全文获取类型
收费全文 | 4747篇 |
免费 | 154篇 |
国内免费 | 8篇 |
专业分类
化学 | 3459篇 |
晶体学 | 10篇 |
力学 | 119篇 |
数学 | 706篇 |
物理学 | 615篇 |
出版年
2020年 | 68篇 |
2019年 | 64篇 |
2018年 | 45篇 |
2017年 | 42篇 |
2016年 | 106篇 |
2015年 | 91篇 |
2014年 | 121篇 |
2013年 | 172篇 |
2012年 | 225篇 |
2011年 | 268篇 |
2010年 | 140篇 |
2009年 | 128篇 |
2008年 | 202篇 |
2007年 | 222篇 |
2006年 | 198篇 |
2005年 | 207篇 |
2004年 | 200篇 |
2003年 | 136篇 |
2002年 | 129篇 |
2001年 | 85篇 |
2000年 | 75篇 |
1999年 | 76篇 |
1998年 | 59篇 |
1997年 | 83篇 |
1996年 | 79篇 |
1995年 | 59篇 |
1994年 | 73篇 |
1993年 | 54篇 |
1992年 | 49篇 |
1991年 | 52篇 |
1990年 | 43篇 |
1989年 | 41篇 |
1988年 | 41篇 |
1987年 | 49篇 |
1986年 | 50篇 |
1985年 | 63篇 |
1984年 | 64篇 |
1983年 | 45篇 |
1982年 | 58篇 |
1981年 | 31篇 |
1980年 | 46篇 |
1979年 | 55篇 |
1978年 | 40篇 |
1977年 | 46篇 |
1976年 | 37篇 |
1975年 | 29篇 |
1974年 | 36篇 |
1972年 | 30篇 |
1968年 | 292篇 |
1967年 | 92篇 |
排序方式: 共有4909条查询结果,搜索用时 15 毫秒
991.
Aldol additions of unprotected carbohydrates to 1.3-dicarbonyl compounds have been described. This transformation is based on a dual activation by tertiary amines and 2-hydroxypyridine. 相似文献
992.
A graph is called t-perfect, if its stable set polytope is defined by non-negativity, edge and odd-cycle inequalities. We characterise the class of all claw-free t-perfect graphs by forbidden t-minors, and show that they are 3-colourable. Moreover, we determine the chromatic number of claw-free h-perfect graphs and give a polynomial-time algorithm to compute an optimal colouring. 相似文献
993.
We provide a criterion for the existence of right approximations in cocomplete additive categories; it is a straightforward generalisation of a result due to El Bashir. This criterion is used to construct adjoint functors in homotopy categories. Applications include the study of (pure) derived categories. For instance, it is shown that the pure derived category of any module category is compactly generated. 相似文献
994.
995.
Alexander M. Breul Christian Pietsch Roberto Menzel Johann Schäfer Anke Teichler Martin D. Hager Jürgen Popp Benjamin Dietzek Rainer Beckert Ulrich S. Schubert 《European Polymer Journal》2012,48(7):1339-1347
Blue emitting dyes bearing a luciferin analogous chromophore were attached to a polystyrene backbone. For this purpose, 4-hydroxy-1,3-thiazoles were functionalized with a styrene unit and polymerized using the reversible addition–fragmentation chain transfer (RAFT) polymerization technique. Two different chain transfer agents were investigated and one monomer was studied in terms of its kinetic behavior. The polymerization kinetics are presented and discussed in detail, showing a controlled polymerization behavior, resulting in well-defined copolymers with polydispersity indices below 1.2. The obtained polymers were characterized by size exclusion chromatography (SEC), 1H NMR, MALDI-TOF MS and UV–vis absorption and fluorescence spectroscopy. In addition, the UV–vis absorption and emission behavior was investigated in thin films. 相似文献
996.
Masaaki Yoshifuji Yuichi Hirano Gregor Schnakenburg Rainer Streubel Edgar Niecke Shigekazu Ito 《Helvetica chimica acta》2012,95(10):1723-1729
1,3‐Di(tert‐butyl)‐2,4‐bis[2,4,6‐tri(tert‐butyl)phenyl]‐1,3‐diphosphacyclobutane‐2,4‐diyl was formed from [2,4,6‐tri(tert‐butyl)phenyl]phosphaacetylene and t‐BuLi. In addition, the X‐ray diffraction analysis was carried out, together with theoretical calculations of the structure and NMR data. 相似文献
997.
The indides Eu2Pd2In and Eu2Pt2In were synthesized from the elements in sealed tantalum tubes in an induction furnace. The samples were characterized by powder X‐ray diffraction. The structures were refined on the basis of single‐crystal X‐ray diffractometer data: HT‐Pr2Co2Al type, C2/c, a = 1035.7(2), b = 592.9(1), c = 823.6(2) pm, β = 104.26(1) °, wR2 = 0.026, 1075 F2 values, 25 variables for Eu2Pd2In and a = 1017.2(2), b = 588.7(1), c = 826.5(1) pm, β = 103.76(1) °, wR2 = 0.062, 706 F2 values, 25 variables for Eu2Pt2In. The indium atoms have four platinum (palladium) neighbors in strongly distorted tetrahedral coordination at Pt–In and Pd–In distances ranging from 273 to 275 pm. These InPd4/2 and InPt4/2 units are condensed via common edges to infinite InPd2 and InPt2 chains, which are surrounded by the europium atoms. The chains form the motif of hexagonal rod packing. 相似文献
998.
