全文获取类型
收费全文 | 866篇 |
免费 | 13篇 |
专业分类
化学 | 635篇 |
晶体学 | 8篇 |
力学 | 21篇 |
数学 | 51篇 |
物理学 | 164篇 |
出版年
2022年 | 6篇 |
2021年 | 8篇 |
2020年 | 9篇 |
2019年 | 15篇 |
2018年 | 11篇 |
2017年 | 9篇 |
2016年 | 16篇 |
2015年 | 9篇 |
2014年 | 13篇 |
2013年 | 31篇 |
2012年 | 32篇 |
2011年 | 58篇 |
2010年 | 32篇 |
2009年 | 33篇 |
2008年 | 50篇 |
2007年 | 56篇 |
2006年 | 57篇 |
2005年 | 55篇 |
2004年 | 45篇 |
2003年 | 36篇 |
2002年 | 45篇 |
2001年 | 6篇 |
2000年 | 15篇 |
1999年 | 5篇 |
1998年 | 13篇 |
1997年 | 12篇 |
1996年 | 18篇 |
1995年 | 5篇 |
1994年 | 14篇 |
1993年 | 5篇 |
1992年 | 16篇 |
1991年 | 8篇 |
1990年 | 12篇 |
1989年 | 10篇 |
1986年 | 6篇 |
1985年 | 13篇 |
1984年 | 7篇 |
1983年 | 8篇 |
1982年 | 6篇 |
1981年 | 13篇 |
1980年 | 6篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1976年 | 5篇 |
1975年 | 3篇 |
1974年 | 4篇 |
1973年 | 4篇 |
1967年 | 3篇 |
1966年 | 3篇 |
排序方式: 共有879条查询结果,搜索用时 31 毫秒
11.
12.
To approach more realistic mechanisms for asymmetric aziridine synthesis from guanidinium ylides and aryl aldehydes, reactions were systematically carried out by using a variety of p-substituted benzaldehydes under modified conditions. Two kinds of reaction mechanisms controlled by the nature of the p-substituents of aryl aldehydes is proposed for the two-steps aziridine synthesis composed of a C-C bond formation by nucleophilic addition of guanidinium ylides to aryl aldehydes (step 1) and the fragmentation of intermediate adducts to aziridine products by intramolecular nucleophilic substitution (step 2). A SNi-like mechanism via cationic-like transition state is proposed for step 2 in the asymmetric synthesis using EDG-substituted benzaldehydes, whereas with EWG-substituted benzaldehydes, a SN2-like mechanism is proposed. Hammett analysis, based on the diastereomeric ratio in the aziridine products, is consistent with the proposed rate-determining steps in these two mechanisms. A second Hammett analysis, based on the enantiomeric ratio of the aziridine products, clearly reveals the difference in the susceptibilities to the electronic substituents effect between step 1 and step 2. 相似文献
13.
Kazuhisha Hiratani Shamsun Naher Michinori Karikomi Kazuo Haga 《Journal of heterocyclic chemistry》2005,42(4):575-582
Novel crownophanes with 27‐ and 28‐membered rings having two hydroxyl groups, two amide groups, and aromatic moieties such as naphthalene, pyridine, and phenyl rings were successfully synthesized by a one‐step reaction from the corresponding macrocyclic polyethers via “tandem Claisen rearrengement” in moderate yields. They can solubilize urea and its derivatives into chloroform solution, while the corresponding macrocyclic polyethers do not solubilize them. According to NMR studies, crownophanes 1 and 2 interact with urea and its derivatives forming 1:1 complexes. 相似文献
14.
Matsuo G Kawamura K Hori N Matsukura H Nakata T 《Journal of the American Chemical Society》2004,126(44):14374-14376
Brevetoxin-B (BTX-B), produced by the red tide organism, Gymnodium breve Davis, is the first member of marine polycyclic ethers to be structurally elucidated and one of the most potent neurotoxins. The structural feature is a trans-fused polycyclic ether ring system with 23 stereocenters. Its unique, complex structure and potent biological activity have attracted the attention of synthetic organic chemists. Total synthesis of BTX-B has been accomplished via the coupling of the ABCDEFG and IJK-ring segments, each ether ring of which was stereoselectively and efficiently constructed on the basis of SmI2-induced intramolecular cyclization, 6-endo-cyclization of hydroxy epoxide, ring-closing olefin metathesis, and SmI2-induced intramolecular Reformatsky-type reaction. Several kinds of double reactions at the left and right sides were efficiently used through the synthesis. 相似文献
15.
Nobuyuki Tanaka 《Journal of Thermal Analysis and Calorimetry》1996,46(3-4):1021-1031
The instantaneous elastic moduli for a nylon-6 monofilament were derived on strain recoveries right after creep, stress relaxation, and rapid elongation,E
c
,E
s
andE
e
, respectively. It was found that during strain recoveryE
s
(>E
e
) andE
e increase monotonically with increasing load,m
1, on the sample. The extrapolated value of Es atm
1=0 g is almost equal to Young's modulus, 4.06 GPa. The value ofE
c also increased with increasingm
1, and atm
1=600 g (1.93 t cm–2) reached about 14 GPa. The endothermic heat change right after creep, stress relaxation or rapid elongation,Q, was negligibly small. For comparison,E
s
,E
c
andQ were also investigated for silicone rubber. It was found thatE
s (53.8 M Pa at the draw ratioD=1.2) decreased abruptly atD=1.3. In the range ofD=1.4–1.9,E
s was only 22.6 MPa. In the case of stress relaxation,Q increased with increasingD from 4 J mol–1 (atD=1.2) to 56 J mol–1 (atD=1.9). FurthermoreE
c (5.58 MPa atm
1=133.8 g (429.4 kg cm–2)) increased gradually with increasing m1 and attained 16.6 MPa atm
1=548.4 g (1.76 t cm–2). In the case of creep,Q was in the range of 0–11.5 J mol–1 and larger when larger loads,m
2 were removed during the later stages of creep.Dedicated to Professor Bernhard Wunderlich on the occasion of his 65th birthdayThe author wishes to thank Mr. Keizi Igarashi and Mr. Tetsuya Yasui for helping in the experiments. 相似文献
16.
