全文获取类型
收费全文 | 18943篇 |
免费 | 368篇 |
国内免费 | 128篇 |
专业分类
化学 | 13379篇 |
晶体学 | 110篇 |
力学 | 357篇 |
数学 | 3258篇 |
物理学 | 2335篇 |
出版年
2022年 | 97篇 |
2021年 | 147篇 |
2020年 | 181篇 |
2019年 | 176篇 |
2018年 | 149篇 |
2017年 | 139篇 |
2016年 | 291篇 |
2015年 | 314篇 |
2014年 | 330篇 |
2013年 | 820篇 |
2012年 | 806篇 |
2011年 | 1042篇 |
2010年 | 541篇 |
2009年 | 460篇 |
2008年 | 970篇 |
2007年 | 990篇 |
2006年 | 1033篇 |
2005年 | 1105篇 |
2004年 | 978篇 |
2003年 | 813篇 |
2002年 | 708篇 |
2001年 | 224篇 |
2000年 | 223篇 |
1999年 | 194篇 |
1998年 | 188篇 |
1997年 | 248篇 |
1996年 | 304篇 |
1995年 | 196篇 |
1994年 | 211篇 |
1993年 | 196篇 |
1992年 | 181篇 |
1991年 | 175篇 |
1990年 | 173篇 |
1989年 | 153篇 |
1988年 | 166篇 |
1987年 | 173篇 |
1986年 | 137篇 |
1985年 | 298篇 |
1984年 | 270篇 |
1983年 | 199篇 |
1982年 | 306篇 |
1981年 | 248篇 |
1980年 | 300篇 |
1979年 | 264篇 |
1978年 | 264篇 |
1977年 | 280篇 |
1976年 | 231篇 |
1975年 | 185篇 |
1974年 | 179篇 |
1973年 | 167篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
The oligoribonucleotides (Ap)7A, (CP)7C, (Gp)7G and (UP)7U were prepared via an automated synthesizer using a silica gel support and the methyl dichlorophosphite procedure. 相似文献
102.
The photoexcitation routes used to produce molecular crystal, triplet states are shown to have important optical and microwave spectral consequences. 2-benzoylpyridine crystals at 4.2 K have T1 → S0 phosphorescence spectra showing line width dependence on whether initial production of the T1 state is through direct T1 → S0 absorption, or through S1 ← S0 absorption followed by S1 → T1 intersystem crossing. Striking differences are seen in the optically detected zero-field resonance spectra. 相似文献
103.
Alexander R. Giaquinto Richard E. Lindstrom James Swarbrick Antonio LoSurdo 《Journal of solution chemistry》1977,6(10):687-701
Solubility and apparent molar volume data are used to demonstrate effects of amide alkylation on amide-water interactions at 25° C. Precise measurements were made of the apparent molar volumes of the amides in binary amide-water mixtures using a dilatometric technique. The results show that the apparent molar volumes of alkyl-substituted amides in water pass through a minimum at an amide concentration which varies inversely with the degree of alkylation. Further studies showed that the solubilities of methyl paraben (methyl-p-hydroxybenzoate) and naphthalene in various amide-water solvent systems increased in characteristic fashion with amide alkylation. 相似文献
104.
The Conversion of Eperuic Acid into Ethers of the enantio-14, 15-Dinorlabdane Series 5 and 6 are strongly odiferous substances of the ambra-type. Their enantiomers 7 and 8 , hitherto unknown, have been synthesized from eperuic acid (4) and their olfactory properties compared with those of 5 and 6 . 4 was esterified by CH2N2 and dehydrogenated with (C6H5Se)2/H2O2 to the α,β-unsaturated ester 9 (61%). Oxidation by KMnO4 in acetone yielded the ketone 3 (60%). Epoxidation followed by treatment with acid converted 3 into the acetals 7 (61%) and 8 (14%). 7 and 8 differ from 5 and 6 in odor intensity, and 6 and 8 show slightly different odor quality. 相似文献
105.
