全文获取类型
收费全文 | 1988篇 |
免费 | 32篇 |
国内免费 | 5篇 |
专业分类
化学 | 1119篇 |
晶体学 | 35篇 |
力学 | 17篇 |
数学 | 349篇 |
物理学 | 505篇 |
出版年
2023年 | 7篇 |
2022年 | 13篇 |
2021年 | 25篇 |
2020年 | 11篇 |
2019年 | 28篇 |
2018年 | 51篇 |
2017年 | 41篇 |
2016年 | 73篇 |
2015年 | 57篇 |
2014年 | 70篇 |
2013年 | 175篇 |
2012年 | 79篇 |
2011年 | 113篇 |
2010年 | 73篇 |
2009年 | 79篇 |
2008年 | 102篇 |
2007年 | 82篇 |
2006年 | 85篇 |
2005年 | 66篇 |
2004年 | 73篇 |
2003年 | 66篇 |
2002年 | 50篇 |
2001年 | 49篇 |
2000年 | 32篇 |
1999年 | 29篇 |
1998年 | 36篇 |
1997年 | 29篇 |
1996年 | 23篇 |
1995年 | 29篇 |
1994年 | 23篇 |
1993年 | 18篇 |
1992年 | 22篇 |
1991年 | 14篇 |
1990年 | 21篇 |
1989年 | 17篇 |
1988年 | 14篇 |
1987年 | 25篇 |
1986年 | 12篇 |
1985年 | 14篇 |
1984年 | 18篇 |
1982年 | 19篇 |
1981年 | 17篇 |
1980年 | 17篇 |
1979年 | 17篇 |
1978年 | 10篇 |
1977年 | 12篇 |
1975年 | 8篇 |
1973年 | 7篇 |
1939年 | 7篇 |
1929年 | 6篇 |
排序方式: 共有2025条查询结果,搜索用时 109 毫秒
941.
Carbon nanofibres and multi-branched carbon nanotubes are synthesized by using a new, proposed in this study method. The acid–base, adsorption and geometric properties of nanomaterials are characterised. Next we present first comparative adsorption and calorimetric studies of phenol uptake from aqueous solutions on mentioned nano-sized carbon materials and on nanotubes having similar diameter. The comparison of carbon nanofibres, multi-branched carbon nanotubes, and multi walled carbon nanotubes shows that for non-porous carbon nanomaterials the concentration of basic surface functionalities determines the mechanism of phenol adsorption. In consequence, larger phenol adsorption is recorded for the most basic nanomaterials (nanofibres and multi walled nanotubes) and smaller for multi branched nanotubes having the smallest surface concentration of basic groups. Possible explanation of differences between enthalpy of phenol displacement on graphite and on studied carbon adsorbents is also given. 相似文献
942.
Barbara Moniuszko‐Szajwaj Milena Masullo Mariusz Kowalczyk Łukasz Pecio Małgorzata Szumacher‐Strabel Adam Cieślak Sonia Piacente Wiesław Oleszek Anna Stochmal 《Helvetica chimica acta》2016,99(5):347-354
Five new triterpenoid saponins, including 3‐O‐β‐d ‐galactopyranosyl‐(1→2)‐[β‐d ‐xylopyranosyl‐(1→3)]‐β‐d ‐glucuronopyranosyl quillaic acid 28‐O‐β‐d ‐glucopyranosyl‐(1→3)‐β‐d ‐xylopyranosyl‐(1→4)‐α‐l ‐rhamnopyranosyl‐(1→2)‐[β‐d ‐xylopyranosyl‐(1→3)‐(4‐O‐acetyl)‐β‐d ‐quinovopyranosyl‐(1→4)]‐β‐d ‐fucopyranoside ( 1 ), 3‐O‐β‐d ‐galactopyranosyl‐(1→2)‐[β‐d ‐xylopyranosyl‐(1→3)]‐β‐d ‐glucuronopyranosyl quillaic acid 28‐O‐(6‐O‐acetyl)‐β‐d ‐glucopyranosyl‐(1→3)‐[β‐d ‐xylopyranosyl‐(1→4)]‐α‐l ‐rhamnopyranosyl‐(1→2)‐[β‐d ‐xylopyranosyl‐(1→3)‐(4‐O‐acetyl)‐β‐d ‐quinovopyranosyl‐(1→4)]‐β‐d ‐fucopyranoside ( 2 ), 3‐O‐β‐d ‐galactopyranosyl‐(1→2)‐[β‐d ‐xylopyranosyl‐(1→3)]‐β‐d ‐glucuronopyranosyl quillaic acid 28‐O‐β‐d ‐xylopyranosyl‐(1→4)‐α‐l ‐rhamnopyranosyl‐(1→2)‐[β‐d ‐xylopyranosyl‐(1→3)‐(4‐O‐acetyl)‐β‐d ‐quinovopyranosyl‐(1→4)]‐β‐d ‐fucopyranoside ( 3 ), 3‐O‐β‐d ‐galactopyranosyl‐(1→2)‐[β‐d ‐xylopyranosyl‐(1→3)]‐β‐d ‐glucuronopyranosyl quillaic acid 28‐O‐β‐d ‐glucopyranosyl‐(1→3)‐β‐d ‐xylopyranosyl‐(1→4)‐α‐l ‐rhamnopyranosyl‐(1→2)‐[(4‐O‐acetyl)‐β‐d ‐quinovopyranosyl‐(1→4)]‐β‐d ‐fucopyranoside ( 4 ), 3‐O‐β‐d ‐galactopyranosyl‐(1→2)‐[β‐d ‐xylopyranosyl‐(1→3)]‐β‐d ‐glucuronopyranosyl quillaic acid 28‐O‐(6‐O‐acetyl)‐β‐d ‐glucopyranosyl‐(1→3)‐[β‐d ‐xylopyranosyl‐(1→4)]‐α‐l ‐rhamnopyranosyl‐(1→2)‐[(4‐O‐acetyl)‐β‐d ‐quinovopyranosyl‐(1→4)]‐β‐d ‐fucopyranoside ( 5 ) together with two known congeners, saponariosides A ( 6 ) and B ( 7 ) were isolated from the roots of Saponaria officinalis L. Their structures were elucidated by extensive spectroscopic methods, including 1D‐ (1H, 13C) and 2D‐NMR (DQF‐COSY, TOCSY, HSQC, and HMBC) experiments, HR‐ESI‐MS, and acid hydrolysis. 相似文献
943.
Krzysztof Tadyszak Agnieszka Boś‐Liedke Jan Jurga Mikołaj Baranowski Radosław Mrówczyński Wojciech Chlewicki Stefan Jurga Tomasz Czechowski 《Magnetic resonance in chemistry : MRC》2016,54(2):136-142
A study concerning the image quality in electron paramagnetic resonance imaging in two‐dimensional spatial experiments is presented. The aim of the measurements was to improve the signal‐to‐noise ratio (SNR) of the projections and the reconstructed image by applying modulation amplitude higher than the radical electron paramagnetic resonance linewidth. Data were gathered by applying four constant modulation amplitudes, where one was below 1/3 (Amod = 0.04 mT) of the radical linewidth (ΔBpp = 0.14 mT). Three other modulation amplitude values were used in this experiment, leading to undermodulated (Amod < 1/3 ΔBpp), partially overmodulated (Amod ~ 1/3 ΔBpp) and fully overmodulated (Amod > > 1/3 ΔBpp) projections. The advantages of an applied overmodulation condition were demonstrated in the study performed on a phantom containing four shapes of 1.25 mM water solution of 2, 2, 6, 6‐tetramethyl‐1‐piperidinyloxyl. It was shown that even when the overmodulated reference spectrum was used in the deconvolution procedure, as well as the projection itself, the phantom shapes reconstructed as images directly correspond to those obtained in undermodulation conditions. It was shown that the best SNR of the reconstructed images is expected for the modulation amplitude close to 1/3 of the projection linewidth, which is defined as the distance from the first maximum to the last minimum of the gradient‐broadened spectrum. For higher modulation amplitude, the SNR of the reconstructed image is decreased, even if the SNR of the measured projection is increased. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
944.
