全文获取类型
收费全文 | 249篇 |
免费 | 12篇 |
国内免费 | 2篇 |
专业分类
化学 | 194篇 |
力学 | 6篇 |
数学 | 27篇 |
物理学 | 36篇 |
出版年
2023年 | 1篇 |
2022年 | 12篇 |
2021年 | 13篇 |
2020年 | 15篇 |
2019年 | 14篇 |
2018年 | 8篇 |
2017年 | 10篇 |
2016年 | 11篇 |
2015年 | 6篇 |
2014年 | 15篇 |
2013年 | 19篇 |
2012年 | 11篇 |
2011年 | 12篇 |
2010年 | 6篇 |
2009年 | 16篇 |
2008年 | 13篇 |
2007年 | 11篇 |
2006年 | 7篇 |
2005年 | 14篇 |
2004年 | 10篇 |
2003年 | 8篇 |
2002年 | 7篇 |
2001年 | 2篇 |
2000年 | 2篇 |
1999年 | 1篇 |
1998年 | 5篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1991年 | 1篇 |
1982年 | 1篇 |
1978年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1968年 | 1篇 |
1966年 | 1篇 |
1916年 | 2篇 |
排序方式: 共有263条查询结果,搜索用时 15 毫秒
131.
van den Belt M Bianciotto OA Costanza R Demers S Diaz S Ferreyra GA Koch EW Momo FR Vernet M 《Photochemistry and photobiology》2006,82(4):865-877
This article describes the use of group model building to facilitate interaction with stakeholders, synthesize research results and assist in the development of hypotheses about climate change at the global level in relation to UV-B radiation and ecosystem service valuation. The objective was to provide a platform for integration of the various research components within a multidisciplinary research project as a basis for interaction with stakeholders with backgrounds in areas other than science. An integrated summary of the scientific findings, along with stakeholder input, was intended to produce a bridge between science and policymaking. We used a mediated modeling approach that was implemented as a pilot project in Ushuaia, Argentina. The investigation was divided into two participatory workshops: data gathering and model evaluation. Scientists and the local stakeholders supported the valuation of ecosystem services as a useful common denominator for integrating the various scientific results. The concept of economic impacts in aquatic and marsh systems was represented by values for ecosystem services altered by UV-B radiation. In addition, direct local socioeconomic impacts of enhanced UV-B radiation were modeled, using data from Ushuaia. We worked with 5 global latitudinal regions, focusing on net primary production and biomass for the marine system and on 3 plant species for the marsh system. Ecosystem service values were calculated for both sectors. The synthesis model reflects the conclusions from the literature and from experimental research at the global level. UV-B is not a significant stress for the marshes, relative to the potential impact of increases in the sea level. Enhanced UV-B favors microbial dynamics in marine systems that could cause a significant shift from primary producers to bacteria at the community level. In addition, synergetic effects of UV-B and certain pollutants potentiate the shift to heterotrophs. This may impact the oceanic carbon cycle by increasing the ratio of respiratory to photosynthetic organisms in surface waters and, thus, the role of the ocean as a carbon sink for atmospheric CO2. In summary, although changes in the marine sector due to anthropogenic influences may affect global climate change, marshes are expected to primarily be affected by climate change. 相似文献
132.
Verschooten L Claerhout S Van Laethem A Agostinis P Garmyn M 《Photochemistry and photobiology》2006,82(4):1016-1023
Adequate photoprotection is essential to control UV-related disorders, including sunburn, photoaging and photocarcinogenisis. Sun avoidance, protection of skin with clothing, and sunscreens are presently the best way of photoprotection, assuming that they are used properly. However, new strategies, which are based on or make use of the endogenous protective response to UV light, may further improve currently used photoprotective means. The addition of repair enzymes and/or antioxidants has a positive effect on skin's recovery from UV-induced DNA-damage. Several botanical agents, mainly vitamins and polyphenols, have shown to influence signal transduction pathways leading to photoprotective effects. Also stimulation of endogenous UV-response pathways via irradiation with a low UV dose or via simulation of UV-induced DNA-damage results in photoprotective effects. Future research in this field and combination of different photoprotective strategies will hopefully lead to improved photoprotection. 相似文献
133.
Marjan Matvejchuk 《International Journal of Theoretical Physics》1998,37(1):103-107
Quantum logics of projections being self-adjointwith respect to a unitary operator on a Hilbert spaceare studied. 相似文献
134.
Marjan Abedi Ali Akbar Khandar Masoumeh Servati Gargari Atash V. Gurbanov Seyed Ali Hosseini Ghodrat Mahmoudi 《无机化学与普通化学杂志》2014,640(11):2193-2197
Two dinuclear mercury(II) iodide compounds, [Hg2(L)(I)4] ( 1 ) and [(L′)Hg(μ‐I)2HgI2]n ( 2 ) [L = N,N′‐bis(phenyl(pyridin‐2‐yl)methylene)propane‐1,2‐diamine and L′ = N‐(phenyl(pyridin‐2‐yl)methylene)propane‐1,2‐diamine] were synthesized and characterized. The molecular structures of [Hg2(L)(I)4] ( 1 ) and [(L′)Hg(μ‐I)2HgI2]n ( 2 ), which were determined by single‐crystal X‐ray diffraction, indicate that each HgII in 1 has a distorted tetrahedral environment around the metal atom with a HgN2I2 chromophore, whereas in 2 one mercury(II) atom adopts a distorted tetrahedral arrangement with a HgI4 chromophore and the other has a distorted square pyramidal environment with HgN3I2 chromophore. In the solid state, compound 2 consists of a 1D coordination polymer structure. 相似文献
135.
