全文获取类型
收费全文 | 1701篇 |
免费 | 62篇 |
国内免费 | 317篇 |
专业分类
化学 | 1516篇 |
晶体学 | 8篇 |
力学 | 2篇 |
综合类 | 17篇 |
物理学 | 58篇 |
综合类 | 479篇 |
出版年
2024年 | 4篇 |
2023年 | 8篇 |
2022年 | 30篇 |
2021年 | 16篇 |
2020年 | 27篇 |
2019年 | 30篇 |
2018年 | 32篇 |
2017年 | 52篇 |
2016年 | 40篇 |
2015年 | 32篇 |
2014年 | 62篇 |
2013年 | 135篇 |
2012年 | 80篇 |
2011年 | 87篇 |
2010年 | 59篇 |
2009年 | 86篇 |
2008年 | 84篇 |
2007年 | 104篇 |
2006年 | 126篇 |
2005年 | 113篇 |
2004年 | 99篇 |
2003年 | 101篇 |
2002年 | 81篇 |
2001年 | 69篇 |
2000年 | 77篇 |
1999年 | 64篇 |
1998年 | 53篇 |
1997年 | 48篇 |
1996年 | 44篇 |
1995年 | 44篇 |
1994年 | 29篇 |
1993年 | 31篇 |
1992年 | 30篇 |
1991年 | 22篇 |
1990年 | 13篇 |
1989年 | 14篇 |
1988年 | 16篇 |
1987年 | 5篇 |
1986年 | 12篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1979年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有2080条查询结果,搜索用时 31 毫秒
61.
Various carboxamides are synthesized from the corresponding carboxylic acids and amines with high product-selectivities using 2-methyl-6-nitrobenzoic or 2,4,6-trichlorobenzoic anhydride in the presence of 4-(dimethylamino)pyridine. 相似文献
62.
《Electroanalysis》2004,16(5):355-359
Water present in undiluted acetic acid can be monitored in situ with Pt and Au microelectrodes using differential pulse and square‐wave voltammetries. Both reduction and oxidation peaks can be obtained. The best quantitative analytical results were obtained for the anodic peak, the Pt microelectrode, and differential pulse voltammetry. The anodic water peak is located at +1.55 V vs. the quasi reference Pt electrode. The voltammetric peak obtained at Pt electrode is apparently not a diffusional one, however, the calibration plot obtained by employing this peak is linear over a wide concentration range, up to 4 mol dm?3. The detection limit has been estimated as 2.3 mM or 0.0043 v/v%. The developed method may be particularly useful, since the Karl Fischer method can not be used to determine water in glacial acetic acid due to the estrification reaction of acetic acid with methanol. A voltammetric wave of undiluted acetic acid could not be obtained in the positive and negative ranges of potential. 相似文献
63.
64.
The apparent molar heat capacities of dilute aqueous solutions of acetic, propanoic and succinic acid and sodium salts of the two monofunctional acids were measured at 300 Kp,2
o
. After subtracting the heat capacity of a point mass, the remaining heat capacity was successfully decomposed into functional group contributions at all temperatures. Together with the results of our previous paper on alcohols and diols the heat capacity contributions of the CH2, CH3, OH, COOH, (COOH)2, and COONa groups are now available and these allow reasonably accurate predictions of the heat capacities of all compounds composed of these groups in this temperature range. 相似文献
65.
Mikael Erlandsson 《Tetrahedron letters》2006,47(32):5829-5832
A new safety catch linker, 3-(4-hydroxymethylphenylsulfanyl)propanoic acid (HMPPA), is described for use in solid phase peptide synthesis. The linker is readily synthesized from commercially available chemicals in a more cost efficient way compared to similar reported linkers. The HMPPA linker is easily attached to an amino derivatized solid support followed by on-resin oxidation of the thioether to sulfoxide, thereby making the linker very stable towards strong acid treatment. Final resin cleavage is performed by reductive acidolysis. 相似文献
66.
E. N. Adamenko L. L. Frolova M. V. Panteleeva A. V. Kuchin 《Chemistry of Natural Compounds》2007,43(1):59-62
Optically active camphordihydro-2,3-pyrazine and camphorquinoxaline were prepared from camphoroquinone enantiomers. It was
shown that (1S,4R)-(+)-camphoroquinone was formed by oxidation of (1S,3R, 4R)-(−)-3-bromocamphor and (1R,4S)-(−)-camphoroquinone from (1R,3S, 4S)-(+)-3-bromocamphor, respectively. Camphor anhydride was a side product (6–10%) of the reaction.
__________
Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 50–52, January–February, 2007. 相似文献
67.
