全文获取类型
收费全文 | 2579篇 |
免费 | 77篇 |
国内免费 | 5篇 |
专业分类
化学 | 1787篇 |
晶体学 | 179篇 |
力学 | 16篇 |
数学 | 337篇 |
物理学 | 342篇 |
出版年
2021年 | 26篇 |
2020年 | 41篇 |
2019年 | 38篇 |
2016年 | 40篇 |
2015年 | 35篇 |
2014年 | 43篇 |
2013年 | 76篇 |
2012年 | 87篇 |
2011年 | 94篇 |
2010年 | 50篇 |
2009年 | 51篇 |
2008年 | 79篇 |
2007年 | 66篇 |
2006年 | 71篇 |
2005年 | 69篇 |
2004年 | 55篇 |
2003年 | 64篇 |
2002年 | 57篇 |
2001年 | 37篇 |
2000年 | 33篇 |
1999年 | 40篇 |
1998年 | 26篇 |
1997年 | 39篇 |
1996年 | 37篇 |
1995年 | 24篇 |
1994年 | 37篇 |
1993年 | 35篇 |
1992年 | 26篇 |
1991年 | 26篇 |
1989年 | 37篇 |
1988年 | 24篇 |
1987年 | 31篇 |
1986年 | 22篇 |
1985年 | 37篇 |
1984年 | 25篇 |
1983年 | 33篇 |
1982年 | 32篇 |
1981年 | 23篇 |
1980年 | 38篇 |
1979年 | 31篇 |
1978年 | 32篇 |
1977年 | 27篇 |
1975年 | 24篇 |
1972年 | 20篇 |
1959年 | 69篇 |
1958年 | 65篇 |
1957年 | 44篇 |
1956年 | 80篇 |
1955年 | 57篇 |
1954年 | 89篇 |
排序方式: 共有2661条查询结果,搜索用时 437 毫秒
51.
In the past the formyloxyl radical, HC(O)O˙, had only been rarely experimentally observed, and those studies were theoretical-spectroscopic in the context of electronic structure. The absence of a convenient method for the preparation of the formyloxyl radical has precluded investigations into its reactivity towards organic substrates. Very recently, we discovered that HC(O)O˙ is formed in the anodic electrochemical oxidation of formic acid/lithium formate. Using a [CoIIIW12O40]5− polyanion catalyst, this led to the formation of phenyl formate from benzene. Here, we present our studies into the reactivity of electrochemically in situ generated HC(O)O˙ with organic substrates. Reactions with benzene and a selection of substituted derivatives showed that HC(O)O˙ is mildly electrophilic according to both experimentally and computationally derived Hammett linear free energy relationships. The reactions of HC(O)O˙ with terminal alkenes significantly favor anti-Markovnikov oxidations yielding the corresponding aldehyde as the major product as well as further oxidation products. Analysis of plausible reaction pathways using 1-hexene as a representative substrate favored the likelihood of hydrogen abstraction from the allylic C–H bond forming a hexallyl radical followed by strongly preferred further attack of a second HC(O)O˙ radical at the C1 position. Further oxidation products are surmised to be mostly a result of two consecutive addition reactions of HC(O)O˙ to the C C double bond. An outer-sphere electron transfer between the formyloxyl radical donor and the [CoIIIW12O40]5− polyanion acceptor forming a donor–acceptor [D+–A−] complex is proposed to induce the observed anti-Markovnikov selectivity. Finally, the overall reactivity of HC(O)O˙ towards hydrogen abstraction was evaluated using additional substrates. Alkanes were only slightly reactive, while the reactions of alkylarenes showed that aromatic substitution on the ring competes with C–H bond activation at the benzylic position. C–H bonds with bond dissociation energies (BDE) ≤ 85 kcal mol−1 are easily attacked by HC(O)O˙ and reactivity appears to be significant for C–H bonds with a BDE of up to 90 kcal mol−1. In summary, this research identifies the reactivity of HC(O)O˙ towards radical electrophilic substitution of arenes, anti-Markovnikov type oxidation of terminal alkenes, and indirectly defines the activity of HC(O)O˙ towards C–H bond activation.The formyloxyl radical, formed electrochemically, is electrophilic, yields anti-Markovnikov oxidation products from alkenes, and is effective for C–H bond activation. 相似文献
52.
