全文获取类型
收费全文 | 1211篇 |
免费 | 33篇 |
国内免费 | 2篇 |
专业分类
化学 | 777篇 |
晶体学 | 25篇 |
力学 | 31篇 |
数学 | 149篇 |
物理学 | 264篇 |
出版年
2023年 | 8篇 |
2022年 | 12篇 |
2021年 | 12篇 |
2020年 | 24篇 |
2019年 | 31篇 |
2018年 | 22篇 |
2017年 | 15篇 |
2016年 | 36篇 |
2015年 | 32篇 |
2014年 | 35篇 |
2013年 | 57篇 |
2012年 | 42篇 |
2011年 | 65篇 |
2010年 | 45篇 |
2009年 | 46篇 |
2008年 | 65篇 |
2007年 | 59篇 |
2006年 | 48篇 |
2005年 | 63篇 |
2004年 | 38篇 |
2003年 | 28篇 |
2002年 | 30篇 |
2001年 | 17篇 |
2000年 | 18篇 |
1999年 | 10篇 |
1997年 | 8篇 |
1996年 | 13篇 |
1994年 | 19篇 |
1993年 | 9篇 |
1992年 | 17篇 |
1991年 | 10篇 |
1990年 | 12篇 |
1989年 | 15篇 |
1988年 | 10篇 |
1987年 | 7篇 |
1986年 | 9篇 |
1985年 | 21篇 |
1984年 | 12篇 |
1983年 | 7篇 |
1982年 | 12篇 |
1981年 | 18篇 |
1980年 | 14篇 |
1978年 | 10篇 |
1977年 | 7篇 |
1975年 | 7篇 |
1974年 | 10篇 |
1973年 | 13篇 |
1964年 | 6篇 |
1963年 | 8篇 |
1938年 | 7篇 |
排序方式: 共有1246条查询结果,搜索用时 15 毫秒
31.
A series of new dicationic dihydrogen complexes of ruthenium of the type cis-[(dppm)(2)Ru(eta(2)-H(2))(L)][BF(4)](2) (dppm = Ph(2)PCH(2)PPh(2); L = P(OMe)(3), P(OEt)(3), PF(O(i)Pr)(2)) have been prepared by protonating the precursor hydride complexes cis-[(dppm)(2)Ru(H)(L)][BF(4)] (L = P(OMe)(3), P(OEt)(3), P(O(i)Pr)(3)) using HBF(4).Et(2)O. The cis-[(dppm)(2)Ru(H)(L)][BF(4)] complexes were obtained from the trans hydrides via an isomerization reaction that is acid-accelerated. This isomerization reaction gives mixtures of cis and trans hydride complexes, the ratios of which depend on the cone angles of the phosphite ligands: the greater the cone angle, the greater is the amount of the cis isomer. The eta(2)-H(2) ligand in the dihydrogen complexes is labile, and the loss of H(2) was found to be reversible. The protonation reactions of the starting hydrides with trans PMe(3) or PMe(2)Ph yield mixtures of the cis and the trans hydride complexes; further addition of the acid, however, give trans-[(dppm)(2)Ru(BF(4))Cl]. The roles of the bite angles of the dppm ligand as well as the steric and the electronic properties of the monodentate phosphorus ligands in this series of complexes are discussed. X-ray crystal structures of trans-[(dppm)(2)Ru(H)(P(OMe)(3))][BF(4)], cis-[(dppm)(2)Ru(H)(P(OMe)(3))][BF(4)], and cis-[(dppm)(2)Ru(H)(P(O(i)Pr)(3))][BF(4)] complexes have been determined. 相似文献
32.
