全文获取类型
收费全文 | 632篇 |
免费 | 46篇 |
国内免费 | 38篇 |
专业分类
化学 | 456篇 |
晶体学 | 5篇 |
力学 | 9篇 |
综合类 | 3篇 |
数学 | 79篇 |
物理学 | 164篇 |
出版年
2023年 | 6篇 |
2022年 | 7篇 |
2021年 | 10篇 |
2019年 | 6篇 |
2018年 | 8篇 |
2017年 | 7篇 |
2016年 | 12篇 |
2015年 | 13篇 |
2014年 | 13篇 |
2013年 | 27篇 |
2012年 | 21篇 |
2011年 | 29篇 |
2010年 | 15篇 |
2009年 | 23篇 |
2008年 | 35篇 |
2007年 | 28篇 |
2006年 | 29篇 |
2005年 | 41篇 |
2004年 | 23篇 |
2003年 | 13篇 |
2002年 | 25篇 |
2001年 | 25篇 |
2000年 | 23篇 |
1999年 | 12篇 |
1998年 | 16篇 |
1997年 | 6篇 |
1996年 | 15篇 |
1995年 | 8篇 |
1994年 | 8篇 |
1993年 | 13篇 |
1992年 | 11篇 |
1991年 | 9篇 |
1990年 | 7篇 |
1989年 | 5篇 |
1988年 | 8篇 |
1987年 | 9篇 |
1986年 | 7篇 |
1985年 | 9篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 10篇 |
1979年 | 10篇 |
1978年 | 11篇 |
1977年 | 6篇 |
1976年 | 8篇 |
1975年 | 6篇 |
1974年 | 4篇 |
1969年 | 6篇 |
排序方式: 共有716条查询结果,搜索用时 15 毫秒
651.
In O(2)-evolving complex Photosystem II (PSII), an unimpeded transfer of electrons from the primary quinone (Q(A)) to the secondary quinone (Q(B)) is essential for the efficiency of photosynthesis. Recent PSII crystal structures revealed the protein environment of the Q(A/B) binding sites. We calculated the plastoquinone (Q(A/B)) redox potentials (E(m)) for one-electron reduction with a full account of the PSII protein environment. We found two different H-bond patterns involving Q(A) and D2-Thr217, resulting in an upshift of E(m)(Q(A)) by 100 mV if the H bond between Q(A) and Thr is present. The formation of this H bond to Q(A) may be the origin of a photoprotection mechanism, which is under debate. At the Q(B) side, the formation of a H bond between D2-Ser264 and Q(B) depends on the protonation state of D1-His252. Q(B) adopts the high-potential form if the H bond to Ser is present. Conservation of this residue and H-bond pattern for Q(B) sites among bacterial photosynthetic reaction centers (bRC) and PSII strongly indicates their essential requirement for electron transfer function. 相似文献
652.
In the course of our efforts to synthesize the sulfur isostere of lysergic acid, we have prepared a number of derivatives of the little known naphtho[1,8-bc]thiophene ring system. Two previously reported compounds, 2-bromo-3,4-dihydro-5-oxo-5H-naphtho[1,8-bc]thiophene and 2-bromo-5-hydroxy-3,4-dihydro-5H-naphtho[1,8-bc] thiophene have been obtained pure for the first time, and the syntheses and spectral data of eighteen new compounds are presented. The new compounds include alcohols, ketones, and halogen derivatives of partially reduced naphtho-[1,8-bc] thiophene, as well as the sulfones of several of these compounds. 相似文献
653.
Liquid-Liquid-and Vapor-Liquid-Equilibria in binary and ternary systems consisting of n-heptane, ethylbenzene and sulfolane were investigated experimentally at 312.57 and 332.45 K in three different types of apparatus using analytical or synthetic methods, static or dynamic operation and measuring temperatures or differential temperatures, pressures or differential pressures. Densities in binary mixtures consisting of n-heptane-ethyl-benzene, sulfolane—ethylbenzene were measured with an oscillating U-tube densimeter as a function of concentration at 293.25 and 303.6 K.Communicated at the Festsymposium celebrating Dr. Henry V. Kehiaian's 60th birthday, Clermont-Ferrand, France, 17–18 May 1990. 相似文献
654.
