全文获取类型
收费全文 | 4055篇 |
免费 | 72篇 |
国内免费 | 46篇 |
专业分类
化学 | 2826篇 |
晶体学 | 25篇 |
力学 | 69篇 |
数学 | 747篇 |
物理学 | 506篇 |
出版年
2022年 | 19篇 |
2021年 | 37篇 |
2020年 | 26篇 |
2019年 | 47篇 |
2018年 | 26篇 |
2017年 | 38篇 |
2016年 | 82篇 |
2015年 | 53篇 |
2014年 | 67篇 |
2013年 | 200篇 |
2012年 | 180篇 |
2011年 | 203篇 |
2010年 | 104篇 |
2009年 | 88篇 |
2008年 | 249篇 |
2007年 | 264篇 |
2006年 | 265篇 |
2005年 | 268篇 |
2004年 | 205篇 |
2003年 | 144篇 |
2002年 | 139篇 |
2001年 | 50篇 |
2000年 | 67篇 |
1999年 | 55篇 |
1998年 | 40篇 |
1997年 | 48篇 |
1996年 | 76篇 |
1995年 | 52篇 |
1994年 | 54篇 |
1993年 | 41篇 |
1992年 | 44篇 |
1991年 | 33篇 |
1990年 | 35篇 |
1989年 | 28篇 |
1988年 | 35篇 |
1987年 | 38篇 |
1986年 | 47篇 |
1985年 | 63篇 |
1984年 | 67篇 |
1983年 | 55篇 |
1982年 | 50篇 |
1981年 | 56篇 |
1980年 | 63篇 |
1979年 | 38篇 |
1978年 | 50篇 |
1977年 | 42篇 |
1976年 | 31篇 |
1975年 | 31篇 |
1974年 | 30篇 |
1973年 | 38篇 |
排序方式: 共有4173条查询结果,搜索用时 15 毫秒
81.
Two new sesquiterpenoids of the unprecedented neoiemnane ring system, and one eremophflane-derived sesquiterpene diol have been isolated from the pacific soft-coral Lemnalia africana. The structure of the novel neolemnane compounds, 4 and 5, were confirmed by X-ray crystallography, and the diol, 6, has been described based upon spectral analysis and chemical modification. 相似文献
82.
Abdel-Magid AF Carson KG Harris BD Maryanoff CA Shah RD 《The Journal of organic chemistry》1996,61(11):3849-3862
Sodium triacetoxyborohydride is presented as a general reducing agent for the reductive amination of aldehydes and ketones. Procedures for using this mild and selective reagent have been developed for a wide variety of substrates. The scope of the reaction includes aliphatic acyclic and cyclic ketones, aliphatic and aromatic aldehydes, and primary and secondary amines including a variety of weakly basic and nonbasic amines. Limitations include reactions with aromatic and unsaturated ketones and some sterically hindered ketones and amines. 1,2-Dichloroethane (DCE) is the preferred reaction solvent, but reactions can also be carried out in tetrahydrofuran (THF) and occasionally in acetonitrile. Acetic acid may be used as catalyst with ketone reactions, but it is generally not needed with aldehydes. The procedure is carried out effectively in the presence of acid sensitive functional groups such as acetals and ketals; it can also be carried out in the presence of reducible functional groups such as C-C multiple bonds and cyano and nitro groups. Reactions are generally faster in DCE than in THF, and in both solvents, reactions are faster in the presence of AcOH. In comparison with other reductive amination procedures such as NaBH(3)CN/MeOH, borane-pyridine, and catalytic hydrogenation, NaBH(OAc)(3) gave consistently higher yields and fewer side products. In the reductive amination of some aldehydes with primary amines where dialkylation is a problem we adopted a stepwise procedure involving imine formation in MeOH followed by reduction with NaBH(4). 相似文献
83.
The crystal structures of the high-temperature modifications of sodium and silver orthophosphates have been determined using powder neutron diffraction (PND) data. II-Na3PO4 adopts the space group Fm3m with
at 400°C. The PO3−4 group is centered around the origin, but it shows high orientational disorder. The sodium ions occupy the
and
sites. II-Ag3PO4, at 650°C, is similar with
. The structure of I-Ag3PO4 at room temperature
has been re-examined by single-crystal X-ray diffraction. The derived model, with R=0.019 for 116 independent reflections, is in agreement with the latest work reported in the literature. The structure of I-Ag3PO4 at 375°C, as determined by PND, has
, and displays no gross modifications from that observed at 25°C, although the anisotropic nature of the silver sites is markedly more pronounced at this higher temperature. The cation mobility is discussed in relation to the high-temperatures structures. 相似文献
Full-size image (<1K)
84.
