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21.
Chisholm MH Macintosh AM Huffman JC Wu D Davidson ER Clark RJ Firth S 《Inorganic chemistry》2000,39(16):3544-3550
Mo2(OtBu)6 and Mo2(NMe2)6 each react with (S,S,S)-triisopropanolamine (2 equiv) in benzene to yield dimolybdenum bis((S,S,S)-isopropanolaminate(3-)), Mo2[(OC-(S)-HMeCH2)3N]2 (M identical to M), as a blue crystalline solid. Cell parameters at -160 degrees C: a = 17.389(6) A, b = 10.843(3) A, c = 10.463(3) A, beta = 125.28(1) degrees, Z = 2 in space group C2. The molecular structure involves an Mo2 unit inside an O6N2 distorted cubic box. The Mo2 axis is disordered about three positions with occupancy factors of ca. 45%, 45%, and 10%. Despite this disorder, the molecular structure is shown to contain a central Mo identical to Mo unit of distance 2.15(3) A coordinated to two triolate ligands which each have two chelating arms and one that spans the Mo identical to Mo bond. The local Mo2O6N2 moiety has approximate C2h symmetry, and the Mo-N distances are long, 2.4 A. The 1H and 13C(1H) NMR spectra recorded in benzene-d6 are consistent with the geometry found in the solid-state structure. The blue color arises from weak absorptions, epsilon approximately 150 dm3 mol-1 cm-1, at 580 and 450 nm in the visible region of the electronic absorption spectrum. Raman spectra recorded in KCl reveal pronounced resonance effects with excitation wavelengths of 488.0, 514.5, and 568.2 nm, particularly for the 322 cm-1 band, which can probably be assigned to nu(Mo identical to Mo). The electronic structure of this compound is investigated by B3LYP DFT calculations, and a comparison is made with the more typical ethane-like (D3d) Mo2(OR)6 compounds is presented. The distortion imposed on the molecule by the triisopropanolaminate(3-) ligands removes the degeneracy of the M-M pi molecular orbitals. The HOMO and SHOMO are both M-M pi and M-O sigma* in character, while the LUMO is M-M pi* and the SLUMO is predominantly M-O sigma* with metal sp character. The calculated singlet-singlet transition energies are compared with those implicit in the observed electronic spectrum. 相似文献
22.
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
23.
Diane C. Rawlings Martin Gouterman Ernest R. Davidson David Feller 《International journal of quantum chemistry》1985,28(6):823-842
Ab initio CI calculations are reported on the lowest doublet, quartet, and sextet states of [FeIII(P)(NH3)2]+. The low-spin ground state is calculated as (dxy2 (dπ)3 with dxy(dπ)4 higher by 0.15 eV. The near-ir bands at ~1 eV observed in low-spin ferriheme proteins are attributed to (π → dπ) transitions. The lowest high-spin state is 6A1g, and the near-ir transitions of the high-spin ferriheme proteins observed at ~1.2 eV are attributed to higher 6[tripsextet] excited states [i.e., ring triplet, metal sextet]. The 30-ps “triplet” transient populated with low quantum yield observed in laser-flash studies on FeIII(TPP)CI [TPP = tetrapbenylporphyrin] may be an 1[tripsextet] state. 相似文献
24.
Ernest R. Davidson 《International journal of quantum chemistry》1974,8(1):83-89
It is shown that the turn-over rule which is used to simplify matrix elements between spin-projected Slater determinants may also be used to simplify formation of matrix elements between any orthonormal set of spin-coupled Slater determinants. The coefficients for the spin-coupling may then be chosen freely to reduce the number of important configurations in the secular equation. 相似文献
25.
