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11.
Ferrocene attached to the surface of a platinum electrode catalyses the electrochemical oxidation of ascorbic acid in acidic buffer solutions. The overpotential for ascorbic acid oxidation is decreased by 150 mV at pH 2.2 compared with reaction at bare platinum; and an increase in anodic current and decrease in cathodic current for the redox reaction of ferrocene occurs on addition of ascorbic acid to the solution. The ferrocene-modified electrode is useful for the voltammetric determination of ascorbic acid in natural fruit juices. The advantages result from the electrocatalytic effect and from the prevention of adsorption of inhibitory substances from solution. 相似文献
12.
Bellomo EG Davidson P Impéror-Clerc M Deming TJ 《Journal of the American Chemical Society》2004,126(29):9101-9105
The aqueous, lyotropic liquid-crystalline phase behavior of the alpha-helical polypeptide, poly(N(epsilon)-2-[2-(2-methoxyethoxy)ethoxy]acetyl-lysine) (1), has been studied using optical microscopy and X-ray scattering. Solutions of optically pure 1 were found to form cholesteric liquid crystals at volume fractions that decreased with increasing average chain length. At very high volume fractions, the formation of a hexagonal mesophase was observed. The pitch of the cholesteric phase could be varied by a mixture of enantiomeric samples L-1 and D-1, where the pitch increased as the mixture approached equimolar. The cholesteric phases could be untwisted, using either magnetic field or shear flow, into nematic phases, which relaxed into cholesterics upon removal of field or shear. We have found that the phase diagram of 1 in aqueous solution parallels that of poly(gamma-benzyl glutamate) in organic solvents, thus providing a useful system for liquid-crystal applications requiring water as solvent. 相似文献
13.
Contributions to the Chemistry of Phosphorus. 123. Synthesis and Properties of the Diphosphagermiranes (t-BuP)2GePh2 and (t-BuP)2GeEt2 The first three-membered P2Ge heterocycles, 1,2-di-tert-butyl-3, 3-diphenyl-1, 2, 3-diphosphagermirane, (t-BuP)2GePh2 (1) , and 1, 2-di-tert-butyl-3, 3-diethyl-1, 2, 3-diphosphagermirane, (t-BuP)2GeEt2 (2) , were synthesized by [2+1] cyclocondensation reactions of K(t-Bu)P—P(t-Bu)K with diphenylgermanium dichloride and diethylgermanium dichloride, respectively. The four-, five-, and six-membered cyclogermaphosphanes (t-BuP)2(GePh2)2 (3) , (t-BuP)3GeR2 ( 6 R = Ph; 7 R = Et), (t-BuP)4GePh2 (5) and (t-BuP)4(GePh2)2 (4) as well as (t-BuP)4 are formed as by-products. The diphosphagermiranes 1 and 2 could be isolated in 93 and 100% purity, respectively, and were unambiguously characterized as compounds with a cyclic P2Ge skeleton. The 31P-NMR parameters of the cyclogermaphosphanes 3—7 are reported. 相似文献
14.
Contributions to the Chemistry of Phosphorus. 105. 1,2,34-Tetraphenyl-1,4-bis(trimethylsilyl)-tetraphosphane and 1,2,3,4-Tetraphenyltetraphosphane 1,2,3,4-Tetraphenyl-1,4-bis(trimethylsilyl)-tetraphosphane, Me3Si? (PPh)4? SiMe3 ( 1 ), is obtained by reacting K2(PPh)4 with trimethylchlorosilane under suitable conditions. Compound 1 disproportionates almost easier than the corresponding triphosphane (Me3Si)2(PPH)3. Of the six possible diastereomers only 1a (erythro, meso, erythro), 1b (erythro, d,l, erythro), 1 d (threo, d,l, threo), and 1 f (erythro, threo, threo) can be detected in solution by 31P-NMR spectroscopy. In consequence of rapid inversion at the P atoms a dynamic equilibrium exists between the different isomers. The assignment of the 31P-NMR-spectroscopically observed spin systems to the corresponding diastereomers results from the dependence of the 1JPP-coupling constants on the dihedral angle between vicinal free electron pairs as well as on the observed frequency distribution. In the alcoholysis of 1 the corresponding hydride H? (PPh)4? H ( 2 ) is formed as the main product. It could be isolated in spite of its instability. At room temperature 2 disproportionates rapidly forming mainly (PPh)4 and H2(PPh)2 (ratio 1:2) at first; later on also H2(PPh)3, H2PPh, and (PPh)5 are found. The corresponding rearrangements follow a four-center mechanism involving predominantly P? P bonds. 相似文献
15.
Contributions to the Chemistry of Phosphorus. 106. Synthesis and Properties of the Diphosphacyclopropane (t-BuP)2CHMe The new 1,2-di-tert-butyl-3-methyl-1,2-diphosphacyclopropane (1,2-di-tert-butyl-3-methyl-diphosphirane), (t-BuP)2CHMe ( 1 ), is obtained by reacting K(t-Bu)P? P(t-Bu)K with 1,1-dichloroethane under suitable conditions. 1 can be isolated by high vacuum distillation and is stable for months when stored under inert gas at room temperature. Particularly, no dimerization to the corresponding 1,2,4,5-tetraphosphacyclohexane takes place. The NMR parameters indicate an increase of the exocyclic bond angles compared to (t-BuP)2CH2. The signs of all CP coupling constants have been determined by spin tickling experiments. The 2J(CCP)-coupling of the methyl group at the ring carbon depends strongly on the dihedral angle. 相似文献
16.
