全文获取类型
收费全文 | 1716篇 |
免费 | 46篇 |
国内免费 | 7篇 |
专业分类
化学 | 1223篇 |
晶体学 | 18篇 |
力学 | 19篇 |
数学 | 46篇 |
物理学 | 463篇 |
出版年
2023年 | 9篇 |
2021年 | 13篇 |
2020年 | 18篇 |
2019年 | 20篇 |
2018年 | 20篇 |
2016年 | 21篇 |
2015年 | 32篇 |
2014年 | 48篇 |
2013年 | 66篇 |
2012年 | 89篇 |
2011年 | 94篇 |
2010年 | 47篇 |
2009年 | 61篇 |
2008年 | 78篇 |
2007年 | 103篇 |
2006年 | 90篇 |
2005年 | 89篇 |
2004年 | 58篇 |
2003年 | 60篇 |
2002年 | 68篇 |
2001年 | 51篇 |
2000年 | 27篇 |
1999年 | 14篇 |
1998年 | 12篇 |
1997年 | 19篇 |
1996年 | 23篇 |
1995年 | 14篇 |
1994年 | 17篇 |
1993年 | 22篇 |
1992年 | 32篇 |
1991年 | 24篇 |
1990年 | 31篇 |
1989年 | 21篇 |
1988年 | 30篇 |
1987年 | 33篇 |
1986年 | 34篇 |
1985年 | 32篇 |
1984年 | 20篇 |
1983年 | 11篇 |
1982年 | 18篇 |
1981年 | 19篇 |
1980年 | 17篇 |
1979年 | 26篇 |
1978年 | 14篇 |
1977年 | 19篇 |
1976年 | 13篇 |
1975年 | 22篇 |
1974年 | 24篇 |
1973年 | 10篇 |
1971年 | 6篇 |
排序方式: 共有1769条查询结果,搜索用时 15 毫秒
31.
Aihara H Alston-Garnjost M Avery RE Barbaro-Galtieri A Barker AR Barnes AV Barnett BA Bauer DA Bengtsson H Bintinger DL Bobbink GJ Bolognese TS Bross AD Buchanan CD Buijs A Cain MP Caldwell DO Clark AR Cowan GD Crane DA Dahl OI Derby KA Eastman JJ Eberhard PH Eisner AM Enomoto R Erné FC Fujii T Gary JW Gorn W Hauptman JM Hofmann W Huth JE Hylen J Kamae T Kaye HS Kees KH Kenney RW Kerth LT Ko W Koda RI Kofler RR Kwong KK Lander RL Langeveld WG Layter JG Linde FL Lindsey CS Loken SC Lu A Lu X 《Physical review letters》1986,57(8):945-948
32.
Abou El-Nasr EA Fujii A Yahagi T Ebata T Mikami N 《The journal of physical chemistry. A》2005,109(11):2498-2504
Geometric structures and excited-state proton dislocation of size-selected salicylic acid clusters (salicylic acid and 5-methoxysalicylic acid) with water were studied by using laser spectroscopic techniques. Fluorescence excitation, dispersed fluorescence, and infrared (IR) spectra of those clusters in supersonic jets were examined for both the electronic ground (S0) and first excited (S1) states. The geometric structures of the clusters were determined on the basis of the IR spectra of the OH stretch region with the help of quantum chemical calculations. The hydroxyl group of the water moiety in the clusters forms a ring involving the carboxylic group of the salicylic acid moiety. The IR spectra in S0 show that the intramolecular hydrogen bond in the salicylic acid moiety is still held upon cluster formation, but the phenolic OH stretch band intensity is remarkably weaken in the clusters. The IR spectra in the S1 state and dispersed fluorescence spectra indicated that the intramolecular excited state proton dislocation is hardly affected by the microsolvation with water, in contrast with the strong suppression of the dislocation in the self-solvation. 相似文献
33.