We have synthesized a 22 residue alanine-based peptide with a tris(bipyridyl)ruthenium(II) amino acid near the middle of the peptide which can act as a photoinducible electron donor. Two histidines spaced i, i + 4 near the C-terminus of the peptide were then cross-linked with a tetraammineruthenium(III) moiety to prenucleate the helix and provide an electron acceptor site. Introduction of the cross-link enhances the average helix content from 67% to 84% at 0 degrees C, as judged by circular dichroism spectroscopy. The temperature dependence of the mean molar residue ellipticity at 222 nm, [THETAV;](222), for the bimetalated peptide was fit to a modified Lifson-Roig helix-coil model to permit extraction of the population of helical conformation at each residue separating the electron donor and acceptor. On average, the residues between the donor and acceptor are 92% helical. Photoinduced electron transfer with a driving force of -1.0 eV and an estimated reorganization energy of 0.82 eV was measured by fluorescence quenching methods in H(2)O and D(2)O, yielding rate constants, k(ET), of 7 +/- 3 x 10(6) s(-)(1) and 5 +/- 1 x 10(6) s(-)(1) at 0 degrees C. Calculation of the electronic coupling matrix element, H(ab), with the Marcus equation yields a value of 0.19 +/- 0.4 cm(-)(1). Analysis in terms of the pathway model for electronic coupling indicates that this magnitude of H(ab) is consistent with the participation of hydrogen bonds in electronic coupling for an isolated alpha-helix. 相似文献
85.
The interaction of aspartic acid with kaolinite was studied by potentiometric titrations and by adsorption measurements both at constant aspartate concentration (but varying pH) and at a constant pH of 5.5. The temperature was 25 degrees C, and the ionic medium 5 mM KNO3. Aspartic acid dissociation constants estimated from titrations agreed with those from the literature. The adsorption of aspartic acid to kaolinite was weak and varied only slightly with pH; 10-18% of 100 microM aspartic acid adsorbed to kaolinite at 100 m(2)L(-1) between pH 3 and 10. Data from the titrations and adsorption experiments were fitted closely by an extended constant-capacitance surface complexation model, in which monodentate outer-sphere complexes formed between deprotonated aspartic acid molecules and protonated sites on the variable-charge edges of the kaolinite crystals. There appeared to be no adsorption to the permanently charged crystal faces. 相似文献
86.
Graham A. Bowmaker Prof. Dr. Michael I. Bruce Brian W. Skelton Neil Somers Allan H. White 《无机化学与普通化学杂志》2007,633(7):1024-1030
The complexes [Cu2Br4]2?, [Cu2I4]2?, [Cu2I2Br2]2?, [Cu2I3Cl]2?, [Ag2Cl4]2? have been characterized as their isomorphous bis(triphenylphosphoranylidene)ammonium ([Ph3PNPPh3]+ = PNP+) salts by single crystal structural determinations. All anions show the centrosymmetric doubly halogen‐bridged forms [XM(μ‐X)2MX]2? with three‐coordinate metal atoms that have been observed in [M2X4]2? complexes with other large organic cations. In [Cu2I2Br2]2? the iodide ligands occupy the bridging positions and the bromide the terminal positions, while in [Cu2I3Cl]2?, obtained in an attempt to prepare [Cu2I2Cl2]2?, two of the iodide ligands occupy the bridging positions with the third iodide and the chloride ligand occupying two statistically disordered terminal positions. In [Ag2Cl4]2? the distortion from ideal trigonal coordination of the metal atom is greater than in the copper complexes, but less than in other previously reported [Ag2Cl4]2? complexes with organic cations. The ν(MX) bands have been assigned in the far‐IR spectra, and confirm previous observations regarding the unexpectedly simple IR spectra of [Cu2X4]2? complexes. 相似文献
87.
William Vidaver Radovan Popovic Douglas Bruce Konrad Colbow 《Photochemistry and photobiology》1981,34(5):633-636
Abstract— Three phases of chlorophyll a fluorescence quenching by O2 are observed in green plants. The effects of various inhibitors on photosynthetic partial processes in chloroplasts were investigated in attempts to (1) localize the O2 -quenching sites and (2) assess possible physiological significance of O2 -quenching. Our results localize the most sensitive (and presumably functionally important) phase to a site between plastoquinone and the photosystem I acceptor, chlorophyll (P700 ), possibly plastocyanin. It is suggested that PC may transfer electrons to oxygen in addition to P700 . 相似文献
88.
