全文获取类型
收费全文 | 439篇 |
免费 | 21篇 |
专业分类
化学 | 363篇 |
晶体学 | 3篇 |
力学 | 3篇 |
数学 | 20篇 |
物理学 | 71篇 |
出版年
2021年 | 4篇 |
2020年 | 6篇 |
2019年 | 12篇 |
2018年 | 3篇 |
2017年 | 3篇 |
2016年 | 12篇 |
2015年 | 11篇 |
2014年 | 7篇 |
2013年 | 13篇 |
2012年 | 24篇 |
2011年 | 17篇 |
2010年 | 9篇 |
2009年 | 8篇 |
2008年 | 16篇 |
2007年 | 16篇 |
2006年 | 27篇 |
2005年 | 14篇 |
2004年 | 14篇 |
2003年 | 10篇 |
2002年 | 7篇 |
2001年 | 4篇 |
2000年 | 4篇 |
1999年 | 4篇 |
1997年 | 5篇 |
1996年 | 6篇 |
1995年 | 3篇 |
1994年 | 5篇 |
1993年 | 6篇 |
1991年 | 4篇 |
1990年 | 7篇 |
1989年 | 11篇 |
1988年 | 10篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 10篇 |
1984年 | 18篇 |
1983年 | 4篇 |
1982年 | 11篇 |
1981年 | 11篇 |
1980年 | 8篇 |
1979年 | 11篇 |
1978年 | 14篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1975年 | 7篇 |
1974年 | 12篇 |
1973年 | 15篇 |
1967年 | 2篇 |
1956年 | 3篇 |
1955年 | 2篇 |
排序方式: 共有460条查询结果,搜索用时 31 毫秒
181.
Irradition of methyl styryl sulphoxide dissolved in MeOH, EtOH, or AcOH induces the photochemical polar addition of the solvents to afford the product, PhCH(OR)CH2SOMe, where R is Me, Et, or Ac. By-products are the corresponding sulphides, PhCH(OR)CH2SMe, and methyl styryl sulphide. Similar adducts of protic solvents are produced upon irradiation of certain vinylic sulphides, methyl phenethynyl sulphide, as well as appropriately substituted propenylbenzenes including o-anethole, o-anol, and o-propenylaniline in soln. 相似文献
182.
Abe M Kawakami T Ohata S Nozaki K Nojima M 《Journal of the American Chemical Society》2004,126(9):2838-2846
Temperature and substituent effects on the stereo- and regioselectivity have been investigated in the photochemical [2 + 2] cycloaddition reaction, the so-called Paternò-Büchi (PB) reaction, of unsymmetrically substituted furans 2a,b (2-methyl- and 3-methylfuran) with aromatic carbonyl compounds 1a,b (benzaldehyde and benzophenone). The regio-random but stereoselective (exo/endo > 97/3) formation of lower substituted oxetane 3a and higher substituted oxetane 4a is found in the reaction with benzaldehyde (1a). The exclusive stereoselectivity is not dependent on the position of methyl substituent on the furan ring and the reaction temperature. The double-bond selection (3a versus 4a) is slightly dependent on the reaction temperature (3a/4a = 55/45 to 40/60). The Eyring plots of the regioselectivity are linear. Contrastively, in the reaction with benzophenone (1b), the double-bond selection (3b versus 4b) largely depends on the reaction temperature. The Eyring plots are not linear, but the inflection points are observed. The transient absorption spectroscopic analyses (picosecond time scale) clarify the intervention of triplet 2-oxabutane-1,4-diyls in the photochemical processes. Computational studies reveal the equilibrium structures of the triplet diradicals, energy barriers between the conformers, and the equilibrium constants. A rational mechanism is herein proposed by the support of both experimental and computational investigations to account for not only the exclusive formation of the exo-configured oxetanes 3a and 4a but also the nonlinear Eyring plots observed in the reaction with 1b. 相似文献
183.
Takashi Okazoe Jun-ichi Hibino Kazuhiko Takai Hitosi Nozakil 《Tetrahedron letters》1985,26(45):5581-5584
The combination of CH2I2-ZN-Ti(OiPr)4 or CH2I2-ZN-Me3Al is effective for the selective methylenation of aldehydes. Seletive methylenation of a ketone group is performed by pretreatment of a substrate with Ti(NET2)4. 相似文献
184.
