全文获取类型
收费全文 | 3436篇 |
免费 | 95篇 |
国内免费 | 12篇 |
专业分类
化学 | 2593篇 |
晶体学 | 42篇 |
力学 | 44篇 |
数学 | 184篇 |
物理学 | 680篇 |
出版年
2022年 | 30篇 |
2021年 | 29篇 |
2020年 | 58篇 |
2019年 | 59篇 |
2018年 | 44篇 |
2017年 | 23篇 |
2016年 | 62篇 |
2015年 | 49篇 |
2014年 | 69篇 |
2013年 | 145篇 |
2012年 | 157篇 |
2011年 | 202篇 |
2010年 | 104篇 |
2009年 | 116篇 |
2008年 | 190篇 |
2007年 | 189篇 |
2006年 | 212篇 |
2005年 | 188篇 |
2004年 | 152篇 |
2003年 | 132篇 |
2002年 | 116篇 |
2001年 | 69篇 |
2000年 | 58篇 |
1999年 | 54篇 |
1998年 | 36篇 |
1997年 | 40篇 |
1996年 | 49篇 |
1995年 | 40篇 |
1994年 | 32篇 |
1993年 | 43篇 |
1992年 | 50篇 |
1991年 | 30篇 |
1990年 | 35篇 |
1989年 | 38篇 |
1988年 | 42篇 |
1987年 | 38篇 |
1986年 | 33篇 |
1985年 | 64篇 |
1984年 | 38篇 |
1983年 | 31篇 |
1982年 | 37篇 |
1981年 | 33篇 |
1980年 | 31篇 |
1979年 | 40篇 |
1978年 | 34篇 |
1977年 | 29篇 |
1976年 | 22篇 |
1975年 | 26篇 |
1974年 | 30篇 |
1973年 | 25篇 |
排序方式: 共有3543条查询结果,搜索用时 15 毫秒
81.
The determination of nickel in various silicate rocks and glasses by photon activation analysis with a linear electron accelerator is described. Simultaneous irradiation of the sample and comparative standards produces the 58Ni(γ, n)57Ni reaction, and a post-irradiation chemical separation is used in conjunction with Ge(Li) γ-spectrometry. Nickel abundances for ten standard silicate rocks and two elementally doped glasses are presented and compared with the data previously published. The method is quite simple and gives good reproducible results for nickel down to sub-p.p.m. levels. 相似文献
82.
Electron-phonon interactions in the charged cubic fluorocarbon, (CF)8 are studied, and compared with those in charged (CH)8 and (CD)8. The A1g mode of 1470 cm(-1) much more strongly couples to the a1g lowest unoccupied molecular orbitals (LUMO) than the A1g mode of 554 cm(-1) in (CF)8. The T2g mode of 1030 cm(-1), the Eg mode of 980 cm(-1), and the A1g mode of 1470 cm(-1) strongly couple to the t2u highest occupied molecular orbitals (HOMO) in (CF)8. The total electron-phonon coupling constants for the monoanion (l(-1)) and monocation (l(+1)) of (CF)8 are estimated to be 0.932 and 0.585 eV, respectively. The logarithmically averaged phonon frequencies for the monoanion (omega(ln,-1)) and monocation (omega(ln,+1)) of (CF)8 are estimated to be 1365 and 998 cm(-1), respectively. The l(-1) and omega(ln,-1) values increase much more significantly by H-F substitution than by H-D substitution in cubane. The larger displacements of carbon atoms in the high frequency vibronic active mode in (CF)8 than those in (CD)8 due to larger atomic mass of fluorine than that of deuterium, and the unchanged electron distributions in the LUMO somewhat localized on carbon atoms as a consequence of H-F and H-D substitution in cubane, are the main reason why the l(-1) and omega(ln,-1) values increase much more significantly by H-F substitution than by H-D substitution. The l(+1) and omega(ln,+1) values less significantly change than the l(-1) and omega(ln,-1) values by H-F substitution as well as by H-D substitution in cubane. This is because the t2u HOMO in (CF)8 and the t2g HOMO in (CH)8 are somewhat localized on fluorine atoms, and thus, the high frequency vibronic active modes in which the displacements of carbon atoms are large cannot necessarily very strongly couple to the HOMO somewhat localized on fluorine atoms in (CF)8. 相似文献
83.
Toshio Yamanaka Mitsuru Ohkubo Fumie Takahashi Masayuki Kato 《Tetrahedron letters》2004,45(13):2843-2845
An efficient synthesis of the orally-active GpIIb/IIIa antagonist FR184764 was achieved. The key intermediate, an optically active ethynyl β-amino ester, was synthesized efficiently by utilizing a lipase catalyzed kinetic resolution step. 相似文献
84.
Convenient synthesis of 7-aryl-3,4,5,6-tetrahydro-2H-pyrido[4,5-b]- and [2,3-b]-1,5-oxazocine-6-ones
Shigeki Seto 《Tetrahedron letters》2004,45(46):8475-8478
A convenient and diversity-oriented method for synthesis of the novel 7-aryl-3,4,5,6-tetrahydro-2H-pyrido[4,5-b]-1,5-oxazocine-6-one skeleton 1 and the very rarely described 7-aryl-3,4,5,6-tetrahydro-2H-pyrido[2,3-b]-1,5-oxazocine-6-one skeleton 2, featuring cyclization using nucleophilic aromatic substitution (SNAr) and Suzuki coupling, is described. 相似文献
85.
