全文获取类型
收费全文 | 4903篇 |
免费 | 85篇 |
国内免费 | 5篇 |
专业分类
化学 | 3490篇 |
晶体学 | 20篇 |
力学 | 108篇 |
数学 | 647篇 |
物理学 | 728篇 |
出版年
2018年 | 46篇 |
2017年 | 33篇 |
2016年 | 65篇 |
2015年 | 60篇 |
2014年 | 65篇 |
2013年 | 169篇 |
2012年 | 137篇 |
2011年 | 178篇 |
2010年 | 136篇 |
2009年 | 117篇 |
2008年 | 170篇 |
2007年 | 188篇 |
2006年 | 172篇 |
2005年 | 186篇 |
2004年 | 163篇 |
2003年 | 154篇 |
2002年 | 134篇 |
2001年 | 93篇 |
2000年 | 108篇 |
1999年 | 82篇 |
1998年 | 71篇 |
1997年 | 68篇 |
1996年 | 80篇 |
1995年 | 80篇 |
1994年 | 69篇 |
1993年 | 98篇 |
1992年 | 75篇 |
1991年 | 65篇 |
1990年 | 54篇 |
1989年 | 56篇 |
1988年 | 59篇 |
1987年 | 57篇 |
1986年 | 66篇 |
1985年 | 91篇 |
1984年 | 65篇 |
1983年 | 50篇 |
1982年 | 62篇 |
1981年 | 66篇 |
1980年 | 58篇 |
1979年 | 66篇 |
1978年 | 57篇 |
1977年 | 61篇 |
1976年 | 44篇 |
1975年 | 43篇 |
1974年 | 47篇 |
1972年 | 33篇 |
1970年 | 34篇 |
1966年 | 33篇 |
1965年 | 47篇 |
1964年 | 54篇 |
排序方式: 共有4993条查询结果,搜索用时 46 毫秒
121.
The distances and orientations among reactant centers in the active site of coenzyme B12-dependent ethanolamine deaminase from Salmonella typhimurium have been characterized in the Co(II)-product radical pair state by using X-band electron paramagnetic resonance (EPR) and two-pulse electron spin-echo envelope modulation (ESEEM) spectroscopies in the disordered solid state. The unpaired electron spin in the product radical is localized on C2. Our approach is based on the orientation-selection created in the EPR spectrum of the biradical by the axial electron-electron dipolar interaction. Simulation of the EPR line shape yielded a best-fit Co(II)-C2 distance of 9.3 A. ESEEM spectroscopy performed at four magnetic field values addressed the hyperfine coupling of the unpaired electron spin on C2 with 2H in the C5' methyl group of 5'-deoxyadenosine and in the beta-2H position at C1 of the radical. Global ESEEM simulations (over the four magnetic fields) were weighted by the orientation dependence of the EPR line shape. A Nelder-Mead direct search fitting algorithm was used to optimize the simulations. The results lead to a partial model of the active site, in which C5' is located a perpendicular distance of 1.6 A from the Co(II)-C2 axis, at distances of 6.3 and 3.5 A from Co(II) and C2, respectively. The van der Waals contact of the C5'-methyl group and C2 indicates that C5' remains close to the radical species during the rearrangement step. The C2-Hs-C5' angle including the strongly coupled hydrogen, Hs, and the C5'-Hs orientation relative to the C1-C2 axis are consistent with a linear hydrogen atom transfer coordinate and an in-line acceptor p-orbital orientation. The trigonal plane of the C2 atom defines sub-spaces within the active site for C5' radical migration and hydrogen atom transfers (side of the plane facing Co(II)) and amine migration (side of the plane facing away from Co(II)). 相似文献
122.
