全文获取类型
收费全文 | 671篇 |
免费 | 65篇 |
国内免费 | 32篇 |
专业分类
化学 | 751篇 |
晶体学 | 3篇 |
力学 | 2篇 |
综合类 | 1篇 |
物理学 | 11篇 |
出版年
2023年 | 13篇 |
2022年 | 15篇 |
2021年 | 26篇 |
2020年 | 26篇 |
2019年 | 33篇 |
2018年 | 23篇 |
2017年 | 16篇 |
2016年 | 46篇 |
2015年 | 29篇 |
2014年 | 31篇 |
2013年 | 102篇 |
2012年 | 24篇 |
2011年 | 29篇 |
2010年 | 41篇 |
2009年 | 22篇 |
2008年 | 39篇 |
2007年 | 37篇 |
2006年 | 32篇 |
2005年 | 35篇 |
2004年 | 18篇 |
2003年 | 32篇 |
2002年 | 15篇 |
2001年 | 13篇 |
2000年 | 5篇 |
1999年 | 5篇 |
1998年 | 8篇 |
1997年 | 10篇 |
1996年 | 8篇 |
1995年 | 5篇 |
1994年 | 5篇 |
1993年 | 6篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1985年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1977年 | 2篇 |
排序方式: 共有768条查询结果,搜索用时 0 毫秒
11.
Borisova I. V. Nechaev M. S. Khrustalev V. N. Zemlyanskii N. N. Ustynyuk Yu. A. 《Russian Chemical Bulletin》2002,51(5):721-753
The review surveys the data on the reactions of phosphorus and arsenic ylides with compounds containing E=X bonds (E = C, Si, Ge, or Sn; X = C or S), cyclic oligomers (R2ES)n (n = 2 or 3), and heavier analogs of carbenes. These reactions give rise to two new classes of heteroorganic betaines containing the (+)E15—C—E14—X(–) (I) and (+)E15—C—E14(–) (II) (E15 = P or As; E14 = Si, Ge, or Sn; X = C or S) structural fragments. Procedures for the synthesis of these compounds, their reactivities, the X-ray diffraction structures, and the electronic structures established by high-level quantum-chemical calculations are considered in detail. The carbon analogs of betaines of type I, viz., compounds bearing the (+)P—C—C—X(–) fragment (III), are also discussed. The latter were long considered as possible intermediates in the reactions of compounds containing the polar C=X bond (X = C, O, S, NR, etc.) with phosphorus ylides (classical Wittig and Corey—Chaykovsky reactions and related processes). 相似文献
12.
Akihiko Kanazawa Tomiki Ikeda Takeshi Endo 《Journal of polymer science. Part A, Polymer chemistry》1993,31(12):3003-3011
Polyesters were prepared which retained phosphonium biocides as counter ions of sodium sulfonate moieties incorporated into the polymers, and surface antibacterial activity of the polyester films against Staphylococcus aureus and Escherichia coli was explored. These films exhibited a high surface antibacterial activity against S. aureus and E. coli, particularly against S. aureus, and the activity was affected by the structure and the compositional ratio of the phosphonium salts. Amount of the released phosphonium salts was very small, so that liberation of the phosphonium biocides can be expected to occur over a long period. Morphological changes of the cells of S. aureus and E. coli in contact with the polyester films were evaluated by scanning electron microscopy. It was found that the surface antibacterial activity of the polyester films was rather bacteriostatic than bactericidal as evidenced by no morphological changes of the bacterial cells in contact with the phosphonium biocides © 1993 John Wiley & Sons, Inc. 相似文献
13.
Ovakimyan M. Zh. Barsegyan S. K. Karapetyan A. A. Panosyan G. A. Indzhikyan M. G. 《Russian Chemical Bulletin》2002,51(9):1744-1747
The reactions of phenylhydrazine and hydroxylamine with (-X-ethyl)triphenylphosphonium salts (X = Ph, Ph3P+Br–) (1, 2) afforded the corresponding -N-ethyl-substituted triphenylphosphonium salts (3, 4). The reaction of triphenyl(2-phenylhydrazinoethyl)phosphonium bromide 3with an aqueous solution of NaOH in benzene afforded a statistical mixture of the nisand transisomers of 2-(diphenylphosphoryl)acetaldehyde phenylhydrazone. (2-Hydroxyaminoethyl)triphenylphosphonium bromide reacted with sodium methoxide to give O-phosphobetaine. 相似文献
14.