Transition Metal Substituted Acylphosphanes and Phosphaalkenes. 17. Synthesis and Structure of the μ-Isophosphaalkyne Complexes [(η5-C5H5)2(CO)2Fe2(μ-CO)(μ-C?PC6H2R3)] (R = Me, iPr, tBu) . Condensation of (η5-C5H5)2(CO)2Fe2(μ-CO)(μ-CSMe)}+SO3CF3? ( 6 ) with 2,4,6-R3C6H2PH(SiMe3) ( 7 ) ( a : R = Me, b : R = iPr, c : R = tBu) affords the complexes (η5-C5H5)2(CO)2Fe2(μ-CO)(η-C?PC6H2R3-2,4,6) ( 9 a–c ) with edge-bridging isophosphaalkyne ligands as confirmed by the x-ray structure analysis of 9 a . 相似文献
53.
Braun T Steffen A Schorlemer V Neumann B Stammler HG 《Dalton transactions (Cambridge, England : 2003)》2005,(20):3331-3336
Treatment of a toluene solution of [PdMe(2)(Cy(2)PCH(2)PCy(2))](1) with pentafluoropyridine in the presence of traces of water affords the generation of the A-frame complexes [(PdMe)(2){mu-kappa(2)(P,P)Cy(2)PCH(2)PCy(2)}(2)(mu-F)][SiMeF(4)]() and [(PdMe)(2){mu-kappa(2)(P,P)Cy(2)PCH(2)PCy(2)}(2)(mu-F)][OC(5)NF(4)](2b). If the reaction is performed in an NMR tube equipped with a PFA inliner, complex 2b is produced, only. Treatment of 1 with pentafluoropyridine in the presence of an excess water yields the pyridyloxy complex [PdMe(OC(5)NF(4))(Cy(2)PCH(2)PCy(2))](3). Compound [(PdMe)(2){mu-kappa(2)(P,P)Cy(2)PCH(2)PCy(2)}(2)(mu-F)][FHF](2c) bearing a bifluoride anion instead of SiMeF(4)(-) or OC(5)NF(4)(-) can be generated by reaction of 1 with substoichiometric amounts of Et(3)N.3HF. The analogous complex [(PdMe)(2){mu-kappa(2)(P,P)Ph(2)PCH(2)PPh(2)}(2)(mu-F)][FHF] (5c) has been synthesized by addition of Ph(2)PCH(2)PPh(2) to a solution of [PdMe(2)(Me(2)NCH(2)CH(2)NMe(2))](4) in THF and subsequent treatment of the reaction mixture with Et(3)N.3HF. The structure of the A-frame complex 5c has been determined by X-ray crystallography. 相似文献
54.
Interpretation of contact angle measurements on two different fluoropolymers for the determination of solid surface tension 总被引:2,自引:0,他引:2
Tavana H Simon F Grundke K Kwok DY Hair ML Neumann AW 《Journal of colloid and interface science》2005,291(2):497-506
Contact angle measurements with a large number of liquids on the semi-fluorinated acryl polymer EGC-1700 films are reported. The surface tension was determined to be gammasv=13.84 mJ/m2 from contact angles of octamethylcyclotetrasiloxane (OMCTS) and decamethylcyclopentasiloxane (DMCPS). Inertness of these two liquids makes them ideal for determination of surface tension of low-energy fluoropolymers. On the other hand, contact angles of many other liquids deviated somewhat from a smooth contact angle pattern that represents the EGC-1700 surface tension. It is argued that noninertness of the molecules of these liquids gives rise to specific interactions with the polymer film, causing the deviations. Furthermore, contact angles of a series of n-alkanes (n-hexane to n-hexadecane) showed systematic deviations from this curve, similar to the trend observed for n-alkanes/Teflon AF 1600 systems studied earlier. Adsorption of vapor of short-chain liquids onto the polymer film caused their contact angles to fall above the gammasv=13.84 mJ/m2 curve, and a parallel alignment of molecules of the long-chain n-alkanes in the vicinity of the solid was the explanation for the deviation of their contact angles below it. It is found that vapor adsorption effect is more significant in the case of Teflon AF 1600, while the alignment of liquid molecules close to the surface is more pronounced for EGC-1700. 相似文献
55.
The effect of the addition of n-butanol (BuOH) and n-hexanol (HexOH) on the micellization of sodium dodecylsulfate (SDS) has been investigated using fluorescence quenching methods.