Dalton GT Viau L Waterman SM Humphrey MG Bruce MI Low PJ Roberts RL Willis AC Koutsantonis GA Skelton BW White AH 《Inorganic chemistry》2005,44(9):3261-3269
Reaction of [WIr3(mu-CO)3(CO)8(eta-C5Me5)] (1c) with [W(C[triple bond]CPh)(CO)3(eta-C5H5)] afforded the edge-bridged tetrahedral cluster [W2Ir3(mu4-eta2-C2Ph)(mu-CO)(CO)9(eta-C5H5)(eta-C5Me5)] (3) and the edge-bridged trigonal-bipyramidal cluster [W3Ir3(mu4-eta2-C2Ph)(mu-eta2-C=CHPh)(Cl)(CO)8(eta-C5Me5)(eta-C5H5)2] (4) in poor to fair yield. Cluster 3 forms by insertion of [W(C[triple bond]CPh)(CO)3(eta-C5H5)] into Ir-Ir and W-Ir bonds, accompanied by a change in coordination mode from a terminally bonded alkynyl to a mu4-eta2 alkynyl ligand. Cluster 4 contains an alkynyl ligand interacting with two iridium atoms and two tungsten atoms in a mu4-eta2 fashion, as well as a vinylidene ligand bridging a W-W bond. Reaction of [WIr3(CO)11(eta-C5H5)] (1a) or 1c with [(eta-C5H5)(CO)2 Ru(C[triple bond]C)Ru(CO)2(eta-C5H5)] afforded [Ru2WIr3(mu5-eta2-C2)(mu-CO)3(CO)7(eta-C5H5)2(eta-C5R5)] [R = H (5a), Me (5c)] in low yield, a structural study of 5a revealing a WIr3 butterfly core capped and spiked by Ru atoms; the diruthenium ethyndiyl precursor has undergone Ru-C scission, with insertion of the C2 unit into a W-Ir bond of the cluster precursor. Reaction of [W2Ir2(CO)10(eta-C5H5)2] with the diruthenium ethyndiyl reagent gave [RuW2Ir2{mu4-eta2-(C2C[triple bond]C)Ru(CO)2(eta-C5H5)}(mu-CO)2(CO)6(eta-C5H5)3] (6) in low yield, a structural study of 6 revealing a butterfly W2Ir2 unit capped by a Ru(eta-C5H5) group resulting from Ru-C scission; the terminal C2 of a new ruthenium-bound butadiyndiyl ligand has been inserted into the W-Ir bond. Reaction between 1a, [WIr3(CO)11(eta-C5H4Me)] (1b), or 1c and [(eta-C5H5)(CO)3W(C[triple bond]CC[triple bond]C)W(CO)3(eta-C5H5)] afforded [W2Ir3{mu4-eta2-(C2C[triple bond]C)W(CO)3(eta-C5H5)}(mu-CO)2(CO)2(eta-C5H5)(eta-C5R5)] [R = H (7a), Me (7c); R5 = H4Me (7b)] in good yield, a structural study of 7c revealing it to be a metallaethynyl analogue of 3. 相似文献
89.
Functionalized, hydrogen-bonding ionic liquids have been successfully evaluated as media for the performance of cofactor-dependent enzyme catalysed oxidations; the effects of incorporating hydroxyl groups into both the cation and anion have been studied and the dependence of activity upon water content has been evaluated. 相似文献
90.
Surface-enhanced resonance Raman scattering (SERRS) from the alpha-diimine complexes [Ru(bpm)(3)](2+) and [Ru(bpz)(3)](2+) is reported for the first time at a roughened silver electrode. In both cases, a possible adsorbate orientation has been proposed involving binding through nitrogen lone pair electrons to the silver surface, based on changes in band positions upon adsorption. The SERRS spectra of [Ru(bpm)(3)](2+) were found to change slightly with a change in applied potential. The relative intensity of the nu(C6C6') band was found to be dependent on both excitation wavelength and applied potential. This was ascribed to an active charge transfer (CT) mechanism operating synergistically with the electromagnetic mechanism. No such CT activity was observed in [Ru(bpz)(3)](2+). It is tentatively suggested that this behavior may arise from the different modes of adsorption of the two complexes. 相似文献