Kortes Richard A. Lin Fu-Tyan Ward Matthew S. Shepherd Rex E. 《Transition Metal Chemistry》2000,25(3):251-259
1:1 and 2:1 palladium(II) complexes of egta4– (egta4– = glycine, N,N-(1,2-ethanediylbis)(oxy-2,1-ethanediyl)bis[N-carboxymethyl]) were prepared by 1:1 and 2:1 addition of K2PdCl4 to K4egta, and examined by 1H-, 13C- and 15N-n.m.r. methods. The 1:1 complex, [Pd(egta)]2– in solution, utilizes a square-planar coordination comprised of two nitrogen and two glycinato carboxylate donors of egta4–, leaving two glycinato carboxylates pendant. The complex has a cis-(R,S) stereochemistry which places both pendant carboxylates below the PdN2O2 square plane and the tether backbone of egta4– in the up, up sense above the same plane. The cis-(R,S) assignment was assisted by computer simulations of the 13C-n.m.r. spectrum for four possible isomers. Only cis-(R,S) and trans-(R,R) calculated 13C-spectra were compatible with the observed 13C-n.m.r. pattern. The HH NOESY spectrum of [Pd(egta)]2– detects long range coupling of the backbone –OCH2CH2O– linkage with both coordinated and pendant glycinato CH2 moieties. The cis-(R,S) isomer's tortional movements allow such contacts whereas a trans-(R,R) isomer does not. The 2:1 complex, [Pd2(egta)(H2O)2] in solution has an extended-chain structure with each palladium(II) center coordinated in the mer-iminodiacetate-like coordination with two bound glycinato-functionalities. 相似文献
106.
The Photochemistry of Open-Chained 2,6- or 2,7-Dien-Carbonyl Compounds On 1n, π*-excitation (λ > 347 nm) citral (5) and the methyl ketone 10 isomerize to compounds A (7, 19) and B (6, 20) , whereas the phenyl ketone 11 changes into the isomer 24 of type E. Evidence is given that the conversions to A and B may arise from the 3n, π*-state of the 2,6-diene-carbonyl compounds. On 1n, π*-excitation (λ = 254 nm) 5 and 10 yield the isomers A (7, 19) and D (18, 22) , but no products of type B. Furthermore, conversion of 10 to the isomer 21 of type C is observed. Selective 1n, π*-excitation (λ = 254 nm) as well as selective 1n, π*-excitation (λ > 347 nm) of the 2,7-diene-carbonyl compounds 12 and 13 give rise to isomerization to the compounds F (25, 28) , exclusively. The intramolecular [2 + 2]-photocycloadditions are shown to be triplet processes. UV.-irradiation (λ > 280 nm) of compounds F (25, 28) furnishes the isomeric products G (26, 29) which photoisomerize to oxetanes of type H (27, 30). 相似文献
107.
J. Justin Gooding Richard G. Compton Colin M. Brennan John H. Atherton 《Journal of colloid and interface science》1996,180(2):605
The mechanism of the dyeing of cotton and nylon cloth by the azo dyes Orange G and Sunset Yellow FCF was investigated using a channel flow cell. The variation in dyeing with flow rate was found to proceed via a mechanism in which the flux of dye entering the cloth relative to the flux of dye to the cloth surfacedecreasedwith increasing flow rate. A mechanism is deduced in which the dye passes from bulk solution, through a porous surface layer within the cloth, before passing into the bulk cloth. Adsorption onto surface sites in this porous layer blocks the passage of further dye into the cloth. Kinetic parameters for such a mechanism are given. 相似文献
108.
Lead tetraacetate (LTA) oxidation of the allylic alcohols 1, 10, 14 and 19 leads to the formation of the epoxides 2, 11, 15 and 20 , products of a novel internal addition reaction of the electron deficient alcohol oxygen to the allylic double bond. In some cases ( 10, 14 ) the formation of a new type of acetoxylated enolethers ( 12, 16 ) is observed. The LTA oxidation of the allylic dienols 21 and 29 gives rise to the formation of the epoxyacetates 25 and 33 , products of a similar internal addition reaction. Furthermore, a variety of cyclization products ( 22, 23, 24, 26, 30, 31, 32 and 34 ) has been isolated whose formation requires an isomerisation of the allylic trans double bond to a cis one. 相似文献
109.
An efficient synthetic sequence toward the C8-C19 region of peloruside A has been developed. The route is highlighted by a selective electrophilic cyclization reaction, a single-step epoxide ring-opening/methylation sequence, and a stereoselective Mukaiyama aldol reaction. [reaction: see text] 相似文献
110.
The CD. spectra of carotenoids in the 220-500 nm region may by inspection be classified as (a) essentially Conservative, (b) intermediate, or (c) essentially nonconservative. A conservative spectrum shows in that spectral region a sequence of 5-6 relatively sharp Cotton effects of alternating sign, the rotatory strengths of which roughly add to zero. In a non-conservative spectrum Δ? has the same sign over the whole region and its absolute value is in general somewhat smaller. In general, typical conservative spectra invert upon isomerization of the molecule from all-trans to mono-cis. Non-conservative spectra do not invert. The model of a chiral polyene of the length of the conjugated carotene chromophore reproduces well the main features of the conservative spectra. The theoretical predictions and the experimental data are shown to conform to the C2-rule [43] [44]. The particular nature of the longest-wavelength transition is interpreted. Based on a summary of the chiroptic data on about 50 naturally occurring compounds, the question is discussed of when conservative spectra arise and when not. 相似文献