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71.
Dendrinou-Samara C Zaleski CM Evagorou A Kampf JW Pecoraro VL Kessissoglou DP 《Chemical communications (Cambridge, England)》2003,(21):2668-2669
Reaction of Mn(ClO4)2 with di-pyridyl ketone oxime, (2-py)2C=NOH, gives the novel cluster [Mn(II)4Mn(III)6Mn(IV)2(mu4-O)2(mu3-O)4(mu3-OH)4(mu3-OCH3)2(pko)12](OH)(ClO4)3 1. It is the only example of a 24-MC-8, and the first metallacrown with ring metal ions in three different oxidation states. Magnetic measurements show antiferromagnetic behavior. 相似文献
72.
Cheng X Kelso C Hornak V de los Santos C Grollman AP Simmerling C 《Journal of the American Chemical Society》2005,127(40):13906-13918
The process by which DNA repair enzymes recognize and selectively excise damaged bases in duplex DNA is fundamental to our mechanistic understanding of these critical biological reactions. 8-Oxoguanine (8-oxoG) is the most common form of oxidative DNA damage; unrepaired, this lesion generates a G:C-->T:A mutation. Central to the recognition and repair of DNA damage is base extrusion, a process in which the damaged base lesion or, in some cases, its partner disengages from the helix and is bound to the enzyme's active site where base excision takes place. The conformation adopted by 8-oxoG in duplex DNA is affected by the base positioned opposite this lesion; conformational changes may also take place when the damaged base binds to its cognate repair enzyme. We performed unrestrained molecular dynamics simulations for several 13-mer DNA duplexes. Oligomers containing G:C and 8oxoG:C pairs adopted Watson-Crick geometries in stable B-form duplexes; 8oxoG showed increased local and global flexibility and a reduced barrier to base extrusion. Duplexes containing the G:A mismatch showed much larger structural fluctuations and failed to adopt a well-defined structure. For the 8oxoG:A mismatch that is recognized by the DNA glycosylase MutY, the damaged nucleoside underwent spontaneous and reproducible anti-->syn transitions. The syn conformation is thermodynamically preferred. Steric hindrance and unfavorable electrostatics associated with the 8oxoG O8 atom in the anti conformation were the major driving forces for this transition. Transition events follow two qualitatively different pathways. The overall anti-->syn transition rate and relative probability of the two transition paths were dependent on local sequence context. These simulations indicate that both the dynamic and equilibrium behavior of the duplex change as a result of oxidation; these differences may provide valuable new insight into the selective action of enzymes on damaged DNA. 相似文献
73.
A tetra L-prolinylmethyl derivative of a tetra-sulfonated calix[4]resorcarene (1) is an effective chiral NMR solvating agent for water-soluble compounds with phenyl, pyridyl, bicyclic aromatic, or indole rings. These aromatic compounds form host-guest complexes with the calix[4]resorcarene in water. Complexation of substrates with the calix[4]resorcarene is likely promoted by hydrophobic effects, and bicyclic substrates have association constants with the calix[4]resorcarene larger than those of similar phenyl-containing compounds. Aromatic resonances of the substrates show substantial upfield shifts because of shielding from the aromatic rings of the calix[4]resorcarene, and several resonances in the 1H NMR spectra typically exhibit enantiomeric discrimination. The extent of enantiomeric discrimination depends in part on interactions of the substituent groups of the substrates with the prolinylmethyl groups of the calix[4]resorcarene. The effectiveness of a calix[4]resorcarene prepared from N-methyl-L-alanine (2) as a chiral NMR discriminating agent is compared to the L-prolinylmethyl derivative. 相似文献
74.
Quadrupole mass spectrometry has been employed to characterize the ionic species in the discharges of pure CH4, CH4/H2, and CH4/Ar systems. For pure methane, the major positive ions in the discharge at low pressure (e.g., 0.15 torr) are CH
3
+
, C2H
3
+
, CH
2
+
, C2H
2
+
, CH
4
+
, C2H
4
+
, and C2H
5
+
at high pressure (e.g., 0.5 torr) the major ions are CH
3
+
, C2H
3
+
, C2H
5
+
, C3H
3
+
, C H3H
7
+
, C4H
7
+
, C5H
7
+
, C6H
5
+
, and C7H
7
+
. The relative abundances of C1 ions decrease with increasing pressure, whereas those of higher-order ions increase with pressure. For 5% CH4 + 95% H2 mixture, in addition to those sampling from the pure methane plasma at the lower pressure, H
n
+
ions have also been detected. For 5% CH4 +95% Ar mixture, the principal ions are CH
3
+
, CH
2
+
, CH+, CH
5
+
, Ar+, and ArH+; the ions containing more than two carbon atoms are negligible. In these discharges, the CH
3
+
and C2H
3
+
are the most important positive ions in C1 and C2 ions, respectively. The ions detected are believed to come from the sheath between the electrode and the luminous plasma, and have high kinetic energy. An ion-molecule reaction mechanism is proposed which can well explain the observed main features of ionic products.Died June 1, 1991. 相似文献
75.
