全文获取类型
收费全文 | 5063篇 |
免费 | 76篇 |
国内免费 | 17篇 |
专业分类
化学 | 2855篇 |
晶体学 | 22篇 |
力学 | 106篇 |
数学 | 533篇 |
物理学 | 1640篇 |
出版年
2020年 | 37篇 |
2019年 | 39篇 |
2018年 | 30篇 |
2016年 | 74篇 |
2015年 | 58篇 |
2014年 | 64篇 |
2013年 | 207篇 |
2012年 | 178篇 |
2011年 | 211篇 |
2010年 | 124篇 |
2009年 | 97篇 |
2008年 | 199篇 |
2007年 | 262篇 |
2006年 | 220篇 |
2005年 | 229篇 |
2004年 | 242篇 |
2003年 | 168篇 |
2002年 | 142篇 |
2001年 | 110篇 |
2000年 | 110篇 |
1999年 | 62篇 |
1998年 | 47篇 |
1997年 | 65篇 |
1996年 | 71篇 |
1995年 | 59篇 |
1994年 | 68篇 |
1993年 | 83篇 |
1992年 | 110篇 |
1991年 | 74篇 |
1990年 | 84篇 |
1989年 | 67篇 |
1988年 | 50篇 |
1987年 | 63篇 |
1986年 | 67篇 |
1985年 | 105篇 |
1984年 | 92篇 |
1983年 | 73篇 |
1982年 | 72篇 |
1981年 | 84篇 |
1980年 | 61篇 |
1979年 | 67篇 |
1978年 | 68篇 |
1977年 | 64篇 |
1976年 | 53篇 |
1975年 | 67篇 |
1974年 | 73篇 |
1973年 | 84篇 |
1972年 | 57篇 |
1971年 | 41篇 |
1970年 | 40篇 |
排序方式: 共有5156条查询结果,搜索用时 0 毫秒
991.
Moran AM Maddox JB Hong JW Kim J Nome RA Bazan GC Mukamel S Scherer NF 《The Journal of chemical physics》2006,124(19):194904
Optoelectronic properties of a polyphenylenevinylene-based oligomer and its paracylophane-linked dimer are studied using a variety of experimental and theoretical techniques. Despite the symmetrical structure and redshifted absorption of the dimer versus the monomer, an exciton picture is not the most appropriate. Electronic structure calculations establish changes in charge density upon optical excitation and show localized excitations that cannot be accounted for by a simple Frenkel exciton model. Visible frequency pump-probe anisotropy measurements suggest that the dimer should be considered as a three-level system with a fast, approximately 130 fs, internal conversion from the higher to lower energy excited electronic state. Signatures of nuclear relaxation processes are compared for electric field-resolved transient grating and two-dimensional photon echo spectra. These measurements reveal that nuclear relaxation occurs on similar time scales for the monomer and dimer. The connection between the spectral phase of four-wave mixing signals and the time dependent width of a nuclear wave packet is discussed. Semiempirical electronic structure and metropolis Monte Carlo calculations show that the dominant line broadening mechanisms for the monomer and dimer are associated with inter-ring torsional coordinates. Together, the theoretical calculations and electric field-resolved four-wave mixing experiments suggest that while the structure of dimer is more rigid than that of monomer, the difference in their rigidities is not sufficient to slow down excited state relaxation of dimer with respect to the monomer. 相似文献
992.
Neverov AA Lu ZL Maxwell CI Mohamed MF White CJ Tsang JS Brown RS 《Journal of the American Chemical Society》2006,128(50):16398-16405
The catalytic ability of a dinuclear Zn2+ complex of 1,3-bis-N1-(1,5,9-triazacyclododecyl)propane (3) in promoting the cleavage of an RNA model, 2-hydroxypropyl-p-nitrophenyl phosphate (HPNPP, 1), and a DNA model, methyl p-nitrophenyl phosphate (MNPP, 4), was studied in methanol solution in the presence of added CH3O- at 25 degrees C. The di-Zn2+ complex (Zn2 :3), in the presence of 1 equiv of added methoxide, exhibits a second-order rate constant of (2.75 +/- 0.10) x 10(5) M(-1) s(-1) for the reaction with 1 at s(s)pH 9.5, this being 10(8)-fold larger than the k2 value for the CH3O- promoted reaction (kOCH3 = (2.56 +/- 0.16) x 10(-3) M(-1) s(-1)). The complex is also active toward the DNA model 4, exhibiting Michaelis-Menten kinetics with a KM and kmax of 0.37 +/- 0.07 mM and (4.1 +/- 0.3) x 10(-2) s(-1), respectively. Relative to the background reactions at s(s)pH 9.5, Zn2 :3 accelerates cleavage of each phosphate diester by a remarkable factor of 1012-fold. A kinetic scheme common to both substrates is discussed. The study shows that a simple model system comprising a dinuclear Zn2+ complex and a medium effect of the alcohol solvent achieves a catalytic reactivity that approaches enzymatic rates and is well beyond anything seen to date in water for the cleavage of these phosphate diesters. 相似文献
993.
