全文获取类型
收费全文 | 2255篇 |
免费 | 57篇 |
国内免费 | 21篇 |
专业分类
化学 | 1535篇 |
晶体学 | 60篇 |
力学 | 31篇 |
数学 | 396篇 |
物理学 | 311篇 |
出版年
2023年 | 24篇 |
2022年 | 28篇 |
2021年 | 44篇 |
2020年 | 46篇 |
2019年 | 54篇 |
2018年 | 25篇 |
2017年 | 15篇 |
2016年 | 43篇 |
2015年 | 55篇 |
2014年 | 44篇 |
2013年 | 118篇 |
2012年 | 131篇 |
2011年 | 149篇 |
2010年 | 76篇 |
2009年 | 56篇 |
2008年 | 134篇 |
2007年 | 109篇 |
2006年 | 131篇 |
2005年 | 135篇 |
2004年 | 96篇 |
2003年 | 83篇 |
2002年 | 92篇 |
2001年 | 32篇 |
2000年 | 18篇 |
1999年 | 26篇 |
1998年 | 26篇 |
1997年 | 33篇 |
1996年 | 39篇 |
1995年 | 25篇 |
1994年 | 24篇 |
1993年 | 21篇 |
1992年 | 24篇 |
1991年 | 13篇 |
1990年 | 21篇 |
1989年 | 21篇 |
1988年 | 15篇 |
1987年 | 15篇 |
1986年 | 14篇 |
1985年 | 19篇 |
1984年 | 28篇 |
1983年 | 16篇 |
1982年 | 21篇 |
1980年 | 22篇 |
1979年 | 18篇 |
1978年 | 19篇 |
1977年 | 20篇 |
1976年 | 16篇 |
1975年 | 15篇 |
1974年 | 18篇 |
1973年 | 13篇 |
排序方式: 共有2333条查询结果,搜索用时 15 毫秒
51.
Becerra R Boganov SE Egorov MP Faustov VI Krylova IV Nefedov OM Promyslov VM Walsh R 《The journal of physical chemistry. A》2008,112(5):849-857
Time-resolved studies of silylene, SiH2, and dimethylsilylene, SiMe2, generated by the 193 nm laser flash photolysis of appropriate precursor molecules have been carried out to obtain rate constants for their bimolecular reactions with dimethylgermane, Me2GeH2, in the gas phase. SiMe2 + Me2GeH2 was studied at five temperatures in the range 299-555 K. Problems of substrate UV absorption at 193 nm at temperatures above 400 K meant that only three temperatures could be used reliably for rate constant measurement. These rate constants gave the Arrhenius parameters log(A/cm3 molecule(-1) s(-1)) = -13.25 +/- 0.16 and E(a) = -(5.01 +/- 1.01) kJ mol(-1). Only room temperature studies of SiH2 were carried out. These gave values of (4.05 +/- 0.06) x 10(-10) cm3 molecule(-1) s(-1) (SiH2 + Me2GeH2 at 295 K) and also (4.41 +/- 0.07) x 10(-10) cm3 molecule(-1) s(-1) (SiH2 + MeGeH3 at 296 K). Rate constant comparisons show the surprising result that SiMe2 reacts 12.5 times slower with Me2GeH2 than with Me2SiH2. Quantum chemical calculations (G2(MP2,SVP)//B3LYP level) of the model Si-H and Ge-H insertion processes of SiMe2 with SiH4/MeSiH3 and GeH4/MeGeH3 support these findings and show that the lower reactivity of SiMe2 with Ge-H bonds is caused by a higher secondary barrier for rearrangement of the initially formed complexes. Full details of the structures of intermediate complexes and the discussion of their stabilities are given in the paper. Other, related, comparisons of silylene reactivity are also presented. 相似文献
52.
