全文获取类型
收费全文 | 7689篇 |
免费 | 333篇 |
国内免费 | 33篇 |
专业分类
化学 | 6353篇 |
晶体学 | 38篇 |
力学 | 86篇 |
数学 | 784篇 |
物理学 | 794篇 |
出版年
2023年 | 59篇 |
2022年 | 197篇 |
2021年 | 302篇 |
2020年 | 202篇 |
2019年 | 181篇 |
2018年 | 140篇 |
2017年 | 146篇 |
2016年 | 276篇 |
2015年 | 283篇 |
2014年 | 303篇 |
2013年 | 464篇 |
2012年 | 599篇 |
2011年 | 631篇 |
2010年 | 453篇 |
2009年 | 400篇 |
2008年 | 523篇 |
2007年 | 521篇 |
2006年 | 447篇 |
2005年 | 406篇 |
2004年 | 336篇 |
2003年 | 262篇 |
2002年 | 237篇 |
2001年 | 82篇 |
2000年 | 62篇 |
1999年 | 77篇 |
1998年 | 51篇 |
1997年 | 55篇 |
1996年 | 44篇 |
1995年 | 53篇 |
1994年 | 26篇 |
1993年 | 23篇 |
1992年 | 18篇 |
1991年 | 22篇 |
1990年 | 24篇 |
1989年 | 13篇 |
1988年 | 7篇 |
1987年 | 17篇 |
1986年 | 13篇 |
1985年 | 19篇 |
1984年 | 19篇 |
1983年 | 13篇 |
1982年 | 14篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1978年 | 6篇 |
1977年 | 2篇 |
1975年 | 4篇 |
1973年 | 2篇 |
1970年 | 2篇 |
1965年 | 2篇 |
排序方式: 共有8055条查询结果,搜索用时 406 毫秒
271.
272.
273.
Maria Ana Castro Adrian E. Roitberg Fabio D. Cukiernik 《Journal of computational chemistry》2013,34(15):1283-1290
A classical molecular mechanics force field, able to simulate coordination polymers (CP) based on ruthenium carboxylates (Ru2(O2CReq)4Lax) (eq = equatorial group containing aliphatic chains, Lax= axial ligand), has been developed. New parameters extracted from experimental data and quantum calculations on short aliphatic chains model systems were included in the generalized AMBER force field. The proposed parametrization was evaluated using model systems with known structure, containing either short or long aliphatic chains; experimental results were reproduced satisfactorily. This modified force field, although in a preliminary stage, could then be applied to long chain liquid crystalline compounds. The resulting atomistic simulations allowed assessing the relative influence of the factors determining the CP conformation, determinant for the physical properties of these materials. © 2013 Wiley Periodicals, Inc. 相似文献
274.
275.
High resolution specific heat measurements have been performed in the temperature range 60-190K. Two very weak anomalies were found around 80K and 140K. Thermal conductivity measurements have also been performed and there was no detectable anomaly. The results are discussed in view of actual proposed interpretations. 相似文献
276.
Paula Avelar Amado Vany Ferraz Denise Brentan da Silva Carlos Alexandre Carollo Ana Hortência Fonsêca Castro 《Natural product research》2018,32(5):610-615
The antioxidant and cytotoxic activities of petroleum ether and methanol extracts, fatty acids and methyl esters from leaves of Smilax brasiliensis were evaluated, and the composition of the extracts was determined. Palmitic, linoleic and linolenic acids were major components of the extracts. For antioxidant activity, all samples exhibited IC50 values lower than BHT (2,6-di-tert-butyl-4-methylphenol). The extracts, fatty acids and methyl esters from S. brasiliensis presented no toxicity to larvae of the brine shrimp, Artemia salina. Among the purified substances, only methyl linolenate showed toxicity (LD50 = 21.47 μg/mL). This study showed, for the first time, the composition of petroleum ether and methanol extracts from S. brasiliensis leaves, as well as the antioxidant and cytotoxic activities of extracts, fatty acids and methyl esters. 相似文献
277.
Quantitative analysis of biogenic polyamines in distilled drinks by direct electrospray ionization tandem mass spectrometry using a nanocontainer
下载免费PDF全文
![点击此处可从《Rapid communications in mass spectrometry : RCM》网站下载免费的PDF全文](/ch/ext_images/free.gif)
278.
Montecrinanes A–C: Triterpenes with an Unprecedented Rearranged Tetracyclic Skeleton from Celastrus vulcanicola. Insights into Triterpenoid Biosynthesis Based on DFT Calculations
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Martín Purino Dr. Alejandro E. Ardiles Dr. Oliver Callies Dr. Ignacio A. Jiménez Prof. Isabel L. Bazzocchi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(22):7582-7591
Three new triterpenoids with an unprecedented 6/6/6/6‐fused tetracyclic carbon skeleton, montecrinanes A–C ( 1 – 3 ), were isolated from the root bark of Celastrus vulcanicola, along with known D:B‐friedobaccharanes ( 4 – 6 ), and lupane‐type triterpenes ( 7 – 12 ). The stereostructures of the new metabolites were elucidated based on spectroscopic (1D and 2D NMR) and spectrometric (HR‐EIMS and HR‐ESIMS) techniques. Their absolute configurations were determined by both NMR spectroscopy, with (R)‐(?)‐α‐methoxyphenylacetic acid as a chiral derivatizing agent, and biogenetic considerations. Biogenetic pathways for montecrinane and D:B‐friedobaccharane skeletons were proposed and studied by DFT methods. The theoretical results support the energetic feasibility of the putative biogenetic pathways, in which the 1,2‐methyl shift from the secondary baccharenyl cation represents a novel and key reaction step for a new montecrinane skeleton. 相似文献
279.
280.
Maria Andrea Mendes Formiga Melo Marco Aurélio Rodrigues de Melo Andrea Suame Gouvea Costa Pontes Ana Flávia Félix Farias Manoel Barbosa Dantas Clediana Dantas Calixto Antonio Gouveia de Souza José Rodrigues de Carvalho Filho 《Journal of Thermal Analysis and Calorimetry》2014,117(2):845-849
Vegetable oils with variable proportions of oleic, linoleic, and linolenic acids are more susceptible to oxidative processes. In this subject, this study evaluates the physical chemical properties and oxidative stability of non-conventional oils such as andiroba, babassu, sesame, oiticica, jatropha, and grape through accelerated oxidation techniques (pressurized differential scanning calorimetry, Rancimat and PetroOxy). It was verified that babassu and andiroba oil do not showed detectable induction period presenting high oxidative stability; moreover, it was observed that the enthalpic events occurred in 1.19, >10, 0.53, 0.49, 0.49, and 0.60 h for the andiroba oil, babassu oil, sesame seeds, jatropha, oiticica oils, and grapes, respectively, stimulating the conclusion of greater stability for the babassu oil. 相似文献