Eduardo García-Mingüens Marcos Ferrndiz-Saperas M. de Gracia Retamosa Carmen Njera Miguel Yus Jos M. Sansano 《Molecules (Basel, Switzerland)》2022,27(14)
The enantioselective 1,3-dipolar cycloaddition between imino esters and (Z)-nitroalkenes bearing a masked amino group in the β-position was studied using several chiral ligands and silver salts. The optimized reaction conditions were directly applied to the study of the scope of the reaction. The determination of the absolute configuration was evaluated using NMR experiments and electronic circular dichroism (ECD). The reduction and hydrolysis of both groups was performed to generate in an excellent enantiomeric ratio the corresponding cis-2,3-diaminoprolinate. 相似文献
15.
Fernando Castañeda Paul Silva M. Teresa Garland Ata Shirazi Clifford A. Bunton 《Phosphorus, sulfur, and silicon and the related elements》2013,188(8):2152-2168
The phosphonium ylidic diesters, methyl and ethyl isopropyl and, methyl and ethyl t-butyl triphenylphosphoranylidene malonates, 1a,b and 2a,b , respectively, have the syn-anti conformation in solution, as in the crystal, and the bulkier alkoxy group is oriented towards phosphorus. The 1 H NMR spectra show that in 1a,b , the isopropyl group is oriented towards the face of a phenyl group, consistent with π shielding in the 1 H signals, and examination of the 1 H coupled 13 C NMR spectra allows assignment of the acyl carbon signals. Computed bond lengths and angles for isolated molecules are similar to those in the crystal, and the geometry and the NMR spectra indicate extensive ylidic resonance. Estimated partial atomic charges on the ester oxygens are more negative when they are oriented towards, rather than away from, phosphorus. 相似文献
16.
Leila S. Boulos Ewies F. Ewies Amin F. M. Fahmy Maysa E. Mohram 《Phosphorus, sulfur, and silicon and the related elements》2013,188(6):790-803
Abstract The reaction of 5(4H)-pyrazolone with phosphorus ylides afforded new triphenylphosphanylidene alkanone derivatives. Moreover, its benzylidene derivative reacted with Wittig–Horner reagents to give the corresponding dialkoxyphosphoryl, alkyl phosphonate, and heterocyclic products. Treatment of pyrazole-4-carbaldehyde with Wittig–Horner reagents and trialkyl phosphites gave the respective alkyl phosphonate adducts. Mechanisms accounting for the formation of the new products are discussed. The biological activity of some of the newly synthesized compounds was also examined. 相似文献
17.
M. M. Sidky M. R. Mahran A. A. El-kateb I. T. Hennawy 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3):409-412
Abstract Trialkyl phosphites react with quinisatin (1) and its monohydrate 3 to give the corresponding phosphate derivatives 2a-c. The same compounds are obtained upon reacting 1 and/or 3 with the proper dialkyl phosphite. Structural reasonings based on IR, MS and NMR spectral data, are presented. 相似文献
18.
Blanka Wladislaw Liliana Marzorati Jonas Gruber 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-4):185-188
Abstract Some new norbornene thioester derivatives of endo configuration are sinthesized. Proofs are provided that thioester group has more preference for endo configuration than the ester group. 相似文献
19.
R. Hekmatshoar S. Souri M. Rahimifard 《Phosphorus, sulfur, and silicon and the related elements》2013,188(8):1605-1609
Protonation of the highly reactive intermediate produced in the reaction between triphenylphosphine and dialkyl acetylenedicarboxylate 1a–c by methyl-2-hydroxy-3-naphthoate lead to vinyl triphenylphosphonium salts which undergo aromatic electrophilic substitution reaction with phenolate-conjugated base to produce 2a–c. 相似文献
20.
Virginie Blot Jean-François Brièr Marion Davoust Stéphanie Minière Vincent Reboul Patrick Metzner 《Phosphorus, sulfur, and silicon and the related elements》2013,188(5-6):1171-1182
Abstract Our work on the thio-Claisen rearrangement mediated by an adjacent sulfinyl group is reviewed. The substrates could easily be prepared on a large scale from diacetone-D-glucose. The rearrangement was effected with a diastereoselectivity of 95:5, in favor of the (S,S) or the (R,R) isomer. An approach to natural bis(lactones) was investigated, using a halolactonization reaction and a second [3,3] sigmatropic shift, again mediated by the sulfinyl group. The second part deals with the catalytic enantioselective benzylidenation of aldehydes, mediated by chiral sulfur ylides. We have introduced simple C 2 symmetric thiolanes for that purpose. The procedure is very practical and enantiomeric excesses up to 96% have been reported for the model of stilbene oxide. A series of ferrocenyl sulfides with planar chirality has also been investigated, leading to unexpected diastereoselectivities and enantiomeric excesses up to 94%. 相似文献