Production of polyacrylonitrile particles by precipitation polymerization in supercritical carbon dioxide 总被引:2,自引:0,他引:2
Masayoshi?OkuboEmail author Syuji?Fujii Hiroshi?Maenaka Hideto?Minami 《Colloid and polymer science》2003,281(10):964-972
Polyacrylonitrile particles were produced by precipitation polymerization of acrylonitrile (AN) without any colloidal stabilizer in supercritical carbon dioxide as a polymerization medium at about 30 MPa for 24 h at 65 °C at different initiator concentrations (0.8–45.2 mmol/l) and at different AN concentrations (10–40% w/v). An increase in the initiator concentration led to increases in the conversion and in the degree of coagulation and to a decrease in the molecular weight. At AN concentration of 20% w/v, micron-sized, relatively monodisperse polyacrylonitrile particles with clean and uneven surfaces were produced. 相似文献
34.
When aqueous solution of aluminium salt involving nitrate, chloride or sulfate was added into sodium fluoride solution, an immediate reaction occured to form cryolite-like precipitate. Examination into the resultant solution and analysis for the precipitate were carried out, leading to the fact that fluorine ion can substitute for bound water molecules around central Al+++ ion, but hardly for bound hydroxide and sulfate ion which should go or be “frozen”, as it is, into cryolite-like precipitate. These ligand substitution reaction seems to contribute to identification of ion species of aluminium in aqueous solution. 相似文献
35.
For the determination of amantadine (1-ADA), 2-adamantanamine (2-ADA), memantine (MEM), and rimantadine (RIM) in melanin binding studies, the simultaneous determination of 1-ADA or 2-ADA, MEM, and RIM is investigated by high-performance liquid chromatographic assay with dansyl chloride as a fluorescent derivative reagent. Dansyl derivatives with fluorescent intensity are detected at an excitation wavelength of 370 nm and an emission wavelength of 506 nm. Retention times of 1-ADA, 2-ADA, MEM, and RIM derivatives are 12.2, 12.2, 15.2, and 16.6 min, respectively. The peak of 1-ADA derivative coelutes with the 2-ADA derivative. The limits of detection for 1-ADA, 2-ADA, MEM, and RIM are 0.014, 0.007, 0.012, and 0.020microM, respectively (signal-to-noise ratio of 3:1). In the intra- and interday assay, the range of standard deviation to the average of 1-ADA, 2-ADA, MEM, and RIM is 4.6-12.7%. Their recovery is also good. The ranking order for synthetic melanin binding among these compounds is RIM > MEM > 2-ADA = 1-ADA. The method is simple, sensitive, and reproducible for simultaneously measuring 1-ADA or 2-ADA, MEM, and RIM. Also, it is useful to investigate their binding kinetics to melanin. 相似文献
36.
A method has been developed for highly sensitive determination of haloperidol in human serum involving a simple extraction procedure followed by gas chromatographic separation. Target components were separated from the extracting solvents with a Van den Berg type solventless sample injector before introduction Into a DB-1 capillary separation column. A surface ionization detector (SID), which has highly selective sensitivity for Substituted amines, was employed for quantitation using bromperidol as an internal standard. Chloroform proved to be the best extracting solvent, yielding a quantitative detection limit of 5 ng/ml (S/N = 2). Comparison of the response to target compounds obtained by the SID, FTD (flame thermionic detector), and FID (flame ionization detector) showed the SID to be superior. 相似文献
37.
Properties of fluorescence and the excitation spectra of fluorescein molecules adsorbed onto a calcinated (773 K) porous Vycor glass have been investigated as a function of the amount of adsorbed dye (the surface coverage, θ = 0.00051 and 0.0098). The fluorescence and fluorescence-excitation spectra of fluorescein adsorbed onto Vycor glass showed the spectrum only due to cation species at θ = 0.00051. On the other hand, the spectra observed at θ = 0.0098 suggested the presence of cation, anion, and dianion species on the surface of Vycor glass. These results indicated the existence of at least two different types of adsorption sites involving the Brønsted acid site on the surfaces of Vycor glass. 相似文献
38.
