全文获取类型
收费全文 | 1212篇 |
免费 | 35篇 |
国内免费 | 5篇 |
专业分类
化学 | 809篇 |
晶体学 | 1篇 |
力学 | 45篇 |
数学 | 225篇 |
物理学 | 172篇 |
出版年
2021年 | 6篇 |
2019年 | 11篇 |
2018年 | 14篇 |
2017年 | 11篇 |
2016年 | 24篇 |
2015年 | 30篇 |
2014年 | 27篇 |
2013年 | 62篇 |
2012年 | 50篇 |
2011年 | 42篇 |
2010年 | 28篇 |
2009年 | 31篇 |
2008年 | 46篇 |
2007年 | 46篇 |
2006年 | 41篇 |
2005年 | 38篇 |
2004年 | 41篇 |
2003年 | 36篇 |
2002年 | 53篇 |
2001年 | 14篇 |
2000年 | 17篇 |
1999年 | 16篇 |
1998年 | 15篇 |
1997年 | 14篇 |
1996年 | 14篇 |
1995年 | 13篇 |
1994年 | 11篇 |
1993年 | 15篇 |
1992年 | 20篇 |
1991年 | 12篇 |
1990年 | 22篇 |
1989年 | 7篇 |
1988年 | 21篇 |
1987年 | 14篇 |
1986年 | 14篇 |
1985年 | 27篇 |
1984年 | 23篇 |
1983年 | 29篇 |
1982年 | 21篇 |
1981年 | 17篇 |
1980年 | 29篇 |
1979年 | 21篇 |
1978年 | 25篇 |
1977年 | 22篇 |
1976年 | 32篇 |
1975年 | 13篇 |
1974年 | 13篇 |
1973年 | 10篇 |
1972年 | 9篇 |
1971年 | 7篇 |
排序方式: 共有1252条查询结果,搜索用时 31 毫秒
971.
The mechanism of degradation of bioresorbable lactic acid- and glycolic acid-based aliphatic polyesters (PLA/GA) is still far from being totally understood although a majority of authors agree to consider in vivo degradation as essentially hydrolytic. In the past years, we have shown that hydrolysis of large size devices is heterogeneous, i.e. faster inside than at the surface because of reaction-diffusion phenomena involving acid-catalyzed ester cleavage reactions and water-soluble macromolecular fragments. The proposed mechanism is recalled together with some direct or indirect consequences, namely induced crystallization at body temperature, size- and formulation-dependence of the degradation rates. From these features, one can conclude that it is now possible to increase or decrease the degradation rate and to turn on or off heterogeneous degradation by using accelerating or braking factors, however, accurate piloting of the hydrolytic degradation of PLA/GA is still not feasible. 相似文献
972.
Henri E. K. Matimba Steen Ingemann Nico M. M. Nibbering 《Journal of the American Society for Mass Spectrometry》1993,4(1):73-81
The gas-phase reactions of negative ions (O-., NH 2 ? , C2H5NH?, (CH3)2N?, C6H 5 t- , and CH3SCH 2 ? ) with fluorobenzene and 1,4-difluorobenzene have been studied with Fourier transform ion cyclotron resonance mass spectrometry. The O?. ion reacts predominantly by (1) proton abstraction, (2) formal H 2 +. abstraction, and (3) attack on an unsubstituted carbon atom. In addition to these processes, attack on a fluorine bearing carbon atom yielding F? and C6H4FO? ions occurs with 1,4-difluorobenzene. Site-specific deuterium labeling reveals the occurrence of competing 1,2-, 1,3-, and 1,4-H 2 +. abstractions in the reaction of O?. with fluorobenzene. Attack of the O?. ion on the 3- and 4-positions in fluorobenzene with formation of the 3- and 4-fluorophenoxide ions, respectively, is preferred to reaction at the 2-position, as indicated by the relative extent of loss of a hydrogen and a deuterium atom in the reactions with labeled fluorobenzenes. The NH 2 ? , C2H5NH?, (CH3)2N?, C6H 5 ? , and CH3SCH 2 ? anions react with fluoroberuene and 1,4-difluorobenzene only by proton abstraction. The relative importance of H+ and D+ abstraction in the reaction of these anions with labeled fluorobenzenes indicates that the 2-position in fluorobenzene is more acidic than the 3- and 4-positions, suggesting that the literature value of the gas-phase acidity of this compound (ΔH acid o = 1620 ± 8 kJ mol?1) refers to the former site. Based on the occurrence of reversible proton transfer between the CH3O? ion and 1,4-difluorobenzene, the ΔH acid o of this compound is redetermined to be 1592 ± 8 kJ mol?1. 相似文献
973.
