全文获取类型
收费全文 | 1251篇 |
免费 | 40篇 |
国内免费 | 7篇 |
专业分类
化学 | 894篇 |
晶体学 | 13篇 |
力学 | 17篇 |
数学 | 153篇 |
物理学 | 221篇 |
出版年
2022年 | 7篇 |
2021年 | 5篇 |
2020年 | 9篇 |
2019年 | 10篇 |
2018年 | 5篇 |
2017年 | 18篇 |
2016年 | 26篇 |
2015年 | 18篇 |
2014年 | 26篇 |
2013年 | 64篇 |
2012年 | 46篇 |
2011年 | 78篇 |
2010年 | 37篇 |
2009年 | 49篇 |
2008年 | 67篇 |
2007年 | 88篇 |
2006年 | 81篇 |
2005年 | 92篇 |
2004年 | 78篇 |
2003年 | 58篇 |
2002年 | 64篇 |
2001年 | 17篇 |
2000年 | 22篇 |
1999年 | 20篇 |
1998年 | 13篇 |
1997年 | 18篇 |
1996年 | 14篇 |
1995年 | 7篇 |
1994年 | 8篇 |
1992年 | 16篇 |
1991年 | 8篇 |
1990年 | 9篇 |
1989年 | 10篇 |
1988年 | 10篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 18篇 |
1984年 | 17篇 |
1983年 | 12篇 |
1982年 | 16篇 |
1981年 | 12篇 |
1980年 | 19篇 |
1979年 | 16篇 |
1978年 | 9篇 |
1977年 | 7篇 |
1976年 | 9篇 |
1975年 | 14篇 |
1974年 | 9篇 |
1973年 | 8篇 |
1971年 | 4篇 |
排序方式: 共有1298条查询结果,搜索用时 421 毫秒
801.
9,10-Diacetyl-2,7-di-tert-butyl-trans-10b,10c-dimethyl-10b,10c-dihydropyrene annelated with a quinoxaline unit (q-DM-DHP, 3) was prepared from 5,13-di-tert-butyl-8,16-dimethyl[2.2]metacyclophane-1,2,9,10-tetraoxide (2) and o-phenylenediamine in two steps. The photochromic isomers of q-DM-DHP 3, closed form 3a and open form 3b, were separated by reverse-phase HPLC, and their isolated UV–visible spectra were estimated. The chiral HPLC analyses of q-DM-DHP 3 indicated their planar chirality, and their CD spectra were measured. The photochromic properties and thermodynamic properties of 3 were also discussed based on their UV–visible, 1H NMR, CD, and fluorescence spectra. 相似文献
802.
Reduction of U(VI)O(2)Cl(4)(2-) in a mixture of 1-ethyl-3-methylimidazolium tetrafluoroborate and its chloride at E°' = -0.996 V vs Fc/Fc(+) and 298 K affords U(V)O(2)Cl(4)(3-), which is kinetically stable and exhibits typical character of U(V) in the UV-vis-NIR absorption spectrum. 相似文献
803.
Daisuke MatsumuraToshimasa Takarabe Takumi TodaTakashi Hayamizu Kiyoto SawamuraKen-ichi Takao Kin-ichi Tadano 《Tetrahedron》2011,67(35):6730-6745
The total syntheses of both natural (+)-spiculoic acid A and (+)-zyggomphic acid, new cytotoxic marine natural products of polyketide origin, have been accomplished for the first time. These syntheses were achieved by the highly stereoselective and high-yielding intramolecular Diels-Alder reaction of a functionalized (E,E,E)-2,7,9-dodecanal derivative to construct the core tetrahydroindan-2-one skeleton. A stereocongener of (+)-spiculoic acid A, i.e., the (2R,5S,6R)-isomer, was also synthesized. The details of these total syntheses are described. 相似文献
804.
Stuart Francis Takao KiyoiMark Reid Keneth DaviesSteven Laats Duncan McArthurHelen Feilden Anna Marie EassonYasuko Kiyoi Grant WishartPeter Ray 《Tetrahedron letters》2011,52(27):3421-3425
Tetrahydro-2H-[2]benzopyrano[3,4-c]pyrrol-3-ones and the related 3a-methyl-2,3,3a,4,5,9b-hexahydro-1H-benzo[e]isoindole analogues were synthesised by an intramolecular Diels-Alder reaction. The observed stereoselectivity was dependent upon the nature of the tethered dienophile as well as the judicious placement of the amide. 相似文献
805.
