全文获取类型
收费全文 | 9070篇 |
免费 | 310篇 |
国内免费 | 78篇 |
专业分类
化学 | 6653篇 |
晶体学 | 41篇 |
力学 | 205篇 |
数学 | 1355篇 |
物理学 | 1204篇 |
出版年
2023年 | 35篇 |
2022年 | 67篇 |
2021年 | 110篇 |
2020年 | 145篇 |
2019年 | 141篇 |
2018年 | 101篇 |
2017年 | 74篇 |
2016年 | 220篇 |
2015年 | 190篇 |
2014年 | 223篇 |
2013年 | 514篇 |
2012年 | 538篇 |
2011年 | 681篇 |
2010年 | 307篇 |
2009年 | 237篇 |
2008年 | 518篇 |
2007年 | 599篇 |
2006年 | 627篇 |
2005年 | 502篇 |
2004年 | 458篇 |
2003年 | 393篇 |
2002年 | 362篇 |
2001年 | 134篇 |
2000年 | 88篇 |
1999年 | 84篇 |
1998年 | 80篇 |
1997年 | 107篇 |
1996年 | 113篇 |
1995年 | 84篇 |
1994年 | 76篇 |
1993年 | 90篇 |
1992年 | 73篇 |
1991年 | 56篇 |
1990年 | 83篇 |
1989年 | 44篇 |
1988年 | 61篇 |
1987年 | 60篇 |
1986年 | 73篇 |
1985年 | 107篇 |
1984年 | 104篇 |
1983年 | 68篇 |
1982年 | 95篇 |
1981年 | 93篇 |
1980年 | 94篇 |
1979年 | 68篇 |
1978年 | 84篇 |
1977年 | 62篇 |
1976年 | 58篇 |
1975年 | 52篇 |
1973年 | 59篇 |
排序方式: 共有9458条查询结果,搜索用时 57 毫秒
351.
Wang Q Barclay JE Blake AJ Davies ES Evans DJ Marr AC McInnes EJ McMaster J Wilson C Schröder M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(14):3384-3396
A novel [NiS4Fe2(CO)6]cluster (1: 'S(4)'=(CH(3)C(6)H(3)S(2))(2)(CH(2))(3)) has been synthesised, structurally characterised and has been shown to undergo a chemically reversible reduction process at -1.31 V versus Fc(+)/Fc to generate the EPR-active monoanion 1(-). Multifrequency Q-, X- and S-band EPR spectra of (61)Ni-enriched 1(-) show a well-resolved quartet hyperfine splitting in the low-field region due to the interaction with a single (61)Ni (I=3/2) nucleus. Simulations of the EPR spectra require the introduction of a single angle of non-coincidence between g(1) and A(1), and g(3) and A(3) to reproduce all of the features in the S- and X-band spectra. This behaviour provides a rare example of the detection and measurement of non-coincidence effects from frozen-solution EPR spectra without the need for single-crystal measurements, and in which the S-band experiment is sensitive to the non-coincidence. An analysis of the EPR spectra of 1(-) reveals a 24 % Ni contribution to the SOMO in 1(-), supporting a delocalisation of the spin-density across the NiFe(2) cluster. This observation is supported by IR spectroscopic results which show that the CO stretching frequencies, nu(CO), shift to lower frequency by about 70 cm(-1) when 1 is reduced to 1(-). Density functional calculations provide a framework for the interpretation of the spectroscopic properties of 1(-) and suggest that the SOMO is delocalised over the whole cluster, but with little S-centre participation. This electronic structure contrasts with that of the Ni-A, -B, -C and -L forms of [NiFe] hydrogenase in which there is considerable S participation in the SOMO. 相似文献
352.
Walter Purcell Andreas Roodt Stephen S Basson Johann G Leipoldt 《Transition Metal Chemistry》1989,14(1):5-6
Summary The crystal structure of (PPh4)2[ReO(OH)(CN)4]·5H2O has been determined from three-dimensional x-ray diffraction data. The light brown crystals are monoclinic, space group P21/n, with cell dimensionsa=16.753(2),b=19.928(2),c=15.338(2) Å and =101,894(1)°,z=4, Dm=1.45(1) g cm–3. The anisotropic refinement of the 6088 observed reflections converged to R=0.077.The [ReO(OH)(CN)4]2– ion has a distorted octahedral geometry. Bond distances: Re =1.70(1), Re–OH=1.90(1) and ReCav=2.12(2) Å. The Re atom is displaced by 0.08 Å out of the plane formed by the four carbon atoms towards the terminal oxo ligand. 相似文献
353.
