全文获取类型
收费全文 | 1589篇 |
免费 | 29篇 |
国内免费 | 5篇 |
专业分类
化学 | 1328篇 |
晶体学 | 9篇 |
力学 | 7篇 |
数学 | 11篇 |
物理学 | 268篇 |
出版年
2022年 | 6篇 |
2021年 | 5篇 |
2020年 | 6篇 |
2019年 | 23篇 |
2018年 | 11篇 |
2017年 | 11篇 |
2016年 | 21篇 |
2015年 | 17篇 |
2014年 | 40篇 |
2013年 | 69篇 |
2012年 | 71篇 |
2011年 | 87篇 |
2010年 | 49篇 |
2009年 | 39篇 |
2008年 | 78篇 |
2007年 | 69篇 |
2006年 | 71篇 |
2005年 | 89篇 |
2004年 | 65篇 |
2003年 | 59篇 |
2002年 | 41篇 |
2001年 | 62篇 |
2000年 | 65篇 |
1999年 | 37篇 |
1998年 | 26篇 |
1997年 | 30篇 |
1996年 | 25篇 |
1995年 | 21篇 |
1994年 | 29篇 |
1993年 | 32篇 |
1992年 | 37篇 |
1991年 | 24篇 |
1990年 | 19篇 |
1989年 | 24篇 |
1988年 | 12篇 |
1987年 | 8篇 |
1986年 | 11篇 |
1985年 | 24篇 |
1984年 | 33篇 |
1983年 | 23篇 |
1982年 | 16篇 |
1981年 | 16篇 |
1980年 | 18篇 |
1979年 | 23篇 |
1978年 | 16篇 |
1977年 | 12篇 |
1976年 | 14篇 |
1975年 | 9篇 |
1974年 | 9篇 |
1973年 | 7篇 |
排序方式: 共有1623条查询结果,搜索用时 15 毫秒
11.
Chemical conversion of some natural oxindoles (pteropodine, isopteropodine and isorhynchophylline) into the corresponding indole alkaloids has been made by way of a sequence of reactions which include formation of iminoethers of the natural oxindoles with Meerwein's reagent, reduction of the iminoethers to 2,3-seco-indoles and cyclization of 2,3-seco-indoles to the desired natural indole alkaloids. Sodium borohydride in acetic acid was found to be a specific reagent for the reduction of oxindole-iminoethers to 2,3-seco-indoles which were the key intermediates in these transformations. Yohimbine-oxindole iminoether was similarly converted to yohimbine and pseudoyohimbine. A number of by-products were obtained and their structures were elucidated. 相似文献
12.
13.
Ionic liquids (ILs) are salts with an extremely low melting point. Substantial efforts have been made to address their low melting point from the enthalpic standpoint (i.e. interionic interactions). However, this question is still open. In this study, we report our findings that entropic (large fusion entropy), rather than enthalpic, contributions are primarily responsible for lowering the melting point in many cases, based on a large thermodynamic dataset. We have established a computational protocol using molecular dynamics simulations to decompose fusion entropy into kinetic (translational, rotational, and intramolecular vibrational) and structural (conformational and configurational) terms and successfully applied this approach for two representatives of ILs and NaCl. It is revealed that large structural contribution, particularly configurational entropy in the liquid state, plays a deterministic role in the large fusion entropy and consequently the low melting point of the ILs.Large structural entropy makes salts liquid at room temperature. 相似文献
14.
15.
16.
Tadashi Okazaki Fumio Sanda Takeshi Endo 《Journal of polymer science. Part A, Polymer chemistry》1996,34(10):2029-2035
Synthesis and radical ring-opening polymerization of vinylcyclopropane bearing six-membered cyclic acetal moiety, 1-vinyl-4,8-dioxaspiro[2.5]octane (1), were carried out. 1 was prepared by the reaction of 1,1-dichloro-2-vinylcyclopropane and 1,3-propanediol in DMF in the presence of a base. Radical polymerization of 1 was carried out in the presence of an appropriate initiator (3 mol % vs. 1) at 60 and 120°C in degassed sealed ampoules for 20 h. A colorless transparent viscous polymer was obtained by the isolation with preparative HPLC. The structure of poly(1) was determined to consist of two 1,5-ring-opened units and a unit bearing no olefinic moiety. The difference of the activation energies for the ring-opening reaction of the cyclopropane ring calculated by the molecular orbital method could explain the selectivity in the direction of the cleavage of the cyclopropane ring. Acid hydrolysis of poly(1) afforded the corresponding polyketone in quantitative conversion. © 1996 John Wiley & Sons, Inc. 相似文献
17.
18.
19.
K. Ishikawa H. Nakano H. Masuda C. Kusano Y. Takahashi T. Hamagishi N. Endo 《Fiber and Integrated Optics》1992,11(4):385-394
A 10-Gbit/s optical receiver is developed using low-capacitance HBT IC technology. The HBT allows a cutoff frequency of 45 GHz and a maximum oscillation frequency of 70 GHz. The receiver contains an automatic gain-control amplifier IC with a gain of 16 dB, a bandwidth of 10.7 GHz, and a decision-circuit IC with an ambiguity of 66 mVp-p at a data rate of 10 Gbit/s. The sensitivity of this receiver is -26.5 dBm at a bit error rate of 10-9. Transmission over a 140-km fiber was achieved successfully. 相似文献
20.
Mitsutoshi Tanimoto Shuji Saito Yasuki Endo Eizi Hirota 《Journal of Molecular Spectroscopy》1983,100(1):205-211
The microwave spectrum of the SiF radical was observed in both and of the ground vibronic state. The SiF radical was produced by a dc discharge either in a mixture or in a transient molecule SiF2 generated by the reaction of SiF4 with heated solid silicon. The latter gave twice as intense a spectrum. A least-squares fit to the observed spectrum showed the rotational constant and the centrifugal distortion constant to be 17 350.2752(63) and 0.03188(13) MHz, respectively, with three standard errors in parentheses applying to the last digits of the constants. The lambda-doubling parameter p0 was found to be negative, ?87.67 MHz, indicating that the 2Σ+ excited state contributions dominate over those of 2Σ?. All four hyperfine coupling constants a, b, c, and d were determined and were employed to discuss the unpaired-electron spin and orbital distributions in the SiF radical. 相似文献