全文获取类型
收费全文 | 1594篇 |
免费 | 25篇 |
国内免费 | 10篇 |
专业分类
化学 | 1114篇 |
晶体学 | 24篇 |
力学 | 22篇 |
数学 | 107篇 |
物理学 | 362篇 |
出版年
2021年 | 13篇 |
2018年 | 9篇 |
2017年 | 8篇 |
2016年 | 22篇 |
2015年 | 24篇 |
2014年 | 14篇 |
2013年 | 74篇 |
2012年 | 73篇 |
2011年 | 64篇 |
2010年 | 42篇 |
2009年 | 43篇 |
2008年 | 78篇 |
2007年 | 84篇 |
2006年 | 91篇 |
2005年 | 115篇 |
2004年 | 66篇 |
2003年 | 78篇 |
2002年 | 59篇 |
2001年 | 33篇 |
2000年 | 31篇 |
1999年 | 24篇 |
1998年 | 25篇 |
1997年 | 18篇 |
1996年 | 19篇 |
1995年 | 14篇 |
1994年 | 11篇 |
1993年 | 12篇 |
1992年 | 21篇 |
1991年 | 15篇 |
1990年 | 27篇 |
1989年 | 21篇 |
1988年 | 12篇 |
1987年 | 12篇 |
1986年 | 16篇 |
1985年 | 25篇 |
1984年 | 32篇 |
1983年 | 19篇 |
1982年 | 31篇 |
1981年 | 26篇 |
1980年 | 21篇 |
1979年 | 19篇 |
1978年 | 20篇 |
1977年 | 30篇 |
1976年 | 15篇 |
1975年 | 15篇 |
1974年 | 14篇 |
1973年 | 13篇 |
1969年 | 8篇 |
1968年 | 10篇 |
1967年 | 11篇 |
排序方式: 共有1629条查询结果,搜索用时 15 毫秒
111.
The synthesis of taurospongin A has been achieved using, as a key step, a pi-allyltricarbonyliron lactone complex to control a highly stereoselective addition of a methyl group to a carbonyl unit located in the side chain of the complex. 相似文献
112.
Iyoda M Kondo T Nakao K Hara K Kuwatani Y Yoshida M Matsuyama H 《Organic letters》2000,2(14):2081-2083
1,8-[1,8-Naphthalenediylbis(4',4-biphenyldiyl)]naphthalene, a very stable strained cyclophane, has been synthesized in moderate yield using the copper-catalyzed coupling of 1, 8-bis(4-(tributylstannyl)phenyl)naphthalene. The X-ray analysis of the titled compound discloses bent p,p'-biphenylylene chains with splayed naphthalene rings, and the p,p'-biphenylylene chains located face-to-face indicate a fairly strong pi-pi interaction. 相似文献
113.
T. Yoshii H. Hara A. Kawamori K. Akabori M. Iwaki S. Itoh 《Applied magnetic resonance》1999,16(4):565-580
Light-induced spin-polarized radical pairs, P700+A1 ? in spinach photosystem (PS) I particles and P680+QA ? in Zn-substituted PS II core complexes, in oriented membranes were studied by pulsed electron paramagnetic resonance (EPR). Based on the determined distance of 25.2 ± 0.2 Å between P700 and A1, the angular dependence of the spin-polarized electron spin echo envelope modulation (ESEEM) spectra on the magnetic field suggests that the angle between $R_{P700 - {\rm A}_1 } $ , the radius-vector connecting P700 and A1, and the membrane normaln was 24 ± 4° in PS I particles. Obtained angle and distance of P700-A1 axis suggested QK side in the molecular geometry of cofactors presented in a recent X-ray crystallography of cyanobacterial PS I reaction center to be an active branch of electron transfer. The distance between P680 and QA was determined to be 27.4 ± 0.3 Å for a nonoriented PS II. The angle between $R_{P680 - Q_{\rm A} } $ , the radius-vector connecting P680 and QA, andn was determined to be 21 ± 5°. The angle of P680-QA axis was close to that of 20° of P870-QA axis reported in X-ray analysis of the purple bacterial reaction center crystal. 相似文献
114.
115.
Kouhei Hatanaka Shigeo Tanimoto Toyonari Sugimoto Masaya Okano 《Tetrahedron letters》1981,22(34):3243-3244
It has been found that the reaction of enol silyl ethers with 2-ethoxy-1,3-dithiolane proceeds smoothly in the presence of zinc chloride catalyst to afford half-protected 1,3-dicarbony1 compounds in moderately good yields. 相似文献
116.
117.
118.
Shinji Fushiya Yoshikazu Sato Shigeo Nozoe Kyosuke Nomoto Tsunematsu Takemoto Sei-ichi Takagi 《Tetrahedron letters》1980,21(32):3071-3072
A new amino acid derivative possessing an iron chelating activity was isolated from root washings of water cultured under iron deficient conditions. The structure of this compound, avenic acid A has been determined as 2(S),3′(S),3″(S)-N-[3-(3-hydroxy-3-carboxypropylamino)-3-carboxypropyl]-homoserine (1) on the basis of chemical and spectroscopic evidence. 相似文献
119.
A new synthetic approach to quinazolines bearing a carboxy group is described. Reaction of 5-carbonyl substituted 1,3,6-tri-methyluracils (I) with dimethyl acetylenedicarboxylate or electron-deficient olefines affords quinazoline derivatives (III-VI) pyrimidine(Z)-dienols (II) formed by base-catalyzed isomerization. 相似文献
120.
Katsuyuki Ogura Hiroshi Sobue Shigeo Nakamura 《Journal of Polymer Science.Polymer Physics》1973,11(10):2079-2088
The glass transition in styrene-based ionomers was investigated by means of infrared spectroscopy and differential scanning calorimetry (DSC). Transition temperatures were determined from the temperature dependence of the peak absorbances of the 1700 and 1745 cm?1 bands. These transition temperatures agreed with glass transition temperatures (Tg) determined by DSC. With increasing degree of ionization, Tg and the enthalpy ΔH of the residual intermolecular hydrogen bonding increased. The values of Tg obtained were analyzed by the theory of Fox and Loshaek for the effect of crosslinks. It is concluded that sodium ions probably from ionic domains and act as crosslinks to reinforce the residual hydrogen bonding and may increase Tg. The absorbance at 1560 cm?1 (νCOO?) did not change at Tg. This suggests that the glass transition observed here is not due to the onset of the mobility in ionic domains, as has been proposed for ethylene-based ionomers on the basis of dielectric measurements. 相似文献