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81.
The naturally occurring butanolides (?)-blastmycinolactol, (+)-blastmycinone, (?)-NFX-2, (+)-antimycinone as well as the four stereoisomers of the butenolide Acaterin were prepared in high enantiomeric purity using hydroxy-vinyl tellurides as starting materials. 相似文献
82.
Renan Cariou Atanas K. Tomov Andrew J.P. White 《Journal of organometallic chemistry》2009,694(5):703-265
Group 4 metal complexes bearing new phenoxy(benzimidazolyl)-imine, -amine and -amide ligands have been synthesized. A series of metal chloride derivatives has been prepared via treatment of MCl4(THF)2 (M = Ti, Zr, Hf) with the in situ generated sodium salt of the (benzimidazolyl)imine phenol 1. Reaction of the pro-ligand 2 with TiCl4(THF)2 afforded the corresponding complex 8 in which the amine proton remains bound to the nitrogen donor. Benzyl complexes of zirconium and hafnium were synthesized via treatment of pro-ligands 1 and 2 with M(CH2Ph)4 precursors. The complexes [NNO]M(CH2Ph)3 (6 M = Zr, 7 M = Hf) were found to undergo benzyl migration from the metal centre to the imine carbon of the ligand backbone giving complexes 11 and 12; the migration follows first order kinetics. The reaction of 1 with Ti(NMe2)4 led to the formation of an unusual C-C coupled product in which a new piperazine ring has formed. Complexes 11 and 12 undergo related transformations, leading to analogous C-C coupled products which were characterized by X-ray crystallography. Deuterium labelling experiments were carried out to determine the mechanistic pathway of the reactions. Chloride and benzyl complexes 3-12 were screened as pre-catalysts for olefin polymerization. 相似文献
83.
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85.
Vecherin SN Ostashev VE Ziemann A Wilson DK Arnold K Barth M 《The Journal of the Acoustical Society of America》2007,122(3):1416
Acoustic travel-time tomography allows one to reconstruct temperature and wind velocity fields in the atmosphere. In a recently published paper [S. Vecherin et al., J. Acoust. Soc. Am. 119, 2579 (2006)], a time-dependent stochastic inversion (TDSI) was developed for the reconstruction of these fields from travel times of sound propagation between sources and receivers in a tomography array. TDSI accounts for the correlation of temperature and wind velocity fluctuations both in space and time and therefore yields more accurate reconstruction of these fields in comparison with algebraic techniques and regular stochastic inversion. To use TDSI, one needs to estimate spatial-temporal covariance functions of temperature and wind velocity fluctuations. In this paper, these spatial-temporal covariance functions are derived for locally frozen turbulence which is a more general concept than a widely used hypothesis of frozen turbulence. The developed theory is applied to reconstruction of temperature and wind velocity fields in the acoustic tomography experiment carried out by University of Leipzig, Germany. The reconstructed temperature and velocity fields are presented and errors in reconstruction of these fields are studied. 相似文献
86.
A CEC monolithic column with strong cation-exchange (SCX) stationary phase based on hydrophilic monomers was prepared by in situ polymerization of acrylamide, methylenebisacrylamide, and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) in a complete organic binary porogenic solvent consisting of DMSO and dodecanol. The sulfonic groups provided by the monomer AMPS on the surface of the stationary phase generate an EOF from anode to cathode, and serve as an SCX stationary phase at the same time. The monolithic stationary phase exhibited normal-phase chromatographic behavior for neutral analytes. For charged analytes, electrostatic interaction/repulsion with the monolith was observed. The strong SCX monolithic column has been successfully employed in the electrochromatographic separation of basic drugs, peptides, and alkaloids extracted from natural products. 相似文献
87.
