全文获取类型
收费全文 | 49257篇 |
免费 | 5069篇 |
国内免费 | 10663篇 |
专业分类
化学 | 45700篇 |
晶体学 | 1876篇 |
力学 | 628篇 |
综合类 | 425篇 |
数学 | 3317篇 |
物理学 | 13043篇 |
出版年
2024年 | 103篇 |
2023年 | 921篇 |
2022年 | 893篇 |
2021年 | 1791篇 |
2020年 | 1842篇 |
2019年 | 1667篇 |
2018年 | 1372篇 |
2017年 | 1741篇 |
2016年 | 1825篇 |
2015年 | 1593篇 |
2014年 | 2131篇 |
2013年 | 4608篇 |
2012年 | 3200篇 |
2011年 | 3452篇 |
2010年 | 2957篇 |
2009年 | 3578篇 |
2008年 | 3406篇 |
2007年 | 3482篇 |
2006年 | 3296篇 |
2005年 | 2878篇 |
2004年 | 2739篇 |
2003年 | 2284篇 |
2002年 | 1900篇 |
2001年 | 1552篇 |
2000年 | 1451篇 |
1999年 | 1174篇 |
1998年 | 1000篇 |
1997年 | 864篇 |
1996年 | 760篇 |
1995年 | 771篇 |
1994年 | 683篇 |
1993年 | 546篇 |
1992年 | 503篇 |
1991年 | 387篇 |
1990年 | 262篇 |
1989年 | 226篇 |
1988年 | 193篇 |
1987年 | 132篇 |
1986年 | 98篇 |
1985年 | 113篇 |
1984年 | 90篇 |
1983年 | 36篇 |
1982年 | 67篇 |
1981年 | 91篇 |
1980年 | 64篇 |
1979年 | 67篇 |
1978年 | 45篇 |
1977年 | 50篇 |
1976年 | 30篇 |
1973年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
951.
Hiroyuki Asanuma Sumie Gotoh Takeshi Ban Makoto Komiyama 《Journal of inclusion phenomena and macrocyclic chemistry》1997,27(3):259-264
Poly(2-vinyl-4,6-diamino-1,3,5-triazine) efficiently binds nucleic acid bases and nucleosides in water by using complementary hydrogen bonding. The binding activity decreases in the order: U, T > A C, G. The corresponding monomer shows virtually no activity, indicating a predominant role of polymer effect for the molecular recognition in water. 相似文献
952.
Afarin Moezzi Marylin M. Olmstead Doris C. Pestana Philip P. Power 《无机化学与普通化学杂志》1995,621(11):1933-1938
The reaction of either Li2PPh or Li2AsPh with the diborane(4) derivative B2(NMe2)2Br2 affords the compounds [PhP(BNMe2)2]2 ( 1 ) or [PhAs(BNMe2)2]2 ( 2 ) in good yield. Both 1 and 2 have cyclic structures featuring non-planar P2B4 or As2B4 six-membered rings which have chair configurations. Although all four borons in each ring have planar coordination, the two phosphorus or arsenic centers have different degrees of pyramidalization. Bond distances within the rings indicate that the B? B, B? P or B? As bonds are single, whereas the exo-B? N bond lengths are consistent with significant π-bonding. The ring structures of 1 and 2 are in sharp contrast to the related boron-nitrogen species (t-BuN)2N4Me4 which has a nido-N2B4 framework. The attempted synthesis of the nitrogen analogue of 1 or 2 by using a similar approach did not result in the isolation of [PhN(BNMe2)2]2, instead the tetramino diborane(4) species [B(NMe2)NHPh]2 ( 3 ), which has a structure similar to other tetramine diborane(4) compounds, was isolated. 相似文献
953.
E. A. Khamidullina A. S. Gromova V. I. Lutsky S. B. Zinchenko A. A. Semenov 《Russian Chemical Bulletin》1996,45(6):1476-1480
From the above-ground part ofThalictrum squarrosum St. ex Willd. (Ranunculaccae), an equilibrium mixture of two triterpene glycosides, squarrosidc B3 (1), 3-O-[O-(-L-rhamno-pyranosyl)-(16)--D-glucopyranosyl]-21 (S),22(),23(R)-3,21,22,30-tctralmydroxy-21, 23-cpoxycycloartlt-24-ene, and squarrosidc B4 (2), its 21(R) epimer, was isolated. Their structures were established on the basis of IR and 2D NMR spectra and FAB mass spectra.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1547–1551, June, 1996.These signals coincide for 5 and 6. 相似文献
954.
