全文获取类型
收费全文 | 3479篇 |
免费 | 16篇 |
国内免费 | 5篇 |
专业分类
化学 | 1708篇 |
晶体学 | 53篇 |
力学 | 307篇 |
数学 | 273篇 |
物理学 | 1159篇 |
出版年
2022年 | 35篇 |
2021年 | 52篇 |
2020年 | 51篇 |
2019年 | 69篇 |
2018年 | 117篇 |
2017年 | 119篇 |
2016年 | 142篇 |
2015年 | 45篇 |
2014年 | 115篇 |
2013年 | 163篇 |
2012年 | 133篇 |
2011年 | 156篇 |
2010年 | 155篇 |
2009年 | 145篇 |
2008年 | 158篇 |
2007年 | 153篇 |
2006年 | 97篇 |
2005年 | 85篇 |
2004年 | 71篇 |
2003年 | 81篇 |
2002年 | 86篇 |
2001年 | 106篇 |
2000年 | 62篇 |
1999年 | 41篇 |
1998年 | 40篇 |
1997年 | 38篇 |
1996年 | 37篇 |
1995年 | 29篇 |
1994年 | 21篇 |
1993年 | 29篇 |
1992年 | 29篇 |
1991年 | 45篇 |
1990年 | 32篇 |
1989年 | 47篇 |
1988年 | 27篇 |
1987年 | 26篇 |
1986年 | 37篇 |
1985年 | 40篇 |
1984年 | 36篇 |
1982年 | 39篇 |
1981年 | 32篇 |
1980年 | 41篇 |
1979年 | 28篇 |
1978年 | 45篇 |
1977年 | 39篇 |
1975年 | 36篇 |
1974年 | 28篇 |
1973年 | 38篇 |
1972年 | 39篇 |
1971年 | 25篇 |
排序方式: 共有3500条查询结果,搜索用时 62 毫秒
91.
A method has been developed for the synthesis of 4-amino-substituted 7-benzyl-2-morpholin-4-yl-5,6,7,8-tetrahydropyrido[3,4-d]pyrimidines
by condensation of ethyl 1-benzyl-3-oxopiperidine-4-carboxylate with morpholine-4-carboxamidine and subsequent reaction of
the 7-benzyl-2-morpholin-4-yl-5,6,7,8-tetrahydro-3H-pyrido[3,4-d]pyrimidin-4-one with trifluoromethanesulfonic anhydride and
secondary amines.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 762–768, May, 2007. 相似文献
92.
I. V. Shcherbakova V. G. Brovchenko E. V. Kuznetsov 《Chemistry of Natural Compounds》1985,21(6):774-777
The reaction of 6,7-dimethoxy-1-methyl-2-benzopyrylium salt with ammonia forms the corresponding isoquinoline, and its reaction with methylamine and benzylamine forms mixtures of the corresponding isoquinolinium salts and naphthylamines. The reduction of 2-benzyl-1-methyl-6,7-dimethoxyisoquinolinium perchlorate with sodium tetrahydroborate has given the corresponding tetrahydroisoquinoline, the hydrogenolysis of which has led to salsolidine. The products have been characterized by elementary analyses and IR and PMR spectroscopy. 相似文献
93.
Yu. V. Kuznetsov L. G. Stolyarova V. P. Lezina L. D. Smirnov 《Russian Chemical Bulletin》1989,38(7):1494-1500
1. | New halogen derivatives of 1-ethyl-2-methyl-, 2-methyl- and 5-hydroxybenzimidazole were obtained. |
2. | A method for the chlorination of aromatic compounds by an alcoholic solution of hydrogen chloride in the presence of hydrogen peroxide was proposed, which enables carrying out the reaction with a high yield. |
3. | The reactivity of the series of 5(6)-hydroxybenzimidazole derivatives during the electrophilic substitution (halogenation) was shown to be as follows: 1-ethyl-2-methyl-5-hydroxybenzimidazole >5(6)-hydroxybenzimidazole 2-methyl-5-hydroxybenzimidazole. |
94.
It is demonstrated with the help of1H and13C NMR spectroscopy and GLC that the formation of the stereoisomers of 2-alkyl-4,5-dimethyl-1,3,2-dioxaborinanes from 2-methyl-1,3-butanediol and acyclic boric acid esters occurs stereospecifically. This conclusion was confirmed by calculations of the optimal geometries and relative energies of the molecules of thecis-andtrans-isomers of model 4,5-dimethyl-1,3,2-dioxaborinanes and the proposed intermediate with a tetravalent boron atom using the method of molecular mechanics.A. V. Bogatskii Physicochemical Institute, National Academy of Sciences of Ukraine, Odessa 270080, Ukraine. Institute of Organic Chemistry, Ufa Science Center, Russian Academy of Science, Ufa 450054, Russia. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 400–404, March, 1998. 相似文献
95.
Conclusions The relationship between the free energies of activation G and reaction Go for proton transfer processes have been analyzed, taking into account the effect of hindered rotation of the reagents. We have shown that the considered effect can considerably affect the shape of the G=f(Go) curve.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 77–81, January 1989. 相似文献
96.
