全文获取类型
收费全文 | 986篇 |
免费 | 80篇 |
国内免费 | 3篇 |
专业分类
化学 | 907篇 |
力学 | 5篇 |
数学 | 56篇 |
物理学 | 101篇 |
出版年
2024年 | 1篇 |
2023年 | 14篇 |
2022年 | 45篇 |
2021年 | 46篇 |
2020年 | 40篇 |
2019年 | 46篇 |
2018年 | 21篇 |
2017年 | 19篇 |
2016年 | 54篇 |
2015年 | 56篇 |
2014年 | 39篇 |
2013年 | 61篇 |
2012年 | 92篇 |
2011年 | 111篇 |
2010年 | 67篇 |
2009年 | 29篇 |
2008年 | 60篇 |
2007年 | 74篇 |
2006年 | 51篇 |
2005年 | 49篇 |
2004年 | 26篇 |
2003年 | 31篇 |
2002年 | 20篇 |
2001年 | 10篇 |
1999年 | 5篇 |
1996年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有1069条查询结果,搜索用时 15 毫秒
971.
Dr. Stefano Corra Dr. Lorenzo Casimiro Prof. Massimo Baroncini Dr. Jessica Groppi Dr. Marcello La Rosa Dr. Marina Tranfić Bakić Prof. Serena Silvi Prof. Alberto Credi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(43):11076-11083
The development of artificial nanoscale motors that can use energy from a source to perform tasks requires systems capable of performing directionally controlled molecular movements and operating away from chemical equilibrium. Here, the design, synthesis and properties of pseudorotaxanes are described, in which a photon input triggers the unidirectional motion of a macrocyclic ring with respect to a non-symmetric molecular axle. The photoinduced energy ratcheting at the basis of the pumping mechanism is validated by measuring the relevant thermodynamic and kinetic parameters. Owing to the photochemical behavior of the azobenzene moiety embedded in the axle, the pump can repeat its operation cycle autonomously under continuous illumination. NMR spectroscopy was used to observe the dissipative non-equilibrium state generated in situ by light irradiation. We also show that fine changes in the axle structure lead to an improvement in the performance of the motor. Such results highlight the modularity and versatility of this minimalist pump design, which provides facile access to dynamic systems that operate under photoinduced non-equilibrium regimes. 相似文献
972.
Jessica Scremin Gabriel J. Mattos Robert D. Crapnell Samuel J. Rowley-Neale Craig E. Banks Elen R. Sartori 《Electroanalysis》2021,33(2):526-536
An electrochemical sensor using glassy carbon electrode modified with carbon black within a poly(allylamine hydrochloride) film is proposed in this work. The novel sensor was characterized by scanning electron microscopy, electrochemical impedance spectroscopy, and cyclic voltammetry using the redox probe Fe(CN)63−/4−. The sensor was applied for the simultaneous determination of dopamine (DA), paracetamol (PAR), amlodipine (AML), and rosuvastatin (RSV). The quantification of all four analytes was carried out by linear sweep voltammetry and presented a linear concentration range for all analytes from 1.0 to 90 μmol L−1, with limit of detection of 0.55, 1.3, 5.7, and 3.0 μmol L−1 for DA, PAR, AML, and RSV, respectively. This sensor was successfully applied in the simultaneous determination of these analytes in environmental, pharmaceutical, and biological samples. 相似文献
973.