The binding constants were calculated using an expression which relates the total concentration of alcohols and the micelle
concentration. The values of K were 4.67 and 17.6 M-1 for BuOH/SDS and HexOH/SDS, similar to values obtained by other methods. The cmc of SDS decreases on addition of alcohols
and goes through a minimum for the BuOH/SDS system. Micellar aggregation numbers (N) were determined from linear plots of Ln (I
0/I) against [Quencher] at low alcohol concentrations. For 15 mM SDS, in the presence of BuOH the N values decrease on addition of alcohol up to 0.2 M. For HexOH, N can be assumed to be constant up to 4.8 mM, after which N decreases. The polarity of the micellar core containing alcohol was evaluated from the I
1/I
3 ratio of monomeric pyrene. The effect of addition of the alcohol causes a decrease in the I
1/I
3, which corresponds to a decrease in the polarity of the pyrene solubilization site.
Received: 28 October 1996 Accepted: 10 January 1997 相似文献
56.
Summary 2-Benzoylamino-6,7-dimethoxy-4H-3,1-benzoxazin-4-one was prepared by thermal treatment of 2-(3-benzoylthioureido)-4,5-dimethoxybenzoic acid and by benzoylation of 2-amino-6,7-dimethoxy-4H-3,1-benzoxazin-4-one. The inactivation of chymotrypsin and human leukozyte elastase by the title compound and 2-benzoylamino-4H-3,1-benzoxazin-4-one is reported.
2-Benzoylamino-6,7-dimethoxy-4H-3,1-benzoxazin-4-on: Synthese und Untersuchung der Inaktivierung von Serin-Proteasen
Zusammenfassung 2-Benzoylamino-6,7-dimethoxy-4H-3,1-benzoxazin-4-on wurde durch thermische Behandlung von 2-(3-Benzoylthioureido)-4,5-dimethoxybenzoesäure und durch Benzoylierung von 2-Amino-6,7-dimethoxy-4H-3,1-benzoxazin-4-on hergestellt. Über die Inaktivierung von Chymotrypsin und humaner Leukozyten-Elastase durch die Titelverbindung und 2-Benzoylamino-4H-3,1-benzoxazin-4-on wird berichtet.相似文献
57.
Gerard de Leeuw John S. Field Raymond J. Haines Beth McCulloch Elsie Meintjies Christiaan Monberg Gillian M. Olivier Praveen Ramdial Clifford N. Sampson Beate Sigwarth Nick D. Steen Kandasamy G. Moodley 《Journal of organometallic chemistry》1984,275(1):99-111
Reaction of [Fe2(CO)9] with a half molar amount of R2PYPR2 (Y = CH2, R = Ph, Me, OMe or OPri; Y = N(Et), R = OPh, OMe or OCH2; Y = N(Me), R = OPri or OEt) leads to the ready formation of a product which on irradiation with ultraviolet light rapidly decarbonylates to the heptacarbonyl derivative [Fe2(μ-CO)(CO)6{μ-R2PYPR2}]. Treatment of the latter with a slight excess of the appropriate ligand results, under photochemical conditions, in the formation of the dinuclear pentacarbonyl complex [Fe2(μ-CO)(C))4{μ-R2PYPR2}2] but under thermal conditions in the formation of the mononuclear species [Fe(CO)3{R2PYPR2}]. Reaction of [Ru3(CO)12] with an equimolar amount of (RO)2PN(R′)P(OR)2 (R′ = Me, R = Pri or Et; R′ = Et, R = Ph or Me) under either thermal or photochemical conditions produces [Ru3(CO)10{μ-(RO)2PN(OR)2}] which reacts further with excess (RO)2PN(R′)P(OR)2 on irradiation with ultraviolet light to afford the dinuclear compound [Ru2(μ-CO)(CO4{μ-(RO)2PN(R′)P(OR)2}2]. The molecular structure of [Ru2(μ-CO)(CO)4{μ-(MeO)2PN(Et)P(OMe)2}2], which has been determined by X-ray crystallography, is described. 相似文献
58.