Thirunavukkarasu K Thirumoorthy K Libuda J Gopinath CS 《The journal of physical chemistry. B》2005,109(27):13272-13282
Nitric oxide (NO) reduction with carbon monoxide (CO) on the Pd(111) surface was studied under isothermal conditions by molecular beam techniques as a function of temperature, NO:CO beam composition, and beam flux. Systematic experiments were performed under transient and steady state conditions. Displacement of adsorbed CO by NO in the transient state of the reaction was observed at temperatures between 375 and 475 K for all the NO:CO compositions studied. NO accumulation occurs on Pd(111) surface under steady state conditions, below 475 K, due to stronger chemisorption of NO. The steady state reaction rates attain a maximum at about 475 K, nearly independent of beam composition. N2 was found to be the major product of the reduction, along with a minor production of N2O. The production of N2 and N2O indicates molecular and dissociative adsorption of NO on Pd(111) at temperatures up to 525 K. Postreaction TPD measurements were performed in order to determine the nitrogen coverage under steady-state conditions. Finally, the results are discussed with respect to the rate-controlling character of the different elementary steps of the reaction system. 相似文献
33.
Schiff-base (SB) derivatives of Ni(II) and Cu(II) porphyrins endowed with various amine functions (R−NH2),n-butylamine,p-anisidine andm-nitroaniline have been prepared from corresponding formyl porphyrins. Protonation studies of these SB derivatives reveal
a marked red shift of the optical absorption bands in the visible region relative to the unprotonated imines. The magnitude
of the observed red shifts in the protonated derivatives, (SBH+) are found to depend on the electron-withdrawing or electron-donating nature of the R group of the amines. The results of
the optical absorption,1H NMR, EPR, and cyclicovoltammetric studies are illustrative of the fact that protonation of the SB derivatives results in
a localized positive charge,
in the periphery of the porphyrin (p) system. The dibenzo-18-crown-6 interspersedbisporphyrin schiff bases have been prepared fromtrans 4,4′-diamino dibenzo-18-crown-6 and formyl porphyrins. The protonation of these SB derivatives is found to proceed in a concerted
fashion. The cation complexation studies by the crown ether entity in thebisporphyrin systems have been investigated using optical absorption, magnetic resonance and electrochemical methods. The redox
characteristics of the protonated dimeric SB porphyrins reveal that the first oxidation step involves a two-electron transfer
reaction. This is important in view of their possible usage in multielectron transfer reactions of biological and catalytic
interest. 相似文献
34.
Manojkumar TK Suh SB Oh KS Cho SJ Cui C Zhang X Kim KS 《The Journal of organic chemistry》2005,70(7):2651-2659
[reaction: see text] We present an ab initio study of the acid-promoted hydrolysis reaction mechanism of N-formylaziridine in comparison with formamide. Since the rate of amide hydrolysis reactions depends on the formation of the tetrahedral intermediate, we focused our attention mainly on the reactant complex, the tetrahedral intermediate, and the transition state connecting these two stationary points. Geometries were optimized using the density functional theory, and the energetics were refined using ab initio theory including electron correlation. Solvent effects were investigated by using polarizable continuum method calculations. The proton-transfer reaction between the O-protonated and N-protonated amides was investigated. In acidic media, despite that the N-protonated species is more stable than the O-protonated one, it is predicted that both N-protonated and O-protonated pathways compete in the hydrolysis reaction of N-formylaziridine. 相似文献
35.
Blum AS Ren T Parish DA Trammell SA Moore MH Kushmerick JG Xu GL Deschamps JR Pollack SK Shashidhar R 《Journal of the American Chemical Society》2005,127(28):10010-10011
Reported in this contribution are the synthesis, characterization, and charge transport properties of wire-like Ru2(ap)4(OPEn), where ap is 2-anilinopyridinate and OPE is -(CCC6H4)nSCH2CH2SiMe3 with n = 1 (1) and 2 (2). Scanning tunneling microscopy (STM) measurements of compound 2 inserted into a SAM of C11 thiol reveal that molecule 2 exhibits (i) the stochastic switching characteristic of wire molecules embedded in insulating SAMs and (ii) higher conductivity than the C11 thiol SAM. More importantly, analysis of the molecular electronic decay constant (beta) exhibits a decrease of at least 15% as compared to purely organic molecular analogues. Hence, the transport characteristics of molecules can be significantly improved for nanoscale electronics through the incorporation of a Ru2 fragment into conjugated backbone. 相似文献
36.