Alnouti Y Lewis SR White CA Bartlett MG 《Rapid communications in mass spectrometry : RCM》2005,19(4):503-508
A simple liquid chromatography/tandem mass spectrometry (LC/MS/MS) method has been developed and validated for the simultaneous determination of zidovudine (AZT) and lamivudine (3TC) in rat plasma, amniotic fluid, placental, and fetal tissues. Samples were processed by acetonitrile precipitation. Chromatography was performed using a C18 column (5 microm, 150 x 3.9 mm i.d). The mobile phase consisted of 30% methanol and 7.5 mM ammonium acetate (pH 6.5). The method was validated in the range of 0.05-25 microg/mL for both 3TC and AZT in the four biological matrices. Finally, the method was applied to a study involving fetal transport following co-administration of these compounds at a dose of 25 mg/kg each in a pregnant rat. 相似文献
76.
Choi JH Lucas D Koshland CP Sawyer RF 《The journal of physical chemistry. B》2005,109(50):23905-23910
The photochemical interaction of 193 nm light with polystyrene nanospheres is used to produce particles with a controlled size and morphology. Laser fluences from 0 to 0.14 J/cm2 at 10 and 50 Hz photofragment nearly monodisperse 110 nm spherical polystyrene particles. The size distributions before and after irradiation are measured with a scanning mobility particle sizer (SMPS), and the morphology of the irradiated particles is examined with a transmission electron microscope (TEM). The results show that the irradiated particles have a smaller mean diameter ( approximately 25 nm) and a number concentration more than an order of magnitude higher than nonirradiated particles. The particles are formed by nucleation of gas-phase species produced by photolytic decomposition of nanospheres. A nondimensional parameter, the photon-to-atom ratio (PAR), is used to interpret the laser-particle interaction energetics. 相似文献
77.
78.
Chlorine trioxide, Cl(2)O(6), reacts with Au metal, AuCl(3), or HAuCl(4).nH(2)O to yield the well-defined chloryl salt, ClO(2)Au(ClO(4))(4). The crystal and molecular structure of ClO(2)Au(ClO(4))(4) was solved by a Rietveld analysis of powder X-ray diffraction data. The salt crystallizes in a monoclinic cell, space group C2/c, with cell parameters a = 15.074(5), b = 5.2944(2), and c = 22.2020(2) A and beta = 128.325(2) degrees. The structure displays discrete ClO(2)(+) ions lying in channels formed by Au(ClO(4))(4)(-) stacks. Au is located in a distorted square planar environment: Au-O = 1.87 and 2.06 A. [ClO(4)] groups are monodentate with ClO(b) = 1.53 and ClO(t) = 1.39 A (mean distances; O(b), oxygen bonded to Au; O(t), free terminal oxygen). A full vibrational study of the Au(ClO(4))(4)(-) anion is supported by DFT calculations. 相似文献
79.
Two- or three-component aza Diels-Alder reactions of Danishefsky's diene with imines or aldehydes and amines in water took place smoothly under neutral conditions in the presence of a catalytic amount of an alkaline salt such as sodium triflate to afford dihydro-4-pyridones in high yields. 相似文献
80.
[reaction: see text] The partial reduction of electron-deficient 2,5-disubstituted pyrroles has been developed into a flexible procedure that gives control of relative stereochemistry by variation of the reduction conditions. After the reaction, the pyrroline products were dihydroxylated at C-3,4 to give either the cis or trans isomers; this flexibility means that a variety of polyhydroxylated pyrrolidines can be prepared in a short sequence. Finally, this method was applied to a synthesis of the naturally occurring glycosidase inhibitor DMDP. 相似文献