Koichiro Fukuda Miyuki Hisamura Nobuyuki Tera 《Journal of solid state chemistry》2007,180(6):1809-1815
A new quaternary layered carbide, Zr2[Al3.56Si0.44]C5, has been synthesized and characterized by X-ray powder diffraction, transmission electron microscopy and thermopower and electrical conductivity measurements. The crystal structure was successfully determined using direct methods, and further refined by the Rietveld method. The crystal is trigonal (space group R3m, Z=3) with lattice dimensions of a=0.331059(5), c=4.09450(5) nm and V=0.38864(1) nm3. The final reliability indices calculated from the Rietveld refinement were Rwp=6.24%, Rp=4.21% and RB=0.82%. The crystal structure is composed of electroconductive NaCl-type ZrC slabs separated by Al4C3-type [Al3.56Si0.44]C3 layers. This material had thermoelectric properties superior to those of the ternary layered carbides Zr2Al3C4 and Zr3Al3C5, with the power factor reaching 7.6×10−5W m−1 K−2. 相似文献
17.
[reaction: see text] Direct asymmetric catalytic Michael reactions have been performed using chiral-amine/acid bifunctional catalysts. Performed with 0.3 equiv of (S)-(+)-1-(2-pyrrolidinylmethyl)pyrrolidine and 0.3 equiv of trifluoroacetic acid as the catalyst, the reaction of alpha,alpha-dialkylaldehydes with (E)-beta-nitrostyrene provided the alpha,alpha-dialkyl Michael products in up to 96% yield with up to 91% ee. With respect to enantioselectivity, l-proline was a poor catalyst of this class of Michael reactions. 相似文献
18.
Morino K Watase N Maeda K Yashima E 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(19):4703-4707
Poly[(4-carboxyphenyl)acetylene] (poly-1) exhibits an intense induced circular dichroism (ICD) in the UV-visible region upon complexation with excess (R)-1-(1-naphthyl)ethylamine ((R)-2), owing to the formation of a predominantly single-handed helical conformation of the polymer backbone. In the presence of a small amount of (R)-2, poly-1 showed a very weak ICD due to the lack of a single-handed helical conformation. However, we have found that the co-addition of the excess bulky, achiral 1-naphthylmethylamine (5) with a small amount of (R)-2 caused a dramatic increase in the ICD magnitude, comparable to the full ICD induced by excess (R)-2. This indicates that an almost single-handed helix can be induced on poly-1 upon complexation with a small amount of (R)-2 assisted by achiral 5. Furthermore, the induced single-handed helical poly-1 could be successfully memorized by the replacement of (R)-2 and 5 with achiral 2-aminoethanol or n-butylamine. 相似文献
19.
Oishi S Kamano T Niida A Odagaki Y Tamamura H Otaka A Hamanaka N Fujii N 《Organic letters》2002,4(7):1051-1054
[reaction: see text] Acyclic psi[(E)-CH=CMe]- and psi[(Z)-CH=CMe]-type dipeptide isosteres were efficiently synthesized. In a key reaction, alpha-alkylation of gamma-mesyloxy-beta-methyl-alpha,beta-unsaturated esters with organocyanocuprates in diethyl ether or tetrahydrofuran preferentially afforded the psi[(E)-CH=CMe]- or psi[(Z)-CH=CMe]-isomer, respectively, via anti-S(N)2' mechanism. 相似文献
20.
Tsutomu Takeichi Nobuyuki Takahashi Rikio Yokota 《Journal of polymer science. Part A, Polymer chemistry》1994,32(1):167-174
Polyimide/polyimide molecular composite (MC) films comprised of a rigid polyimide derived from biphenyltetracarboxylic dianhydride (BPDA) and p-phenylenediamine (PDA) and a flexible polyimide derived from BPDA and bis (3,3'-diaminodiphenyl) acetylene (intA) and/or oxydianiline (ODA) were prepared by blending the polyamic acid solutions in 7 : 3 weight ratio, and then imidizing the blend films. Acetylene content in the flexible polyimide backbone was controlled by the ratio of intA and ODA. Cold-drawing of the blend polyamic acid films, followed by imidization, gives high modulus polyimide/polyimide MC films. The modulus of the MC films increased almost linearly with the draw ratio, reaching 25.5 GPa for the 40% drawn film. Acetylene groups in the flexible polyimide can be thermally cured to crosslink. The onset of exotherm appeared at 340°C on DSC, reaching maximum at 398°C. After the thermal crosslinking, the MC films maintained the high modulus, though elongation became small. Taking advantage of the crosslinkable acetylene units, two MC films were laminated and processed at 400°C for 20 min under 100 kg/cm2 to give a good-quality laminate film. The interface of the two films was strongly bonded through the crosslinking of acetylene groups. Laminate films maintained the high modulus afforded by the cold-drawing. © 1994 John Wiley & Sons, Inc. 相似文献