Gribble GW Switzer FL Bushweller JH Jewett JG Brown JH Dion JL Bushweller CH Byrn MP Strouse CE 《The Journal of organic chemistry》1996,61(13):4319-4327
The (1)H NMR spectra of 10-benzyl-9,11-diphenyl-10-azatetracyclo[6.3.0.0.(4,11)0.(5,9)]undecane (BnPh(2)()) and 10-methyl-9,11-diphenyl-10-azatetracyclo[6.3.0.0.(4,11)0.(5,9)]undecane (MePh(2)()) decoalesce due to slowing inversion at nitrogen and to slowing isolated bridgehead phenyl rotation. The high nitrogen inversion barriers in MePh(2)() (DeltaG() = 12.2 +/- 0.1 kcal/mol at 250 K) and BnPh(2)() (DeltaG() = 10.6 +/- 0.1 kcal/mol at 215 K) are typical of tertiary amines in which at least one C-N-C bond angle is constrained to a small value. Compared to the minuscule rotation barriers about sp(2)-sp(3) carbon-carbon bonds in simple molecular systems, the bridgehead phenyl rotation barriers in MePh(2)() (DeltaG() = 9.8 +/- 0.1 kcal/mol at 210 K) and BnPh(2)() (DeltaG() = 9.8 +/- 0.1 kcal/mol at 210 K) are unusually high. Molecular mechanics calculations (MMX force field) suggest that the origin of the high phenyl rotation barriers lies in the close passage of an o-phenyl proton and a methyl (or benzylmethylene) proton in the transition state. BnPh(2)() crystallized from hexane as white needles in the monoclinic system Pn. Unit cell dimensions are as follows: a = 12.198(1) ?, b = 6.1399(6) ?, c = 14.938(2) ?, beta = 107.470(4) degrees, V = 1067.1(2) ?(3), Z = 2. In the crystal molecular structure, the imine bridge CNC bond angle in BnPh(2)() is constrained to a small value (96 degrees ). The benzylic phenyl group is oriented gauche to the nitrogen lone pair. 相似文献
17.
Contributions to the Chemistry of Phosphorus. 128. Synthesis of the Diphosphastanna-cyclopropane (t-BuP)2Sn(t-Bu)2 The first three-membered P2Sn heterocycle, 1,2,3,3-tetra-tert-butyl-1,2,3-diphosphastanna-cyclopropane (1,2,3,3-tetra-tert-butyl-1,2,3-diphosphastannirane) ( 1 ), has been synthesized by [2+1] cyclocondensation of K(t-Bu)P—P(t-Bu)K with (t-Bu)2SnCl2. 1 is stable at room temperature. Besides, (t-BuP)2[Sn(t-Bu)2]2 ( 2 ), (t-BuP)4Sn(t-Bu)2 ( 3 ), and (t-BuP)4 are formed. In the reaction with Et2SnCl2, the six-membered ring compound [(t-BuP)2SnEt2]2 ( 4 ) is the main-product; the four- and five-membered cyclostannaphosphanes (t-BuP)3SnEt2 ( 5 ) and (t-BuP)3(SnEt2)2 ( 6 ) are also formed. 1 could be isolated in the pure state and has been unambiguously characterized as a three-membered heterocycle with a P2Sn skeleton. The 31P-NMR parameters of the other new cyclostannaphosphanes 2–6 are reported. 相似文献
18.
Contributions to the Chemistry of Phosphorus. 159. On the Reaction of the Diphosphaborirane (t-BuP)2BN(i-Pr)2 with Potassium or Potassium Naphthalenide The reaction of (t-BuP)2BN(i-Pr)2 with potassium or K-naphthalenide in tetrahydrofuran leads to K(t-Bu)P? ;BN(i-Pr)2? P(t-Bu)K ( 1 ) via P? ;P bond cleavage of the three-membered ring skeleton. Above ? 78°C 1 changes into the asymmetric compound K(t-Bu)P? ;P(t-Bu)? BHN(i-Pr)2 ( 2 ). In dimethoxyethane additionally the monometallated diphosphaborirane K(t-Bu)P2BN(i-Pr)2 ( 3 ) is formed. 1 and 3 , which could be isolated free from other phosphorus containing compounds, as well as the corresponding silylphosphanes Me3Si(t-Bu)P? ;BN(i-Pr)2? ;P(t-Bu)SiMe 3 ( 4 ) and Me3Si(t-Bu)P2BN(i-Pr)2 ( 5 ) were characterized by NMR spectroscopy. Protolysis of 3 or 5 leads to a decomposition of the three-membered ring skeleton with formation of H(t-Bu)P? ;PH2. 相似文献
19.
20.
Contributions to the Chemistry of Phosphorus. 144. Synthesis and Properties of the Hexaphospha-3-germaspiro[2.4]heptane (t-BuP)2Ge(t-BuP)4 The cyclocondensation of K(t-Bu)P? P(t-Bu)K with germanium tetrachloride in the molar ratio of 2:1 yields the novel spirocyclic compound 1,2,4,5,6,7-hexa-tert-butyl-1,2,4,5,6,7-hexaphospha-3-germaspiro[2.4]heptane, (t-BuP)2Ge(t-BuP)4 ( 1 ). Besides considerable amounts of (t-BuP)4 are formed and occasionally some (t-BuP)3 can be found. 1 could be isolated in the pure state and has been NMR-spectroscopically characterized as a spirocyclic compound with a P2GeP4 skeleton. 相似文献