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991.
A compound reported earlier (Polyhedron 1989, 8, 2339) as (Bu(n)()(4)N)(2)H(2)[Mo(2)[Mo(CO)(4)(PhPO(2))(2)](2)] has been reexamined. We find that the hydrogen atoms in this formula are not present. Therefore, the complex must be considered as having a central triply bonded Mo(2)(6+) unit, instead of a quadruply bonded Mo(2)(4+) unit. Our conclusion is based on a variety of experimental evidence, including X-ray crystal structures of four crystal forms, as well as the neutron crystal structure of one. This explains the relatively long Mo-Mo bond lengths found in the range 2.1874(7)-2.2225(7) A and the absence of a delta --> delta transition in the visible spectrum. From electrochemistry we also find that the diphosphonate ligand has such an exceptional ability to stabilize higher oxidation states that even common solvents such as CH(2)Cl(2) and C(2)H(5)OH readily oxidize the Mo(2)(4+) unit that is introduced from the Mo(2)(O(2)CCH(3))(4) or [Mo(2)(O(2)CCH(3))(2)(NCCH(3))(6)](BF(4))(2) employed in the preparation. The only chemically reversible wave at E(1/2) = -1.54 V vs Ag/AgCl corresponds to the reduction process Mo(2)(6+) --> Mo(2)(5+). 相似文献
992.
Chin-Hsing Chou Li-Tse Chu Shao-Jung Chiu Chin-Fan Lee Yao-Teng She 《Tetrahedron》2004,60(31):6581-6584
Flash vacuum pyrolysis of arylmethylazides 7a-d gave 2,4-diazapentadienes 5a-d in high yield (76-92%). The thermal cyclization of 5a-d gave cis-imidazolines 1a-d, further heating or Swern oxidation of 1a-d gave dehydrogenated products, imidazoles 2a-d. 相似文献
993.
994.
Water-soluble cyclodextrin polymers for enhanced relative recovery of hydrophobic analytes during microdialysis sampling 总被引:2,自引:0,他引:2
Microdialysis relative recovery (RR) enhancement using different water-soluble, epichlorohydrin-based cyclodextrin polymers (CD-EPS) was studied in vitro for different analytes, amitryptiline, carbamazepine, hydroquinone, ibuprofen, and 4-nitrophenol. When compared to the native CDs (alpha, beta, and gamma) on a per mole basis, the CD-EPS enhanced microdialysis RR was either statistically greater or the same. beta-CD-EPS was more highly retained than native beta-CD by a 20 000 Da molecular weight cutoff (MWCO) polycarbonate membrane, but showed no statistical difference for loss across a 100 000 Da MWCO polyethersulfone membrane (PES). When the same weight percent of beta-CD or beta-CD-EPS was included in the microdialysis perfusion fluid, the beta-CD-EPS produced a higher microdialysis RR than native beta-CD for all analytes across the PES membrane. However, enhancements for the PC membrane were statistically insignificant when beta-CD and beta-CD-EPS were compared on a per mole basis. These results suggest that CD-EPS may be used as effective enhancement agents during microdialysis sampling and for some membranes provide the additional advantage of being retained more than native CDs. 相似文献
995.
Voltammetric behavior of sodium 7‐methoxyl‐4′‐hydroxylisoflavone‐3′‐sulfonate (SMHS) in the aqueous solution from pH 1 to 5 was studied by linear sweep voltammetry, cyclic voltammetry and normal pulse voltammetry. Experimental results showed that in 0.2 mol*L?1 sodium citrate‐hydrochloric acid buffer solution (pH=4.65), SMHS caused only one reduction wave at ?1.34 V (vs. saturated calomel electrode, SCE), which was an h‐reversible adsorptive wave of SMHS protonized involving one electron and one proton. The peak current of SMHS on linear sweep voltammogram was proportional to its concentration in the range of 8.0 × 10 ?8.0·10 mol*L?1 (r = 0.995). and the detection limit was 5.0·10?‐6mol*L?1. The method was applied to determination of SMHS, in synthetic samples. In addition, its scavenging effect on superoxide anion radical was studied by the auto‐oxidation of pyrogallol in HCI‐tris buffer solution (pH = 8.2) in order to explain its peculiar biological effects. The experimental results proved that SMHS has antioxidant quality, and it is an efficient free radical scavenger of superoxide anion radical. 相似文献
996.
Li S Cao P Colorado R Yan X Wenzl I Shmakova OE Graupe M Lee TR Perry SS 《Langmuir : the ACS journal of surfaces and colloids》2005,21(3):933-936
Compositionally mixed, self-assembled monolayers (SAMs) derived from 16,16,16-trifluorohexadecanethiol and a normal alkanethiol, either hexadecanethiol or pentadecanethiol, were formed on Au(111) substrates. The relative composition of the films was determined using X-ray photoelectron spectroscopy and was found to approximately equal the equimolar composition of the isooctane solution from which they were formed. The frictional properties of the mixed films were measured on the nanometer scale using atomic force microscopy and were observed to decrease when the chain length of the CH(3)-terminated component was shortened by one methylene unit (i.e., when hexadecanethiol was replaced by pentadecanethiol). For comparison, the frictional properties of a mixed-chain-length CH(3)-terminated SAM derived from hexadecanethiol and pentadecanethiol in a 1:1 ratio was also examined. In contrast to the mixed CF(3)/CH(3) system, the latter mixed-chain-length system exhibited relatively higher friction when compared to single-component SAMs derived solely from either hexadecanethiol or pentadecanethiol. For both types of mixed films, the change in frictional properties that occurs as a result of modifying the position of neighboring terminal groups with respect to the surface plane is discussed in terms of the influence of local packing environments on interfacial energy dissipation (friction). 相似文献
997.
