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911.
The kinetics of the aminolysis of aryl thiocarbamates [ATC: H2NC(=O)SC6H4Z] with benzylamines (XC6H4CH2NH2) in acetonitrile at 10.0 degrees C have been studied. The rate order with variation of the non-leaving amino group, RNH, in RNHC(=O)SC6H4Z is NH2 < PhNH < EtNH indicating that the polar (sigma*) and steric (E(s)) effects of the RNH group are insignificant, and the strength of push to expel the leaving group in the tetrahedral transition state is the sole, important effect. The strong push provided by the NH2 group, the negative rhoXZ(-0.38) value, the size of betaZ(-0.54), and failure of the reactivity-selectivity principle are all consistent with the concerted mechanism. The kinetic isotope effects involving deuterated amine nucleophiles (XC6H4CH2ND2) are normal (k(H)/k(D)approximately 1.40-1.73) suggesting a hydrogen-bonded cyclic transition state. 相似文献
912.
Yamashita DS Dong X Oh HJ Brook CS Tomaszek TA Szewczuk L Tew DG Veber DF 《Journal of combinatorial chemistry》1999,1(3):207-215
To more rapidly prepare members of the 1,3-bis(acylamino)-2-butanone class of cysteine protease inhibitors, a solid-phase synthesis was developed. 1-Azido-3-amino-2,2-dimethoxybutane (4), which has the two amino groups differentiated and the ketone protected as a a ketal, served as a surrogate for the 1,3-diamino-2-butanone core. Amine (4) was coupled to the BAL-resin-linked carboxylic acids derived from alpha-amino acid esters. Evaluation of a small combinatorial array by measuring inhibition constants (Ki,appS) against cathepsins K, L, and B provided some structure-activity relationship trends with respect to selectivity and potency. Novel, potent inhibitors of cathepsins K and L were identified. 相似文献
913.
Y.G. Shul K.S. Oh J.C. Yang K.T. Jung 《Journal of Sol-Gel Science and Technology》1997,8(1-3):255-259
Low temperature phase transformation of alkoxide-derived titania was studied under an oxygen flow containing water vapor.
The addition of hexylene glycol to the titanium alkoxide solution was effective in transforming rutile phase at low temperature
(300°C). Hexylene glycol seems to form a bidentate ligand structure with TEOT in solution state. This bidentate structure
promoted the rutile phase at low temperature, under O2-steam treatment. This modification of TiO2 gel structure could be monitored by FT-IR and XANES. 相似文献
914.
Kwang Seok Oh Byung Gwon Lee Man Seok Han Yong Gun Shul 《Reaction Kinetics and Catalysis Letters》2002,77(1):51-58
The effect of calcination conditions on MoO3/SiO2 catalysts for the synthesis of methylphenyl carbonate was investigated in terms of catalytic activities and surface properties. The calcination temperature was varied in the range of 300 to 800oC. These calcination conditions have shown a close relationship with the catalyst activities. The optimal calcination temperature of MoO3/SiO2was found to be around 550-600oC. The catalysts were characterized by XRD, SEM, FT-IR and XPS analysis. 相似文献
915.
Yoon UC Kwon HC Hyung TG Choi KH Oh SW Yang S Zhao Z Mariano PS 《Journal of the American Chemical Society》2004,126(4):1110-1124
The results of studies designed to obtain information about the factors that control the chemical efficiencies/regioselectivities and quantum yields of single electron transfer (SET)-promoted reactions of acceptor-polydonor systems are reported. Photochemical and photophysical investigations were carried out with bis-donor tethered phthalimides and naphthalimides of general structure N-phthalimido- and N-naphthalimido-CH2CH2-D-CH2CH2-NMsCH2-E (E = SiMe3 or CO2NBu4 and D = NMs, O, S, and NMe). These substrates contain common terminal donor groups (NMsCH2SiMe3 or NMsCH2CO2NBu4) that have known oxidation potentials and cation radical fragmentation rates. Oxidation potentials and fragmentation rates at the other donor site in each of these substrates are varied by incorporating different heteroatoms and/or substituents. Photoproduct distribution, reaction quantum yield, and fluorescence quantum yield measurements were made. The results show that photocyclization reactions of alpha-trimethylsilylmethansulfonamide (E = SiMe3)- and alpha-carboxymethansulfonamide (E = CO2NBu4)-terminated phthalimides and naphthalimides that contain internal sulfonamide, ether, and thioether donor sites (D = NMs, O, or S) are chemically efficient (80-100%) and that they take place exclusively by a pathway involving sequential photoinduced SET (zwitterionic biradical desilylation or decarboxylation) biradical cyclization. In contrast, photoreactions of alpha-trimethylsilylmethansulfonamide- and alpha-carboxymethansulfonamide-terminated phthalimides and naphthalimides that that contain an internal tertiary amine donor site (D = NMe) are chemically inefficient and follow a pathway involving alpha-deprotonation at the tertiary amine radical cation center in intermediate, iminium radical-containing, zwitterionic biradicals. In addition, the quantum efficiencies for photoreactions of alpha-trimethylsilylmethansulfonamide- and alpha-carboxymethansulfonamide-terminated phthalimides are dependent on the nature of the internal donor (eg., phi = 0.12 for D = NMs, E = SiMe3; phi = 0.02 for D = S, E = SiMe3; phi = 0.04 for D = NMe, E = SiMe3). The results of this effort are discussed in terms of how the relative energies of interconverting zwitterionic biradical intermediates and the energy barriers for their alpha-heterolytic fragmentation reactions influence the chemical yields and quantum efficiencies of SET promoted photocyclization reactions of acceptor-polydonor substrates. 相似文献
916.
