全文获取类型
收费全文 | 3512篇 |
免费 | 136篇 |
国内免费 | 13篇 |
专业分类
化学 | 2760篇 |
晶体学 | 27篇 |
力学 | 26篇 |
数学 | 190篇 |
物理学 | 658篇 |
出版年
2023年 | 27篇 |
2022年 | 27篇 |
2021年 | 48篇 |
2020年 | 75篇 |
2019年 | 60篇 |
2018年 | 25篇 |
2017年 | 16篇 |
2016年 | 75篇 |
2015年 | 73篇 |
2014年 | 85篇 |
2013年 | 170篇 |
2012年 | 205篇 |
2011年 | 228篇 |
2010年 | 116篇 |
2009年 | 108篇 |
2008年 | 245篇 |
2007年 | 233篇 |
2006年 | 242篇 |
2005年 | 239篇 |
2004年 | 219篇 |
2003年 | 161篇 |
2002年 | 173篇 |
2001年 | 59篇 |
2000年 | 63篇 |
1999年 | 55篇 |
1998年 | 40篇 |
1997年 | 44篇 |
1996年 | 31篇 |
1995年 | 24篇 |
1994年 | 25篇 |
1993年 | 21篇 |
1992年 | 33篇 |
1991年 | 22篇 |
1990年 | 17篇 |
1989年 | 22篇 |
1988年 | 24篇 |
1987年 | 20篇 |
1985年 | 40篇 |
1984年 | 31篇 |
1983年 | 19篇 |
1982年 | 27篇 |
1981年 | 25篇 |
1980年 | 22篇 |
1979年 | 22篇 |
1978年 | 13篇 |
1977年 | 20篇 |
1976年 | 13篇 |
1975年 | 11篇 |
1974年 | 13篇 |
1973年 | 15篇 |
排序方式: 共有3661条查询结果,搜索用时 0 毫秒
941.
942.
Kiso Y Kuzawa K Saito Y Yamada T Nagai M Jung YJ Min KS 《Analytical and bioanalytical chemistry》2002,374(7-8):1212-1217
Phosphate is generally analyzed by color intensity of phosphoantimonylmolybdenum blue (PAMB): especially by a spot test based on spectrophotometric or visual determination. Visual determination is one of the simplest and most inexpensive methods; however, it is believed to be of low precision. In this work, we have developed an alternative phosphate spot test, where phosphate concentration is measured by the color band length of PAMB formed in a detection tube similar to a gas detection tube. The color band is formed by the entrapment of the hydrophobic ion pair of PAMB and quaternary ammonium ions in the column; we demonstrated that this color band length correlates quantitatively with the phosphate concentration. In order to optimize the measurement performance, systematic investigations have been carried out for various experimental parameters such as phosphate concentration, preparation conditions of the column packing material, the type of support material (either silica-gel or PVC), the solution feeding conditions (filtration or penetration), and volume of the sample solution. The effects from some co-existing components were also studied for the measurement of real samples.Determination of phosphate concentrations in actual wastewaters was successfully carried out by this method, indicating that the quantification range achievable was of 3-18 mg PO(4) L(-1) without dilution prior to the analysis. No interference was observed from suspended solid (SS) and organic pollutants (COD, TOC) during the wastewater analysis. Because of the simplicity and rapid processing of the samples (not requiring any special instruments) this newly developed method can be applied to the on-site analysis of wastewater. 相似文献
943.
The 1,4-elimination reaction of 1-allyloxy-4-methoxy-(2Z)-alkenes with n-butyllithium is shown to proceed in a marked preference to the [2,3] Wittig rearrangement to afford the allylic (1Z,3E)-dienyl ethers in high stereoselectivities. The synthetic utility of this method is demonstrated by the Claisen rearrangement of the dienyl ethers thus obtained. 相似文献
944.
Photochemically inactive diarylethene derivatives having a N-(O-hydroxyphenyl) group underwent photochromic reaction when they were esterified by the addition of acetic anhydride. 相似文献
945.