A hybrid approach combining energy density analysis with the interaction energy decomposition method
We propose a new analysis technique for characterizing molecular interactions that combines an energy decomposition scheme, such as the Kitaura-Morokuma decomposition method, with energy density analysis, which partitions the total energy of the system into atomic contributions. The combined scheme, termed Interaction-EDA, enables us to estimate the local contribution of interaction energy components, such as electrostatic, exchange, polarization, and charge transfer. The evaluation of the local interaction energy is rather important in large systems. For a numerical assessment, the Interaction-EDA method is applied to the process of CO adsorption on Si(100) - (2 x 1) surface. 相似文献
955.
CCl2自由基与H2O分子反应动力学研究 总被引:2,自引:0,他引:2
用213 nm激光光解CCl4产生CCl2自由基,用LP LIF技术测定了室温下基态CCl2自由基与H2O分子的反应速率常数为(5.45±0.95)×10-14 cm3•molecule-1•s-1.在G2MP2理论水平上计算了CCl2+H2O反应的最低单重态势能面,揭示了插入与加成 消除两种反应机理,得到了三个可能的产物通道:HCl+HClCO、HCl+trans ClCOH以及HCl+cis ClCOH.并用RRKM TST和传统过渡态理论计算了这三个通道的分支比及其温度效应.结果说明在低温下(273 K),插入机理的产物通道的分支比远大于加成 消除机理的产物通道, HCl+HClCO是主要产物,分支比为77.4%,其次是HCl+cis ClCOH,分支比为22.6%.而在高温下(3000 K),加成 消除机理的反应通道大于插入机理, HCl+trans ClCOH分支比为82.3%. 相似文献
956.
Perczel A Jákli I McAllister MA Csizmadia IG 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(11):2551-2566
Folding properties of small globular proteins are determined by their amino acid sequence (primary structure). This holds both for local (secondary structure) and for global conformational features of linear polypeptides and proteins composed from natural amino acid derivatives. It thus provides the rational basis of structure prediction algorithms. The shortest secondary structure element, the beta-turn, most typically adopts either a type I or a type II form, depending on the amino acid composition. Herein we investigate the sequence-dependent folding stability of both major types of beta-turns using simple dipeptide models (-Xxx-Yyy-). Gas-phase ab initio properties of 16 carefully selected and suitably protected dipeptide models (for example Val-Ser, Ala-Gly, Ser-Ser) were studied. For each backbone fold most probable side-chain conformers were considered. Fully optimized 321G RHF molecular structures were employed in medium level [B3LYP/6-311++G(d,p)//RHF/3-21G] energy calculations to estimate relative populations of the different backbone conformers. Our results show that the preference for beta-turn forms as calculated by quantum mechanics and observed in Xray determined proteins correlates significantly. 相似文献
957.
采用浸渍法,制备了用于合成吲哚的Cu/SiO2催化剂,研究了不同载体和活性组分Cu对苯胺和乙二醇一步合成吲哚反应的催化活性,发现γ-Al2O3、活性炭、Na-Y,SiO2-MgO、MCM41、SiO26种载体中,SiO2是最好的载体;催化剂活性组分Cu含量为0.78mmol/gSiO2时,吲哚的收率高达88%,考察了水蒸汽、氢气、反应温度及接触时间等因素对反应性能的影响,并对催化剂进行了TG测试,得到了Cu/Sio2催化剂催化合成吲哚适应反应条件。 相似文献
958.
Ti2(OMe)4/SiO2催化剂的制备及其合成碳酸二甲酯的反应性能 总被引:4,自引:2,他引:4
采用表面改性和离子交换法制备了SiO2负载的Ti2(OMe)4双核桥联配合物催化剂,用IR、TPD和微反技术考察了催化剂的表面构造及CO2和CH3OH在催化剂表面上的化学吸附和反应性能。结果表明:双核桥联配合物Ti2(OMe)4以Ti-O-Si键锚定在SiO2表面上;CO2在催化剂表面存在桥式和甲氧碳酸酯基两种吸附态,其中甲氧碳酸酯基吸附态是生成DMC的关键物种;CH3OH在催化剂上只有一种分子吸附态。在150℃以下,CO2和CH3OH在Ti2(OMe)4/SiO2催化剂表面上高选择地生成碳酸二甲酯。 相似文献
959.
960.
用光电流作用谱、光电流-电势图等光电化学方法研究了ITO(铟锡氧化物导电玻璃)/TiO2 /Q-PbS(量子点PbS)膜电极的光电转换性质。结果表明,由于量子尺寸的效应,在膜电极制备中,随着ITO/TiO2电极在饱和Pb(CH3COO)2溶液中浸泡时间的不同,所制备的Q-PbS颗粒大小不同,禁带宽度随着浸泡时间的增大而减小,浸泡时间为40 s、在80 ℃烘干下制备的Q-PbS的禁带宽度为1.68 eV,其价带位置为-5.072 eV。Q-PbS修饰ITO/TiO2电极可使光电流发生明显的红移,从而提高宽禁带半导体的光电转换效率。 相似文献