全文获取类型
收费全文 | 1252篇 |
免费 | 60篇 |
国内免费 | 4篇 |
专业分类
化学 | 912篇 |
晶体学 | 7篇 |
力学 | 27篇 |
数学 | 76篇 |
物理学 | 294篇 |
出版年
2023年 | 12篇 |
2022年 | 8篇 |
2021年 | 19篇 |
2020年 | 17篇 |
2019年 | 17篇 |
2018年 | 13篇 |
2017年 | 9篇 |
2016年 | 34篇 |
2015年 | 29篇 |
2014年 | 38篇 |
2013年 | 63篇 |
2012年 | 84篇 |
2011年 | 101篇 |
2010年 | 63篇 |
2009年 | 57篇 |
2008年 | 78篇 |
2007年 | 80篇 |
2006年 | 84篇 |
2005年 | 66篇 |
2004年 | 55篇 |
2003年 | 50篇 |
2002年 | 42篇 |
2001年 | 23篇 |
2000年 | 27篇 |
1999年 | 18篇 |
1998年 | 11篇 |
1997年 | 9篇 |
1996年 | 21篇 |
1995年 | 8篇 |
1994年 | 16篇 |
1993年 | 14篇 |
1992年 | 17篇 |
1991年 | 10篇 |
1990年 | 8篇 |
1989年 | 10篇 |
1987年 | 5篇 |
1986年 | 9篇 |
1985年 | 15篇 |
1984年 | 7篇 |
1983年 | 7篇 |
1982年 | 10篇 |
1981年 | 8篇 |
1980年 | 7篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1975年 | 5篇 |
1974年 | 2篇 |
1973年 | 4篇 |
排序方式: 共有1316条查询结果,搜索用时 31 毫秒
981.
The fragmentation process of ionized 1,4-dioxane and the reactions between the C3H6O+* ions, one of the major fragments, and various reactants (including acetonitrile, formaldehyde, ethylene, and propene) have been studied experimentally with mass spectrometry. In the present work, G3(MP2) calculations were carried out to investigate these processes theoretically. In agreement with experiment, isomers CH3OCHCH2+* (1) and *CH2CH2OCH2+ (2) were found to be the C3H6O+* ions fragmented from ionized 1,4-dioxane, with 2 being the major product. The mechanisms of the formation of 1 and 2 were successfully established. In addition, the characteristic reactivities, as well as the corresponding reaction mechanisms, of both isomers were rationalized with the aid of calculations. Finally, a minor reaction between isomer 2 and propene was identified, and the presence of the product of this reaction was found to be useful in explaining the aforementioned mass spectrometric data. 相似文献
982.
A series of leflunomide (1a) analogues were examined for antiinflammatory activity using the carrageenan-induced paw edema assay. Some of the compounds were significantly more potent than leflunomide, particularly those with electron-donating or negative inductive groups situated in the phenyl rings. In contrast, all the nonsubstituted compounds or with further chain-extension in the 4-position of the rings led to a decrease in activity. The LD(50) values of the most active compounds (1d, g-j) in male ICR mice were significantly greater than those of either 1a or its active metabolite 2 and therefore merit further study. 相似文献
983.
In addition to pyridines, α‐chloroformylarylhydrazine hydrochloride 1 can also react with some N‐heterocyclic compounds. The cycloaddition of 1 with isoquinoline was achieved to obtain 3 . The production of 4, 5, 6 given by cycloaddition of 1 with pyridazine was de pendent on the reaction condition. Some heterocyclic compounds bearing an X‐C=N (X:S, N) group on the ring can react with 1 to gain the derivatives of 2,4‐dihydro‐1,2,4‐triazol‐3‐one. 7, 8, 9 and 10 were given by reaction of 1 with 1,3,5‐triazine, 1,4,5,6‐tetrahydropyrimidine, 1,3‐thiazole and 2‐amino‐1,3‐thiazole, respectively. The reactions for 2‐amino‐1,3,4‐thiadiazole and 3‐amino‐1,2,4‐triazole had the same product 11 . 相似文献
984.
A chemically addressable, bistable [2]rotaxane, which incorporates a dumbbell-shaped component containing both secondary dialkylammonium and 1,2-bis(pyridinium)ethane recognition sites for its ring component, dibenzo[24]crown-8 (DB24C8), has been assembled. (1)H NMR spectroscopy has demonstrated that deprotonation (and reprotonation) of the secondary dialkylammonium (dialkylamine) recognition site induces the DB24C8 ring to move away from this site to the 1,2-bis(pyridinium)ethane one (and back again) in a discrete manner, particularly when the experiment is performed in CDCl(3) solution. 相似文献
985.
Pi-Tai Chou Chun-Yen Chen Chiu-Ting Cheng Shih-Chieh Pu Kun-Chan Wu Yi-Ming Cheng Chih-Wei Lai Yi-Hsuan Chou Hsin-Tien Chiu 《Chemphyschem》2006,7(1):222-228
Syntheses of CdTe/CdSe type-II quantum dots (QDs) using CdO and CdCl2 as precursors for core and shell, respectively, are reported. Characterization was made via near-IR interband emission, transmission electron microscopy (TEM), energy dispersive spectroscopy (EDX), and X-ray diffraction (XRD). Femtosecond fluorescence upconversion measurements on the relaxation dynamics of the CdTe core (in CdTe/CdSe) emission and CdTe/CdSe interband emission reveal that as the size of the core increases from 5.3, 6.1 to 6.9 nm, the rate of photoinduced electron separation decreases from 1.96, 1.44 to 1.07 x10(12) s(-1). The finite rates of the initial charge separation are tentatively rationalized by the small electron-phonon coupling, causing weak coupling between the initial and charge-separated states. 相似文献
986.