[reaction: see text] Palladium-catalyzed coupling reaction of N,N-dimethylformamide with aryl or alkenyl halides successfully proceeded in the presence of phosphoryl chloride to afford the corresponding tertiary amides in good yields. 相似文献
185.
Shiotsuka M Yamamoto Y Okuno S Kitou M Nozaki K Onaka S 《Chemical communications (Cambridge, England)》2002,(6):590-591
An Ru(II)-Au(I)-Ru(II) triad has been synthesized from [Ru(bpy)2(3-ethynylphenanthroline)]2+ with Au(tht)Cl and characterized by spectroscopic means such as NMR and ESI-MS; the Ru(II)-Au(I)-Ru(II) triad shows an intense emission at 620 nm upon excitation at 360 nm, which suggests an efficient energy transfer from the Au site to Ru sites via extended pi-conjugation through the ethynyl units. 相似文献
186.
T. Nozaki M. Iwamoto K. Usami K. Mukai A. Hiraiwa 《Journal of Radioanalytical and Nuclear Chemistry》1979,52(2):449-459
Oxygen in silicon nitride films on silicon wafers was analyzed by activation with the16O(3He, p)18F reaction. By3He bombardment of samples propertly arranged under consideration of the18F recoil effect, total oxygen was reliably determined and its predominant part was estimated to be located whether on film
surface, in film interior, or on film-substrate interface. Sample films with 0.1 to 2 μm thicknesses were found to contain
0.2 to 2 μg/cm2 of oxygen in locations varying with preparation conditions. This method has been compared with ESCA and other methods for
surface analysis. 相似文献
187.
Takashi Nozaki James C. W. Chien 《Journal of polymer science. Part A, Polymer chemistry》1991,29(12):1807-1817
Solvay type S –VCl3 catalyst has 7% of catalytically active vanadium sites ([C*]) with kp (rate constant of propagation) = 31 (M s)?1 for ethylene polymerization. Addition of a comonomer, propylene of 4-methylpentene-1 (4-MP) significantly raised the ethylene polymerization activity. S –VCI3 catalyst has very small amounts of catalytically active vanadium for propylene polymerizations: [C] = 0.19% with kp,i = 857 (M s)?1 and [C] = 0.45% with kp,a = 23 (M s)?1 for isospecific and nonspecific sites, respectively. Addition of a conomer, ethylene or 4-MP. lowered the propylene polymerization activity. S –VCI3 is more easily reduced to the divalent ion by AIR3 than S –TiCl3. Methyl-p-toluate moderates the reducting power of AIR3; it increase the productivity and stereoselectivity of the S –YiCl3 catalyst, VCI3 supported on MgCl2 (CW–V catalyst) has enhanced rate constant of propylene polymerization but has the opposite effects on the S –TiCl3 Catalyst. VCI3 supported on MgCl2 (CW–V catalyst) has enhances rate constant of propylene polymerization but only a minute fraction of the supported vanadiums are catalytically active: [C] = 0.019% and kp,i = 1580 (Ms)?1, [C] = 0.057% and kp,i = 58 (M s)?1. This is compared with far greater number of catalytically active titanium sites in the TiCl3 supported on MgCl2 catalyst: [C] = 6% and kp,i = 200 (M s)?1, [C] = 6% and kp,a = 16(M s)?1. Therefore, both the S –VCI3 and CW–V catalysts are highly stereoselective but low in efficiency with respect to the utilization of the vanadium ion in the catalysis. 相似文献
188.