F. F.? Knapp Jr. S.?Mirzadeh A. L.?Beets M.?Du 《Journal of Radioanalytical and Nuclear Chemistry》2005,263(2):503-509
Summary Greater availability of therapeutic radioisotopes is required to meet the demands for increasing clinical applications in nuclear medicine, oncology and interventional cardiology. Because of the need for very high specific activity products, methods other than direct neutron capture reactions (n,γ-elastic and n,n’-inelastic routes) are required to insure that the highest specific activity - and hopefully no carrier added (nca) - radioisotopes are available. Two major methods to obtain nca radioisotopes from reactors are through the use of radionuclide generator systems using reactor-produced parents and the formation of desired radioisotopes through beta-decay of reactor-produced species. In this paper we describe our recent development of new approaches to obtain nca 177Lu from the decay of reactor-produced 177Yb and free of the long-lived 177mLu (T1/2 = 160 d) radiocontaminant. We also describe preliminary results of our new “indirect” method for the production of 195mPt via decay of 195mIr, reactor-produced by neutron irradiation of highly enriched 193Ir. 相似文献
655.
Summary Interferences in the flameless determination of lead using the HGA 72 were studied for HCl, HNO3, H2SO4, H3PO4, HClO4, HF and for a number of cations. In certain instances interferents in just 1000-fold concentration over lead are shown to cause deviations in the lead signal. Many of the interferences are matrix-dependent themselves and can be reduced by matrix variations. Optimization of the thermal pretreatment can be used effectively to reduce random error and/or systematic errors. The thermal stability of lead compounds in the graphite furnace was sufficient for charring procedures up to 900° C for 20 s.
Statistische Untersuchungen über Störungen bei der flammenlosen Atomabsorptionsspektralphotometrie
Zusammenfassung Interferenzen, die bei der flammenlosen Bestimmung von Blei in der HGA 72 auftreten, wurden bei HCl, HNO3, H2SO4, H3PO4, HClO4, HF und einigen Kationen studiert. In gewissen Fällen reicht ein nur 1000 facher Überschuß des Störions, um Abweichungen vom Signal des Bleistandards hervorzurufen. Da das Auftreten von Interferenzen auch oft matrixabhängig ist, können Matrixvariationen zur Unterdrückung von Interferenzen eingesetzt werden. Die Optimierung des Temperaturprogrammes wird erfolgreich zur Beeinflussung des Zufallsfehlers und/ oder systematischer Fehler herangezogen. In der Graphitrohrküvette reichte die Stabilität der Bleiverbindung aus, um beim Veraschungsschritt die Temperatur bis auf 900° C (20 s) zu steigern.
Part I: Z. Anal. Chem. [13].Part II: Z. Anal. Chem. [14]. 相似文献
656.
Lithium fluoroarylamidinates [(Ar(F)C(NSiMe(3))(2)Li)(n).xD] (Ar(F) = 4-CF(3)C(6)H(4), n = 2, D = OEt(2), x = 1 (2a); n = 1, D = TMEDA, x = 1 (4a); Ar(F) = 2-FC(6)H(4), n = 2, D = OEt(2), x = 1 (2b); Ar(F) = 4-FC(6)H(4), n = 2, D = OEt(2), x = 2 (2c); Ar(F) = 2,6-F(2)C(6)H(3), n = 2, D = OEt(2), x = 1 (2d); n = 2, D = 2,6-F(2)C(6)H(3)CN, x = 2 (3d); Ar(F) = C(6)F(5), n= 2, D = OEt(2), x = 1 (2e), n = 1, D = TMEDA, x = 1 (4e); n = 1, x = 2, D = OEt(2) (5e); D = THF (6e)) were prepared by the well-known method from LiN(SiMe(3))(2) and the corresponding nitrile in diethyl ether or by addition of the appropriate donor D to the respective diethyl ether complexes. Depending on the substituents at the aryl group and on the donors D, three different types of structures were confirmed by X-ray crystallography. Hydrolysis of 2e gave C(6)F(5)C(NSiMe(3))N(H)SiMe(3) (7e) and C(6)F(5)C(NH)N(H)SiMe(3) (8e). The lithium fluoroarylamidinates 2a-2d react with Me(3)SiCl to give the corresponding tris(trimethylsilyl)fluoroarylamidines Ar(F)C(NSiMe(3))N(SiMe(3))(2) (9a-9d). Attempts to prepare C(6)F(5)C(NSiMe(3))N(SiMe(3))(2) from 2e and Me(3)SiCl failed; however, the unprecedented cage [[C(6)F(5)C(NSiMe(3))(2)Li](4)LiF] (10e) in which a fluoride center is surrounded by a distorted trigonal bipyramid of five Li atoms was obtained from this reaction. 相似文献
657.
