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991.
The magneto-structural correlation between a Mn(ll) ion, coordinated in an octahedral environment, and two nitronyl nitroxide
radical ligands in trans- and cis-metal-radical complexes is investigated by the broken symmetry (BS) approach within density functional theory (DFT).
The dependences of coupling constants J on three structural parameters: (i) bond angle θ (Mn-O-N (nitroxide)); (ii) rotating angle ψ, defined by the nitronyl nitroxide radical plane rotating around the axial Mn-O (nitroxide); (iii) bond distance R (Mn-O (nitroxide)) are directly calculated. Our calculations showed that both trans- and cis-Mn(ll)-radical complexes behave a stronger antiferromagnetic interaction, consistent with experiments. In view of
molecular orbital theory, the direct exchanges, including σ-type and π-type exchanges, are responsible for the magnetic exchange
pathways. There is a preferable linear correlation between the calculated coupling constants J and the overlap integral squares S b between the local magnetic orbitals at the various rotating angle ψ at the fixed bond angle θ and bond distance R, in both trans- and cis-Mn(ll)-radical complexes. 相似文献
992.
Courtney?L.?Sherman Jennifer?S.?BrodbeltEmail author Alan?P.?Marchand Bhaskar?Poola 《Journal of the American Society for Mass Spectrometry》2005,16(7):1162-1171
The self-assembly of ligand-metal-ligand sandwich complexes involving a novel quinoxaline-containing crown ether, 1, was studied by electrospray ionization mass spectrometry (ESI-MS). Donor-acceptor π-stacking interactions between the electron-poor
quinoxaline group of 1 and electron-rich benzene groups from benzo- or dibenzo-18-crown-6 were found to significantly enhance the formation of mixed-ligand
sandwich complexes with a free energy gain of up to 17 kJ/mol relative to sandwich formation involving macrocycles without
such aromatic functionalities. The relative intensities of the sandwich complexes were greatest with the alkali metals Na+, K+, and Rb+ as well as with the ammonium ion in equimolar concentrations with the macrocycles. The preferential formation of the mixed-ligand
sandwich complexes demonstrated that donor-acceptor π-stacking interactions contribute to the assembly of molecular structures
and can be monitored by ESI-MS. 相似文献
993.
Low-energy CAD product-ion spectra of various molecular species of phosphatidylserine (PS) in the forms of [M−H]− and [M−2H+Alk]− in the negative-ion mode, as well as in the forms of [M+H]+, [M+Alk]+, [M−H+2Alk]+, and [M−2H+3Alk]+ (where Alk=Li, Na) in the positive-ion mode contain rich fragment ions that are applicable for structural determination.
Following CAD, the [M−H]− ion of PS undergoes dissociation to eliminate the serine moiety (loss of C3H5NO2) to give a [M−H−87]− ion, which equals to the [M−H]− ion of a phoshatidic acid (PA) and give rise to a MS3-spectrum that is identical to the MS2-spectrum of PA. The major fragmentation process for the [M−2H+Alk]− ion of PS arises from primary loss of 87 to give rise to a [M−2H+Alk−87]− ion, followed by loss of fatty acid substituents as acids (RxCO2H, x=1,2) or as alkali salts (e. g., RxCO2Li, x=1,2). These fragmentations result in a greater abundance of [M−2H+Alk−87−R2CO2H]− than [M−2H+Alk−87−R1CO2H]− and a greater abundance of [M−2H+Alk−87−R2CO2Li]− than [M−2H+Alk−87−R1CO2Li]−; while further dissociation of the [M−2H+Alk−87−R2(or 1)CO2Li]− ions gives a preferential formation of the carboxylate anion at sn-1 (R1CO2−) over that at sn-2 (R2CO2−). Other major fragmentation process arises from differential loss of the fatty acid substituents as ketenes (loss of Rx′CH=CO, x=1,2). This results in a more prominent [M−2H+Alk−R2′CH=CO]− ion than [M−2H+Alk−R1′CH=CO]− ion. Ions informative for structural characterization of PS are of low abundance in the MS2-spectra of both the [M+H]+ and the [M+Alk]+ ions, but are abundant in the MS3-spectra. The MS2-spectrum of the [M+Alk]+ ion contains a unique ion corresponding to internal loss of a phosphate group probably via the fragmentation processes involving
rearrangement steps. The [M−H+2Alk]+ ion of PS yields a major [M−H+2Alk−87]+ ion, which is equivalent to an alkali adduct ion of a monoalkali salt of PA and gives rise to a greater abundance of [M−H+2Alk−87−R1CO2H]+ than [M−H+2Alk−87−R2CO2H]+. Similarly, the [M−2H+3Alk]+ ion of PS also yields a prominent [M−2H+3Alk−87]+ ion, which undergoes consecutive dissociation processes that involve differential losses of the two fatty acyl substituents.
Because all of the above tandem mass spectra contain several sets of ion pairs involving differential losses of the fatty
acid substituents as ketenes or as free fatty acids, the identities of the fatty acyl substituents and their positions on
the glycerol backbone can be easily assigned by the drastic differences in the abundances of the ions in each pair. 相似文献
994.