The molecular geometry of perfluoro(methyloxirane) has been studied using gas-phase electron diffraction data, effective least-squares refinement of the structure being achieved with the aid of constraints to limit the number of variable parameters. With the CCF3 bond constrained to be 0.078 Å longer than the ring CC, the refined bond- length values CF (av.) = 1.323(2), CO (av.) = 1.410(8), and CC (ring) = 1.467(7) Å (g values, with e.s.d. in parentheses) were obtained; the angles between ring bonds and substituent CF bonds were CCF (av.) 121(1) and OCF (av.) 114(1)o, the corresponding parameters involving the bulkier CCF3 fragment being larger by 3o in each case [∠CCCF3 124(1)o∠OCCF3 117(2)o]. The remaining refined parameters were ∠CCF(of CF3) = 110.6(4)o and τ , a torsion angle defining the orientation of the CF bonds of the CF3 group with respect to ring bonds, = 29(2)o. Dependent bond angles possessed the values 62.7 (COC), 58.7 [OCC (ring)], 108.3 [FCF (CF3 group)], 114 [FCF (ring CF2)], and 111o (FCCF3). 相似文献
53.
Ronald E. Banks Ismail M. Farhat Roy Fields Robin G. Pritchard Mohamad M. Saleh 《Journal of fluorine chemistry》1985,28(3):325-340
The nitrosoarenes ArNO (Ar = C6H5, 2-MeC6H4, 2,4,6- Me3C6H2 and C6F5) have been condensed with 4-(dichloroamino)- tetrafluoropyridine to provide the azoxy-compounds pyFN()Ar (pyF = 2,3,5,6-tetrafluoro-4-pyridyl); de-oxygenation of the first three with triphenylphosphine or triethyl phosphite gave the corresponding azo-compounds, and the reverse reaction was achieved in the case of pyFNNC6H2Me3-2,4,6 using peroxytrifluoroacetic acid. Thermolysis of 4-azidotetrafluoropyridine in the presence of pentafluoronitrosobenzene provided the perfluorinated azoxy-compound pyFN()C6F5. X-Ray methods have been used to determine the molecular geometry of pyFN()C6H2Me3-2,4,6. 相似文献
54.
A three step synthesis of 2-pentafluorosulfanylnaphthalene is reported. Initial addition of SF5Cl to benzobarralene was followed by elimination to form 2-pentafluorosulfanylbenzobarralene. Heating of this compound with 3,6-bis-(2-pyridyl)-1,2,4,5-tetrazine led to elimination of the ethylene bridge via a sequence of cycloadditions and retro-cycloadditions to form the title compound. 相似文献
55.
Byrd JN Michels HH Montgomery JA Côté R Stwalley WC 《The Journal of chemical physics》2012,136(1):014306
Electronic structure calculations have been carried out for all possible alkali tetramers that can be formed from X(2) + X(2) → X(2)X(2), X(2) + Y(2) → X(2)Y(2), and XY + XY → X(2)Y(2) alkali dimer association reactions. Vibrationally stable rhombic (D(2h)) and planar (C(s)) structures are found for all possible tetramers formed from the alkali metals, Li to Cs. All tetramer formation reactions (from ground state singlet homonuclear or heteronuclear dimers) are found to be exothermic with binding energies ranging from 6282 cm(-1) for Li(2)Li(2) to 1985 cm(-1) for Cs(2)Cs(2). Extensive calculations, carried out at long-range for several reactant pairs, indicate that there are barrier-less pathways for the formation of tetramers from dimer association reactions. At low temperatures, direct formation of tetramers is unlikely, owing to the large exothermicity associated with these association reactions, but atom exchange reactions (X(2) + Y(2) ? XY + XY) are possible for some species. 相似文献
56.
Runjiang Song Yonggui Robin Chi 《Angewandte Chemie (International ed. in English)》2019,58(26):8628-8630
N‐Heterocyclic carbene catalyzed radical reactions are challenging and underdeveloped. In a recent study, Ohmiya, Nagao and co‐workers found that aldehyde carbonyl carbon centers can be coupled with alkyl radicals under NHC catalysis. An elegant aspect of this study is the use of a redox‐active carboxylic ester that behaves as an single‐electron oxidant to convert the Breslow intermediate into a radical adduct and concurrently release an alkyl radical intermediate as a reaction partner. 相似文献
57.