Michael Charwath Prof. Dr. Kurt Utvary Joseph M. Kanamueller 《Monatshefte für Chemie / Chemical Monthly》1977,108(5):1093-1097
Prolonged heating of (CH3NPF3)2 in a sealed tube at 120°C yields (CH3N)4P3F7 and PF5. Under the experimental conditions these compounds react to (CH3N)4P3F6
+PF6
–. Reaction of (CH3NPF3)2 in the presence of PF5 leads to a marked decrease in the formation of the ionic compound. The influence of the fluoride donating saltsMF (M=Li, Na, K, Cs) is investigated.
Mit 1 Abbildung
Früher: Alkyliminophosphorsäuretrihalogenide, 1. und 2. Mitt. 相似文献
Mit 1 Abbildung
Früher: Alkyliminophosphorsäuretrihalogenide, 1. und 2. Mitt. 相似文献
123.
Hans-Peter Abicht Peter Lehniger Kurt Issleib 《Journal of organometallic chemistry》1983,250(1):609-616
The complexes of type R2)2 (M = Pd, Pt) are readily deprotonated by n-Buli under various conditions yielding μ-C-bis-dilithiated species. The resulting carbanions are attacked by the electrophiles Mel, Me3SiCl and Ph2PCl to form the corresponding disubstituted derivatives. The reaction with AuCl . PPh3 yields heterobimetallic complexes with two different MC σ-bonds. The compounds obtained are analytically and spectroscopically characterized. 相似文献
124.
Interpolation methods such as the nudged elastic band and string methods are widely used for calculating minimum energy pathways and transition states for chemical reactions. Both methods require an initial guess for the reaction pathway. A poorly chosen initial guess can cause slow convergence, convergence to an incorrect pathway, or even failed electronic structure force calculations along the guessed pathway. This paper presents a growing string method that can find minimum energy pathways and transition states without the requirement of an initial guess for the pathway. The growing string begins as two string fragments, one associated with the reactants and the other with the products. Each string fragment is grown separately until the fragments converge. Once the two fragments join, the full string moves toward the minimum energy pathway according to the algorithm for the string method. This paper compares the growing string method to the string method and to the nudged elastic band method using the alanine dipeptide rearrangement as an example. In this example, for which the linearly interpolated guess is far from the minimum energy pathway, the growing string method finds the saddle point with significantly fewer electronic structure force calculations than the string method or the nudged elastic band method. 相似文献
125.
Individual hydration water molecules in aqueous protein solutions have been observed using experimental schemes for homonuclear two-dimensional and heteronuclear three-dimensional NMR experiments in H2O solution, which do not require suppression of the solvent line by presaturation. In these experiments, the location of the hydration waters is determined from their nuclear Overhauser effects (NOE s) with individual hydrogen atoms of distinct amino acid residues. In the basic pancreatic trypsin inhibitor (BPTI ), four internal water molecules that had been reported in three different crystal forms were also found to be in the same locations in the solution structure, with lifetimes with respect to exchange of the water protons in excess of 0.3 ns. Additional NOE s with polypeptide protons located on the protein surface may involve either hydration water molecules or hydroxyl protons of amino acid side chains. Their total number is small compared to the number of NOE s expected from the hydration water molecules identified in the crystal structures of BPTI . 相似文献
126.
Improved ultrasonic extraction procedure for the determination of polycyclic aromatic hydrocarbons in sediments 总被引:6,自引:0,他引:6
The aim of this work was to optimize an ultrasonic extraction procedure for the determination of polycyclic aromatic hydrocarbons (PAHs) in sediments and to compare it with the reflux procedure using methanolic potassium hydroxide. Sample extracts were purified with a miniaturized silica gel chromatographic column and analyzed by gas chromatography-mass spectrometry (GC-MS). Ultrasonication using n-hexane-acetone (1:1, v/v) solvent mixture on dried homogenized marine sediment gave better precision (smaller relative standard deviation (RSD) values) and comparable quantities of individual PAH's compared to the reflux procedure. Ultrasonication with the n-hexane-acetone (1:1, v/v) mixture, utilizing four 15 min extraction cycles, was found to be sufficient for extracting PAHs from wet sediments. The optimized ultrasonic extraction procedure extracted aliphatic and aromatic hydrocarbons from the National Institute of Standards and Technology SRM 1941a with recoveries greater than 90%. The major advantages of ultrasonication compared to the reflux method are the lower extraction times, simplicity of the apparatus and extraction procedure. The optimized ultrasonication procedure has been used in our laboratory to extract hydrocarbons from naturally wet sediments from rivers, and coastal and marine areas. 相似文献
127.