N. V. Kuznetsov 《Journal of Mathematical Sciences》1982,18(3):398-404
Combining ideas of convolution due to Rankin with spectral considerations of Selberg, the author proposes a new approach to obtaining mean values for certain number-theoretic functions f(n). This approach is illustrated for the examples of functions f(n)=(Mn2+N),(n)(Mn+N), where (n) is the number of divisors of n.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 76, pp. 159–166, 1978. 相似文献
97.
Y. M. Shulga A. A. Startsev Y. I. Yermakov B. N. Kuznetsov Y. G. Borod'ko 《Reaction Kinetics and Catalysis Letters》1977,6(3):377-383
By the XPS method it has been shown that in the catalysts obtained by interacting W(-C4H7)4 with SiO2 several oxidation states of the supported tungsten are realized together with a higher coverage of the silica surface with tungsten compounds, as compared with the catalysts prepared by impregnation of SiO2 with an aqueous solution of (NH4)2 WO4.
, , W(-C4H7)4 SiO2, , , SiO2 (NH4)2 WO4.相似文献
98.
A. N. Startsev B. N. Kuznetsov Yu. I. Yermakov 《Reaction Kinetics and Catalysis Letters》1975,3(3):321-327
The influence of preparation and treatment conditions on the catalytic activity in propylene disproportionation has been studied on catalysts prepared by interaction of W(-C4H7)4 with SiO2. The influence of the prehydration temperature of silica gel and the temperature of the treatment of the catalyst with hydrogen has been found to be appreciable. Comparison between the activity of the catalysts and the average oxidation state of tungsten ions suggests that the oxidation state of tungsten ions in the active catalyst component is +4.
, W(-C4H7)4 SiO2 . . , , 4.相似文献
99.
Yu. I. Yermakov B. N. Kuznetsov E. M. Chelganov 《Reaction Kinetics and Catalysis Letters》1980,14(1):37-41
A sharp change in the catalytic properties of rhodium in benzene hydrogenation, propane hydrogenolysis and n-hexane conversion has been found for catalysts prepared via interaction between Rh(CO)2Cl2 and silica modified by Sn, Pb, Mo and W ions.
, Rh(CO)2Cl2 , Sn, Pb, Mo W, , , -.相似文献
100.
Makarycheva-Mikhailova AV Bokach NA Kukushkin VY Kelly PF Gilby LM Kuznetsov ML Holmes KE Haukka M Parr J Stonehouse JM Elsegood MR Pombeiro AJ 《Inorganic chemistry》2003,42(2):301-311
Pt(IV)-mediated addition of the sulfimide Ph2S = NH and the mixed sulfide/sulfimides o- and p-[PhS(=NH)](PhS)-C6H4 by the S=NH group to the metal-bound nitriles in the platinum(IV) complexes [PtCl4(RCN)2] proceeds smoothly at room temperature in CH2Cl2 and results in the formation of the heterodiazadiene compounds [PtCl4[NH=C(R)N=SR'Ph]2] (R' = Ph, R = Me, Et, CH2Ph, Ph; R' = o- and p-(PhS)C6H4; R = Et). While trans-[PtCl4(RCN)2] (R = Et, CH2Ph, Ph) reacting with Ph2S=NH leads exclusively to trans-[PtCl4[NH=C(R)N=SPh2]2], cis/trans-[PtCl4(MeCN)2] leads to cis/trans mixtures of [PtCl4[NH=C(Me)N=SPh2]2] and the latter have been separated by column chromatography. Theoretical calculations at both HF/HF and MP2//HF levels for the cis and trans isomers of [PtCl4[NH=C(Me)N=SMe2]2] indicate a higher stability for the latter. Compounds trans-[PtCl4[E-NH=C(R)N=SPh2]2] (R = Me, Et) and cis-[PtCl4[E-NH=C(Me)N=SPh2][Z-NH=C(Me)N=SPh2]] have been characterized by X-ray crystallography. The complexes [PtCl4[NH=C(R)N=SPh2]2] undergo hydrolysis when treated with HCl in nondried CH2Cl2 to achieve the amidines [PtCl4[NH=C(NH2)R]2] the compound with R = Et has been structurally characterized) and Ph2SO. The heterodiazadiene ligands, formed upon Pt(IV)-mediated RCN/sulfimide coupling, can be liberated from their platinum(IV) complexes [PtCl4[NH=C(R)N=SR'Ph]2] by reaction with Ph2PCH2CH2PPh2 (dppe) giving free NH=C(R)=SR'Ph and the dppe oxides, which constitutes a novel route for such rare types of heterodiazadienes whose number has also been extended. The hybrid sulfide/sulfimide species o- and p-[PhS(=NH)](PhS)C6H4 also react with the Pt(II) nitrile complex [PtCl2(MeCN)2] but the coupling--in contrast to the Pt(IV) species--gives the chelates [PtCl2[M-I=C(Me)N=S(Ph)C6H4SPh]]. The X-ray crystal structure of [PtCl2[M-I=C(Me)N=S(Ph)C6H4SPh-o]] reveals the bond parameters within the metallacycle and shows an unusual close interaction of the sulfide sulfur atom with the platinum. 相似文献