Berry JF Cotton FA Fewox CS Lu T Murillo CA Wang X 《Dalton transactions (Cambridge, England : 2003)》2004,(15):2297-2302
Structural studies of pentachromium and pentacobalt extended metal atom chain (EMAC) systems are presented in which the metal chains are helically wrapped by either the pentadentate dianion tripyridyldiamide (tpda) or its diethyl-substituted analogue bis(4-ethyl-2-pyridylamido)pyridine (etpda). The compound Cr(5)(tpda)(4)(NCS)(2), has alternating long and short Cr-Cr distances, contrary to recent reports that describe it as symmetrical with essentially equally spaced chromium atoms. The linear Cr(5)(10+) chain is composed of two Cr(2)(4+) quadruply bonded units and an isolated high spin Cr(II) unit. The new compounds Cr(5)(etpda)(4)Cl(2), [Cr(5)(etpda)(4)FCl]PF(6), and Co(5)(etpda)(4)(NCS)(2)() employ the etpda ligand as "insulation" around a central pentametal-atom "wire." Compound, like all other oxidized pentachromium compounds, has very disparate alternating short-long-short-long Cr-Cr distances, 2.032(3) A, 2.560(6) A, 1.873(5) A, and 2.509(4) A. Compound shows a nearly uniform spacing of the Co atoms, although the outer Co-Co distances (2.279[4] A) are slightly longer than the inner ones (2.239[4] A). 相似文献
998.
Xiaoping Gao Chaozhi Wan Tianjing He Junqing Li Huwen Xin Fan-chen Liu 《Chemical physics letters》1984,112(5):465-468
The enhanced fluorescence and enhanced Raman spectra of dye-1555 molecules adsorbed on silver chloride sols are detected for the first time. The enhanced fluorescence is attributed to the double resonance effect proposed by Chew and Wang. The enhanced Raman spectra indirectly support the energy-transfer model of spectral sensitization of dyes, one of two competing mechanisms of spectral sensitization in the photographic process. 相似文献
999.
阴离子交换色谱中淋洗液流速对测定灵敏度影响的讨论 总被引:3,自引:0,他引:3
研究了阴离子交换色谱法定量分析时淋洗液流速对测定灵敏度影响的规律。使用3种不同型号的阴离子交换柱,详细地研究了3种总离子浓度不同的样品中淋洗液流速对峰高和峰面积的影响。实验表明,若标准溶液和样品溶液中的阴离子总浓度相差较大,将在不同程度上影响分析结果的准确度。初步探讨了上述现象的机理。通过紫外(210nm)和电导检测结果的对比观察,说明灵敏度的变化不完全如文献中提到的由抑制器抑制不足或NO-2质子化所致。在相同条件下改变柱温后的实验结果表明也不是离子交换过程中离子传质速度比较慢引起离子交换不完全的原因。提出的其它机理假设仍需进一步研究证实。 相似文献
1000.
Zhou Lu Bhaskar Chilukuri Chi Yang Abdel-Monem M. Rawashdeh Ravi K. Arvapally Sammer M. Tekarli Xiaoping Wang Christian T. Cardenas Thomas R. Cundari Mohammad A. Omary 《Chemical science》2020,11(41):11179
An efficient strategy for designing charge-transfer complexes using coinage metal cyclic trinuclear complexes (CTCs) is described herein. Due to opposite quadrupolar electrostatic contributions from metal ions and ligand substituents, [Au(μ-Pz-(i-C3H7)2)]3·[Ag(μ-Tz-(n-C3F7)2)]3 (Pz = pyrazolate, Tz = triazolate) has been obtained and its structure verified by single crystal X-ray diffraction – representing the 1st crystallographically-verified stacked adduct of monovalent coinage metal CTCs. Abundant supramolecular interactions with aggregate covalent bonding strength arise from a combination of M–M′ (Au → Ag), metal–π, π–π interactions and hydrogen bonding in this charge-transfer complex, according to density functional theory analyses, yielding a computed binding energy of 66 kcal mol−1 between the two trimer moieties – a large value for intermolecular interactions between adjacent d10 centres (nearly doubling the value for a recently-claimed Au(i) → Cu(i) polar-covalent bond: Proc. Natl. Acad. Sci. U.S.A., 2017, 114, E5042) – which becomes 87 kcal mol−1 with benzene stacking. Surprisingly, DFT analysis suggests that: (a) some other literature precedents should have attained a stacked product akin to the one herein, with similar or even higher binding energy; and (b) a high overall intertrimer bonding energy by inferior electrostatic assistance, underscoring genuine orbital overlap between M and M′ frontier molecular orbitals in such polar-covalent M–M′ bonds in this family of molecules. The Au → Ag bonding is reminiscent of classical Werner-type coordinate-covalent bonds such as H3N: → Ag in [Ag(NH3)2]+, as demonstrated herein quantitatively. Solid-state and molecular modeling illustrate electron flow from the π-basic gold trimer to the π-acidic silver trimer with augmented contributions from ligand-to-ligand’ (LL′CT) and metal-to-ligand (MLCT) charge transfer.A stacked Ag3–Au3 bonded (66 kcal mol−1) complex obtained crystallographically exhibits charge-transfer characteristics arising from multiple cooperative supramolecular interactions. 相似文献