全文获取类型
收费全文 | 19821篇 |
免费 | 756篇 |
国内免费 | 99篇 |
专业分类
化学 | 13441篇 |
晶体学 | 126篇 |
力学 | 425篇 |
数学 | 2942篇 |
物理学 | 3742篇 |
出版年
2023年 | 126篇 |
2022年 | 177篇 |
2021年 | 276篇 |
2020年 | 381篇 |
2019年 | 402篇 |
2018年 | 236篇 |
2017年 | 238篇 |
2016年 | 611篇 |
2015年 | 567篇 |
2014年 | 650篇 |
2013年 | 1061篇 |
2012年 | 1140篇 |
2011年 | 1285篇 |
2010年 | 752篇 |
2009年 | 667篇 |
2008年 | 1056篇 |
2007年 | 1044篇 |
2006年 | 968篇 |
2005年 | 915篇 |
2004年 | 803篇 |
2003年 | 672篇 |
2002年 | 621篇 |
2001年 | 327篇 |
2000年 | 309篇 |
1999年 | 288篇 |
1998年 | 252篇 |
1997年 | 237篇 |
1996年 | 274篇 |
1995年 | 261篇 |
1994年 | 229篇 |
1993年 | 233篇 |
1992年 | 212篇 |
1991年 | 183篇 |
1990年 | 185篇 |
1989年 | 181篇 |
1988年 | 156篇 |
1987年 | 131篇 |
1986年 | 124篇 |
1985年 | 185篇 |
1984年 | 167篇 |
1983年 | 141篇 |
1982年 | 168篇 |
1981年 | 163篇 |
1980年 | 137篇 |
1979年 | 143篇 |
1978年 | 166篇 |
1977年 | 124篇 |
1976年 | 109篇 |
1975年 | 127篇 |
1974年 | 108篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
131.
The mass spectral fragmentation of carbazole was reinvestigated using metastable ion studies, exact mass measurements and 1,8-dideuteriocarbazole. 1,8-Dideuteriocarbazole was prepared by successive metalations and deuterations of carbazole. The 1H- and 13C-nmr spectra of 1,8-dideuteriocarbazole confirmed earlier assignments for carbazole. 相似文献
132.
The folding landscapes of polypeptides and proteins exhibit a hierarchy of local minima. The causes range from proline isomerization all the way down to microstructure in the free energy caused by residual frustration inherent in even the best 20 amino acid design. The corresponding time scales range from hours to submicroseconds. The smallest microstructures are difficult to detect. We have measured the folding/unfolding kinetics of the engineered trpzip2 peptide at different tryptophan fluorescence wavelengths, each yielding a different rate. Wavelength-dependent folding kinetics on 0.1-2 mus time scales show that different microstructures with a range of solvent exposure and local dynamics are populated. We estimate a lower limit for the roughness of the free energy surface based on the range of rates observed. 相似文献
133.
Rudin M Mueggler T Allegrini PR Baumann D Rausch M 《Analytical and bioanalytical chemistry》2003,377(6):973-981
Modern drug development requires technologies that allow rapid translation from the preclinical to the clinical stage. It is obvious that non-invasive imaging modalities such as magnetic resonance imaging (MRI) will play a central role in this regard. This article reviews the use of structural and functional MRI readouts for characterization of central nervous system (CNS) disorders and evaluation of the efficacy of potential CNS drugs. Examples comprise dementia of Alzheimer's type, cerebral ischemia, and neuroinflammation covering both clinical and preclinical aspects. In these examples MRI has been used to obtain relevant structural information on brain atrophy, on the location and extent of ischemic brain areas, and on the number and distribution of demyelinated plaques. These structural data are complemented by readouts assessing the functional consequences associated with the pathomorphological changes. In the last decade, MRI has evolved into a standard tool for the development of CNS drugs. With regard to target-specific/molecular imaging applications MRI is limited by its inherently low sensitivity; complementary imaging modalities utilizing optical and radionuclear reporter systems will thus be required. 相似文献
134.
The effect of the parent basis set on the basis set superposition error caused by bond functions is investigated systematically. An important difference between BSSE at the SCF and correlated levels is pointed out. Three new basis sets are defined, denoted 6-311 + G(d,p)B, 6-311 + G(2d,p)B, and 6-311 + G(2df,p)B. BSSE for the first-row hydrides seems to increase uniformly with increasing atomic number of the central atom. Expansion of the valence part of the basis set from 6-31G to 6-311G, as well as adding f functions, has a significant effect on the BSSE. Additional BSSEs incurred by bond functions are less than or equal to 1 kcal/mol for the 6-311 + G(2df,p)B basis set. For the dissociation energies of the first-row hydride species, agreement with experiment within only a few kcal/mol can be obtained even without resorting to isogyric reaction cycles. For high-quality calculations, adding bond functions seems to have definite advantages over expanding the polarization space beyond the [2d1f] level. 相似文献
135.