Keitaro Ishii Takuya Sone Yukio Shimada Takahide Shigeyama Masahiro Noji Shigeo Sugiyama 《Tetrahedron》2004,60(48):10887-10898
The photochemical C,C-bond cleavage of N-benzyl β-aziridinylacrylonitrile 1 and acrylate 2 and the subsequent [3+2] cycloaddition with electron-deficient alkenes afforded head-to-head adducts selectively and efficiently. Irradiation of N-phenyl aziridine 3 with acrylonitrile gave adducts, but photoreaction of N-benzoyl aziridine 4 and thermal reactions of 3 and 4 with alkenes yielded C(γ),N-cleaved products instead of cycloadducts. N-trityl aziridine 5 also reacted with electron-deficient alkenes, affording 2,3-cis-pyrrolidine derivatives exclusively. A formal synthesis of a pyrrolizidine alkaloid, isoretronecanol (27), starting from 5 was achieved in a convenient manner. 相似文献
946.
Yasukawa T Satoh T Miura M Nomura M 《Journal of the American Chemical Society》2002,124(43):12680-12681
Benzoyl chlorides efficiently react with 2 equiv of dialkylacetylenes as well as diphenylacetylene in the presence of an iridium catalyst accompanied by decarbonylation to produce 1,2,3,4-tetrasubstituted naphthalenes in good yields. Use of 2-naphthoyl chlorides selectively affords the corresponding anthracene derivatives. 相似文献
947.
Alessandra Fernicola Stefania Panero Bruno Scrosati Masahiro Tamada Hiroyuki Ohno 《Chemphyschem》2007,8(7):1103-1107
A series of ionic liquids (ILs) are prepared by neutralizing tertiary amines with N,N-bis(trifluoromethanesulfonyl)imide (HTFSI). As demonstrated by thermal and electrochemical characterizations, these ILs have very good temperature stability and a high ionic conductivity, that is, of the order of 10(-2) S cm-1. By incorporating these ILs into a poly(vinylidenfluoride-co-hexafluoropropylene) polymer matrix, membranes with a high melting temperature, high decomposition point and with an ionic conductivity of about 10(-2) S cm-1 at 140 degrees C, are obtained. These IL-based, proton-conducting membranes are proposed as new polymer electrolytes for high-temperature polymer electrolyte membrane fuel cells (PEMFCs). 相似文献
948.
Prakash TP Manoharan M Kawasaki AM Lesnik EA Owens SR Vasquez G 《Organic letters》2000,2(25):3995-3998
[structure] Oligonucleotides with two novel modifications, 2'-O-?2-[N, N-(dimethyl)aminooxy]ethyl? (2'-O-DMAOE) and 2'-O-?2-[N, N-(diethyl)aminooxy]ethyl? (2'-O-DEAOE), have been synthesized. These modifications exhibit high binding affinity to target RNA (and not to DNA) and enhance the nuclease stability of oligonucleotides considerably with t(1/2) > 24 h as a phosphodiester. 相似文献
949.
Intramolecular insertion of a C-C double bond into a C-C single bond was achieved by treatment of cyclobutanone bearing an o-styryl group at the 3-position with a catalytic amount of a cationic rhodium(I)-dppp complex. Initially, rhodium is inserted between the carbonyl carbon and the alpha-carbon of the cyclobutanone. Intramolecular coordination of the vinyl group results in its migratory insertion into the C-Rh linkage. Reductive elimination affords benzobicyclo[3.2.1]octan-3-one. Notably, a ring-opened alpha,beta-unsaturated ketone was obtained when dppe was used instead of dppp. In this reaction, rhodium cleaved the bond between the alpha sp3 carbon and the beta sp3 carbon of the cyclobutanone. The coordinating vinyl group directs this new regioselectivity of cleavage observed with the dppe ligand. 相似文献
950.
Cycloadditions of oxetanes with isocyanates were markedly accelerated by the complex of Bu2SnI2 and Ph3P=O which could be readily handled and hardly caused a trimerization of isocyanates, yielding oxazin-2-ones under neutral and mild reaction conditions. 相似文献