全文获取类型
收费全文 | 3357篇 |
免费 | 138篇 |
国内免费 | 31篇 |
专业分类
化学 | 2379篇 |
晶体学 | 23篇 |
力学 | 46篇 |
数学 | 443篇 |
物理学 | 635篇 |
出版年
2023年 | 18篇 |
2022年 | 53篇 |
2021年 | 84篇 |
2020年 | 66篇 |
2019年 | 50篇 |
2018年 | 50篇 |
2017年 | 48篇 |
2016年 | 109篇 |
2015年 | 121篇 |
2014年 | 116篇 |
2013年 | 223篇 |
2012年 | 215篇 |
2011年 | 240篇 |
2010年 | 156篇 |
2009年 | 154篇 |
2008年 | 212篇 |
2007年 | 199篇 |
2006年 | 185篇 |
2005年 | 158篇 |
2004年 | 117篇 |
2003年 | 116篇 |
2002年 | 108篇 |
2001年 | 77篇 |
2000年 | 103篇 |
1999年 | 63篇 |
1998年 | 28篇 |
1997年 | 41篇 |
1996年 | 66篇 |
1995年 | 34篇 |
1994年 | 26篇 |
1993年 | 26篇 |
1992年 | 24篇 |
1991年 | 26篇 |
1990年 | 16篇 |
1989年 | 13篇 |
1988年 | 15篇 |
1987年 | 7篇 |
1986年 | 17篇 |
1985年 | 16篇 |
1984年 | 13篇 |
1983年 | 10篇 |
1982年 | 19篇 |
1981年 | 7篇 |
1980年 | 10篇 |
1979年 | 14篇 |
1978年 | 12篇 |
1977年 | 7篇 |
1976年 | 11篇 |
1974年 | 7篇 |
1973年 | 7篇 |
排序方式: 共有3526条查询结果,搜索用时 15 毫秒
991.
Vélez E Quijano J Gaviria J Roux MV Jiménez P Temprado M Martín-Valcárcel G Pérez-Parajón J Notario R 《The journal of physical chemistry. A》2005,109(34):7832-7838
The enthalpies of combustion and vaporization of 3-buten-1-ol and 3-butyn-1-ol have been measured by static bomb combustion calorimetry and correlation gas chromatography techniques, respectively, and the gas-phase enthalpies of formation, Delta(f)H degrees (m)(g), have been determined, the values being -147.3 +/- 1.8 and 16.7 +/- 1.6 kJ mol(-1), for 3-buten-1-ol and 3-butyn-1-ol, respectively. High level calculations at the G2 and G3 levels have also been carried out. Relationships between the enthalpies of formation of 1-alkanols, 1-alkenols and 1-alkynols and with the corresponding hydrocarbons have been discussed. From the calculated contributions to Delta(f)H degrees (m)(g) for the substitutions of CH(3) by CH(2)OH, CH(3)CH(2) by CH(2)=CH and CH(3)CH(2) by CH triple bond C, we have estimated the Delta(f)H degrees (m)(g) values for 3-buten-1-ol and 3-butyn-1-ol, in excellent agreement with the experimental ones. Delta(f)H degrees (m)(g) values for 1-alkenols and 1-alkynols up to 10 carbon atoms have also been estimated. 相似文献
992.
R. Baggio O. González M. T. Garland J. Manzur V. Acuña A. M. Atria E. Spodine O. Peña 《Journal of chemical crystallography》1993,23(9):749-753
The title compound, Cu2C17H16O2.5Cl2, is a copper(II) complex, in which each dimeric unit has two copper atoms bridged by a pyrazolato ligand, a di(2-pyridyl)]methane ligand, and an oxygen atom from the modified methylene group that links the two pyridine rings (O-C-O-CH2CH3). The complex crystallizes in the monoclinic groupP21/n,a=7.7330(1),b=19.589(6),c=13.123(2)Å,=95.020(0)°,V=1980.3(7)Å3,Z=4,F
w=514.3,D
Do
=1.725 g cm–3(MoK)=0.71073 Å,F(000)=1032,=2.44 mm–1, R=0.044 for 2209 observed reflections,Rw=0.047,s=0.086. Each dimer is linked to the next unit by long chlorine bonds (Cu(2)-Cl(1)=2.846(2)Å). These tetrameric units form zigzagged chains through copperchlorine bonds of 3.380(2)Å. The copper(II) complex presents antiferromagnetic behavior withT
m=161 K. 相似文献
993.
Alberto Escarpa Concepcin Prez‐Cabrera María Cristina Gonzlez 《Journal of separation science》2000,23(11):637-643
A fast high‐performance liquid chromatography method was used for analysis of prominent flavan‐3‐ols and flavonols in vegetables. Gradient elution with phosphoric acid‐acetonitrile mixtures and phosphoric acid‐methanol mixtures allowed fast and complete separation of the studied phenolic compounds within analysis times less than 10 min. The development of two elution gradients using methanol and acetonitrile as modifiers proved to be an excellent approach for the verification of the real polyphenolic composition in vegetables samples because the two optimized methods allowed the separation of the same number of compounds in the same elution order. Diode‐array detection was employed for the provisional identification of phenolic compounds that were not available as standards. We preferred methanol as a modifier because it was less toxic and cheaper than acetonitrile. Detection limits ranged between 0.12 and 0.59 μg mL–1. High recoveries of phenolics from fresh vegetables were measured in all studied cases, independent of the phenolic structure, matrix, and vegetable in question. High levels of procyanidins between 150 and 450 mg kg–1 were found in all studied vegetables. Quantification of quercetin and kaempferol glycosides was only possible in marrow and onion, respectively. 相似文献
994.
