps: recursive partition, pruning and amalgamation), is reviewed. This is seen as constructing a partition with maximal information content about a parameter to be predicted, followed by simplification by the elimination of ‘negligible’ information. The measure of information is defined for an exponential family outcome as a deviance difference, and appropriate modifications of pruning and amalgamation rules are discussed. It is further shown how the proposed approach makes it possible to develop tree-growing for situations usually treated by generalized linear models (GLIM). In particular, Poisson and logistic regression can be tree-structured. Moreover, censored survival data can be treated, as in GLIM, by observing a formal equivalence of the likelihood under random censoring and an appropriate Poisson model. Three examples are given of application to Poisson, binary and censored survival data. 相似文献
69.
70.
5‐Amino‐4‐methyl‐2‐phenyl‐6‐substitutedfuro[2,3‐d]pyrimidines ( 2a‐c ) were reacted with 2,5‐dimethoxytetrahydrfuran to afford the pyrrolyl derivatives 3a‐c . Compound 3a was chosen as intermediate for the synthesis of poly fused heterocycles incorporated furopyrimidines moiety 4–11 . Some of the synthesized compounds were screened for their antibacterial and antifungal activities. 相似文献