Corden VA Duhme-Klair AK Hostachy S Perutz RN Reddig N Becker HC Hammarström L 《Inorganic chemistry》2011,50(3):1105-1115
A heteroditopic ligand H(2)-L consisting of a dihydroxybenzene (catechol)-unit linked via an amide bond to a pyridyl-unit and its methyl-protected precursor Me(2)-L were synthesized, characterized, and their photophysical properties investigated. The three accessible protonation states of the ligand, H(3)-L(+), H(2)-L, and H-L(-), showed distinct (1)H NMR, absorption and emission spectroscopic characteristics that allow pH-sensing. The spectroscopic signatures obtained act as a guide to understand the signaling mechanism of the luminescent pH and molybdate sensor [Re(bpy)(CO)(3)(H(2)-L)](+). It was found that upon deprotonation of the 2-hydroxy group of H(2)-L, a ligand-based absorption band emerges that overlaps with the Re(dπ)→bpy metal-to-ligand charge transfer (MLCT) band of the sensor, reducing the quantum yield for emission on excitation in the 370 nm region. In addition, deprotonation of the catechol-unit leads to quenching of the emission from the Re(dπ)→bpy (3)MLCT state, consistent with photoinduced electron transfer from the electron-rich, deprotonated catecholate to the Re-based luminophore. Finally, reaction of 2 equiv of [Re(bpy)(CO)(3)(H(2)-L)](+) with molybdate was shown to give the zwitterionic Mo(VI) complex [MoO(2){Re(CO)(3)(bpy)(L)}(2)], as confirmed by electrospray ionization (ESI) mass spectrometry and X-ray crystallography. The crystal structure determination revealed that two fully deprotonated sensor molecules are bound via their oxygen-donors to a cis-dioxo-MoO(2) center. 相似文献
58.
Summary Salts of the anions [SnX5]–, [SnX4Cl, [SnX3Cl2]–, [SnX3]–, [PbX3]–, [SbX4]–, [SbX3Cl]–, [SbX2Cl2]–, [BiX4]–, [AuCl2]–, [AuX2]–, [AuXCl]–, [AuX4]–, [Au2X6]2– and [PtX4]2–, where X– = C6F5S–, have been isolated and characterised. The neutral SbX3 and BiX3 species, have also been isolated and shown to be pyramidal monomers (19F.n.m.r., i.r., and Raman spectral evidence). Various physical properties of the complexes prepared, as well as their stereochemistries (where these could be ascertained), are similar to those of the known corresponding halogeno compounds of these elements. These results further demonstrate the pseudo-halide nature of the pentafluorothiophenoxide ion.Author to whom all correspondence should be directed at: Laboratoire de Chimie de Coordination, Uniyersité Louis Pasteur, 67008 Strasbourg, France. 相似文献
59.
Two-dimensional electrophoresis of the total polypeptides in ripe red grape berries. 总被引:1,自引:0,他引:1
The total polypeptide composition of mature grape berries was analyzed by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis and two-dimensional electrophoresis (isoelectric focusing in the first dimension followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the second dimension), followed by Coomassie Blue and nitrate silver staining, respectively. Adapted methods for total protein preparation of grapes and for two-dimensional gel electrophoretic separation of polypeptides are presented. The grape patterns presented up to 52 fractions with Mrs ranging from 15,000 to 110,000. The polypeptides displayed pIs from 4.6 to 7.3. A group of spots from Mr 28,000 to 83,000 and with a pI from 4.6 to 5.4 was strongly silver stained. The Mr 28,000 spot, pI 4.6, was revealed to be a complex of four fractions. Reproducible separations were obtained with the different carrier ampholyte mixtures tested. 相似文献
60.