全文获取类型
收费全文 | 5041篇 |
免费 | 40篇 |
国内免费 | 7篇 |
专业分类
化学 | 2384篇 |
晶体学 | 122篇 |
力学 | 159篇 |
综合类 | 3篇 |
数学 | 594篇 |
物理学 | 1826篇 |
出版年
2019年 | 46篇 |
2018年 | 66篇 |
2017年 | 68篇 |
2016年 | 73篇 |
2015年 | 51篇 |
2014年 | 81篇 |
2013年 | 203篇 |
2012年 | 155篇 |
2011年 | 183篇 |
2010年 | 105篇 |
2009年 | 141篇 |
2008年 | 169篇 |
2007年 | 178篇 |
2006年 | 143篇 |
2005年 | 129篇 |
2004年 | 137篇 |
2003年 | 112篇 |
2002年 | 118篇 |
2001年 | 104篇 |
2000年 | 98篇 |
1999年 | 87篇 |
1998年 | 53篇 |
1997年 | 57篇 |
1996年 | 67篇 |
1995年 | 60篇 |
1994年 | 69篇 |
1993年 | 54篇 |
1992年 | 71篇 |
1991年 | 73篇 |
1990年 | 78篇 |
1989年 | 57篇 |
1988年 | 42篇 |
1987年 | 59篇 |
1986年 | 61篇 |
1985年 | 60篇 |
1984年 | 81篇 |
1983年 | 87篇 |
1982年 | 79篇 |
1981年 | 87篇 |
1980年 | 98篇 |
1979年 | 71篇 |
1978年 | 90篇 |
1977年 | 95篇 |
1976年 | 70篇 |
1975年 | 52篇 |
1974年 | 48篇 |
1973年 | 43篇 |
1972年 | 35篇 |
1970年 | 33篇 |
1969年 | 33篇 |
排序方式: 共有5088条查询结果,搜索用时 15 毫秒
61.
R. G. Aflyatunova N. A. Aliev Ch. Sh. Kadyrov M. R. Yagudaev 《Chemistry of Natural Compounds》1984,19(4):478-482
The reactions of benzoxazolinone and benzoxazolinethione with alkyl -chlorovinyl ketones have been studied. The reaction products are N-(-acylvinyl)benzoxazolinones and N-(-acylvinyl)benzoxazolinethiones. The reaction of alkali-metal salts of benzoxazolinone with alkyl -chlorovinyl ketones leads to the opening of the oxazolinone ring. The structures of the compounds synthesized have been studied by IR, mass, and PMR spectroscopy.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 507–511, July–August, 1983. 相似文献
62.
The activation of18O and19F by the reactions18O(γ, p)17N and19F(γ, 2p)17N was determined as a function of the maximum energy of the bremsstrahlung beam. By the use of this method it is possible
to measure oxygen or fluorine by counting the delayed neutrons emitted by17N with a half-life of 4.2 sec.
相似文献
63.
Hyaluronic acid-copper(II) complexes were studied by ultraviolet and electron spin resonance spectroscopy (ESR), as well as by rheological measurements. Spin-Hamiltonian parameters were obtained from experimental spectra measured at 200 K by non-linear optimization method. These showg
x
g
y
<g
z
and indicate the
ground electronic state of copper ion, which is characteristic for carboxylato-copper(II) complexes. The non-axial symmetry of theg-tensor suggest rhombic crystal field symmetry. Formation of HA-Cu(II) complex is realized through the interaction of copper ions with carboxyl groups of the HA macromolecule. This complex is characteristic of the network-like rheological properties of its aqueous solution. 相似文献
64.
Anionic polymerization of methyl methacrylate in THF and toluene at ?70° was studied by means of i.r. spectroscopy. The reaction mixtures show characteristic absorption at 1600–1750 cm?1 caused by the stretching vibrations of the propagating centre as well as by the groups obtained in side reactions (metal splitting, addition to a carbonyl group, cyclization). Sec-butyllithium is more effective in THF than n-butyllithium, ethyllithium and fluorenyllithium. When polymerizing in toluene, small amounts of a donor (THF or HMPT) cause formation of active centres similar to those in pure THF. The “living” ends of oligo-MMA form 1:1 complexes with electron acceptors such as trialkylaluminium and triethylboron. 相似文献
65.