Complexes of type C6H7(+)·L (L = N2 and CO2) studied by explicitly correlated coupled cluster theory
Complexes of the benzenium ion (C(6)H(7)(+)) with N(2) or CO(2) have been studied by explicitly correlated coupled cluster theory at the CCSD(T)-F12x (x = a, b) level [T. B. Adler et al., J. Chem. Phys. 127, 221106 (2007)] and the double-hybrid density functional B2PLYP-D [T. Schwabe and S. Grimme, Phys. Chem. Chem. Phys. 9, 3397 (2007)]. Improved harmonic vibrational wavenumbers for C(6)H(7)(+) have been obtained by CCSD(T?)-F12a calculations with the VTZ-F12 basis set. Combining them with previous B2PLYP-D anharmonic contributions we arrive at anharmonic wavenumbers which are in excellent agreement with recent experimental data from p-H(2) matrix isolation IR spectroscopy [M. Bahou et al., J. Chem. Phys. 136, 154304 (2012)]. The energetically most favourable conformer of C(6)H(7)(+)·N(2) shows a π-bonded structure similar to C(6)H(7)(+)·Rg (Rg = Ne, Ar) [P. Botschwina and R. Oswald, J. Phys. Chem. A 115, 13664 (2011)] with D(e) ≈ 870 cm(-1). For C(6)H(7)(+)·CO(2), a slightly lower energy is calculated for a conformer with the CO(2) ligand lying in the ring-plane of the C(6)H(7)(+) moiety (D(e) ≈ 1508 cm(-1)). It may be discriminated from other conformers through a strong band predicted at 1218 cm(-1), red-shifted by 21 cm(-1) from the corresponding band of free C(6)H(7)(+). 相似文献
999.
The molybdenum(V) complex [Mo(O)Cl(3)dppe] [dppe = 1,2-bis(diphenylphosphino)ethane] is considered as a model system for a combined study of the electronic structure using UV/vis absorption and magnetic circular dichroism (MCD) spectroscopy. In order to determine the signs and MCD C-term intensities of the chlorido → molybdenum charge-transfer transitions, it is necessary to take the splitting of the excited doublet states into sing-doublet and trip-doublet states into account. While transitions to the sing-doublet states are electric-dipole-allowed, those to the trip-doublet states are electric-dipole-forbidden. As spin-orbit coupling within the manifold of sing-doublet states vanishes, configuration interaction between the sing-doublet and trip-doublet states is required to generate the MCD C-term intensity. The most prominent feature in the MCD spectrum of [Mo(O)Cl(3)dppe] is a "double pseudo-A term", which consists of two corresponding pseudo-A terms centered at 27000 and 32500 cm(-1). These are assigned to the ligand-to-metal charge-transfer transitions from the p(π) orbitals of the equatorial chlorido ligands to the Mo d(yz) and d(xz) orbitals. On the basis of the theoretical expressions developed by Neese and Solomon (Inorg. Chem. 1999, 38, 1847-1865), a general treatment of the MCD C-term intensity of these transitions is presented that explicitly considers the multideterminant character of the excited states. The individual MCD signs are determined from the corresponding transition densities derived from the calculated molecular orbitals of the title complex (BP86/LANL2DZ). 相似文献
1000.
Makal A Benedict J Trzop E Sokolow J Fournier B Chen Y Kalinowski JA Graber T Henning R Coppens P 《The journal of physical chemistry. A》2012,116(13):3359-3365
The excited-state structure of [Cu(I)[(1,10-phenanthroline-N,N') bis(triphenylphosphine)] cations in their crystalline [BF(4)] salt has been determined at both 180 and 90 K by single-pulse time-resolved synchrotron experiments with the modified polychromatic Laue method. The two independent molecules in the crystal show distortions on MLCT excitation that differ in magnitude and direction, a difference attributed to a pronounced difference in the molecular environment of the two complexes. As the excited states differ, the decay of the emission is biexponential with two strongly different lifetimes, the longer lifetime, assigned to the more restricted molecule, becoming more prevalent as the temperature increases. Standard deviations in the current Laue study are very much lower than those achieved in a previous monochromatic study of a Cu(I) 2,9-dimethylphenanthroline substituted complex ( J. Am. Chem. Soc. 2009 , 131 , 6566 ), but the magnitudes of the shifts on excitation are similar, indicating that lattice restrictions dominate over the steric effect of the methyl substitution. Above all, the study illustrates emphatically that molecules in solids have physical properties different from those of isolated molecules and that their properties depend on the specific molecular environment. This conclusion is relevant for the understanding of the properties of molecular solid-state devices, which are increasingly used in current technology. 相似文献