Zareen Ahkter Andrew J. Edwards Scott L. Ingham Jack Lewis Ana M. Martin Castro Paul R. Raithby Gregory P. Shields 《Journal of Cluster Science》2000,11(1):217-226
The reaction of the di-gold cation [Au2(dppx)]2+ with the heptanuclear cluster dianion [Os7(CO)20]2– affords the mixed metal cluster [Os7(CO)20{Au2(dppx)}] (x=m (1), e (2), b (3)). On standing, in solution, this complex undergoes decarbonylation to give the cluster [Os7(CO)19{Au2(dppx)}] (x=m (4), e (5), b (6)). The complexes have been characterised spectroscopically, and an X-ray structure determination of the dppm derivative shows that it contains a metal core based on an Os7 edge-bridged bicapped tetrahedron with the two
3-Au atoms capping adjacent triangular Os3 faces of the central tetrahedron. In an analogous reaction, the carbido anion [Os7(H)C(CO)19]– affords the neutral cluster [Os7C(CO)19{Au2(dppm)}] (7) when treated with [Au2(dppm)]2+ in the presence of base. 相似文献
136.
Martin R. Bryce Peter J. Skabara Adrian J. Moore Andrei S. Batsanov Judith A.K. Howard Vanessa J. Hoy 《Tetrahedron》1997,53(52):1002-17794
Reactions of tetrathiafulvalenyllithium (TTF-Li) with a range of aldehydes and ketones have been explored for the first time, to provide an efficient route to mono-functionalised TTF derivatives of general formulae TTF-CH(OH)R (R = alkyl, phenyl, tetrathiafulvalenyl and ferrocenyl) and TTF-CR(OH)R′ (R = alkyl, phenyl, R′ = alkyl, phenyl, tetrathiafulvalenyl, ferrocenyl, and R-R′ = fluorenyl). Subsequent transformations involving the reactive alcohol group of some of these compounds are reported. The structures of the three title compounds have been established by single-crystal X-ray analysis. 相似文献
137.
The compounds CdCr2S4 and CoCr2S4 have been hot pressed into disks that are highly transparent in the infrared. Stoichiometry has been altered by varying the ratio, where M2+ is Cd2+ or Co2+. The effects of nonstoichiometry on optical transmission were determined. Excess M2+ attenuates the transmission much more than excess Cr3+. 相似文献
138.
Jean M. J. Tronchet Bernard Gentile Alain P. Bonenfant Olivier R. Martin 《Helvetica chimica acta》1979,62(3):696-699
Further examples of O-isopropylidenaldehydosugars in the furanose series Formyl-bearing furanose derivatives 1–3 (α-D -xylo or α-D -erythro configurations) and 5–8 (configurations α-D -ribo, α-D -lyxo, β-L -threo and D -arabino) have been prepared by classical synthetic steps and their properties mainly spectroscopic, reported. The coupling constant between the formyl and the vicinal proton is always small. Like other members of the series, these new aldehydosugars constitute very useful and flexible synthetic intermediates. 相似文献
139.
The influence of potassium addition on the morphological properties of Fe/MgO and Ni/MgO has been investigated: KNO3 addition to precursors results in a decrease of the reducibility and in a smaller dispersity of the metallic phase. A loss of potassium is observed at reduction temperatures in excess of 773 K. Silica-supported Ni behaves differently: silica inhibits the potassium volatilization and nickel reducibility is enhanced. This illustrates how promoter effects can be influenced by the nature of support.
Fe/MgO Ni/MgO: KNO3 . , 773 K. , : . .相似文献
140.
Fluorescence of the Xe2 continuum between 1460 and 1800 Å has been observed during excitation of Xe2 molecules with CO fourth positive bands from a microwave discharge lamp. Fluorescence spectra at different Xe and added gas pressures show that in collisions with Xe or Kr atoms, the excited Xe2 diatoms are efficiently relaxed to low vibrational levels of the 1Σu+ and 3Σu+ potentials without being quenched noticeably. With Kr the rate constant for relaxation out of the primarily excited high vibrational levels is found to be about 3×10?11 cm3 molecule?1 s?1. 相似文献