全文获取类型
收费全文 | 125004篇 |
免费 | 1338篇 |
国内免费 | 461篇 |
专业分类
化学 | 68465篇 |
晶体学 | 2035篇 |
力学 | 4955篇 |
综合类 | 5篇 |
数学 | 12103篇 |
物理学 | 39240篇 |
出版年
2016年 | 1272篇 |
2015年 | 982篇 |
2014年 | 1319篇 |
2013年 | 4776篇 |
2012年 | 3376篇 |
2011年 | 4383篇 |
2010年 | 2733篇 |
2009年 | 2513篇 |
2008年 | 3923篇 |
2007年 | 3977篇 |
2006年 | 4037篇 |
2005年 | 3963篇 |
2004年 | 3453篇 |
2003年 | 3150篇 |
2002年 | 3053篇 |
2001年 | 3570篇 |
2000年 | 2674篇 |
1999年 | 2195篇 |
1998年 | 1884篇 |
1997年 | 1872篇 |
1996年 | 1802篇 |
1995年 | 1772篇 |
1994年 | 1545篇 |
1993年 | 1533篇 |
1992年 | 1753篇 |
1991年 | 1745篇 |
1990年 | 1677篇 |
1989年 | 1683篇 |
1988年 | 1684篇 |
1987年 | 1674篇 |
1986年 | 1587篇 |
1985年 | 2135篇 |
1984年 | 2262篇 |
1983年 | 1887篇 |
1982年 | 2212篇 |
1981年 | 2027篇 |
1980年 | 2073篇 |
1979年 | 2058篇 |
1978年 | 2186篇 |
1977年 | 2090篇 |
1976年 | 2115篇 |
1975年 | 2053篇 |
1974年 | 1895篇 |
1973年 | 2102篇 |
1972年 | 1282篇 |
1971年 | 975篇 |
1970年 | 912篇 |
1969年 | 923篇 |
1968年 | 1050篇 |
1967年 | 1101篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Harding WW Tidgewell K Schmidt M Shah K Dersch CM Snyder J Parrish D Deschamps JR Rothman RB Prisinzano TE 《Organic letters》2005,7(14):3017-3020
[reaction: see text] Two new neoclerodane diterpenes, salvinicins A (4) and B (5), were isolated from the dried leaves of Salvia divinorum. The structures of these compounds were elucidated by spectroscopic techniques, including (1)H and (13)C NMR, NOESY, HMQC, and HMBC. The absolute stereochemistry of these compounds was assigned on the basis of single-crystal X-ray crystallographic analysis of salvinicin A (4) and a 3,4-dichlorobenzoate derivative of salvinorin B. 相似文献
962.
Experiments have been carried at magnetic-field strengths of 9.4, 14.1, and 17.6 T to explore the evolution of intermolecular multiple quantum coherences in the nonlinear regime where the system evolves for times that are much greater than the characteristic time of action of the long-range dipolar field, tau(d). The results show the expected Bessel function form of the recorded signal as a function of time of evolution, with evident zeros and sign changes. As expected, the rate of signal evolution increases at higher-field strengths as a result of the increased equilibrium magnetization. A numerical method for calculating the evolution of magnetization under the action of the distant dipolar field, relaxation, and diffusion that is based on Fourier analysis of the magnetization distribution has been applied to the correlated two-dimensional spectroscopy revamped by asymmetric z-gradient echo detection sequence in the nonlinear regime and shown to produce results that are in good agreement with experimental data acquired at different magnetic fields and rates of spatial modulation. Experiments and simulations have also been used to explore the evolution of magnetization in a mixture of two interacting spin species in the nonlinear regime. 相似文献
963.
H. Binder B. Kohlstrunk U. Brenn W. Schwieger G. Klose 《Colloid and polymer science》1998,276(12):1098-1109
Microcrystals of the metal silicate hydrate ilerite orient macroscopically on the surface of a ATR-crystals and thus, are
accessible for infrared linear dichroism measurements. We present first results which indicate that the alkyl chain packing
and the orientation of the polar group of dodecyltrimethylammoniumbromide (DTAB) intercalated between silicate layers can
be determined in terms of infrared order parameters. The properties of DTAB can be modulated by the relative humidity of the
surrounding atmosphere and by temperature. Upon heating DTAB undergoes a phase transition from a paraffin-like solid to a
fluid phase. The former is characterized by the orthorhombic perpendicular packing of the frozen alkyl chains with tilted
long axes. The interactions between the ionic groups of the surfactant and that of the host matrix stabilize the lamellar
arrangement of DTAB in the crystalline and in the fluid phases.