Tada T Nozaki D Kondo M Hamayama S Yoshizawa K 《Journal of the American Chemical Society》2004,126(43):14182-14189
The electrical conductances of dithiolates of polyacene (PA(n)DTs) and polyphenanthrene (PPh(n)DTs), which are typical carbon ladder compounds, are calculated by means of the Landauer formulation combined with density functional theory, where n is the number of benzene rings involved. Surface Green function used in the Landauer formulation is calculated with the Slater-Koster parameters. Attention is turned to the wire-length dependence of the conductances of PA(n)DTs and PPh(n)DTs. The damping of conductance of PA(n)DTs is much smaller than that of PPh(n)DTs because of the small HOMO-LUMO gaps of PA(n)DTs. PA(n)DTs are thus good molecular wires for nanosized electronic devices. Conductance oscillation is found for both molecular wires when n is less than 7. The electrical conductance is enhanced in PA(n)DTs with even-numbered benzene rings, whereas it is enhanced in PPh(n)DTs with odd-numbered benzene rings. The observed conductance oscillation of PA(n)DTs and PPh(n)DTs is due to the oscillation of orbital energy and electron population. Other pi-conjugated oligomers (polyacetylene-DT, oligo(thiophene)-DT, oligo(meso-meso-linked zinc(II) porphyrin-butadiynylene)-DT, oligo(p-phenylethynylene)-DT, and oligo(p-phenylene)-DT) are also studied. In contrast to PA(n)DTs and PPh(n)DTs, the five molecular wires show ordinary exponential decays of conductance. 相似文献
189.
Sakai Y Yoza K Kato CN Nomiya K 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(17):4077-4083
The preparation and structural characterization of the novel polyoxoanion [(alpha-1,2,3-P(2)W(15)Ti(3)O(62))(4)[mu(3)-Ti(OH)(3)](4)Cl](45-) (1 a; abbreviated to [TiO(6)](16); FW approximately 16 000) which consists of four tri-Ti(IV)-1,2,3-substituted alpha-Dawson substructures, four Ti(OH)(3) bridging groups, and one encapsulated Cl(-) ion, are described. A water-soluble, all-inorganic composition compound of the tetrameric Ti-O-Ti-bridged anhydride form, Na(x)H(45-x)[1 a].y H(2)O (1; x=16-19, y=60-70), which was afforded by the reaction of the tri-lacunary Dawson polyoxotungstate Na(12)[B-alpha-P(2)W(15)O(56)].19 H(2)O with an excess of TiCl(4) in aqueous solution, was obtained as analytically pure, homogeneous colorless crystals. Single-crystal X-ray structure analysis revealed that 1 a was an inorganic, giant "tetrapod"-shaped molecule (inscribed to a sphere with a diameter of approximately 32 A) with approximately T(d) symmetry, in which the 16 edge- and/or corner-shared TiO(6) octahedra were contained. This number of TiO(6) octahedra was larger than that found in other titanium(IV)-substituted polyoxotungstates. Complex 1 was characterized by complete elemental analysis, TG/DTA, FTIR, UV/Vis absorption, and solution ((31)P and (183)W) NMR spectroscopy. The longest wavelength band in the UV/Vis absorption spectra of 1 in water was attributed to the O-->Ti(IV) ligand-to-metal charge-transfer (LMCT) transition: the wavelength of the LMCT band increased linearly as the number of TiO(6) octahedra contained in the Keggin and Dawson polyoxoanions increased. The Ti(n) chromophores formed in the Keggin and Dawson polyoxotungstates are water-soluble analogues of solid TiO(2) or SrTiO(3) as light-semiconductors and photocatalysts. 相似文献
190.
A triphenylphosphine-catalyzed cyclization of alpha-keto esters, alpha-keto nitriles, or alpha,alpha,alpha-trifluoroacetophenone with dimethyl acetylenedicarboxylate is reported to produce highly functionalized alpha,beta-unsaturated gamma-butyrolactones in moderate yields. Thus treating a mixture of methyl 4-nitrophenylglyoxylate and dimethyl acetylenedicarboxylate with 20 mol % of triphenylphosphine afforded 5,5'-disubstituted 3-methoxy-4-(methoxycarbonyl)-2(3H)-furanone in 94% yield. In the reaction of alpha-keto esters R(1)COCOOMe, an electron-withdrawing R(1) substituent is required for satisfactory reactivity. On the other hand, electron-donating R(1) substituents give higher yields with alpha-keto nitriles R(1)COCN. Another electron-deficient carbonyl compound, alpha,alpha,alpha-trifluoroacetophenone, gave the corresponding lactone in good yield. The use of an alpha-hydroxy ketone as an electrophilic carbonyl compound with more than 1 equiv of triphenylphosphine produced dihydrofuran derivatives. One equivalent of triphenylphosphine oxide was obtained as a major product. An intramolecular Wittig reaction is proposed as a reaction mechanism. 相似文献