Friedrichs G Colberg M Fikri M Huang Z Neumann J Temps F 《The journal of physical chemistry. A》2005,109(21):4785-4795
The determination of rate constants for fast chemical reactions from nonexponential cavity ringdown profiles requires a consideration of the interfering laser bandwidth effect that arises if the line width of the ringdown probe laser exceeds the absorption line width of the detected species. The deconvolution of the kinetics and the bandwidth effect can be accomplished with the extended simultaneous kinetics and ringdown (eSKaR) model presented by Guo et al. (Guo, et al. Phys. Chem. Chem. Phys. 2003, 5, 4622). We present a detailed validation of this eSKaR model by a corresponding investigation of the well-known rate constant for the reaction NH2 + NO. Line profiles of the pulsed ringdown probe laser and the NH2 absorption line were determined from forward modeling of experimental ringdown profiles and verified by narrow-bandwidth laser absorption measurements. In addition, the rate constant for the title reaction was evaluated using the eSKaR model and also by means of a conventional pump-probe approach with variable time delays between the photolysis (pump) and ringdown (probe) laser pulses. The resulting room temperature rate constant for the NH2 + NO reaction, k1= (8.5 +/- 1.0) x 10(12) cm(3) mol(-1) s(-1), and the room temperature pressure broadening coefficient of NH2, = 2.27 GHz/bar, measured on the A2A1<-- X2B1 transition at wavelengths around lambda = 597 nm, were found to be in excellent agreement with the available literature data. 相似文献
59.
Nair S Dimeo RM Neumann DA Horsewill AJ Tsapatsis M 《The Journal of chemical physics》2004,121(10):4810-4819
The methyl rotational tunneling spectrum of p-xylene confined in nanoporous zeolite crystals has been measured by inelastic neutron scattering (INS) and proton nuclear magnetic resonance (NMR), and analyzed to extract the rotational potential energy surfaces characteristic of the methyl groups in the host-guest complex. The number and relative intensities of the tunneling peaks observed by INS indicate the presence of methyl-methyl coupling interactions in addition to the methyl-zeolite interactions. The INS tunneling spectra from the crystals (space group P2(1)2(1)2(1) with four crystallographically inequivalent methyl rotors) are quantitatively interpreted as a combination of transitions involving two coupled methyl rotors as well as a transition involving single-particle tunneling of a third inequivalent rotor, in a manner consistent with the observed tunneling energies and relative intensities. Together, the crystal structure and the absence of additional peaks in the INS spectra suggest that the tunneling of the fourth inequivalent rotor is strongly hindered and inaccessible to INS measurements. This is verified by proton NMR measurements of the spin-lattice relaxation time which reveal the tunneling characteristics of the fourth inequivalent rotor. 相似文献
60.
Synthesis and Structure of Highly Functionalized 2, 3‐Dihydro‐1H‐1, 3, 2‐diazaboroles A series of differently substituted 2, 3‐dihydro‐1H‐1, 3, 2‐diazaboroles has been prepared by various methods. 1, 3‐Di‐tert‐butyl‐2‐trimethylsilylmethyl‐1H‐1, 3, 2‐diazaborole ( 7 ), 2‐isobutyl‐1, 3‐bis(1‐cyclohexylethyl)‐1H‐1, 3, 2‐diazaborole ( 8 ), 1, 3‐bis‐(1‐cyclohexylethyl)‐2‐trimethylsilylmethyl‐1H‐1, 3, 2‐diazaborole ( 9 ) 1, 3‐bis(1‐methyl‐1‐phenyl‐propyl)‐2‐trimethylsilylmethyl‐1H‐1, 3, 2diazaborole ( 10 ) and 2‐bromo‐1, 3‐bis(1‐methyl‐1‐phenyl‐propyl)‐1H‐1, 3, 2‐diazaborole ( 11 ) were formed by reaction of the corresponding 1, 4‐diazabutadienes with the boranes Me3SiCH2BBr2, iBuBBr2 and BBr3 followed by reduction of the resulting borolium salts [R1 = tBu, Me(cHex)CH, [Me(Et)Ph]C; R2 = Me3SiCH2, iBu, Br] with sodium amalgam. Treatment of 11 and 12 with silver cyanide afforded the 2‐cyano‐1, 3, 2‐diazaboroles 13 and 14 . An alternative route to compound 8 is based on the alkylation of 2‐bromo‐1, 3, 2‐diazaborole 12 with isobutyllithium. Equimolar amounts of 13 and isobutyllithium give rise to the formation of 15 . The new compounds were characterized by 1H‐, 13C‐, 11B‐NMR, IR and mass spectra. The molecular structures of 7 and meso ‐10 were confirmed by x‐ray structural analysis. 相似文献