The hydroxyindolenine 2 , upon treatment with Ac2O in pyridine, rearranged into the bridged 16-spiro compound 3 . Compound 3 can be considered to be a synthon of the carbocation at C(16) of ibogaine ( 7 ) since the solvolysis of 3 in EtOH/H2O gave a mixture of the 16-ethoxy- and 16-hydroxyibogaine derivatives 5 and 6 , respectively. 相似文献
995.
996.
M. M. C. Lpez-Gonzlez M. J. Callejo Cudero J. M. Barrales-Rienda 《Journal of polymer science. Part A, Polymer chemistry》1997,35(16):3409-3429
A detailed structural analysis of the copolyesterification in bulk without any external catalyst at 160°C between o-phthalic anhydride (P), oleic acid (O), and trimethylolpropane (T) with a mol ratio ([ COOH]/[ OH]) = 0.70 has been carried out by high resolution 13C nuclear magnetic resonance (13C-NMR) (DMSO-d6 and CDCl3 solutions), 1H nuclear magnetic resonance (1H-NMR) (CDCl3 solution), and by volumetry. The use of CDCl3 as NMR solvent has allow us to identify several signals that have been assigned to trimethylolpropane monoesters with different esterification degrees in the o-phthalate residue. Identically, we have detected signals assignable to monoesters at the chain ends in structures with different chain lengths. These monoesterified structures have been also confirmed by analyzing samples modified by the diazomethane addition. These modified samples have been also used to determine free acid groups as their methoxylic derivatives by 1H-NMR in CDCl3 solution. We have not observed any detectable signs of gelation nor products produced by secondary reactions. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 3409–3429, 1997 相似文献
997.
Anglica Serpa Guerra Catalina Gmez Hoyos Jorge Velsquez-Cock Lina Vlez Piedad Gan Robin Zuluaga 《Molecules (Basel, Switzerland)》2022,27(3)
Color is an important characteristic of food products. This characteristic is related to consumer acceptability. To use the entire rhizome of Curcuma longa (CL) as a food colorant, a novel gel alike stable suspension (CLS) was previously developed using cellulose nanofibers (CNFs). Therefore, the present study was conducted to evaluate the CLS as a color additive on a stirred yogurt. Three concentrations of CLS were studied (0.1, 0.125, and 0.15 wt. %) and compared to yogurt without CLS. The obtained yogurts were characterized through the determination of pH, titratable acidity, syneresis, color and curcumin content after 1, 7, 14, and 21 days of storage. Additionally, rheological and sensory measurements were performed on the samples after one day of storage. Results show that the addition of CLS does not affect the pH and titratable acidity of the samples, but all the yogurts showed an increase in their syneresis during the storage time, showing a breakdown of the gel structure. Furthermore, the CLS suspension has the ability to impart a yellow color to yogurts, a characteristic that was stable during storage. Finally, the addition of 1 wt. % or 1.25 wt. % of CLS allows the development of a yogurt with adequate sensory perception. 相似文献
998.
999.
Miguel Vzquez Manuel R. Bermejo Matilde Fondo Ana M. García‐Deibe Ana M. Gonzlez Rosa Pedrido Jesús Sanmartín 《无机化学与普通化学杂志》2002,628(5):1068-1074
The electrochemical reaction of the bis‐bidentate Schiff base H2FTs [N, N′‐bis(2‐tosylaminobenzylidene)‐1, 2‐diaminobencene] with cobalt, nickel, copper, zinc and cadmium, lead to the isolation of neutral [M(FTs)] complexes. All of them were characterized by elemental analyses, mass spectrometry, IR and 1H NMR spectroscopy and magnetic measurements, where appropriate. Recrystallization of the nickel complex yields single crystals of [Ni(FTs)]·CH3CN ( 1 ). The x‐ray characterization shows a distorted square‐planar environment for the nickel atom, with the Schiff base acting as a tetradentate N4 donor. Complex 1 can be described as a mononuclear single‐stranded helical compound, with spontaneous resolution of the P enantiomer upon crystallisation. 相似文献
1000.
Ignacio Gonzlez M. Eugenia Muoz Antonio Santamaría J. Adolfo Pomposo Hans Grande Javier Rodríguez‐Parra 《Macromolecular rapid communications》2002,23(12):659-663
An improved method, based on mechanical stirring and a controlled solvent evaporation process, is proposed to obtain PANI(CSA)0.5/m‐cresol solutions, which lead to gels with conductivities in the range 3 to 150 S/cm, preserving the elasticity associated to the gel network. Such high conductivity values have only been found for brittle films cast from PANI solutions. According to our experimental results, there is no justification for an analogy between the elastic modulus and the conductivity of physical networks. 相似文献