945.
Dr. Arkadiusz Listkowski Dr. Natalia Masiera Michał Kijak Dr. Roman Luboradzki Dr. Barbara Leśniewska Prof. Jacek Waluk 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(20):6324-6333
Porphycene, a porphyrin isomer, is an efficient fluorophore. However, four-fold meso substitution with alkyl groups decreases the fluorescence quantum yield by orders of magnitude. For aryl substituents, this effect is small. To explain this difference, we have synthesized and studied a mixed aryl-alkyl-substituted compound, 9,20-diphenyl-10,19-dimethylporphycene, as well as the 9,20-diphenyl and 9,20-dimethyl derivatives. Analysis of the structural, spectroscopic, and photophysical data of the six porphycenes, combined with quantum chemical calculations, shows a clear correlation between the strength of the intramolecular NH⋅⋅⋅N hydrogen bonds and the efficiency of the radiationless depopulation of the lowest-excited singlet state. This result led us to propose a model in which the delocalization of the inner protons in the cavity of the macrocycle is responsible for the nonradiative deactivation channel. The applicability of the model is confirmed by the literature data for other alkyl- or aryl-substituted porphycenes. The finding of a correlation between structural and emissive characteristics enables a rational design of porphycenes with desired photophysical properties. 相似文献
946.
947.
Piotr Szcześniak Olga Staszewska-Krajewska Bartłomiej Furman Jacek Mlynarski 《Tetrahedron: Asymmetry》2017,28(12):1765-1773
A comparison of three different catalytic systems for the efficient, asymmetric synthesis of N-({(3R,4R)-4-[(benzyloxy)methyl]pyrrolidin-3-yl}methyl)-N-(2-methylpropyl)benzenesulfonamide 1 (BZN) is described. The presented strategy is based on the organocatalytic Michael addition of aldehyde 2 to trans-nitroalkene 3, and subsequent reductive cyclization. High yields, enantio-, and diastereoselectivities were achieved in the Michael addition by application of a POSS- or Wang resin-supported Hayashi–Jørgensen catalyst. 相似文献
948.
949.
Jakub Ostapko Dr. Krzysztof Nawara Michał Kijak Dr. Joanna Buczyńska Dr. Barbara Leśniewska Dr. Mariusz Pietrzak Grażyna Orzanowska Prof. Jacek Waluk 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(48):17311-17320
Among seven possible nitrogen‐in constitutional isomers of porphyrin only one, porphycene, has been obtained so far in the free, unsubstituted form. Herein, the synthesis of another isomer, parent hemiporphycene ( HPc ), and its thorough structural, spectral, photophysical, electrochemical, and theoretical characterization are reported. Most of the properties of HPc are intermediate between those of porphyrin and porphycene, as evidenced by the values of inner‐cavity dimensions, orbital‐energy splittings, absorption coefficients, magnetic circular dichroism parameters, NH‐stretching frequencies, fluorescence quantum yields, tautomerization rates, and redox potentials. The largest differences arise with respect to tautomerism, due to the low symmetry of HPc and inequivalence of the four nitrogen atoms that define the inner cavity. Two trans tautomers are observed, separated in energy by about 1 kcal mol?1. Tautomerization from the higher‐ to the lower‐energy form is detected in the lowest‐excited singlet state and occurs at a rate that is about four orders of magnitude lower than that observed for porphycene. Hemiporphycene is a very good model for the investigation of inequivalent intramolecular H‐bonds present in one molecule; two such bonds in HPc reveal unusual characteristics, and the bond strength results from the interplay between the N ??? N distance and the N?H?N angle. 相似文献
950.