Parr JA Li G Fedorov I McCaffery AJ Reisler H 《The journal of physical chemistry. A》2007,111(31):7589-7598
The state-to-state vibrational predissociation (VP) dynamics of the hydrogen-bonded ammonia-acetylene dimer were studied following excitation in the asymmetric CH stretch. Velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the asymmetric CH stretch fundamental, ammonia fragments were detected by 2 + 1 REMPI via the B1E' <-- X1A1' and C'1A1' <-- X1A1' transitions. The fragments' center-of-mass (c.m.) translational energy distributions were determined from images of selected rotational levels of ammonia with one or two quanta in the symmetric bend (nu2 umbrella mode) and were converted to rotational-state distributions of the acetylene co-fragment. The latter is always generated with one or two quanta of bending excitation. All the distributions could be fit well when using a dimer dissociation energy of D0 = 900 +/- 10 cm(-1). Only channels with maximum translational energy <150 cm(-1) are observed. The rotational excitation in the ammonia fragments is modest and can be fit by temperatures of 150 +/- 50 and 50 +/- 20 K for 1nu2 and 2nu2, respectively. The rotational distributions in the acetylene co-fragment pair-correlated with specific rovibrational states of ammonia appear statistical as well. The vibrational-state distributions, however, show distinct state specificity among channels with low translational energy release. The predominant channel is NH3(1nu2) + C2H2(2nu4 or 1nu4 + 1nu5), where nu4 and nu5 are the trans- and cis-bend vibrations of acetylene, respectively. A second observed channel, with much lower population, is NH3(2nu2) + C2H2(1nu4). No products are generated in which the ammonia is in the vibrational ground state or the asymmetric bend (1nu4) state, nor is acetylene ever generated in the ground vibrational state or with CC stretch excitation. The angular momentum (AM) model of McCaffery and Marsh is used to estimate impact parameters in the internal collisions that give rise to the observed rotational distributions. These calculations show that dissociation takes place from bent geometries, which can also explain the propensity to excite fragment bending levels. The low recoil velocities associated with the observed channels facilitate energy exchange in the exit channel, which results in statistical-like fragment rotational distributions. 相似文献
974.
975.
976.
977.
Jessica Cano Alberto Rodriguez Emilio Aicart Elena Junquera 《Journal of inclusion phenomena and macrocyclic chemistry》2007,59(3-4):279-285
The molecular encapsulation of two tricyclic antidepressants (TCA) drugs, amitriptyline and imipramine, by a glycosidic receptor,
6-hydroxypropyl-β-cyclodextrin (HPBCD), has been carried out in water solution by means of conductometric studies at different
temperatures ranging from 15 °C to 45 °C. Conductivity measurements of aqueous solutions of the drug were performed: (i) in
the absence of HPBCD, as a function of drug concentration; and (ii) in the presence of HPBCD, as a function of HPBCD concentration.
Both drugs, amitriptyline and imipramine, form inclusion complexes characterized by a 1:1 stoichiometry and an association
constant () in the range of 500–1200 M−1. The ionic molar conductivities at infinite dilution of the free () and complexed () drugs have been calculated from these conductivity data as well. From the dependency of the association constant on temperature,
changes on the enthalpy, ΔH
0, entropy, ΔS
0, and heat capacity at constant pressure, , have been determined. This thermodynamic information, which reveals that the complexes formed by HPBCD and the antidepressant
drugs, AMYTPH+ and IPRH+, are enthalpy driven at T ≥ 25 °C but entropy driven at T < 25 °C, points to the contribution of van der Waals interactions, hydrophobic effect and solvent reorganization, as the
main driven forces promoting the interaction. The analysis of these association processes was also used to elucidate the potential
viability of using HPBCD as a vector of these antidepressant drugs. 相似文献
978.
Prema D Wiznycia AV Scott BM Hilborn J Desper J Levy CJ 《Dalton transactions (Cambridge, England : 2003)》2007,(42):4788-4796
The synthesis of two 2-formylquinolines is reported via the Skraup method followed by SeO(2) oxidation. Each aldehyde is condensed with (1R,2R)-diaminocyclohexane and (R)-BINAM, yielding four enantiomerically-pure bis(imine-quinoline) ligands. The neutral ligands are reacted with ZnCl(2) to give complexes with bis(bidentate) coordination of ZnCl(2) units. X-Ray structural characterization of three complexes shows them to have a single-stranded helical motif, with M helicity, except in one case where a 1:1 mixture of M and P helices is seen. The ligands and complexes are further characterized spectroscopically by solution (1)H and (13)C NMR, UV-vis and ECD. 相似文献
979.
The synthesis of macrocyclic two-armed diketopiperazine receptors and their binding properties toward peptides is described. Macrocyclization with short linkers led to receptors with significantly modified binding properties compared to their flexible open chain parent receptors, whereas with long linkers the original binding selectivities were largely retained. 相似文献
980.
[reaction: see text] The first synthesis of the pyrrolidinone core of the polyene beta-lactone antibiotic KSM-2690 B is described. An ammonia-free Birch reductive aldol reaction utilizing acetaldehyde is one of the key steps, together with a ruthenium-catalyzed alkene isomerization reaction. 相似文献