首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   220篇
  免费   5篇
化学   158篇
晶体学   2篇
力学   4篇
数学   8篇
物理学   53篇
  2022年   1篇
  2021年   2篇
  2020年   1篇
  2019年   1篇
  2018年   4篇
  2017年   5篇
  2016年   2篇
  2015年   5篇
  2014年   8篇
  2013年   8篇
  2012年   11篇
  2011年   12篇
  2010年   8篇
  2009年   2篇
  2008年   11篇
  2007年   15篇
  2006年   15篇
  2005年   7篇
  2004年   6篇
  2003年   10篇
  2002年   8篇
  2001年   9篇
  2000年   10篇
  1998年   5篇
  1997年   3篇
  1996年   3篇
  1995年   2篇
  1994年   8篇
  1993年   7篇
  1992年   1篇
  1991年   4篇
  1990年   1篇
  1989年   2篇
  1988年   1篇
  1987年   1篇
  1986年   1篇
  1985年   5篇
  1982年   1篇
  1981年   2篇
  1979年   2篇
  1976年   1篇
  1975年   3篇
  1974年   4篇
  1973年   2篇
  1972年   1篇
  1971年   2篇
  1967年   1篇
  1966年   1篇
排序方式: 共有225条查询结果,搜索用时 0 毫秒
31.
Ellison DK  Tait W 《Talanta》1988,35(9):734-736
Osteryoung square-wave voltammetric analysis is a rapid technique that can be used to determine nicarbazin residues in chicken tissue. The method is reproducible and can be applied to samples containing as little as 1-10 mug/g nicarbazin. This level of sensitivity is more than adequate for meeting governmental regulations. Osteryoung square-wave voltammetry is significantly faster than classical differential pulse polarography.  相似文献   
32.
Photodetachment cross sections of trapped electrons are expressed analytically by a one-parameter model with an initial 1s bound state and a final plane-wave state. The theoretical cross-section curve, location of the peak maximum, and band width are discussed. Calculated photoabsorption profiles compare fairly well with experimental data. Comment is made concerning the ω?8/3 photon-energy dependence for the bound-continuum band shape previously proposed in resolution of the absorption spectrum of trapped electrons into bound-bound and bound-continuum bands.  相似文献   
33.
The mini-computer and control electronics of a Bruker 113v Fourier Transform Spectrometer (FTS), capable of 0.03 cm–1 resolution for frequencies up to 4000cm–1, have been replaced by an inexpensive IBM-PC 486-based system. The original performance is exceeded. JCAMP-DX standard is used for data exchange. The system can be adapted to any FTS or similar instrument which requires time domain data acquisition.  相似文献   
34.
The fractional Stokes–Einstein relation postulates a direct relationship between conductivity and shear flow. Like viscosity, the electrical resistivity of a glass-forming liquid exhibits a non-Arrhenius scaling with temperature. However, while both viscosity and resistivity are non-Arrhenius, here we show that these two properties follow distinct functional forms. Through analysis of 821 unique silicate liquids, we show that viscosity is best represented using the Mauro–Yue–Ellison–Gupta–Allan (MYEGA) model, whereas the resistivity of the same compositions more closely follows the Avramov–Milchev (AM) equation. Our results point to two fundamentally different mechanisms governing viscous flow and conductivity and therefore cast doubt on the general validity of the fractional Stokes–Einstein relation.  相似文献   
35.
Fluorescence methodologies have been utilized to examine micropolarity, intramolecular motion, and singlet quenching in the intraparticle void volume of zeolites X, Y, and ultrastable Y (USY) interfaced with bathing polar solvents. Micropolarity was assessed from the 3-to-1 band ratio (III/I) of the fluorescence spectrum of pyrene (PY) and from lambda(max) of the fluorescence spectrum of 1-pyrenecarboxaldehyde (1-PCA). In zeolites bathed in anhydrous solvents, both PY and 1-PCA reported increased micropolarity according to the trend USY < bulk solvent < NaX approximately NaY. For example, in NaY (USY), III/I ranged from 0.44 (0.98) in acetonitrile to 0.52 (1.34) in n-hexanol, compared to 0.60, 1.06, and 1.62 in bulk acetonitrile (ACN), n-hexanol, and n-hexane, respectively. The polarity studies reveal that the ionic nature of NaX and NaY and the hydrophobic nature of USY strongly influence the microenvironment of the arene despite the presence of desorbing polar solvents. Constraints on intramolecular motion were examined in polar-solvated NaX through measurements of the fluorescence lifetime of trans-stilbene. Lifetimes ranged from 113 ps in NaX-ACN to 671 ps in NaX-tert-butyl alcohol. The latter value is close to that observed in bulk glycerol. Diffusion-controlled quenching of PY fluorescence by O2 and a series of nitrocompounds dissolved in solvents bathing the zeolite was examined by a time-resolved approach. For all of the quenchers and solvents studied, quenching was more efficient in USY compared to NaX and NaY. Interestingly, the rate of O2 quenching in USY-MeOH was only 12 times lower than that in bulk MeOH. In contrast, in NaY-MeOH and NaX-MeOH the rate of O2 quenching was too low to be measured. The rate constants in these systems were therefore taken as the rate constant for diffusion-controlled quenching of trapped electrons measured previously. These values were 600 times and 10(5) times lower than the rate of fluorescence quenching in USY-MeOH, respectively. The O2 quenching studies show that dispersive interactions of polar solvents with the cavity walls dominate in USY because of the hydrophobic nature of the USY surface. In NaX and NaY, stronger ion-dipole and hydrogen bonding interactions dominate and lead to more restricted access and lowered quenching efficiency. Perrin (or static) quenching of pyrene fluorescence was also examined to infer the concentration of nitromethane (NM) in the void volume of NaX and NaY bathed in MeOH, ACN, or H2O. The results indicate that access of NM to the interior of NaY is more inhibited in ACN compared to MeOH, presumably because of the higher dipole moment of ACN and its resulting stronger association with the zeolite surface. At similar levels of static quenching equated to a similar NM concentration in the zeolite, dynamic quenching by NM varied by no more than a factor of 2 in all systems compared. This implies that the rate of NM diffusion in solvated zeolite interiors is similar regardless of zeolite or solvent properties. In contrast to O2 diffusion in zeolites, NM exhibits a high dipole moment and can therefore migrate through polar-solvated zeolite apertures by adsorbing to the zeolite. Overall, the results of this study show a close relationship between the behavior of probes and quenchers in the confines of polar-solvated zeolite interiors and the chemical properties of the zeolite. Differences between weakly and strongly interacting surfaces are revealed clearly in the results.  相似文献   
36.
