全文获取类型
收费全文 | 604篇 |
免费 | 58篇 |
国内免费 | 58篇 |
专业分类
化学 | 419篇 |
晶体学 | 6篇 |
力学 | 57篇 |
综合类 | 1篇 |
数学 | 38篇 |
物理学 | 199篇 |
出版年
2024年 | 4篇 |
2023年 | 7篇 |
2022年 | 21篇 |
2021年 | 13篇 |
2020年 | 31篇 |
2019年 | 16篇 |
2018年 | 16篇 |
2017年 | 10篇 |
2016年 | 28篇 |
2015年 | 26篇 |
2014年 | 28篇 |
2013年 | 36篇 |
2012年 | 42篇 |
2011年 | 65篇 |
2010年 | 36篇 |
2009年 | 34篇 |
2008年 | 47篇 |
2007年 | 44篇 |
2006年 | 30篇 |
2005年 | 27篇 |
2004年 | 19篇 |
2003年 | 11篇 |
2002年 | 24篇 |
2001年 | 14篇 |
2000年 | 6篇 |
1999年 | 5篇 |
1998年 | 13篇 |
1997年 | 3篇 |
1996年 | 7篇 |
1995年 | 3篇 |
1993年 | 1篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 7篇 |
1988年 | 8篇 |
1987年 | 7篇 |
1986年 | 6篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1979年 | 2篇 |
1977年 | 3篇 |
1975年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有720条查询结果,搜索用时 15 毫秒
191.
In an attempt to synthesize a new pyrimidine complex of copper(H) in a solution reaction of CuBr2 with 2,2‘-bis(hexahydropydmidine) and hydro-chloric acid, we unexpectedly obtained a bright yellow chip-like crystal of CuCl4(H2tn) [H2tn=(H3NCH2CH2CH2NH3)^2 ]. Its structure was determined by single-crystal X-ray diffraction analysis. The crystal belongs to orthorhombic system, space group Pnma, with cell parameters: a=0.7216(2)nm, b=1.8308(6)nm, c=0.7553(3)nm, V=0.9953(6)nm^3, Z=4, F(000)=564, Mr=281.49, Dc=1.879 g/cm^3. u(Mo Ka)=3.204 mm^-1, R1=0.0248, wR2=0.0575. The analysis of the crystal structure indicates that the complex has a three-dimensional network structure, which is formed by hydrogen bonds and electrostatic interaction. 相似文献
192.
Lewis FD Zhang L Liu X Zuo X Tiede DM Long H Schatz GC 《Journal of the American Chemical Society》2005,127(41):14445-14453
The structure and properties of oligonucleotide conjugates possessing stilbenedicarboxamide chromophores at both ends of a poly(dA):poly(dT) base-pair domain of variable length have been investigated using a combination of spectroscopic and computational methods. These conjugates form capped hairpin structures in which one stilbene serves as a hairpin linker and the other as a hydrophobic end-cap. The capping stilbene stabilizes the hairpin structures by ca. 2 kcal/mol, making possible the formation of a stable folded structure containing a single A:T base pair. Exciton coupling between the stilbene chromophores has little effect on the absorption bands of capped hairpins. However, exciton-coupled circular dichroism (EC-CD) can be observed for capped hairpins possessing as many as 11 base pairs. Both the sign and intensity of the EC-CD spectrum are sensitive to the number of base pairs separating the stilbene chromophores, as a consequence of the distance and angular dependence of exciton coupling. Calculated spectra obtained using a static vector model based on canonical B-DNA are in good agreement with the experimental spectra. Molecular dynamics simulations show that conformational fluctuations of the capped hairpins result in large deviations of the averaged spectra in both the positive and negative directions. These results demonstrate for the first time the ability of B-DNA to serve as a helical ruler for the study of electronic interactions between aligned chromophores. Furthermore, they provide important tests for atomistic theoretical models of DNA. 相似文献
193.
Lewis FD Wu Y Zhang L Zuo X Hayes RT Wasielewski MR 《Journal of the American Chemical Society》2004,126(26):8206-8215
The synthesis, steady-state spectroscopy, and transient absorption spectroscopy of DNA conjugates possessing both stilbene electron donor and electron acceptor chromophores are described. These conjugates are proposed to form nicked DNA dumbbell structures in which a stilbenedicarboxamide acceptor and stilbenediether donor are separated by variable numbers of A-T or G-C base pairs. The nick is located either adjacent to one of the chromophores or between two of the bases. Thermal dissociation profiles indicate that stable structures are formed possessing as few as two A-T base pairs. Circular dichroism (CD) spectra in the base pair region are characteristic of B-DNA duplex structures, whereas CD spectra at longer wavelengths display two bands attributed to exciton coupling between the two stilbenes. The sign and intensity of these bands are dependent upon both the distance between the chromophores and the dihedral angle between their transition dipoles [Deltaepsilon approximately Rda(-2) sin(2theta)]. Pulsed laser excitation of the stilbenediamide results in creation of the acceptor-donor radical ion pair, which decays via charge recombination. The dynamics of charge separation and charge recombination display an exponential distance dependence, similar to that observed previously for systems in which guanine serves as the electron donor. Unlike exciton coupling between the stilbenes, there is no apparent dependence of the charge-transfer rates upon the dihedral angle between donor and acceptor stilbenes. The introduction of a single G-C base pair between the donor and acceptor results in a change in the mechanism for charge separation from single step superexchange to hole hopping. 相似文献
194.
论弹性波绕射与动应力集中问题 总被引:7,自引:1,他引:6
本文综述了弹性波绕射与动应力集中问题的复变函数解法,给出了单连通问题、多连通问题及多裂纹问题的具体求解过程及算式. 相似文献
195.
196.
利用Guassian94程序和QCISD/6-311++G方法得到He-HF和He-FH两种线性平衡结构,计算出了这两种弱结合分子的结构参数、离解能和二阶力常数。在此基础上,利用多体项展式方法民出He-HF(X^1Σ^+)体系的分析势能函数,得到的势能面准确地再现了He-HF和He-FH两种线性结构。 相似文献
197.
198.
论半无限长杆对有限长梁的横向弹性冲击问题 总被引:3,自引:0,他引:3
研究了半无限长弹性杆对有限长弹性梁的横向冲击问题,采用小高——中原对受冲击接触面的准静态处理方法,给出了描述这一问题的基本方程,并用Laplace变换方法对它进行了求解,文中通过数值的Durbin反演结果展示了杆梁间冲击问题的一些特点 相似文献
199.
为了评估鲁西南沿黄县区(东明县、牡丹区、鄄城县、郓城县)玉米中铅、镉、砷、铬重金属的污染风险,采集该地区玉米果穗,利用电感耦合等离子体质谱(ICP-MS)仪测定其玉米籽粒中铅、镉、砷、铬的含量,并使用单因子污染指数法、内梅罗综合多因子指数法和目标危险系数法开展玉米籽粒重金属污染评价和健康风险评价。结果表明,鲁西南沿黄县区玉米籽粒中重金属铅、镉、砷、铬的含量均未超出GB 2762—2017中规定的限量,重金属含量以Cr最高;鲁西南沿黄县区P综和TTHQ均小于1,说明玉米籽粒中的重金属元素含量为清洁安全等级,食用该地区玉米对人体的健康风险较低。 相似文献
200.