The correlation function for density fluctuations in a monatomic fluid obeys a formally exact kinetic equation containing a memory function. A previously derived short time approximation (STA) for this memory function is tested by comparing its predictions with the results of molecular dynamic simulations of a dense Lennard-Jones fluid at a variety of temperatures. This approximation takes into account the contribution to the correlation function of uncorrelated repulsive binary collisions. The qualitative changes of predicted correlation functions with temperature and wave vector are generally correct. The major exception to this is the transverse current correlation function for small wave vector. The quantitative accuracy is better at short times than long times and better at high temperatures than low temperatures. The major failing of the STA is its underestimation of the amplitudes of the negative dips in the current autocorrelation functions and of the temperature dependence of the amplitudes of the dips. Despite its deficiencies in predicting the time dependence of current correlation functions, the STA gives accurate results for the self-diffusion coefficient and the shear viscosity coefficient at the highest temperatures studied. 相似文献
37.
Investigation of the use of B3LYP zero-point energies and geometries in the calculation of enthalpies of formation 总被引:1,自引:0,他引:1
Larry A. Curtiss Krishnan Raghavachari Paul C. Redfern John A. Pople 《Chemical physics letters》1997,270(5-6):419-426
The use of B3LYP/6–31G* zero-point energies and geometries in the calculation of enthalpies of formation has been investigated for the enlarged G2 test set of 148 molecules [J. Chem. Phys. 106 (1997) 1063]. A scale factor of 0.96 for the B3LYP zero-point energies gives an average absolute deviation nearly the same as scaled HF/6–31G* zero-point energies for G2, G2(MP2), and B3LYP/6–311 + G(3df,2p) enthalpies. A scale factor of 0.98, which has been recommended in some studies, increases the average absolute deviation by about 0.2 kcal/mol. Geometries from B3LYP/6–31G* are found to do as well as MP2/6–31G* geometries in the calculation of the enthalpies of formation. 相似文献
38.
Prema Ranganathan Wilmer K. Fife Martel Zeldin 《Journal of polymer science. Part A, Polymer chemistry》1990,28(10):2711-2717
4-Vinylpyridinium trifluoromethanesulfonate monomers substituted at nitrogen with H, O, CH3, C2H5, C6H13, and C12H25 were synthesized and characterized spectroscopically. Thermal analyses (DSC and TGA) were carried out on all the compounds. The solid monomers (N? H, N? CH3, N? C6H13, and N? C12H25) exhibited endothermic melting followed by exothermic polymerization and exothermic decomposition (>400°C). Liquid N? C2H5 monomer revealed only exothermic polymerization and decomposition. The N? O polymer underwent thermal decomposition below 300°C. The N–C12H25 homopolymer, prepared from monomer in the DSC or in bulk, displayed an unusual thermal transition at 250°C, which has been attributed to a polymer backbone reorientation leading to side-chain ordering of the dodecyl groups. 相似文献
39.
K. D. Singh Mudher K. Krishnan N. C. Jayadevan 《Journal of Radioanalytical and Nuclear Chemistry》1993,176(2):175-184
Chemical characterization of rubidium uranium(IV) trisulfate, Rb2U(SO4)3, a new chemical assay standard for uranium requires accurate analysis of rubidium. A gravimetric and an X-ray fluorescence method (XRF) for the determination of rubidium in this compound are described. In the gravimetric method, rubidium is determined as Rb2Na[Co(NO2)6].H2O without separating uranium with a precision of the order of ±0.5%. In the XRF method, the concentration ratio of rubidium to uranium, CRb/CU, is determined in the solid samples by the binary ratio method using calibration between intensity ratios (IRb/IU) and concentration ratios (CRb/CU). The concentration of rubidium is derived using the uranium value which is known with a precision better than ±0.05%. The XRF method has a precision better than ±0.8% for rubidium determination. 相似文献
40.
Krishnan Raghavachari Michael J. Frisch John A. Pople Paul von R. Schleyer 《Chemical physics letters》1982,85(2):145-149
Calculations with an extended polarized basis set and Møller-Plesset perturbation theory including triple substitution correlation corrections in the fourth-order treatment indicate that singlet ethylidene (CH3CH:) is not a local minimum on the C2H4 potential energy surface. Rearrangement to ethylene occurs without actuation. Barriers for hydrogen scrambling and for 1,1-hydrogen elimination are estimated. 相似文献