Patrick Gurin Nouara Yahi Fodil Azzaz Henri Chahinian Jean-Marc Sabatier Jacques Fantini 《Molecules (Basel, Switzerland)》2022,27(12)
We analyzed the epitope evolution of the spike protein in 1,860,489 SARS-CoV-2 genomes. The structural dynamics of these epitopes was determined by molecular modeling approaches. The D614G mutation, selected in the first months of the pandemic, is still present in currently circulating SARS-CoV-2 strains. This mutation facilitates the conformational change leading to the demasking of the ACE2 binding domain. D614G also abrogated the binding of facilitating antibodies to a linear epitope common to SARS-CoV-1 and SARS-CoV-2. The main neutralizing epitope of the N-terminal domain (NTD) of the spike protein showed extensive structural variability in SARS-CoV-2 variants, especially Delta and Omicron. This epitope is located on the flat surface of the NTD, a large electropositive area which binds to electronegatively charged lipid rafts of host cells. A facilitating epitope located on the lower part of the NTD appeared to be highly conserved among most SARS-CoV-2 variants, which may represent a risk of antibody-dependent enhancement (ADE). Overall, this retrospective analysis revealed an early divergence between conserved (facilitating) and variable (neutralizing) epitopes of the spike protein. These data aid in the designing of new antiviral strategies that could help to control COVID-19 infection by mimicking neutralizing antibodies or by blocking facilitating antibodies. 相似文献
974.
The dielectric loss factor of epoxy resin cured by amine is measured all along the curing, performed under a microwave field (2,45 GHz). A linear hypothesis based on the dielectric contributions of all known constituents allows to show a polar intermediate state; the formation of this state is reversible. An interpretation taking into account hydrogen bonds is proposed. The evolution of dielectric constants during curing brings evidences that microwaves are a peculiarly convenient means of energy transfer to chemical processes as cross-linking reactions. 相似文献
975.
Travelling waves of densities of binary fluid mixtures are investigated near a critical point. The free energy is considered in a non-local form taking account of the density gradients. The equations of motions are applied to a universal form of the free energy near critical conditions and can be integrated by a rescaling process where the binary mixture is similar to a single fluid. Nevertheless, density solution profiles obtained are not necessarily monotonic. As indicated in Appendix, the results might be extended to other topics like in finance or biology. 相似文献
976.
977.
Henri Doucet Takeshi Ohkuma Kunihiko Murata Tohru Yokozawa Masami Kozawa Eiji Katayama Anthony F. England Takao Ikariya Ryoji Noyori 《Angewandte Chemie (International ed. in English)》1998,37(12):1703-1707
A turnover number (TON) of 2 400 000 and a turnover frequency (TOF) of 63 s−1 are achieved with the chiral RuII complex 1 (R=p-CH3C6H4) in the asymmetric hydrogenation of acetophenone. Carbonyl-selective asymmetric hydrogenation of α,β-unsaturated ketones proceeds in the presence of these RuII catalysts, and 4-substituted cyclohexanones are selectively converted into cis alcohols. 相似文献
978.
Christian Girard Henri B. Kagan 《Angewandte Chemie (International ed. in English)》1998,37(21):2922-2959
Who would have thought before 1986 that an enantiomerically impure catalyst could give a product in an asymmetric synthesis with an enantiomeric excess higher than that of the catalyst? Until then it was assumed that the ee value of the product (eeprod) from an asymmetric synthesis was linearly correlated to the ee value of the chiral auxiliary (eeaux)—in fact a large deviation is possible (see diagram). These nonlinear effects are not only of academic interest since they have a variety of practical uses, which are highlighted in this review. 相似文献
979.
980.
Henri Cartan 《Mathematische Annalen》1932,106(1):540-573
Sans résumé 相似文献