全文获取类型
收费全文 | 1073篇 |
免费 | 48篇 |
国内免费 | 12篇 |
专业分类
化学 | 729篇 |
晶体学 | 11篇 |
力学 | 10篇 |
数学 | 135篇 |
物理学 | 248篇 |
出版年
2024年 | 3篇 |
2023年 | 6篇 |
2022年 | 16篇 |
2021年 | 29篇 |
2020年 | 30篇 |
2019年 | 37篇 |
2018年 | 36篇 |
2017年 | 30篇 |
2016年 | 56篇 |
2015年 | 26篇 |
2014年 | 40篇 |
2013年 | 126篇 |
2012年 | 60篇 |
2011年 | 60篇 |
2010年 | 49篇 |
2009年 | 51篇 |
2008年 | 47篇 |
2007年 | 33篇 |
2006年 | 31篇 |
2005年 | 34篇 |
2004年 | 48篇 |
2003年 | 22篇 |
2002年 | 28篇 |
2001年 | 10篇 |
2000年 | 12篇 |
1999年 | 4篇 |
1998年 | 15篇 |
1997年 | 4篇 |
1996年 | 13篇 |
1995年 | 6篇 |
1994年 | 19篇 |
1993年 | 12篇 |
1992年 | 15篇 |
1991年 | 10篇 |
1990年 | 7篇 |
1989年 | 14篇 |
1988年 | 10篇 |
1987年 | 3篇 |
1985年 | 8篇 |
1984年 | 9篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 10篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1977年 | 6篇 |
1975年 | 4篇 |
1972年 | 3篇 |
1971年 | 4篇 |
1969年 | 2篇 |
排序方式: 共有1133条查询结果,搜索用时 234 毫秒
71.
A new homologous series of thermotropic liquid crystalline poly(azomethine-ether)s was synthesized by solution polycondensation of various diformyl-alpha,omega-diphenoxyalkanes, I-VIII with 2,5-bis(m-aminobenzylidene)cyclopentanone, XI and 2,6-bis(m-aminobenzylidene)cyclohexanone, XII. The inherent viscosities of the polymers were in the range 0.34-0.75 dI g-1. The mesomorphic properties were studied as a function of the diphenoxyalkane spacer length. Analyses by DSC and optical polarized microscopy demonstrated that the poly(azomethine-ether)s form nematic mesophases over wide temperature ranges. The poly(azomethine-ether)s based on the cyclohexanone moiety had better thermal stabilities than the corresponding poly(azomethine-ether)s based on cyclopentanone. 相似文献
72.
Vivek T. Humne Kamal Hasanzadeh Pradeep D. Lokhande 《Research on Chemical Intermediates》2013,39(2):585-595
Selective O-deallylation of dihydropyrazoles has been achieved by use of iodine (10 mol%) in PEG-400 as ecofriendly solvent. Iodine (10 mol%) in dimethyl sulfoxide at 100 °C also afforded O-deallylation with aromatization compatible with highly reactive N-allyl and formyl groups. The function of iodine in the synthesis of substituted pyrazoles under different conditions is described. 相似文献
73.
Singh Mhaveer Kamal Yunus T.-K. Tamboli Ennus T. Parveen Rabea Ansari Shahid H. Ahmad Sayeed 《平面色谱法杂志一现代薄层色谱法》2013,26(3):267-273
JPC – Journal of Planar Chromatography – Modern TLC - A novel HPTLC method has been developed for the estimation of glabridin in Licorice rhizome and its Unani polyherbal formulation... 相似文献
74.
A fast, simple, and efficient protocol for the direct conversion of alcohols and aldehydes to methyl ester has been developed using TsNBr2 without any catalyst. The one pot reaction proceeds in the presence of a base at room temperature in methanol, to produce the corresponding methyl ester in high yield within a short time. 相似文献
75.
Dr. Mohammad Kamal Hossain Dr. Qasem Ahmed Drmosh Amar Kamal Mohamedkhair 《化学:亚洲杂志》2021,16(13):1807-1819
A new route has been developed to design plasmonic pollen grain-like nanostructures (PGNSs) as surface-enhanced Raman scattering (SERS)-active substrate. The nanostructures consisting of silver (Ag) and gold (Au) nanoparticles along with zinc oxide (ZnO) nanoclusters as spacers were found highly SERS-active. The morphology of PGNSs and those obtained in the intermediate stage along with each elemental evolution has been investigated by a high-resolution field emission scanning electron microscopy. The optical band gaps and crystal structure have been identified by UV-vis absorption and X-ray powder diffraction (XRD) measurements, respectively. For PGNSs specimen, three distinct absorption bands related to constituent elements Ag, Au, and ZnO were observed, whereas XRD peaks confirmed the existence of Ag, Au, and ZnO within the composition of PGNSs. SERS-activity of PGNSs was confirmed using Rhodamine 6G (R6G) as Raman-active dyes. Air-cooled solid-state laser kits of 532 nm were used as excitation sources in SERS measurements. SERS enhancement factor was estimated for PGNSs specimen and was found as high as 3.5×106. Finite difference time domain analysis was carried out to correlate the electromagnetic (EM) near-field distributions with the experiment results achieved under this investigation. EM near-field distributions at different planes were extracted for s-, p- and 45° of incident polarizations. EM near-field distributions for such nanostructures as well as current density distributions under different circumstances were demonstrated and plausible scenarios were elucidated given SERS enhancements. Such generic fabrication route as well as correlated investigation is not only indispensable to realize the potential of SERS applications but also unveil the underneath plasmonic characteristics of complex SERS-active nanostructures. 相似文献
76.
