全文获取类型
收费全文 | 4536篇 |
免费 | 76篇 |
国内免费 | 34篇 |
专业分类
化学 | 2783篇 |
晶体学 | 38篇 |
力学 | 76篇 |
数学 | 467篇 |
物理学 | 1282篇 |
出版年
2019年 | 34篇 |
2018年 | 21篇 |
2017年 | 24篇 |
2016年 | 67篇 |
2015年 | 43篇 |
2014年 | 57篇 |
2013年 | 183篇 |
2012年 | 179篇 |
2011年 | 227篇 |
2010年 | 95篇 |
2009年 | 87篇 |
2008年 | 207篇 |
2007年 | 223篇 |
2006年 | 201篇 |
2005年 | 201篇 |
2004年 | 190篇 |
2003年 | 167篇 |
2002年 | 151篇 |
2001年 | 139篇 |
2000年 | 127篇 |
1999年 | 67篇 |
1998年 | 61篇 |
1997年 | 47篇 |
1996年 | 89篇 |
1995年 | 72篇 |
1994年 | 94篇 |
1993年 | 104篇 |
1992年 | 99篇 |
1991年 | 58篇 |
1990年 | 56篇 |
1989年 | 80篇 |
1988年 | 56篇 |
1987年 | 51篇 |
1986年 | 59篇 |
1985年 | 73篇 |
1984年 | 56篇 |
1983年 | 34篇 |
1982年 | 56篇 |
1981年 | 53篇 |
1980年 | 57篇 |
1979年 | 49篇 |
1978年 | 55篇 |
1977年 | 66篇 |
1976年 | 44篇 |
1975年 | 43篇 |
1974年 | 50篇 |
1973年 | 49篇 |
1972年 | 31篇 |
1971年 | 26篇 |
1970年 | 24篇 |
排序方式: 共有4646条查询结果,搜索用时 46 毫秒
991.
Abachi S Derrick M Kooijman P Musgrave B Price L Repond J Sugano K Blockus D Brabson BB Brom J Jung C Ogren H Rust DR Snyder A Cork B Akerlof C Chapman J Errede D Ken MT Kesten P Meyer DI Neal H Nitz D Thun R Tschirhart R Baringer P Bylsma BG Debonte R Low EH McIlwain RL Miller DH Ng CR Rangan K Shibata E 《Physical review D: Particles and fields》1990,41(7):2045-2056
992.
Bowcock T Kinoshita K Pipkin FM Procario M Wilson R Wolinski J Xiao D Ammar R Baringer P Coppage D Haas P Lam H Jawahery A Park CH Kubota Y Nelson JK Perticone D Poling R Chen W Dominick J McIlwain RL Miller DH Ng CR Schaffner SF Shibata EI Yao W Sparks K Thorndike EH Alam MS Kim IJ Li WC Lou XC Sun CR Fulton R Hempstead M Jensen T Johnson DR Kagan H Kass R Morrow F Whitmore J Bortoletto D Goldberg M Horwitz N Mestayer MD Moneti GC Sharma V Shipsey IP Skwarnicki T Csorna SE Letson T Alexander J 《Physical review D: Particles and fields》1990,41(3):805-808
993.
Summary Thep-tert-butylcalix[n]arenes (n=4,6,7,8) andp-tert-butyldihomooxacalix[4]arene have been separated by reverse phase liquid chromatography. Optimum conditions have been obtained on a Spherisorb ODS1, 5 m C18 column by isocratic ambient elution with acetonitrile-methyltert-butyl ether. Calibration plots have been obtained from purified calixarenes and the reliability of the method is confirmed from test mixtures of calixarenes of known composition. 相似文献
994.
9,10-Phenanthrenequinone and acenaphthenequinone are shown to act as simple redox-dependent receptors toward aromatic ureas in CH(2)Cl(2) and DMF. Reduction of the o-quinones to their radical anions greatly increases the strength of hydrogen bonding between the quinone carbonyl oxygens and the urea N-hydrogens. This is detected by large positive shifts in the redox potential of the quinones with no change in electrochemical reversibility upon addition of urea guests. Cyclic voltammetric studies with a variety of possible guests show that the effect is quite selective. Only guests with two strong hydrogen donors, such as O-H bonds or amide N-H bonds, that are capable of simultaneously interacting with both carbonyl oxygens give large shifts in the redox potential of the quinones. The electronic character and conformational preference of the guest are also shown to significantly affect the magnitude of the observed potential shift. In the presence of strong proton donors the electrochemistry of the quinone becomes irreversible indicating that proton transfer has taken place. Experiments with compounds of different acidity show that the pK(a) of the protonated quinone radical is about 15 on the DMSO scale, >4 pK(a) units smaller than that of 1,3-diphenylurea. This is further proof that hydrogen bonding and not proton transfer is responsible for the large potential shifts observed with this and similar guests. 相似文献
995.
We have used the ring-polymer molecular dynamics method to calculate approximate Kubo-transformed velocity autocorrelation functions and self-diffusion coefficients for low-pressure liquid para-hydrogen at temperatures of 25 and 14 K. The resulting diffusion coefficients are shown to be consistent with experimental shear viscosities and the established finite-size relation D(L) approximately = D(infinity)-2.837k(B)T6pietaL, where k(B) is the Boltzmann constant, T the absolute temperature, eta the shear viscosity, and L the length of the (cubic) simulation cell. The diffusion coefficients D(L) obtained in simulations with finite system sizes are therefore too small. However, the extrapolation to infinite system size corrects this deficiency and leads to excellent agreement with experimental results. This both demonstrates the influence of system-size effects on quantum mechanical diffusion coefficients and provides further evidence that ring-polymer molecular dynamics is an accurate as well as practical way of including quantum effects in condensed phase molecular dynamics. 相似文献
996.