[reaction: see text] Addition of diverse nucleophiles to the unsaturated lactone 2 that results from hetero Diels-Alder reaction of Brassard's diene 1 with aldehydes leads to an efficient and general approach to the synthesis of 2,3-dihydro-4H-pyran-4-ones 3. 相似文献
917.
The preparation of a series of neutral ligands containing ether and amide groups is described. These ligands as well as related ones bearing other diamide groups are shown to selectively chelate Group II A cations by picrate extraction from water to methylene dichloride. This result was also confirmed by atomic absorption measurement. The changes in UV absorption of aromatic rings and amide groups in the ligands upon titration with metal salts in methanol allow the estimation of the ordering of cation binding.
Neutrale Diamid-Ionophore — Phenylendioxyacetamide
Zusammenfassung Die Darstellung einer Reihe von neutralen Liganden mit Ether- und Amidgruppen wird beschrieben. Sowohl diese Liganden — als auch verwandte mit anderen Diamidgruppen — bilden mit Kationen der Gruppe II A selektiv Chelate, wie durch Pikratextraktion aus wäßriger Lösung gezeigt wurde. Dieses Ergebnis wurde auch durch Messungen der Atomabsorption bestätigt. Die Änderungen in der UV-Absorption der aromatischen Ringe und der Amidgruppen in Methanol erlauben eine Abschätzung der Größenordnung der Kationenbindung.相似文献
918.
Joel?R.?Kimmel Oh?Kyu?Yoon Ignacio?A.?Zuleta Oliver?Trapp Richard?N.?ZareEmail author 《Journal of the American Society for Mass Spectrometry》2005,16(7):1117-1130
Hadamard transform (HT) time-of-flight mass spectrometry (TOFMS) is a multiplexing technique that offers high duty cycle for
the mass analysis of continuous ion sources. The multiplexing advantage is maximized when spectral noise is independent of
signal intensity. For conditions in which shot noise predominates, the variance in each peak is a function of the population
of all measured species. We develop expressions for the performance of a HT-TOF mass spectrometer based on Poissonian statistics
for the arrival times of ions at the detector. These expressions and complementary probabilistic simulations are used to estimate
the magnitude of the baseline noise as a function of mass spectral features and acquisition conditions. Experiment validates
the predictions that noise depends on the total number of ions in the acquired spectrum, and the achieved signal-to-noise
ratio for a given species depends on its relative population. We find that for HT-TOFMS experiments encoded with an n-order binary off-on sequence that contains N=2
n
−1 elements, the peak height precision, which is the peak intensity divided by its standard deviation, is greater than that
of an equivalent conventional TOF experiment by a factor of √N/2 times the square root of the fractional abundance of the
peak of interest. Thus, HT-TOFMS is superior to conventional TOF for all species whose fractional abundance F
i exceeds 2/N, which for a typical N value of 2047 corresponds to F
i > 0.001. HT-TOF mass spectra collected at 2500 per second demonstrates the method’s capability of monitoring transient processes
not possible by conventional means. 相似文献
919.
920.
Activation of the extra cellular signal regulated kinase (ERK) pathway is involved in both proliferation and growth arrest of cells depending on intensity and duration of stimuli. In this study, we have elucidated differential regulation of the zinc-stimulated p21(CiP/WAF1) and cyclin D1 activation by inhibition of phosphoinositide 3-kinase (PI3K). In HT-29 colorectal cancer cells, the ERK activities were increased by zinc, which was accompanied by the induction of p21(Cip/WAF1) and cyclin D1. However, in the HT-29 cells pre-treated with PI3K inhibitor, LY294002, zinc induced further the p21(CiP/WAF) induction whereas abrogated cyclin D1 induction. In addition, the induction of p21(Cip/WAF1) expression completely inhibited the incorporation of bromodeoxyuridine (BrdU) into the nucleus, indicating that p21(CiP/WAF1) is an important indicator for ERK-dependent growth arrest. These studies suggest presence of an inter-related regulatory mechanism of cell proliferation by ERK and PI3K pathways. 相似文献