Marjan A. Rafiee 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2014,88(13):2359-2362
Quadrupolar parameters of nuclei can be used as a tool to understand the electronic structure of the compounds. Magnesium hydride (MgH2) is a potential hydrogen storage material due to its outstanding hydrogen capacity, however, its high thermodynamic stability is unfavorable for dehydrogenation processes. Understanding the bonding nature of Mg and H is essential for improving its dehydrogenation performance. In this work the charge density distribution in MgH2 is studied. For this purpose, using calculated NQCCs of hydrogen atoms, the electronic structure of α-MgH2 with several high pressure forms of MgH2 were compared. The results show that in the high pressure phases (β, γ, and δ) some hydrogens have very small NQCC and therefore these hydrogens form weaker bond with Mg. In other words, easier condition for dehydrogenation in pressure-induced forms is expected. The electric field gradient (EFG) at the site of quadrupolar nuclei were calculated to obtain NQCC parameters using Gaussian 03 at B3LYP/6-31G level of theory. The selected level and basis set give the rather acceptable qualitative NQCCs of hydrogen atoms. 相似文献
136.
Formation and Characterisation of the Silver Hydride Nanocluster Cation [Ag3H2((Ph2P)2CH2)]+ and Its Release of Hydrogen 下载免费PDF全文
Dr. Marion Girod Marjan Krstić Dr. Rodolphe Antoine Dr. Luke MacAleese Prof. Jérome Lemoine Athanasios Zavras Dr. George N. Khairallah Prof. Vlasta Bonačić‐Koutecký Prof. Philippe Dugourd Prof. Richard A. J. O'Hair 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(50):16626-16633
Multistage mass spectrometry and density functional theory (DFT) were used to characterise the small silver hydride nanocluster, [Ag3H2L]+ (where L=(Ph2P)2CH2) and its gas‐phase unimolecular chemistry. Collision‐induced dissociation (CID) yields [Ag2HL]+ as the major product while laser‐induced dissociation (LID) proceeds via H2 formation and subsequent release from [Ag3H2L]+, giving rise to [Ag3L]+ as the major product. Deuterium labelling studies on [Ag3D2L]+ prove that the source of H2 is from the hydrides and not from the ligand. Comparison of TD‐DFT absorption patterns obtained for the optimised structures with action spectroscopy results, allows assignment of the measured features to structures of precursors and products. Molecular dynamics “on the fly” reveal that AgH loss is favoured in the ground state, but H2 formation and loss is preferred in the first excited state S1, in agreement with CID and LID experimental findings. This indicates favourable photo‐induced formation of H2 and subsequent release from [Ag3H2L]+, an important finding in context of metal hydrides as a hydrogen storage medium, which can subsequently be released by heating or irradiation with light. 相似文献
137.
A glycosylated 2,2-bis(methylol)propionic acid (bis-MPA) derivative as a versatile glycodendritic building block was synthesized by direct O-glycosylation of benzyl protected bis-MPA with d-glucose pentaacetate. After removal of the benzyl protection group by hydrogenation, the obtained bis-O-glycosylated bis-MPA unit with free carboxyl group was applied for modification of trityl protected chitosan by amidation. The removal of the 6-O-trityl protection group on chitosan backbone under acidic conditions yields acetyl protected glycosylated chitosan that forms gel in water. The acetyl protection group of d-glucose was removed under alkaline conditions to form glycosylated chitosan which is water-soluble at physiological conditions and, thus, it is suitable for further investigation as a potential carrier in drug delivery systems. 相似文献
138.
An Isocyanide‐Based Multicomponent Reaction: New Route for Synthesis of Isoxazolinedione Derivatives
Azizollah Habibi Fahimeh Vafadarnejad Marjan Akbari Armand 《Journal of heterocyclic chemistry》2013,50(4):887-890
An efficient and novel multicomponent reaction for synthesis of new isoxazolinedione has been described. This protocol involves reaction of two molecules of isocyanide, aliphatic oxime, alkylidene substituted Meldrum's acid, and water (moisture). Elemental analyses, IR, 1H NMR, and 13C NMR spectroscopic data of products are consistent with the structure defined by X‐ray diffraction analysis. 相似文献
139.
Firouz Matloubi Moghaddam Mostafa Kiamehr Mohammad Reza Khodabakhshi Marjan Jebeli Javan Shaghayegh Fathi Alexander Villinger Viktor O. Iaroshenko Peter Langer 《Helvetica chimica acta》2013,96(11):2103-2114
An efficient one‐pot, three‐component synthesis of novel dispiro[oxindole‐3,3′‐pyrrolidines] by 1,3‐dipolar cycloaddition of azomethine ylides, in situ generated by reaction of 1,2‐diones with sarcosine and subsequent decarboxylation, with a series of (E)‐3‐benzylidene‐2,3‐dihydro‐1H‐indol‐2‐ones is reported. Molecular complexity is generated in only one synthetic step. All reactions proceed with excellent regioselectivity and in good‐to‐excellent yields. The workup is easy, the reaction times are short, and no catalyst is required. 相似文献
140.