Pappenfus TM Chesterfield RJ Frisbie CD Mann KR Casado J Raff JD Miller LL 《Journal of the American Chemical Society》2002,124(16):4184-4185
A terthiophene-based quinodimethane, 3',4'-dibutyl-5,5' '-bis(dicyanomethylene)-5,5' '-dihydro-2,2':5',2' '-terthiophene (1) was synthesized and crystallized. Compound 1 has a planar quinoid geometry that is stabilized by dicyanomethylene groups at each end of the molecule. In the crystal each molecule is part of a dimerized face-to-face pi-stack, with intermolecular spacings of 3.47 and 3.63 A, respectively. Cyclic voltammetry showed that 1 could be reversibly reduced and oxidized in methylene chloride solution. Thin film transistors (TFTs) were prepared by vacuum evaporation of 1 onto SiO2(300 nm)/Si substrates, followed by evaporation of Ag source and drain contacts. The doped Si substrate served as the gate electrode. X-ray diffraction and atomic force microscopy indicate the films are polycrystalline, with the long axes of the molecules approximately perpendicular to the substrate. The TFT measurements revealed n-channel conduction in films of 1, with room-temperature electron field effect mobilities as high as 0.005 cm2/Vs. The butyl side chains give 1 appreciable solubility in a range of common solvents, and preliminary TFT results on films cast from chlorobenzene show electron mobility as high as 0.002 cm2/Vs. These results indicate that pi-stacked quinoidal thiophene oligomers are a promising new class of soluble n-channel organic semiconductors. 相似文献
997.
[reaction: see text] Highly regioselective, catalytic asymmetric reductive coupling reactions of 1,3-enynes and ketones have been achieved using catalytic amounts of Ni(cod)(2) and a P-chiral, monodentate ferrocenyl phosphine ligand. These couplings represent the first examples of catalytic, intermolecular reductive coupling of alkynes and ketones, enantioselective or otherwise, and afford synthetically useful 1,3-dienes possessing a quaternary carbinol stereogenic center in up to 70% ee. 相似文献
998.
The crystal structure of cobalt orthophosphate has been refined by full-matrix least-squares procedures using automatic diffractometer data to a residual R = 0.039 (Rw = 0.058). The space group is , with a = 5.063(1), b = 8.361(2), c = 8.788(2) Å, and β = 121.00(2)°. Co3(PO4)2 is isotypic with the previously reported γ-Zn3(PO4)2 and Mg3(PO4)2. Cobalt ions occupy two distinct coordination polyhedra, one five and one six-coordinated, in a ratio of two to one. The structure is described in detail. 相似文献
999.
Miller JH Fan-Chiang CC Straatsma TP Kennedy MA 《Journal of the American Chemical Society》2003,125(20):6331-6336
Molecular dynamics (MD) simulations were carried out on the DNA oligonucleotide GGGAACAACTAG:CTAGTTGTTCCC in its native form and with guanine in the central G(19):C(6) base pair replaced by 8-oxoguanine (8oxoG). A box of explicit water molecules was used for solvation, and Na(+) counterions were added to neutralize the system. The direction and magnitude of global bending were assessed by a technique used previously to analyze simulations of DNA containing a thymine dimer. The presence of 8oxoG did not greatly affect the magnitude of DNA bending; however, bending into the major groove was significantly more probable when 8oxoG replaced G(19). Crystal structures of glycosylases bound to damaged-DNA substrates consistently show a sharp bend into the major groove at the damage site. We conclude that changes in bending dynamics that assist the formation of this kink are a part of the mechanism by which glycosylases of the base excision repair pathway recognize the presence of 8oxoG in DNA. 相似文献
1000.
Yuyuan Xiao Xiao-Kun Zhao Tianpin Wu Jeffrey T. Miller Han-Shi Hu Jun Li Wenliang Huang Paula L. Diaconescu 《Chemical science》2021,12(1):227
Inverse-sandwich samarium and ytterbium biphenyl complexes were synthesized by the reduction of their trivalent halide precursors with potassium graphite in the presence of biphenyl. While the samarium complex had a similar structure as previously reported rare earth metal biphenyl complexes, with the two samarium ions bound to the same phenyl ring, the ytterbium counterpart adopted a different structure, with the two ytterbium ions bound to different phenyl rings. Upon the addition of crown ether to encapsulate the potassium ions, the inverse-sandwich samarium biphenyl structure remained intact; however, the ytterbium biphenyl structure fell apart with the concomitant formation of a divalent ytterbium crown ether complex and potassium biphenylide. Spectroscopic and computational studies were performed to gain insight into the electronic structures and bonding interactions of these samarium and ytterbium biphenyl complexes. While the ytterbium ions were found to be divalent with a 4f14 electron configuration and form a primarily ionic bonding interaction with biphenyl dianion, the samarium ions were in the trivalent state with a 4f5 electron configuration and mainly utilized the 5d orbitals to form a δ-type bonding interaction with the π* orbitals of the biphenyl tetraanion, showing covalent character.Inverse-sandwich samarium and ytterbium biphenyl complexes were synthesized and characterized by X-ray crystallography. Combined experimental and computational studies indicated that they have distinct electronic structures and bonding interactions. 相似文献