Markham George D. Bock Cindy L. Trachtman Mendel Bock Charles W. 《Structural chemistry》1999,10(4):263-276
Ab initio molecular orbital and density functional methods have been used to study the potential energy surfaces of the substituted acetic acids HX—CH2—COOH, where X is one of the Group VIA Chalcophiles S, Se, or Te. The various conformers adopted by these compounds provide information regarding the energetic importance of nonbonded
and
hydrogen bonding interactions involving oxygen atoms with different hybridizations. Density functional and ab initio molecular orbital methods yield similar structural and energetic trends for these compounds. Calculations show that the structure of the lowest-energy conformer of each of these acids has the X—C—C—O backbone substantially twisted from planarity, similar to that previously observed for the corresponding aldehydes, HX—CH2—CHO. In the twisted acid structures the shortest
distance is within about 0.1 Å of the sum of the X and O van der Waals radii, which reduces overcrowding of the lone pairs of electrons on these atoms. In conformers where the heavy atom backbone is planar, one of the
distances is significantly shorter than the sum of the van der Waals radii, and the total molecular energy of these conformers is higher than that of the twisted forms. The variation of X—H vibrational frequencies among conformers reflects the extent of X—H hydrogen bonding, and indicates that formation of this hydrogen bond is not the dominant factor in determining the lowest-energy conformation. When X is oxygen (HO—CH2—COOH), the lowest-energy conformer is also nonplanar, whereas for the corresponding aldehyde, HO—CH2—CHO, the lowest-energy conformer is a planar structure with CS symmetry. The conformational preferences of these simple species provide reference points for inter- and intramolecular interactions in more complex systems of biological interest. 相似文献
68.
The title compound, {[n-Bu2Sn(O2CCH2CS2NC4H8)]2O}2, has been synthesized by the reaction of (tetrahydro-pyrrodithiolocarbamoylthio)acetic acid with the di-n-bubyltin oxide in 1∶1 molar ratio. The complex has been characterized by elemental analysis, IR and NMR. The crystal structure of it has been determined by X-ray single crystal diffraction. And the results showed that the crystal belongs to triclinic system with space group P1 and some crystal parameters: a=1.220 2(9) nm, b=1.315 8(10) nm, c=1.380 4(10) nm, α=111.215(9)°, β=99.357(9)°, γ=96.075(10)°, V=2.006(2) nm3, Z=1, F(000)=908, μ=1.489 mm-1, Dc=1.474 g·cm-3, R1=0.037 5, wR2=0.0839. The complex has a centrosymmetric dimer structure mode with a four-membered central endo-cyclic Sn2O2 unit in which two bridged oxygen atoms both connect with an exo-cyclic tin atom which has a distorted octahedron. Each of the endo-cyclic tin atoms exhibits a distorted trigonal bipyramid coordination geometry with an additional weak coordination carboxylate oxygen. Four carboxylate ligands are divided into two types. And two of them are bidentate and connecting to each of exo-cyclic tin atoms by using both oxygen atoms, whereas the others bridge to each pair of exo-and endo-cyclic tin atoms utilizing one oxygen atom only. CCDC: 220513. 相似文献
69.
《Electroanalysis》2004,16(21):1777-1784
The surface of boron‐doped diamond (BDD) electrode is modified by the polymer film for the first time. The cationic polymer film of N,N‐dimethylaniline (DMA) is electrochemically deposited on BDD electrode surface. This polymer (PDMA) film‐coated BDD electrode is used as a sensor which selectively detect dopamine (DA) in the presence of ascorbic acid (AA). This electrode also can detect both DA and its metabolite, 3,4‐dihydroxy phenyl acetic acid (DOPAC) in the presence of AA in the range of the physiological concentrations of these species. Favorable ionic interaction (i.e., electrostatic attraction) between the PDMA film and AA or DOPAC lowers their oxidation potentials and enhances the current response for AA and DOPAC compared to that at the bare electrode. The PDMA film also shows a hydrophobic interaction with DA and DOPAC. In cyclic voltammetric measurements, the PDMA film‐coated electrode can successfully separate the oxidation potentials for AA and DA coexisting in the same solution and the separation is about 200 mV. AA oxidizes at more negative potential than DA. In square‐wave voltammetry, the sensitivity of the PDMA film‐coated BDD electrode for DA in the presence of higher concentration of AA is higher than that of the PDMA film‐coated glassy carbon electrode. The hydrodynamic amperometric experiments confirm that the oxidation of AA is not affected by the oxidized product of DA and vice versa. So, unlike the bare electrode the catalytic oxidation of AA by the oxidized DA is eliminated at the PDMA film‐coated BDD electrode. The sensitivities of the modified electrode for AA, DA and DOPAC, which are present in the same solution with their physiological concentration ratios, are calculated to be 0.070, 0.363 and 0.084 μA μM?1, respectively. The modified electrode exhibits a stable and sensitive response to DA. 相似文献
70.
将Fe2O3纳米粉体经一定浓度的H2SO4浸泡活化后制成纳米固体超强酸SO42-/Fe2O3,将其用于催化合成乙酸乙酯以考察其活性。利用均匀设计分析了超强酸制备过程及酯化反应过程中各因素的影响,研究结果表明较好的制备条件是:H2SO4浓度:2.5mol·L-1;浸泡时间:1h;活化温度:167℃;活化时间:1h,此时获得的固体超强酸SO42-/Fe2O3的粒径小于50nm。当催化剂用量为冰乙酸质量的5%,n(乙醇)∶n(冰乙酸)为3∶1,反应3.5h后乙酸的转化率高于80%。该催化剂经H2SO4溶液浸泡、活化再生后可重新使用,推断出其酸强度H0<-14.5。 相似文献