V. V. Burlakov A. V. Letov P. Arndt W. Baumann A. Spannenberg Ch. Fischer L. I. Strunkina M. Kh. Minacheva Ya. S. Vygodskii U. Rosenthal V. B. Shur 《Journal of molecular catalysis. A, Chemical》2003,200(1-2):63-67
Zwitterionic titanoxanes {Cp[η5-C5H4B(C6F5)3]Ti}2O (I) and {(η5-iPrC5H4)[η5-1,3-iPrC5H3B(C6F5)3]Ti}2O (II), which contain two positively charged Ti(IV) centres in the molecule, are able to catalyse the ring-opening polymerization of -caprolactone (-CL) in toluene solution and in bulk. The process proceeds with a noticeable rate even at room temperature and accelerates strongly on raising the temperature to 60 °C. The best results have been obtained on carrying out the reaction in bulk. Under these conditions, the use of I as a catalyst (-CL:I = 1000:1) gives at 60 °C close to quantitative yield of poly--CL with the molecular mass of 197 000. An increase in the -CL:I ratio to 6000:1 increases the molecular mass of poly--CL to 530 000. Tetrahydrofuran (THF) is also polymerized under the action of I albeit with a lesser rate. However, the molecular mass of the resulting poly-THF can reach rather big values under optimal conditions (up to 217 000 at 20 °C and the THF:I ratio of 770:1). A rise in the reaction temperature from 20 to 60 °C results here to a decrease in the efficiency of the process. Titanoxane II is close to I in its catalytic activity in the -CL polymerization but it is much less active in the polymerization of THF. Propylene oxide (PO), in contrast to -CL and THF, gives with I only liquid oligomers in wide temperature and PO:I molar ratio ranges (−30 to +20 °C, PO:I = 500–2000:1). γ-Butyrolactone and 1-methyl-2-pyrrolidone are not polymerized under the action of I at room temperature. The reactions found are the first examples of catalysis of the cationic ring-opening polymerization by zwitterionic metallocenes of the group IVB metals. 相似文献
66.
Thilo Berkenbusch Andy Ch. Laungani Reinhard Brückner Manfred Keller 《Tetrahedron letters》2004,45(52):9517-9520
4-tert-Butyl-2-cyclohexen-1-one dimerizes in THF solution via its kinetic enolate, leading to di-tert-butylditwistane 8 in up to 36% yield (−78 °C → room temp., protonolysis, flash chromatography). X-ray crystallography shows that 8 incorporates one R and one S enantiomer of the starting ketone; none of the diastereomeric ditwistanes epi-8, epi’-8 or iso-8 was isolated. This means that the formation of 8 proceeds with mutual kinetic resolution and 100% induced diastereoselectivity. 相似文献
67.
Chandrasekhar S Narsihmulu Ch Reddy NR Sultana SS 《Chemical communications (Cambridge, England)》2004,(21):2450-2451
We demonstrate for the first time, L-proline as a chiral catalyst for transfer aldol reaction between aldehydes and diacetone alcohol. 相似文献
68.
B. Roduit Ch. Borgeat B. Berger P. Folly B. Alonso J. N. Aebischer F. Stoessel 《Journal of Thermal Analysis and Calorimetry》2005,80(1):229-236
Summary An advanced kinetic study on the thermal behaviour of pyrotechnic ignition mixtures has been carried out by differential scanning calorimetry using different B/KNO3 mixtures (50:50, 30:70, 20:80) as a model reaction. The experimental conditions applied (isochoric conditions/closed crucibles and isobaric conditions/open crucibles) as well as the composition of the mixtures noticeably influences the relative thermal stabilities of the energetic materials. The kinetic study focused on the prediction of the thermal stability of the different mixtures both in extended temperature ranges and under temperature conditions at which ordinary investigation would be very difficult. Using advanced numerical tools [1], thermal ageing and influence of the complex thermal environment on the heat accumulation conditions were computed. This can be done for any surrounding temperature profile such as isothermal, non-isothermal, stepwise, modulated, shock, adiabatic conditions and additionally for temperature profiles reflecting real atmospheric temperature changes (yearly temperature profiles of different climates with daily minimal and maximal fluctuations). Applications of accurate decomposition kinetics enabled the determination of the time to maximum rate under adiabatic conditions (TMRad) with a precision given by the confidence interval of the predictions. This analysis can then be applied for the examination of the effects of the surrounding temperature for safe storage or transportation conditions (e.g. determination of the safe transport or storage temperatures). 相似文献
69.
A method has been developed for obtaining 3-vinylbenzoxazolinones and 3-vinylbenzoxazolinethione. It has been shown that it is also possible to obtain 3-methylbenzoxazolinethione by this method in good yield (76%). 相似文献
70.
A three-component coupling was used to prepare a series of 1,4-disubstituted-1,2,3-triazoles from the corresponding acetylated Baylis-Hillman adducts, sodium azide and terminal alkynes. This one-pot reaction further increases the efficacy of ‘Click’ synthesis and diversifies the preparation of multi-functional 1,4-disubstituted-1,2,3-triazoles. 相似文献