The planarity of the second stable conformer of 1,3‐butadiene, the archetypal diene for the Diels–Alder reaction in which a planar conjugated diene and a dienophile combine to form a ring, is not established. The most recent high level calculations predicted the species to adopt a twisted, gauche structure owing to steric interactions between the inner terminal hydrogens rather than a planar, cis structure favored by the conjugation of the double bonds. The structure cis‐1,3‐butadiene is unambiguously confirmed experimentally to indeed be gauche with a substantial dihedral angle of 34°, in excellent agreement with theory. Observation of two tunneling components indicates that the molecule undergoes facile interconversion between two equivalent enantiomeric forms. Comparison of experimentally determined structures for gauche‐ and trans‐butadiene provides an opportunity to examine the effects of conjugation and steric interactions.  相似文献   
37.
We have studied the oxidation of self-assembled monolayers (SAMs) of alkanes and alkenes with a thermal beam of OH radicals. The target films were produced by bonding alkane thiols and alkene thiols to a gold surface and the SAMs are mounted in a vacuum chamber at a base pressure of 10-9 Torr. Hydroxyl radicals were produced by a corona discharge in an Ar/H2O2/water mixture. The resultant molecular beam was scanned by an electrostatic hexapole and the OH radicals [4 (+/- 1) x 1011 OH radicals cm-2 sec-1] were focused onto the target SAM. All of the hydroxyl radicals impinging on the SAM surface are rotationally (J' ' 相似文献   
38.
The amphiphilic gadolinium complex MS-325 ((trisodium-{(2-(R)-[(4,4-diphenylcyclohexyl) phosphonooxymethyl] diethylenetriaminepentaacetato) (aquo)gadolinium(III)}) is a contrast agent for magnetic resonance angiography (MRA). MS-325 consists of two slowly interconverting diastereoisomers, A and B (65:35 ratio), which can be isolated at pH > 8.5 (TyeklAr, Z.; Dunham, S. U.; Midelfort, K.; Scott, D. M.; Sajiki, H.; Ong, K.; Lauffer, R. B.; Caravan, P.; McMurry, T. J. Inorg. Chem. 2007, 46, 6621-6631). MS-325 binds to human serum albumin (HSA) in plasma resulting in an extended plasma half-life, retention of the agent within the blood compartment, and an increased relaxation rate of water protons in plasma. Under physiological conditions (37 degrees C, pH 7.4, phosphate buffered saline (PBS), 4.5% HSA, 0.05 mM complex), there is no statistical difference in HSA affinity or relaxivity between the two isomers (A 88.6 +/- 0.6% bound, r1 = 42.0 +/- 1.0 mM(-1) s(-1) at 20 MHz; B 90.2 +/- 0.6% bound, r1 = 38.3 +/- 1.0 mM(-1) s(-1) at 20 MHz; errors represent 1 standard deviation). At lower temperatures, isomer A has a higher relaxivity than isomer B. The water exchange rates in the absence of HSA at 298 K, kA298 = 5.9 +/- 2.8 x 10(6) s(-1), kB298 = 3.2 +/- 1.8 x 10(6) s(-1), and heats of activation, DeltaHA = 56 +/- 8 kJ/mol, DeltaHB = 59 +/- 11 kJ/mol, were determined by variable-temperature 17O NMR at 7.05 T. Proton nuclear magnetic relaxation dispersion (NMRD) profiles were recorded over the frequency range of 0.01-50 MHz at 5, 15, 25, and 35 degrees C in a 4.5% HSA in PBS solution for each isomer (0.1 mM). Differences in the relaxivity in HSA between the two isomers could be attributed to the differing water exchange rates.  相似文献   
39.
40.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号