Received: 14 January 1998 Accepted: 27 July 1998 相似文献
964.
S. Herrmann W. Vonau F. Gerlach H. Kaden 《Analytical and bioanalytical chemistry》1998,362(2):215-217
A new sensor module is developed for the in-situ control of waters, especially surface waters. Complex miniaturized sensor technology is used for the determination of the parameters pO2, pH, pNO3, pCl, pPO4, pNH3 or pNH4 and conductivity. Miniaturization of sensors results in small sample volumes and small volumes of calibration solution required. Results of testing this sensor module are described. The special features of this apparatus are its compact construction and its optimum functioning capability under flow conditions. 相似文献
965.
The near-diffusion-controlled reactions of hydroxyl radical, hydrated electron, and hydrogen atom with platinum macrocyclic complex ions in aqueous media have been studied using pulse radiolysis in conjunction with UV-visible absorption and conductivity detection. The hydrated electron and hydrogen atom react with trans-[Pt(cyclam)(Cl)(2)](2+) where cyclam is 1,4,8,11-tetraazacyclotetradecane to yield platinum(III) transients that exhibit intense absorption peaks in the 280-300 nm region; however in the case of the H-atom, the reaction involves a competition between chloride abstraction and a minor process, suggested to be attack on the organic ligand. The platinum(III) products are kinetically labile toward loss of chloro ligands, but these reactions are reversible in the presence of added KCl. The reactions of hydroxyl radical with [Pt(cyclam)](2+) and with [Pt(tmc)](2+), where tmc is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, lead to platinum(III) intermediates absorbing in the 250-300 nm region. Depending on the presence or absence of added KCl and on the pH, the platinum(III) cyclam systems can react to form a product(s) exhibiting absorption peaks near 330 and 455 nm, and this species is proposed to be a long-lived amidoplatinum(III) complex. In support of this proposal is the observation that the tmc system does not give rise to a similar visible-absorbing product. The interrelations of the cyclam-based transients through acid-base, chloro-substitution and water-elimination processes are discussed. 相似文献
966.
A convenient direct transformation of -nitrobenzyl 6-(1′-hydroxyethyl)-1-azabicyclo-(3.2.0)heptane-3,7-dione-2-carboxylate to -formimidoyl thienamycin utilizing the silylated derivative of -formimidoyl cysteamine is described. 相似文献
967.
H. E. Zaugg J. E. Leonard R. W. Denet D. L. Arendsen 《Journal of heterocyclic chemistry》1974,11(5):797-802
The previously discovered neighboring group reaction has been extended to the synthesis of chroman derivatives (i.e., 4, 5, 6) containing geminal methyls in the 2-position, a feature common to certain physiologically active natural chromans. In two instances, cyclic ortho ester by-products (8), not observed in previous work, were formed as a result of the intramolecular trapping of tetrahedral intermediates. Reasons for the incursion of this unexpected side reaction are discussed. 相似文献
968.
The high frequency oscillator provides an excellent means of detecting the end-points of titrations performed with ethylenediaminetetraacetic acid or its salts. The determination of various divalent metals is reported, based on both direct and indirect titrations. Because of the great sensitivity of the method it is possible to determine cobalt, nickel, copper (II), zinc, cadmium, lead and manganese (II) in concentration ranges of 1/1000 to 1/6000M. 相似文献
969.
970.
Dullens RP Claesson EM Kegel WK 《Langmuir : the ACS journal of surfaces and colloids》2004,20(3):658-664
We report a single step preparation of monodisperse fluorescent poly(methyl)methacrylate (PMMA) lattices cross-linked with ethylene glycol dimethacrylate with radii in the range 150-1000 nm using dispersion polymerization. The particles are applied as fluorescent cores in core-shell PMMA particles for confocal microscopy (Dullens et al. Langmuir 2003, 19, 5963). Contrary to un-cross-linked particles, these cross-linked colloids are stable in good solvents for PMMA as well. Therefore we studied the properties of the cross-linked PMMA particles in the good solvents tetrahydrofuran (THF), chloroform, and toluene using light scattering and confocal scanning laser microscopy. We show that the particles swell instantaneously and that their volume can increase up to more than seven times their volume in poor solvents. Further, it is very likely that the particles are charged in THF. 相似文献