Klaus Burger Gabor Radics Lothar Hennig Christoph B?ttcher Jan Spengler Fernando Albericio 《Monatshefte für Chemie / Chemical Monthly》2005,5(3):763-776
Starting from L-malic, L-citramalic, and rac. thiomalic acids routes to L-homoisoserine, 2-methyl-L-homoisoserine and rac. homoisocysteine have been developed. The new orthogonally protected and carboxy-activated building blocks are GABA as well as -hydroxy and -mercapto acid derivatives, suitable for the construction of peptide and depsipeptide surrogates. 相似文献
995.
996.
Renata Toczy?owska 《Analytica chimica acta》2005,540(1):167-172
Back-side contact Au and Ag microelectrodes were used as transducers to construct planar all-solid-state electrodes suitable for flow-through analysis. The microsensors were based on plasticized PVC potassium-selective membranes containing ion-electron conducting polymer—polypyrrole doped with di(2-ethylhexyl) sulfosuccinate. The proposed technique allowed simple construction of microsensors in one step, by membrane solution casting directly on the surface of the planar metallic transducers. The performance of the microsensors based on Au and Ag transducers were determined and compared with planar sensors based on internal electrolyte immobilized in polyHEMA. The addition of the polypyrrole to the membrane composition did not influence on the selectivity, reproducibility and long-term stability of the microsensors but improved their standard potential stability in time in comparison with coated-wire type sensors. Moreover, all-solid-state microsensors based on Au transducers exhibited better signal stability than Ag based sensors. 相似文献
997.
998.
Kameo S Nakai K Kurokawa N Kanehisa T Naganuma A Satoh H 《Analytical and bioanalytical chemistry》2005,381(8):1514-1519
Mercury vapor is effectively absorbed via inhalation and easily passes through the blood–brain barrier; therefore, mercury poisoning with primarily central nervous system symptoms occurs. Metallothionein (MT) is a cysteine-rich metal-binding protein and plays a protective role in heavy-metal poisoning and it is associated with the metabolism of trace elements. Two MT isoforms, MT-I and MT-II, are expressed coordinately in all mammalian tissues, whereas MT-III is a brain-specific member of the MT family. MT-III binds zinc and copper physiologically and is seemed to have important neurophysiological and neuromodulatory functions. The MT functions and metal components of MTs in the brain after mercury vapor exposure are of much interest; however, until now they have not been fully examined. In this study, the influences of the lack of MT-I and MT-II on mercury accumulation in the brain and the changes of zinc and copper concentrations and metal components of MTs were examined after mercury vapor exposure by using MT-I, II null mice and 129/Sv (wild-type) mice as experimental animals. MT-I, II null mice and wild-type mice were exposed to mercury vapor or an air stream for 2 h and were killed 24 h later. The brain was dissected into the cerebral cortex, the cerebellum, and the hippocampus. The concentrations of mercury in each brain section were determined by cold vapor atomic absorption spectrometry. The concentrations of mercury, copper, and zinc in each brain section were determined by inductively coupled plasma mass spectrometry (ICP-MS). The mercury accumulated in brains after mercury vapor exposure for MT-I, II null mice and wild-type mice. The mercury levels of MT-I, II null mice in each brain section were significantly higher than those of wild-type mice after mercury vapor exposure. A significant change of zinc concentrations with the following mercury vapor exposure for MT-I, II null mice was observed only in the cerebellum analyzed by two-way analysis of variance. As for zinc, the copper concentrations only changed significantly in the cerebellum. Metal components of metal-binding proteins of soluble fractions in the brain sections were analyzed by size-exclusion high-performance liquid chromatography (HPLC) connected with ICP-MS. From the results of HPLC/ICP-MS analyses, it was concluded that the mercury components of MT-III and high molecular weight metal-binding proteins in the cerebellum of MT-I, II null mice were much higher than those of wild-type mice. It was suggested that MT-III is associated with the storage of mercury in conditions lacking MT-I, and MT-II. It was also suggested that the physiological role of MT-III and some kind of high molecular weight proteins might be impaired by exposure to mercury vapor and lack of MT-I and MT-II. 相似文献
999.
This report describes a new type of intra-intermolecular criss-cross cycloadditions. Thermal reactions of unsymmetrical allenylazines in the presence of alkynes led to three fused five-membered heterocycles in some cases. In the case of unsymmetrical substituted alkynes, a regioselectivity was observed. The molecular structures of all products are discussed. One X-ray crystal structure is also reported. 相似文献
1000.
Many aspects of the behavior of surfactants have not been well understood due to the coupling of many different mechanisms.
Computer simulation is, therefore, attractive in the sense that it can explore the effect of different mechanisms separately.
In this paper, the shapes, structures and sizes of sodium dodecylbenzenesulfonate (SDBS) micelles under different concentrations
in an oil/water mixture were studied via molecular dynamics (MD) simulations using a simplified atomistic model which basically
maintains the hydrophile and lipophile properties of the surfactant molecules. Above the critical micellar concentration (cmc),
surfactant molecules aggregate spontaneously to form a wide variety of assemblies, from spherical to rodlike, wormlike and
bilayer micelles. Changes in their ratios of the principle moments of inertia (g1/g3, g2/g3) indicated the transition of micelle shapes at different concentrations. The aggregation number of micelle is found to have
a power-law dependence on surfactant concentration. 相似文献