The 13C NMR resonance and one-bond 1H-13C coupling constants of coenzyme B12 enriched in 13C in the cobalt-bound carbon have been observed in the complex of the coenzyme with the B12-dependent ribonucleotide reductase from Lactobacillus leichmannii. Neither the 13C NMR chemical shift nor the 1H-13C coupling constants are significantly altered by binding of the coenzyme to the enzyme. The results suggest that ground-state Co-C bond distortion is not utilized by this enzyme to activate coenzyme B12 for C-Co bond homolysis. 相似文献
994.
995.
Koch KM Brown PB Rothman DL de Graaf RA 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2006,182(1):66-74
When homogenizing the static magnetic field over extended in vivo volumes, significant residual inhomogeneity can remain after spherical harmonic shim optimization. This is due to the low spatial orders of shims available on in vivo MR systems and the presence of higher-order inhomogeneity in the vicinity of anatomic air cavities. Mediation of this problem through the development of higher-order spherical harmonic shims is severely impeded by bore space limitations. Sample-specific passive shims are not limited to low-order spatial compensation and offer an alternative means to increased homogenization. Here, we present a novel construction protocol for sample-specific passive shims comprised of both diamagnetic (bismuth) and paramagnetic (zirconium) materials. A prototype shim is constructed and shown to significantly homogenize the mouse brain at 9.4 T. Further homogenization capabilities are simulated through alteration of the shim construction. 相似文献
996.
Pfordresher PQ Brown S Meier KM Belyk M Liotti M 《The Journal of the Acoustical Society of America》2010,128(4):2182-2190
There has been a recent surge of research on the topic of poor-pitch singing. However, this research has not addressed an important distinction in measurement: that between accuracy and precision. With respect to singing, accuracy refers to the average difference between sung and target pitches. Precision, by contrast, refers to the consistency of repeated attempts to produce a pitch. A group of 45 non-musician participants was asked to vocally imitate unfamiliar 5-note melodies, as well as to sing a series of familiar melodies from memory (e.g., Happy Birthday). The results showed that singers were more accurate than they were precise, and that a majority of participants could justifiably be categorized as imprecise singers. Accuracy and precision measures were correlated with one another, and conditional-probability analyses suggested that accuracy predicted precision more so than the converse. Finally, performance differences across groups of singers were greater for the imitation of unfamiliar tone sequences than for the recall of familiar melodies. 相似文献
997.
Udovydchenkov IA Rypina II Brown MG 《The Journal of the Acoustical Society of America》2010,127(5):EL185-EL191
The discrete form of the mode filtering problem is considered. The relevant equations constitute a linear inverse problem. Solutions to problems of this type are subject to a well-known trade-off between resolution and precision. But unlike the typical linear inverse problem, the correctly formulated mode filtering problem is subject to an energy conservation constraint. This letter focuses on the importance of satisfying, approximately at least, the energy conservation constraint when mode filtering is performed. 相似文献
998.
Keith A. Brown Daniel J. Eichelsdoerfer Xing Liao Shu He Chad A. Mirkin 《Frontiers of Physics》2014,9(3):385-397
Dip-pen na.nolithography (DPN) is a useful method for directly printing materials on surfaces with sub-50nm resolution. Because it, involves the physical transport of materials from a scanning probe tip to a surface and the subsequent chemical interaction of that material with the surface, there are many factors to consider when attempting to understand DPN. In this review, we overview the physical and chemical processes that are known to play a role in DPN, Through a detailed review of the literature, we classify inks into three general categories based on their transport properties, and highlight the myriad ways that. DPN can be used to perform chemistry at the tip of a scanning probe. 相似文献
999.
1000.
Daniel L. Reger Mark D. Smith Kenneth J. Brown 《Journal of chemical crystallography》2009,39(8):545-548
Abstract The cation in the solid state structure of [Fe{HC(3,5-Me2pz)3}2][Fe2OCl6] (pz = pyrazolyl ring) contains an octahedral iron(II) center with an average Fe–N bond distance of 2.17 Å, indicating that
the iron(II) is in the high-spin form. While M–N bond distances of this length with κ3-[HC(3,5-Me2pz)3] ligands generally cause tilting of pz to open up the “bite” angle, in this structure the average tilting angle is only 3.1°,
a much lower value than observed in earlier structures of this same cation paired with different anions. The crystals are
monoclinic, space group P21/n, with a = 11.1671(12) Å, b = 10.8091(11) Å, c = 17.4385(17) Å, α = γ = 90°, β = 95.685(2)°, and Z = 2.
Graphical Abstract The cation in the solid state structure of [Fe{HC(3,5-Me2pz)3}2][Fe2OCl6] (pz = pyrazolyl ring) contains an octahedral iron(II) center in the high-spin form with unusual orientations of the pyrazolyl
rings.
相似文献