João Vitor Soares Guilherme Dal Poggetto Renan V. Viesser Uenifer R. Couto Cláudio F. Tormena 《Magnetic resonance in chemistry : MRC》2022,60(5):481-488
Long-range proton-fluorine coupling constants (nJHF) are helpful for the structure elucidation of fluorinated molecules. However, their magnitude and sign can change with the relative position of coupled nuclei and the presence of substituents. Here, trans-4-tert-butyl-2-fluorocyclohexanone was used as a model compound for the study of the transmission of 4JHF. In this compound, the 4JH6axF was measured to be +5.1 Hz, which is five times larger than the remaining 4JHF in the same molecule (4JH4F = +1.0 Hz and 4JH6eqF = +1.0 Hz). Through a combination of experimental data, natural bond orbital (NBO) and natural J-coupling (NJC) analyses, we observed that stereoelectronic interactions involving the π system of the carbonyl group are involved in the transmission pathway for the 4JH6axF. Interactions containing the π system as an electron acceptor (e.g., σC6 H6ax → π*C═O and σC F → π*C═O) increase the value of the 4JH6axF, while the interaction of the π system as an electron donor (e.g., πC═O → σ*C F) decreases it. Additionally, the carbonyl group was shown not to be part of the transmission pathway of the diequatorial 4JH6eqF coupling in cis-4-tert-butyl-2-fluorocyclohexanone, revealing that there is a crucial symmetry requirement that must be fulfilled for the π system to influence the value of the 4JHF in these systems. 相似文献
88.
The trifluoromethyl anion (CF3−) displays 13C NMR chemical shift (175.0 ppm) surprisingly larger than neutral (CHF3, 122.2 ppm) and cation (CF3+, 150.7 ppm) compounds. This unexpected deshielding effect for a carbanion is investigated by density functional theory calculations and decomposition analyses of the 13C shielding tensor into localized molecular orbital contributions. The present work determines the shielding mechanisms involved in the observed behaviour of the fluorinated anion species, shedding light on the experimental NMR data and demystify the classical correlation between electron density and NMR chemical shift. The presence of fluorine atoms induces the carbon lone pair to create a paramagnetic shielding on the carbon nucleus. 相似文献
89.
Prof. Dr. Renan B. Campos Valmir B. Silva Leociley R. A. Menezes Fernanda M. M. Ocampos Juliano M. Fernandes Prof. Dr. Andersson Barison Prof. Dr. Alfredo R. M. Oliveira Prof. Dr. Dean J. Tantillo Prof. Dr. Elisa S. Orth 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(22):5017-5026
The harmful impact caused by pesticides on human health and the environment necessitates the development of efficient degradation processes and control of prohibited stocks of such substances. Organophosphates (OPs) are among the most used agrochemicals in the world and their degradation can proceed through several possible pathways. Investigating the reactivity of OPs with nucleophilic species allows one to propose new and efficient catalyst scaffolds for use in detoxification. In light of the remarkable catalytic activity of imidazole (IMZ) at promoting dephosphorylation processes of OPs, the reactivity of 4(5)-hydroxymethylimidazole (HMZ) with diethyl-2,4-dinitrophenylphosphate (DEDNPP) and Paraoxon are evaluated by combining experimental and theoretical approaches. It is observed that HMZ is an efficient and regiospecific catalyst with reactivity modulated by competing tautomers. To propose an optimal IMZ-based catalyst, quantum chemical calculations were performed for monosubstituted 4(5)IMZ derivatives that might cleave DEDNPP. Both inductive effects and hydrogen bonding by the substituents are shown to influence barriers and mechanisms. 相似文献
90.
Marisa C. Oliveira Renan A. P. Ribeiro Elson Longo Maurício R. D. Bomio Fabiana V. Motta Sergio R. de Lazaro 《International journal of quantum chemistry》2020,120(1):e26054
Identifying the forces that drive a temperature-induced phase transition is always challenging in the prospect of the first-principles methods. Herein, we perform a first-principles study of the temperature effects on structural, energetic, electronic, and vibrational properties of four BaTiO3 polymorphs using quasi-harmonic approximations. Study of the stability between these four phases, which we break into contributions arising from the vibration of the lattice, electronic structure, and volume expansion/contraction, is helpful to confirm the sequence of phase transitions as cubic → tetragonal → orthorhombic → rhombohedral, as well as its transition temperatures. A general mechanism was proposed based on the combination between structural distortions at [TiO6] clusters, vibrational characteristics, and electronic structure. These findings confirm the power of quasi-harmonic approximations to disclose the main fingerprints associated with both thermic and mechanical phase transitions, serving as a guide for further theoretical studies. 相似文献