Kyoichiro Shibatani Kiyoshi Fujii 《Journal of polymer science. Part A, Polymer chemistry》1970,8(7):1647-1656
The reaction of stereoisomers of pentane-2, 4-diol and heptane-2, 4, 6-triol with formaldehyde was investigated as a model for the formalization reaction of poly(vinyl alcohol) in order to determine effect of the stereochemical configuration of the polyol molecules on the reaction. The isotactic (meso) diol portion reacted with formaldehyde to give cis-formal several times faster than did the syndiotactic (dl) diol portion to give trans-formal at 30–80°C. In the reaction of heterotactic (meso-dl) triol which provides both the isotactic and syndiotactic diol portions in a molecule, the proportion of trans-formal in the total formal decreased as the reaction proceeded. This shows that the formation of cis-formal is also favored thermodynamically to a greater extent, and hence the intramolecular migration of trans-formal to cis-formal did occur during the reaction. The rates of hydrolysis of formals of the diols were compared with those of the triols in order to see the effect of a hydroxyl group adjacent to the formal ring on the reaction. No appreciable rate difference was observed between the dimer and trimer models both in cis- and trans- formals. Therefore it was deduced from these results that the increase of the rate of hydrolysis of poly(vinyl formal) with the increase of hydroxyl groups along the polymer chain is a characteristic of macromolecules that is not observed in the low molecular weight models. 相似文献
39.
Otaka A Yukimasa A Watanabe J Sasaki Y Oishi S Tamamura H Fujii N 《Chemical communications (Cambridge, England)》2003,(15):1834-1835
Gamma-acetoxy-alpha,beta-enoates were easily reduced by samarium diiodide (SmI2) in THF to generate samarium dienolates which were kinetically trapped with ease at their alpha-positions by electrophiles (proton, aldehydes or ketones) to yield (E)-alkene dipeptide isosteres or gamma-amino acid derivatives in high chemical yields. 相似文献
40.
Otaka A Watanabe J Yukimasa A Sasaki Y Watanabe H Kinoshita T Oishi S Tamamura H Fujii N 《The Journal of organic chemistry》2004,69(5):1634-1645
A samarium diiodide (SmI(2))-mediated reduction of gamma,gamma-difluoro-alpha,beta-enoates (15, 29, and 34) was successfully applied to the synthesis of (Z)-fluoroalkene dipeptide isosteres (23, 30, and 35), which have served as potential dipeptide mimetics. Reduction of the gamma,gamma-difluoro-alpha,beta-enoates by SmI(2) proceeded via successive two-electron transfers to form dienolate species which upon kinetically controlled trapping with t-BuOH yielded Xaa-Gly-type fluoroalkene isosteres exemplified by 23, 30, and 35. Replacement of the t-BuOH kinetic trapping agent with aldehydes or ketones provided access to alpha-substituted fluoroalkene isosteres (43 and 45) through aldol reactions of Sm-dienolates with the carbonyl compounds. Of particular note, the use of the SmI(2)-HCHO reagent system with chiral enoate 34 provided D-Phe-psi[(Z)-CF[double bond]CH]-D/L-Ser isosteres (45), which could be converted to enantiomerically pure isosteres (49-52) that bore a variety of side chain functionalities at the alpha-position. This was achieved by a sequence of manipulations consisting of beta-lactone formation followed by chromatographic separation and ring-opening with soft nucleophiles. Included in the present work is the first utilization of a Rh-catalyzed Reformatsky reaction of chiral imines for the stereoselective preparation of alpha,alpha-difluoro-beta-amino acid derivatives (28 and 33). The appropriate choice of reagents (carbonyl compounds for kinetic trapping or ring-opening nucleophiles and imines for Reformatsky reactions) allows the presented methodology to yield various fluoroalkene isosteres possessing a wide range of side chain functionalities. 相似文献