全文获取类型
收费全文 | 838篇 |
免费 | 33篇 |
国内免费 | 4篇 |
专业分类
化学 | 690篇 |
晶体学 | 8篇 |
力学 | 7篇 |
数学 | 23篇 |
物理学 | 147篇 |
出版年
2022年 | 4篇 |
2021年 | 13篇 |
2020年 | 20篇 |
2019年 | 21篇 |
2018年 | 9篇 |
2017年 | 3篇 |
2016年 | 22篇 |
2015年 | 12篇 |
2014年 | 22篇 |
2013年 | 38篇 |
2012年 | 50篇 |
2011年 | 44篇 |
2010年 | 16篇 |
2009年 | 32篇 |
2008年 | 40篇 |
2007年 | 51篇 |
2006年 | 54篇 |
2005年 | 60篇 |
2004年 | 52篇 |
2003年 | 34篇 |
2002年 | 36篇 |
2001年 | 10篇 |
2000年 | 9篇 |
1999年 | 19篇 |
1998年 | 15篇 |
1997年 | 16篇 |
1996年 | 6篇 |
1995年 | 9篇 |
1994年 | 6篇 |
1993年 | 8篇 |
1992年 | 9篇 |
1991年 | 8篇 |
1990年 | 12篇 |
1989年 | 8篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 4篇 |
1985年 | 12篇 |
1984年 | 10篇 |
1983年 | 11篇 |
1982年 | 5篇 |
1981年 | 8篇 |
1980年 | 4篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1974年 | 5篇 |
1973年 | 3篇 |
排序方式: 共有875条查询结果,搜索用时 625 毫秒
31.
One-dimensional potential energy curves for the isomerization of protonated Schiff base of retinal (PSBR) in bacteriorhodopsin (bR), i.e., isomerization from all-trans- to 13-cis-forms, have been calculated by means of time-dependent density functional theory (TD-DFT) calculations, in order to elucidate the mechanism of initial step in photo-absorption. The transition state of the isomerization in the first excited state is located at theta(13-14)=58 degrees , where theta(13-14) means twist angle around the C(13)=C(14) double bond of PSBR The potential barrier is formed by the avoided crossing between S(1) (B(u)-like) and S(2) (A(g)-like) states. The mechanism of the isomerization was discussed on the basis of theoretical results. 相似文献
32.
Hiroto Nagaoka Kaoru Kobayashi Toshiaki Okamura Yasuji Yamada 《Tetrahedron letters》1987,28(52):6641-6644
The sequential Michael reaction of (S)-4,5-di-O-isopropylidenepent-2- enoate
and
with lithium enolate
afforded diastereo- and enantioselectively bicyclo[2.2.2]octane
and
(or
), respectively. The adducts were efficiently converted into both enantiomeric keto aldehydes (−)-
and (+)-
. 相似文献
33.
Okuyama Y Nakano H Takahashi K Hongo H Kabuto C 《Chemical communications (Cambridge, England)》2003,(4):524-525
Easily prepared, chiral sulfideoxathiane ligands are described which give excellent enantioselectivity (up to 99% ee) in the Pd-catalyzed allylic alkylation of 1,3-diphenyl-2-propenyl acetate with a range of alkyl malonate nucleophiles. 相似文献
34.
Hiroto Tachikawa Hiroshi Kawabata Kazumi Matsushige 《Journal of organometallic chemistry》2005,690(12):2895-2904
The structures and electronic states of phenyl-capped terthiophene (denoted by P3T) and the ionic species of P3T have been investigated by means of density functional theory (DFT) and direct MO dynamics calculations. P3T is one of the high-performance molecular devices, which has been utilized as a semi-conductor. The calculations indicated that the neutral P3T has a non-planar structure whose the phenyl rings in both ends of thiophene chain are largely deviated from the molecular plane. The cation and anion radicals, dication and dianion were considered as its ionic states. The structure for cation radical of P3T is close to more planar than that of neutral P3T. The structures for anion radical, dication and dianion take a pure planar structure. The first excitation energy of neutral P3T is calculated to be 2.90 eV at the TD-B3LYP/6-31G(d)//B3LYP/6-311+G(d) level, while the P3T cation and anion radicals have lower excitation energies (1.22 and 1.10 eV, respectively). The direct MO dynamics calculation showed that neutral, cation and anion hold near planar structure at 300 K. On the other hand, oligothiophene (n = 5) and its ionic species are strongly deformed from the planar structure, and thiophene rings in both ends of chain rotate rapidly by thermal activation. The mechanism of the electron conductivity in P3T was discussed on the basis of theoretical results. 相似文献
35.
Hiroto Yoshida 《Tetrahedron》2007,63(22):4793-4805
A variety of aldehydes, ketones, benzoquinones, or sulfonylimines were found to couple with arynes and isocyanides, giving iminodihydroisobenzofuran or iminoisoindoline derivatives of structural diversity in a straightforward manner. Nucleophilic addition of isocyanides to arynes, a first step of the three-component coupling, was proved to be reversible by the reaction of an unsymmetrical aryne. 相似文献
36.