Summary Three independent decomposition methods for organic matter are compared for use in the determination of arsenic by the hydride-AAS method. Matrix interferences in the arsine evolution cause systematic errors.
Bestimmung von Arsen im ng/g- und g/g-Bereich in organischem und biologischem Material
Zusammenfassung Drei unabhängige Aufschlußmethoden zwecks Arsenbestimmung nach der Hydrid-Atomabsorptionsmethode wurden verglichen. Störungen seitens der Matrix bei der AsH3-Entwicklung sind eine der Ursachen für systematische Fehler.相似文献
658.
An exciton-phonon system of a linear aggregate is investigated in the intermediate-coupling regime. For sufficiently large intermolecular coupling. the variational approach predicts a discontinuous transition from a mobile exciton at weak exciton-phonon coupling to an immobile exciton at strong coupling. Fully converged numerical calculations exhibit strong deviations from the variational approach at intermediate exciton-phonon coupling strength and do not show a discontinuous transition. In the strong-coupling regime, results from the variational approach (which are closely related to the small polaron theory) deviate considerably from numerically converged calculations and an appropriate perturbation approximation. 相似文献
659.
Webb LJ Nemanick EJ Biteen JS Knapp DW Michalak DJ Traub MC Chan AS Brunschwig BS Lewis NS 《The journal of physical chemistry. B》2005,109(9):3930-3937
Hydrogen-terminated, chlorine-terminated, and alkyl-terminated crystalline Si(111) surfaces have been characterized using high-resolution, soft X-ray photoelectron spectroscopy from a synchrotron radiation source. The H-terminated Si(111) surface displayed a Si 2p(3/2) peak at a binding energy 0.15 eV higher than the bulk Si 2p(3/2) peak. The integrated area of this shifted peak corresponded to one equivalent monolayer, consistent with the assignment of this peak to surficial Si-H moieties. Chlorinated Si surfaces prepared by exposure of H-terminated Si to PCl5 in chlorobenzene exhibited a Si 2p(3/2) peak at a binding energy of 0.83 eV above the bulk Si peak. This higher-binding-energy peak was assigned to Si-Cl species and had an integrated area corresponding to 0.99 of an equivalent monolayer on the Si(111) surface. Little dichloride and no trichloride Si 2p signals were detected on these surfaces. Silicon(111) surfaces alkylated with CnH(2n+1)- (n = 1 or 2) or C6H5CH2- groups were prepared by exposing the Cl-terminated Si surface to an alkylmagnesium halide reagent. Methyl-terminated Si(111) surfaces prepared in this fashion exhibited a Si 2p(3/2) signal at a binding energy of 0.34 eV above the bulk Si 2p(3/2) peak, with an area corresponding to 0.85 of a Si(111) monolayer. Ethyl- and C6H5CH2-terminated Si(111) surfaces showed no evidence of either residual Cl or oxidized Si and exhibited a Si 2p(3/2) peak approximately 0.20 eV higher in energy than the bulk Si 2p(3/2) peak. This feature had an integrated area of approximately 1 monolayer. This positively shifted Si 2p(3/2) peak is consistent with the presence of Si-C and Si-H surface functionalities on such surfaces. The SXPS data indicate that functionalization by the two-step chlorination/alkylation process proceeds cleanly to produce oxide-free Si surfaces terminated with the chosen alkyl group. 相似文献
660.
Cation-pi interactions between aromatic residues and cationic amino groups in side chains and have been recognized as noncovalent bonding interactions relevant for molecular recognition and for stabilization and definition of the native structure of proteins. We propose a novel type of cation-pi interaction in metalloproteins; namely interaction between ligands coordinated to a metal cation--which gain positive charge from the metal--and aromatic groups in amino acid side chains. Investigation of crystal structures of metalloproteins in the Protein Data Bank (PDB) has revealed that there exist quite a number of metalloproteins in which aromatic rings of phenylalanine, tyrosine, and tryptophan are situated close to a metal center interacting with coordinated ligands. Among these ligands are amino acids such as asparagine, aspartate, glutamate, histidine, and threonine, but also water and substrates like ethanol. These interactions play a role in the stability and conformation of metalloproteins, and in some cases may also be directly involved in the mechanism of enzymatic reactions, which occur at the metal center. For the enzyme superoxide dismutase, we used quantum chemical computation to calculate that Trp163 has an interaction energy of 10.09 kcal mol(-1) with the ligands coordinated to iron. 相似文献