[Ph3PNSbPh3Cl] ( 1 ) was prepared by oxidative addition of ClNPPh3 to triphenylstibine in dichloromethane solution. The compound is characterized by IR spectroscopy and by an X‐ray structure determination. 1 crystallizes in the monoclinic space group P21/c with four formula units per unit cell. Lattice dimensions at 193 K: a = 925.3(1), b = 1777.2(1), c = 1825.5(1) pm, β = 94.07(1)°, R1 = 0.0228. 1 forms monomeric molecules with tetrahedrally coordinated phosphorus and trigonal‐bipyramidally coordinated antimony atom, the atoms N and Cl being in axial positions. The bond lengths PN and SbN are 155.0(2) and 198.4(2) pm, respectively, the PNSb angle is 138.6(1)°. 1 reacts with iron trichloride to give the known phosphoraneiminato complex [FeCl2(NPPh3)]2. 相似文献
128.
Wolfgang Hnle Walter Schmettow Karl Peters Jen‐Hui Chang Hans Georg von Schnering 《无机化学与普通化学杂志》2004,630(12):1858-1862
The novel ternary polyphosphides M4P21I (M = K, Rb) have been synthesized from the elements in single crystalline form, representing further examples for the formation of mixed crystals between simple salts and binary phosphides. They form as ruby‐red platelets and dark‐red prisms, respectively, and are only slightly sensitive to moisture and oxygen. The compounds are isotypic (Ccmm (no 63); Z = 4; oP104; K4P21I: a = 12.853Å; b = 21.795Å; c = 9.748Å; 1168 hkl, R = 0.033; Rb4P21I: a = 13.281Å; b = 21.868Å; c = 9.771Å; 777 hkl, R = 0.053) and feature corrugated 2D networks formed from two different types of polymerized P7 units. The networks form large cavities filled by M+ and I‐ ions. Zigzag chains of condensed trigonal M6 prisms, centered by the I‐ anions, separate the polyphosphide nets. The mean homoatomic P‐P bond length (d = 2.216Å) corresponds to a P‐P single bond. However, the individual P‐P distances vary with position and function (2.126 ‐ 2.247Å) and these are compared with those of the isolated P21‐3 anion. 相似文献
129.
Kurt Girstmair 《Monatshefte für Mathematik》1984,98(3):193-209
The subject of this article are a priori constructions of primitive elements in field extensions. Consider the totality of all separable polynomialsf of degreen over a fieldK with rootsx
1,...,x
n
and prescribed Galois groupG. A vector (b
1,...,b
n
)K
n
is called stably primitive (forG), if, for each suchf,b
1
x
1+...+b
n
x
n
generates the splitting field off. We develop representation theoretical devices to investigate the set of stably primitive vectors geometrically. A fundamental observation is that is either very large or very small (or even empty). These two cases are illustrated by various examples. Moreover, criteria are given to decide which case holds. For a number of groups where is recognized to be small we show. 相似文献
130.
Kurt Sturm 《Zeitschrift für Physik B Condensed Matter》1978,29(1):27-32
The real part of the effective dielectric function is derived analytically in the pseudopotential approximation. For anyk-direction, the theory allows the evaluation of the plasmon dispersion,
p
(k), which is found to be anisotropic in general due to band structure effects. Application of the theory to Al yields an isotropic shift atk=0 of
p
(0)
v
– 0.3 eV and explains the recently observed anisotropic behaviour of the dispersion very well fork < 0.6k
F
. 相似文献