Cp(2)Ti[P(OEt)(3)](2)-promoted reactions of 2-(alk-1-yn-1-yl)-2-(trialkylsilyl)-1,3-dithianes (RS)(2)C(Si)CCR with terminal olefins and carbonyl compounds produced (trialkylsilylethynyl)cyclopropanes and 1-(trialkylsilyl)alk-3-en-1-ynes, respectively. These compounds were suggest to be produced via the formation of intermediary titanium alpha-(trialkylsilylethynyl)carbene complexes Cp(2)Ti=C(R)CCSi in preference to their regioisomers, alpha-(trialkylsilyl)alkynylcarbene complexes Cp(2)Ti=C(Si)CCR. 相似文献
37.
38.
Iwaoka M Komatsu H Katsuda T Tomoda S 《Journal of the American Chemical Society》2004,126(16):5309-5317
To investigate the nature of nonbonded Se...O interactions, three series of 2-substituted benzeneselenenyl derivatives [2-(CHO)C6H4SeX (1), 2-(CH2OH)C6H4SeX, (2), 2-(CH2OiPr)C6H4SeX (3); X = Cl, Br, CN, SPh, SeAr, Me] were synthesized. The 17O NMR absorption observed for 17O-enriched aldehydes 1 appeared upfield relative to benzaldehyde (PhCHO), while the opposite downfield shifts relative to benzyl alcohol (PhCH2OH) were observed for 17O-enriched alcohols 2 and ethers 3. The magnitude of both the upfield and the downfield shifts became larger as the electron-withdrawing ability of a substituent X increased. Quantum chemical calculations at the B3LYP level revealed that for all model compounds the most stable conformer has an intramolecular nonbonded Se.O interaction. Thus, the relative 17O NMR chemical shifts (DeltadeltaO) for 1-3 would reflect the strengths of the Se...O interactions. The natural bond orbital (NBO) analysis demonstrated that the stabilization energy due to an nO --> sigma Se-X orbital interaction (ESe...O) correlates with the Se...O atomic distance on a single curve irrespective of the type of the O atom. On the other hand, the atoms in molecules (AIM) analysis showed that the nonbonded Se...O interactions can be characterized by the presence of a bond critical point, the total energy density (HSe...O) of which decreases with strengthening of the interaction. The results suggested that Se...O interactions have a dominant covalent character rather than an electrostatic one. 相似文献
39.
Hiroshi Hata Dr. Shigeru Yamaguchi Goro Mori Satomi Nakazono Taisuke Katoh Keishi Takatsu Satoru Hiroto Hiroshi Shinokubo Prof. Dr. Atsuhiro Osuka Prof. Dr. 《化学:亚洲杂志》2007,2(7):849-859
Highly regioselective and efficient borylation of a variety of porphyrins has been achieved by reaction with bis(pinacolato)diboron through C? H bond activation under iridium catalysis on the basis of the synthetic protocol developed by Miyaura, Hartwig, and Smith. A boryl group can be selectively introduced at sterically uncongested positions in the peripheral aryl groups of porphyrin substrates whose peripheral β‐positions are sterically hindered. Curiously, β substituents adjacent to the aryl group to be borylated have unexpectedly large effects on the regioselectivity, because the iridium catalyst can discriminate between subtle steric differences. Chemoselective borylation was also achieved for several functionalized porphyrins. This borylation protocol can be applied to various monomeric and oligomeric functional porphyrins, hence offering an efficient route to elaborate multiporphyrin‐based molecular constructs. 相似文献
40.
T Abe K Horiuchi H Kikuchi T Aritsuka Y Takata E Fukushi Y Fukushi J Kawabata K Ueno S Onodera N Shiomi 《Chemistry Central journal》2012,6(1):89
ABSTRACT: BACKGROUND: Sugar beet molasses is a viscous by-product of the processing of sugar beets into sugar. The molasses is known to contain sucrose and raffinose, a typical trisaccharide, with a well-established structure. Although sugar beet molasses contains various other oligosaccharides as well, the structures of those oligosaccharides have not been examined in detail. The purpose of this study was isolation and structural confirmation of these other oligosaccharides found in sugar beet molasses. RESULTS: Four oligosaccharides were newly isolated from sugar beet molasses using high-performance liquid chromatography (HPLC) and carbon-Celite column chromatography. Structural confirmation of the saccharides was provided by methylation analysis, matrix-assisted laser desorption/ionaization time of flight mass spectrometry (MALDI-TOF-MS), and nuclear magnetic resonance (NMR) measurements. CONCLUSION: The following oligosaccharides were identified in sugar beet molasses: beta-D-galactopyranosyl-(1- > 6)-beta-D-fructofuranosyl-(2 <-> 1)-alpha-D-glucopyranoside (named beta-planteose), alpha-D-galactopyranosyl-(1- > 1)-beta-D-fructofuranosyl-(2 <-> 1)-alpha-D-glucopyranoside (named1-planteose), alpha-D-glucopyranosyl-(1- > 6)-alpha-D-glucopyranosyl-(1 <-> 2)-beta-D-fructofuranoside (theanderose), and beta-D-glucopyranosyl-(1- > 3)-alpha-D-glucopyranosyl-(1 <-> 2)-beta-D-fructofuranoside (laminaribiofructose). 1-planteose and laminaribiofructose were isolated from natural sources for the first time. 相似文献