Datu Agasi Mohd Kamal Norizam Salamt Siti Sarah Mohamad Zaid Mohd Helmy Mokhtar 《Molecules (Basel, Switzerland)》2021,26(9)
Tea is one of the most widely consumed beverages worldwide after water, and green tea accounts for 20% of the total tea consumption. The health benefits of green tea are attributed to its natural antioxidants, namely, catechins, which are phenolic compounds with diverse beneficial effects on human health. The beneficial effects of green tea and its major bioactive component, (−)-epigallocatechin-3-gallate (EGCG), on health include high antioxidative, osteoprotective, neuroprotective, anti-cancer, anti-hyperlipidemia and anti-diabetic effects. However, the review of green tea’s benefits on female reproductive disorders, including polycystic ovary syndrome (PCOS), endometriosis and dysmenorrhea, remains scarce. Thus, this review summarises current knowledge on the beneficial effects of green tea catechins on selected female reproductive disorders. Green tea or its derivative, EGCG, improves endometriosis mainly through anti-angiogenic, anti-fibrotic, anti-proliferative and proapoptotic mechanisms. Moreover, green tea enhances ovulation and reduces cyst formation in PCOS while improving generalised hyperalgesia, and reduces plasma corticosterone levels and uterine contractility in dysmenorrhea. However, information on clinical trials is inadequate for translating excellent findings on green tea benefits in animal endometriosis models. Thus, future clinical intervention studies are needed to provide clear evidence of the green tea benefits with regard to these diseases. 相似文献
77.
Mokhtar A. Abd ul-Malik Adel M. Kamal El-Dean Shaban M. Radwan Remon M. Zaki 《Journal of heterocyclic chemistry》2021,58(11):2067-2077
A new series of pyrazolopyrazinoselenolotriazolopyrimidines was synthesized by a facile method based on condensation of 5-amino-3-methyl-1-phenyl-1H-pyrazolo[3,4-b]selenolo[3,2-e]pyrazine-6-carbonitrile ( 3 ) with triethyl orthoformate followed by intramolecular cyclization with hydrazine to afford 7-amino-8-imino-3-methyl-1-phenyl-1,8-dihydro-7H-pyrazolo[3″,4″:5′,6′]pyrazino[2′,3′:4,5] selenolo[3,2-d]pyrimidine ( 5 ). The latter compound was utilized as a multipurpose precursor for the construction of other new triazoles fused to the pyrazolopyrazino- selenolopyrimidine moiety. Alternatively, acetylation and chloro-acetylation of compound 3 using acetic anhydride and chloroacetyl chloride yielded the acetyl amino 11 and chloroacetamido 12 derivatives, respectively. Compound 12 underwent nucleophilic substitution upon reaction with morpholine to provide the morpholinyl acetamide 13 . Furthermore, the pyrazolopyridoselenolopyrazine ring system 14 was synthesized by the reaction of the o-amino-carbonitrile 3 with malononitrile. Assignment of the chemical structures for the new compounds was confirmed depending on elemental and spectral techniques. On the other hand, most of the synthesized compounds revealed promising results against various bacterial and fungal strains. 相似文献
78.
79.
Unexpected dimerization of 1,3‐dimethyl‐5‐methylenebarbituric acid revealed by a combined experimental and computational study 下载免费PDF全文
Bernd Doser Kamal Sweidan Norbert Kuhn Christian Ochsenfeld 《Journal of Physical Organic Chemistry》2015,28(5):354-357
A comparison of experimental and calculated 13C‐nuclear magnetic resonance chemical shifts reveals the molecular structure of a dimer that was obtained by an unexpected dimerization of 1,3‐dimethyl‐5‐methylenebarbituric acid. Furthermore, the puckering angle of the cyclobutane unit linking the six‐membered rings is discussed in detail. The influence of substituents on 1,3‐position of the cyclobutane ring on the puckering angle is demonstrated based on 1,1,3,3‐tetramethylcyclobutane. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
80.
M. Sc. Vincent Eschenbrenner‐Lux Dipl.‐Biol. Philipp Küchler Dr. Slava Ziegler Dr. Kamal Kumar Prof. Herbert Waldmann 《Angewandte Chemie (International ed. in English)》2014,53(8):2134-2137
The imino Diels–Alder reaction is an efficient method for the synthesis of aza‐heterocycles. While different stereo‐ and enantioselective inverse‐electron‐demand imino Diels–Alder (IEDIDA) reactions have been reported before, IEDIDA reactions including electron‐deficient dienes are unprecedented. The first enantioselective IEDIDA reaction between electron‐poor chromone dienes and cyclic imines, catalyzed by zinc/binol complexes is described. The novel reaction provides a facile entry to a natural product inspired collection of ring‐fused quinolizines including a potent modulator of mitosis. 相似文献