A long-range corrected (LC) time-dependent density functional theory (TDDFT) incorporating relativistic effects with spin-orbit couplings is presented. The relativistic effects are based on the two-component zeroth-order regular approximation Hamiltonian. Before calculating the electronic excitations, we calculated the ionization potentials (IPs) of alkaline metal, alkaline-earth metal, group 12 transition metal, and rare gas atoms as the minus orbital (spinor) energies on the basis of Koopmans' theorem. We found that both long-range exchange and spin-orbit coupling effects are required to obtain Koopmans' IPs, i.e., the orbital (spinor) energies, quantitatively in DFT calculations even for first-row transition metals and systems containing large short-range exchange effects. We then calculated the valence excitations of group 12 transition metal atoms and the Rydberg excitations of rare gas atoms using spin-orbit relativistic LC-TDDFT. We found that the long-range exchange and spin-orbit coupling effects significantly contribute to the electronic spectra of even light atoms if the atoms have low-lying excitations between orbital spinors of quite different electron distributions. 相似文献
806.
A review into the aza-Diels-Alder reaction, mainly concentrating on literature examples that form piperidin-4-ones from the reaction of imines and electron rich dienes or enones, either through a Lewis acidic/Br?nsted acid approach or through the use of an organocatalyst. This review questions whether the mechanism of the aza-Diels-Alder reaction is step wise as opposed to concerted when using oxygenated dienes. 相似文献
807.
Yasui T Omoto Y Osato K Kaji N Suzuki N Naito T Watanabe M Okamoto Y Tokeshi M Shamoto E Baba Y 《Lab on a chip》2011,11(19):3356-3360
We developed a new passive-type micromixer based on the baker's transformation and realized a fast mixing of a protein solution, which has lower diffusion constant. The baker's transformation is an ideal mixing method, but there is no report on the microfluidic baker's transformation (MBT), since it is required to fabricate the complicated three-dimensional (3D) structure to realize the MBT device. In this note, we successfully fabricate the MBT device by using precision diamond cutting of an oxygen-free copper substrate for the mould fabrication and PDMS replication. The MBT device with 10.4 mm mixing length enables us to achieve complete mixing of a FITC solution (D = 2.6 × 10(-10) m(2) s(-1)) within 51 ms and an IgG solution (D = 4.6 × 10(-11) m(2) s(-1)) within 306 ms. Its mixing speed is 70-fold higher for a FITC solution and 900-fold higher for an IgG solution than the mixing speed by the microchannel without MBT structures. The Péclet number to attain complete mixing in the MBT device is estimated to be 6.9 × 10(4). 相似文献
808.
Morikawa T Li X Nishida E Nakamura S Ninomiya K Matsuda H Hamao M Muraoka O Hayakawa T Yoshikawa M 《Chemical & pharmaceutical bulletin》2011,59(7):889-895
Five new triterpene saponins perennisosides VIII (1), IX (2), X (3), XI (4), and XII (5) were isolated from the MeOH-eluated fraction of the methanolic extract from the flowers of Bellis perennis. The MeOH-eluted fraction of the methanolic extract from the flowers of B. perennis was found to inhibit gastric emptying in olive oil-loaded mice at a dose of 200 mg/kg, per os (p.o.). The stereostructures of 1-5 were elucidated on the basis of chemical and spectroscopic evidence. 相似文献
809.
Horiuchi T Takeda Y Haginoya N Miyazaki M Nagata M Kitagawa M Akahane K Uoto K 《Chemical & pharmaceutical bulletin》2011,59(8):991-1002
The design, synthesis, and evaluation of novel thieno[2,3-d]pyrimidin-4-yl hydrazone analogues as cyclin-dependent kinase 4 (CDK4) inhibitors are described. In continuing our program aim to search for potent CDK4 inhibitors, the introduction of a thiazole group at the hydrazone part has led to marked enhancement of chemical stability. Furthermore, by focusing on the optimization at the C-4' position of the thiazole ring and the C-6 position of the thieno[2,3-d]pyrimidine moiety, compound 35 has been identified with efficacy in a xenograft model of HCT116 cells. In this paper, the potency, selectivity profile, and structure-activity relationships of our synthetic compounds are discussed. 相似文献
810.
Tanaka N Mizoshita N Maegawa Y Tani T Inagaki S Jorapur YR Shimada T 《Chemical communications (Cambridge, England)》2011,47(17):5025-5027
A novel spirobifluorene-bridged allylsilane precursor, which can be easily purified by silica gel chromatography, was prepared by using a new molecular building block for allylsilane sol-gel precursors (MBAS) and successfully converted into a highly fluorescent periodic mesoporous organosilica film. 相似文献