Keisuke Kato Chie Matsuba Hiroyuki Takayama Tomoyuki Mochida Tat'yana A. Peganova Oleg V. Gusev 《Tetrahedron》2006,62(42):9988-9999
The oxidative cyclization-carbonylation of meso-2-alkyl-2-propargylcyclohexane-1,3-diols mediated by Pd(II) with chiral bisoxazoline (box ligand) afforded bicyclic-β-alkoxyacrylates. Based on a ligand screening, 2,2′-isopropylidenebis[(4S,5R)-4,5-di(2-naphthyl)-2-oxazoline] ligand has been developed. The products with a chiral quaternary carbon were obtained in 71-100% yields with 85-95% ee. 相似文献
86.
Tsubaki H Tohyama S Koike K Saitoh H Ishitani O 《Dalton transactions (Cambridge, England : 2003)》2005,(2):385-395
Intramolecular pi-pi and CH-pi interactions between the bpy and PR3 ligands of fac-[Re(bpy)(CO)3(PR3)]+ affect their structure, and electrochemical and spectroscopic properties. Intramolecular CH-pi interaction was observed between the alkyl groups on the phosphine ligand (R =nBu, Et) and the bpy ligand, and intramolecular pi-pi and CH-pi interactions were both observed between the aryl group(s) on the phosphorus ligand (R =p-MeOPh, p-MePh, Ph, p-FPh, OPh) and the bpy ligand, while no such interactions were found in the trialkylphosphite complexes (R = OiPr, OEt, OMe). The intramolecular interactions distort the pyridine rings of the bpy ligand as long as 3.7 x 10(-2)A in crystals. Molecular orbital calculations of the bpy ligand suggest that this distortion decreases the energy gap between its pi and pi* orbitals. An absorption band attributed to the pi-pi*(bpy) transition of the distorted rhenium complexes, measured in a KBr pellet, was red-shifted by 1-5 nm compared to the complexes without the distorted bpy ligand. Even in solution, similar red shifts of the pi-pi*(bpy) absorption were observed. The redox potential E1/2(bpy/bpy*-) of the complexes with the trialkylphosphine and triarylphosphine ligand are shifted positively by 110-120 mV and 60-80 mV respectively, compared with those derived from the electron-attracting property of the phosphorus ligand. In contrast with these properties, three nu(CO) IR bands, which are sensitive to the electron density on the central rhenium because of pi-back bonding, were shifted to higher energy, and a Re(I/II)-based oxidation wave was observed at a more positive potential according to the electron-attracting property of the phosphorus ligand. 相似文献
87.
Haru Ogawa Yoshinari Taketugu Taiji Imoto Yōichi Taniguchi Hidefumi Kato 《Tetrahedron letters》1979,20(36):3457-3460
Rearrangement of 2,4-di--pentadienals was described as a consequence of the successive alkyl substitutions on the C-2 and C-4 positions. 1H-FT nmr spectroscopy suggested that the reaction involves ketene as an intermediate. 相似文献
88.
Tetsuro Kato Ryosuke Nemoto Hisashi Mori Ryoetsu Abe Katsuo Unno Akio Goto Hideyuki Murota Masaoki Harada Motofumi Homma 《Applied biochemistry and biotechnology》1984,10(1-3):199-211
To achieve targeted distribution of anticancer drugs with sustained activity, ferromagnetic ethylcellulose microcapsules containing
an anticancer drug, mitomycin C (FM-MMC-mc), were prepared by a method based on phase separation principles. Two prototypes
of FM-MMC-mc were made: one with the drug as the core and zinc ferrite on its capsular surface (outer type); the other with
both the drug and zinc ferrite as the core (inner type). Both preparations provided a sustained-release property and a sensitive
response to conventional magnetic force, although certain differences in the release rate of drug, magnetic responsiveness,
and particle size were found between the two dosage forms. Animal studies showed that the magnetic microcapsules could be
magnetically controlled in the artery and urinary bladder. VX2 tumors in the rabbit hind limb and urinary bladder were successfully
treated with magnetic control of FM-MMC-mc. Pharmacokinetic study revealed that the targeting of the microcapsules markedly
enhanced the drug absorption into the surrounding tissues for a prolonged period of time. The results indicate the feasibility
and effectiveness of the magnetic microcapsules as a targeted drug delivery system. 相似文献
89.
Tadahiro Kato Motoyuki Tanaka Shin‐suke Takagi Keita Nakanishi Masahiro Hoshikawa 《Helvetica chimica acta》2004,87(1):197-214
The synthesis of kempa‐6,8‐dien‐3β‐ol ( 4a ), as a synthetic leading model of the natural product 4b , was carried out starting from intermediate 12 , the synthetic route of which has been developed previously (Scheme 1). The conversion of 12 to the model compound 4a involved the elaboration of three structure modifications by three processes, Tasks A, B, and C (see Scheme 2). Task A was achieved by epoxy‐ring opening of 41 with Me3SiCl (Scheme 9), and Task B being performed by oxidation at the 13‐position, followed by hydrogenation, and then epimerization (Schemes 4 and 5). The removal of the 2‐OH group from 12 (Task C) was achieved via 30b according to Scheme 6, whereby 30b was formed exclusively from 30a / 31a 1 : 1 (Scheme 7). In addition, some useful reactions from the synthetic viewpoint were developed during the course of the present experiments. 相似文献
90.
Parallel G-quadruplexes formed from oligonucleotide sequences, d(TTAGn), where n = 3-5, have been shown to form a dimer through end-to-end stacking of 3'-terminal G-tetrads. The monomers and dimers of the G-quadruplexes are in dynamic equilibrium with an exchange rate of approximately 1 s-1. A thermodynamic study demonstrated that the dimerization of the G-quadruplexes is largely enthalpic in origin. 相似文献