The reaction between 1.5 equiv of elemental iodine and rare earth metals in powder form in THF at room temperature gives the rare earth triiodides LnI(3)(THF)(n)() in good yields. Purification by Soxhlet extraction of the crude solids with THF reliably gives the THF adducts LnI(3)(THF)(4) [Ln = La, Pr] and LnI(3)(THF)(3.5) [Ln = Nd, Sm, Gd, Dy, Er, Tm, Y] as microcrystalline solids. X-ray crystallography reveals that the early, larger lanthanide iodide PrI(3)(THF)(4) crystallizes as discrete molecules having a pentagonal bipyramidal structure, whereas the later, smaller lanthanide iodides LnI(3)(THF)(3.5) [Ln = Nd, Gd, Y] crystallize as solvent-separated ion pairs [LnI(2)(THF)(5)][LnI(4)(THF)(2)] in which the cations adopt a pentagonal bipyramidal geometry and the anions adopt an octahedral geometry in the solid state. 相似文献
354.
Treatment of several divalent transition-metal trifluoromethanesulfonates [M(II)(OTf)2; M(II) = Mn, Co, Ni] with [NEt4][Tp*Fe(III)(CN)3] [Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate] in DMF affords three isostructural rectangular clusters of {[Tp*Fe(III)(CN)3M(II)(DMF)4]2[OTf]2} x 2DMF (M(II) = Mn, 3; Co, 4; Ni, 5) stoichiometry. Magnetic studies of 3-5 indicate that the Tp*Fe(CN)3(-) centers are highly anisotropic and exhibit antiferromagnetic (3 and 4) and ferromagnetic (5) exchange to afford S = 4, 2, and 3 spin ground states, respectively. ac susceptibility measurements suggest that 4 and 5 exhibit incipient single-molecule magnetic behavior below 2 K. 相似文献
355.
We report a novel source of tunable, coherent radiation in the vacuum ultraviolet, which use resonantly enhanced difference-frequency mixing in xenon gas Efficiencies are high (o.4%) at modest input laser intensities. With existing dye lasers the range of tunability will span the 152–200 nm region. 相似文献
356.
Stephen T. L. Choy 《Analysis Mathematica》1981,7(4):235-237
S — E, F — . C(S, E) S E. : C(S, E) F C(S) . , C(S, E) C(S). 相似文献
357.
Stephen Pierce 《Journal of Geometry》1981,16(1):30-40
Let V be an n-dimensional regular quadratic space over a field K of characteristic not 2. Assume n 4. Let W be a regular hyperplane and v a nonzero vector orthogonal to W. Suppose every regular hyperplane in W is universal. If is an isometry of V not leaving W invariant, then , together with the isometries of W, generate the orthogonal group of V, with one exception.The work of the author was partially supported by NSERC Grant A-7862. 相似文献
358.
A pH titration study shows that 6(A)-((2-(bis(2-aminoethyl)amino)ethyl)amino)-6(A)-deoxy-beta-cyclodextrin (betaCDtren) forms binary metallocyclodextrins, [M(betaCDtren)](2+), for which log(K/dm(3) mol(-)(1)) = 11.65 +/- 0.06, 17.29 +/- 0.05, and 12.25 +/- 0.03, respectively, when M(2+) = Ni(2+), Cu(2+), and Zn(2+), where K is the stability constant in aqueous solution at 298.2 K and I = 0.10 mol dm(-)(3) (NaClO(4)). The ternary metallocyclodextrins [M(betaCDtren)Trp](+), where Trp(-) is the tryptophan anion, are characterized by log(K/dm(3) mol(-)(1)) = 8.2 +/- 0.2 and 8.1 +/- 0.2, 9.5 +/- 0.3 and 9.4 +/- 0.2, and 8.1 +/- 0.1 and 8.3 +/- 0.1, respectively, where the first and second values represent the stepwise stability constants for the complexation of (R)- and (S)-Trp(-), respectively, when M(2+) = Ni(2+), Cu(2+), and Zn(2+). From comparisons of stabilities and UV-visible spectra, the binary and ternary metallocyclodextrins appear to be six-coordinate when M(2+) = Ni(2+) and Zn(2+) and five-coordinate when M(2+) = Cu(2+). The factors affecting the stoichiometries and stabilities of the metallocyclodextrins, are discussed and comparisons are made with related systems. 相似文献
359.