Treatment of trans-[RuIII(16-TMC)Cl2]Cl (16-TMC = 1,5,9,13-tetramethyl-1,5,9,13-tetraazacyclohexadecane) with zinc metal in aqueous solution under nitrogen resulted in the formation of trans-[RuII(16-TMC)(N2)Cl]PF6 (1) which has been characterized by X-ray crystal analysis. The Ru---N2 and N---N bond distances in 1 are 1.921(6) and 1.005(10) Å, respectively. 相似文献
987.
Ying-Nan Chiu 《中国化学会会志》1985,32(3):201-206
It is shown that time-reversal (doubly-) degenerate, many-electron states in molecules of point-group symmetry C3, C4, C6, S4, and S6 and T etc., can have non-vanishing matrix elements over a time-odd (electric dipole-electric dipole) polarizability operator contributing to optical rotation. In agreement with well-known results for Kramers' doublets, the optical rotations of the two separated and oriented states of this doublet have opposite signs in this polarizability mechanism, and they have the same sign in the time-even pseudoscalar mechanism which is the usual natural optical rotation of chiral molecules. These results are proven, in an alternative formulation using time-reversal in a second-order process, to hold regardless of even or odd numbers of spins—in contrast to the first-order processes such as the Jahn-Teller effect. The universality of time-reversal in spin, orbital and rotational angular momentum, in point and continuous groups, is show in a unified treatment with consistent phases. It was shown also how time-reversal symmetry can resolve the ambiguities in lower point-groups and determine relationships for which the point-group symmetry is powerless. 相似文献
988.
Cubic shells and spherical nanoparticles of β‐SiC were produced at 1273 K by processing the ceramic precursors formed from the reactions between vapors of organochlorosilanes, Me2SiCl2, MeSiCl3, MeSiHCl2, and PhSiCl3, and liquid Na at 523‐723 K. From Me2SiCl2, a flexible linear polycarbosilane precursor was synthesized and covered the NaCl byproduct surface to form a cubic shape. Hollow cubic β‐SiC shells were produced after the NaCl templates were removed. From MeSiCl3, a rigid cross‐linked polycarbosilane was produced and phase segregated from the NaCl byproduct. The precursor was transformed into nanoparticles without special morphology. MeSiHCl2 produced a cross‐linked polysilane precursor at low temperatures, which can be converted into a mixture of β‐SiC and Si nanoparticles. At high temperatures, the polysilane converted to polycarbosilane and produced hollow cubic β‐SiC shells. The carbon‐rich PhSiCl3 generated cube‐like particles as the final product, which contained β‐SiC and carbon. 相似文献
989.
This paper describes the synthesis and loading of silica and polystyrene-acrylic based nanocapsules with small molecules. The nanocapsules are used for delivering defined packages of stimuli to single cells with both high spatial and temporal resolutions. To introduce molecules into the capsules, we characterized two approaches. The first approach is based on a base-swell process in which the shell of the capsule is swelled so small molecules can diffuse into the interior of the capsule and be trapped inside once the capsules are de-swelled. The second approach is based on a dry-swell-dry process in which the solution containing the molecules of interest and the nanocapsules is physically dried to promote more molecules to enter into the interior of the capsule. We characterized both methods by monitoring the content of and the release from individual capsules with confocal microscopy and wide-field imaging. To illustrate the biological applications of such nanocapsules, we used optical trapping to position a single carbachol-loaded capsule adjacent to a single CHO cell that has been transfected with muscarinic acetylcholine (M1) receptors, released the carbachol from the capsule with a single 3-ns N2 laser pulse, and then monitored the subsequent intracellular signaling triggered by the binding of carbachol to the M1 receptors. 相似文献
990.
trans, trans‐Farnesyl diphosphate (FPP) serves as a universal substrate for a large family of sesquiterpene cyclases that are responsible for biosynthesis of more than 300 structurally diverse sesquiterpenes in nature. A new FPP substrate analogue, 12‐fluoro‐farnesylphosphonophosphate (12‐F‐F‐CH2PP), was synthesized in this paper for applications on kinetic and mechanistic studies of the enzyme family. Trichodiene synthase (TS), a sesquiterpene cyclase, catalyzes the conversion of trans, trans‐farnesyl diphosphate (FPP) to trichodiene. 12‐F‐F‐CH2PP was tested as a potential inhibitor of TS. Inactivation and inhibition kinetic experiments showed that 12‐F‐F‐CH2PP was not a mechanism‐based inactivator for TS; instead, a mixed‐type reversible inhibition was observed with inhibition constants Ki1 = 2.33 ± 0.50 μM and Ki2 = 25.80 ± 7.70 μM, values close to those previously determined for farnesylphosphonophosphate, Ki1 = 3.25 μM and Ki2 = 9.10 μM. Although 12‐F‐F‐CH2PP did not irreversibly inactivate TS, this new analogue serves as a potential active‐site directed inactivator and mechanistic probe of other sesquiterpene cyclases and FPP‐utilizing enzymes, which utilize FPP as a common acyclic substrate. 相似文献