The ergosterol content of soil can be used as an indicator of fungal activity. A method has been developed for the extraction and determination of ergosterol in organic soils, as part of a study to assess the correlation between fungal activity and the sequestration of metal pollutants. The moisture content of the soil affected the extraction process. Four consecutive extractions with methanol removed >95% of the ergosterol that can be obtained from the fresh sample (63% moisture) by exhaustive extraction. By freeze drying the soils prior to extraction (a) up to 35% more ergosterol was extracted after a single extraction, (b) >90% of the recoverable ergosterol was collected in two extractions and (c) the repeatability of the extraction was improved. Storage of soil extracts in the absence of light prevents degradation of ergosterol. A previously reported method for determination of ergosterol by HPLC has been improved by modification of the eluant composition. With 46% methanol/46% acetonitrile/8% dichloromethane, ergosterol was eluted with good resolution approximately 8 min after injection of 20 mul of the extract. The detection limit of the HPLC method was 0.5 mug/ml ergosterol, equivalent to 0.06 mug/g in 25 g fresh soil. Changes in ergosterol contents of peaty soil treated with fungicide, and in samples of the peaty podzol and a humus iron podzol in the vicinity of fungal fruiting bodies, have been determined. 相似文献
26.
Inspired by the determination of the structure of the nitrogenase enzyme cofactor by Rees et al., the binding of an N2 molecule to some model iron-sulfur compounds was investigated usingab initio calculations. Side-on and end-on coordination to one two and four iron centers were investigated. In most cases, the N2 is loosely bound and retains its internal triple bond, but a few examples are found where the N2 is “activated” and has a longer N-N bond length. 相似文献
27.
Experiments in a positive-polarity, wire/plate electrode establish the effects of the concentration of octamethylcyclotetrasiloxane (150–1100 ppm) and the operating current (0.5–2.55 μA/cm length of wire) on the rate of deposition of silicon dioxide on the high voltage wire. The wire is 100 μm radius tungsten and the wire-to-plate spacing is 1.5 cm. Analyses of the deposit with X-ray diffraction, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy show that it is amorphous silicon dioxide. The deposition rate increases linearly with increasing silicone concentration and corona current. For the concentrations of silicone likely to present in indoor air, the gas-phase processes limit the rate of deposition. 相似文献
28.
R. A. English S. G. Davidson Z. L. Miskovic F. O. Goodman A. T. Amos B. L. Burrows 《Progress in Surface Science》1996,53(2-4):323-330
A recursive Green-function (RGF) technique is used to study the surface states of an electrified solid. The concatenating nature of the method leads to a continued-fraction form of the RGF, which can be expressed as a ratio of Bessel functions. The surface density of states (SDOS), derived from the RGF, reveals a quasi-Stark-ladder distribution of the energy levels at the surface atom. The dependence of the SDOS on the applied field and surface perturbation parameters is discussed. 相似文献
29.
A microwave-assisted digestion procedure using HNO3, HF, and H2O2 has been developed for analysis of elements in ambient particulate matter (PM). The samples are collected on cellulose filters and analyzed by inductively coupled plasma mass spectrometry (ICP-MS). The ICP-MS is calibrated with external standards, and recovery of analytes is tested with NIST SRM 1648 Urban Dust. This method has been used to quantify the airborne concentrations of a large number of elements, including Ag, As, Ba, Be, Ca, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, K, Li, Mg, Mn, Mo, Ni, Pb, Rb, Se, Sb, Sr, Ti, Tl, V, and Zn. For the majority of these elements, recovery of the NIST SRM is within 15% of the certified values. 相似文献
30.
Inductively coupled plasma mass spectrometry (ICP/MS) is utilized as a detector for several organotin species separated by high-performance liquid chromatography. Detection limits obtained by ICP/MS are 3 orders of magnitude lower than those obtained with inductively coupled plasma atomic emission spectrometry (ICP/AES) detection under the same chromatographic conditions. Chromatographic detection limits are higher than conventional solution nebulization for the same compound by a factor of 20. Ion-exchange chromatography yields linear response over 3 orders of magnitude, while ion pair chromatography gives a linear response of only 2 orders of magnitude as a result of poor resolution. The relative standard deviation for the injection of 20 ng of tin compounds is less than 10%. 相似文献