Multidentate naphthyridine-based ligands were used to prepare a series of diiron(II) complexes. The compound [Fe(2)(BPMAN)(mu-O(2)CPh)(2)](OTf)(2) (1), where BPMAN = 2,7-bis[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine, exhibits two reversible oxidation waves with E(1/2) values at +310 and +733 mV vs Cp(2)Fe(+)/Cp(2)Fe, as revealed by cyclic voltammetry. Reaction with O(2) or H(2)O(2) affords a product with optical and M?ssbauer properties that are characteristic of a (mu-oxo)diiron(III) species. The complexes [Fe(2)(BPMAN)(mu-OH)(mu-O(2)CAr(Tol))](OTf)(2) (2) and [Fe(2)(BPMAN)(mu-OMe)(mu-O(2)CAr(Tol))](OTf)(2) (3) were synthesized, where Ar(Tol)CO(2)(-) is the sterically hindered ligand 2,6-di(p-tolyl)benzoate. Compound 2 has a reversible redox wave at +11 mV, and both 2 and 3 react with O(2), via a mixed-valent Fe(II)Fe(III) intermediate, to give final products that are also consistent with (mu-oxo)diiron(III) species. The paddle-wheel compound [Fe(2)(BBAN)(mu-O(2)CAr(Tol))(3)](OTf) (4), where BBAN = 2,7-bis(N,N-dibenzylaminomethyl)-1,8-naphthyridine, reacts with dioxygen to yield benzaldehyde via oxidative N-dealkylation of a benzyl group on BBAN, an internal substrate. In the presence of bis(4-methylbenzyl)amine, the reaction also produces p-tolualdehyde, revealing oxidation of an external substrate. A structurally related compound, [Fe(2)(BEAN)(mu-O(2)CAr(Tol))(3)](OTf) (5), where BEAN = 2,7-bis(N,N-diethylaminomethyl)-1,8-naphthyridine, does not undergo N-dealkylation, nor does it facilitate the oxidation of bis(4-methylbenzyl)amine. The contrast in reactivity of 4 and 5 is attributed to a difference in accessibility of the substrate to the diiron centers of the two compounds. The M?ssbauer spectroscopic properties of the diiron(II) complexes were also investigated. 相似文献
360.
Lock JS May BL Clements P Lincoln SF Easton CJ 《Organic & biomolecular chemistry》2004,2(9):1381-1386
The preparation of 6(A)-deoxy-6(A)-(6-(2-(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)acetamido)hexylamino)-alpha-cyclodextrin, 3, 6(A)-deoxy-6(A)-(6-(2-(1,4,7,10,13-pentaoxa-16-azacyclooctadecan-16-yl)acetamido)hexylamino)-alpha-cyclodextrin, 4, and their beta-cyclodextrin analogues, 5 and 6, are described. (1)H (600 MHz) ROESY NMR spectra of the C(6) substituted beta-cyclodextrins, 5 and 6, are consistent with the intramolecular complexation of their azacyclopentadecanyl- and azacyclooctadecanyl(acetamido)hexylamino substituents in the beta-cyclodextrin annulus in D(2)O at pD = 8.5 whereas those of their alpha-cyclodextrin analogues, 3 and 4 are not complexed in the alpha-cyclodextrin annulus. This is attributed to the monoazacoronand components of the substituents being able to pass through the beta-cyclodextrin annulus whereas they are too large to pass through the alpha-cyclodextrin annulus. However, the substituents of 3 and 4 are intermolecularly complexed by beta-cyclodextrin to form pseudo [2]-rotaxanes. Metallocyclodextrins are formed by 5 through complexation by the monoazacoronand substituent component for which log (K/dm(3) mol(-1))= <2, 6.34 and 5.38 for Ca(2+), Zn(2+) and La(3+), respectively, in aqueous solution at 298.2 K and I= 0.10 mol dm(-3)(NEt(4)ClO(4)). 相似文献