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31.
We report the synthesis of a series of poly(p-phenylene ethynylene)s (PPEs) with high ionization potentials and associated high excited-state electron affinities. Their photophysical properties were investigated using steady-state and time-resolved fluorescence techniques. The ionization potentials of the polymer thin films were determined using ultraviolet photoelectron spectroscopy (UPS), and those with the highest ionization potentials displayed high sensitivity for the detection of electron-donating aromatic compounds. The effects of sterics, chemical structure, and electronic properties on the polymers' sensory responses were investigated by fluorescence quenching experiments in both solution and solid thin films. In addition, we report that in some cases the excited-state charge-transfer complexes (exciplexes) of the PPEs with analytes were observed. These latter effects provide promising opportunities for the formation of sensitive and selective chemical sensors. 相似文献
32.
Fry BG Wüster W Ryan Ramjan SF Jackson T Martelli P Kini RM 《Rapid communications in mass spectrometry : RCM》2003,17(18):2047-2062
The evolution of the venomous function of snakes and the diversification of the toxins has been of tremendous research interest and considerable debate. It has become recently evident that the evolution of the toxins in the advanced snakes (Colubroidea) predated the evolution of the advanced, front-fanged delivery mechanisms. Historically, the venoms of snakes lacking front-fanged venom-delivery systems (conventionally grouped into the paraphyletic family Colubridae) have been largely neglected. In this study we used liquid chromatography with mass spectrometry (LC/MS) to analyze a large number of venoms from a wide array of species representing the major advanced snake clades Atractaspididae, Colubrinae, Elapidae, Homalopsinae, Natricinae, Psammophiinae, Pseudoxyrhophiinae, Xenodontinae, and Viperidae. We also present the first sequences of toxins from Azemiops feae as well as additional toxin sequences from the Colubrinae. The large body of data on molecular masses and retention times thus assembled demonstrates a hitherto unsuspected diversity of toxins in all lineages, having implications ranging from clinical management of envenomings to venom evolution to the use of isolated toxins as leads for drug design and development. Although definitive assignment of a toxin to a protein family can only be done through demonstrated structural studies such as N-terminal sequencing, the molecular mass data complemented by LC retention information, presented here, do permit formulation of reasonable hypotheses concerning snake venom evolution and potential clinical effects to a degree not possible till now, and some hypotheses of this kind are proposed here. The data will also be useful in biodiscovery. 相似文献
33.
Ab initio molecular orbital structures and energies of B2F4, B2Cl4, N2O4, and C2O have been calculated for both perpendicular D2d and planar D2h rotamers. The experimental trend toward greater preference for the D2d forms in going from B2F4 to B2Cl4 is reproduced. N2O4 favors the planar conformation, although the rotation barrier is overestimated at the theoretical levels used. The oxalate dianion is calculated to be more stable in the D2d conformation; the experimental planar arrangement in the solid may be due to crystal packing forces. The preferences for one conformation over another are small; analysis indicates that different effects may predominate in each case: π stabilization for B2F4, hyperconjugation for B2Cl4, lone-pair interactions for N2O4, and electrostatic repulsions for C2O. 相似文献
34.
The potential generated by a plastic-membrane calcium ion-selective electrode (i.s.e.) is shown to be indirectly measurable by a non-zero current method based on bipolar pulse conductance. Linear current—voltage curves are obtained using 0–5-V pulses; the current axis intercept is related to the i.s.e. potential. A simple electrical contact (e.g., platinum or stainless steel) can be used instead of a poised reference electrode as the counter electrode in this two-electrode system. Long-term exposure of the i.s.e. to calcium solutions causes an upward drift in the measured current. This drift is minimized by avoiding long exposure times to solution, rinsing the electrode between measurements, and constructing current—voltage curves for determination of the current axis intercepts. Voltage pulses lasting 100 μs are optimum for this method. Shorter pulses are subject to error from capacitive charging currents, and longer pulses yield poorer precision, and degrade the electrode through faradaic reactions. The measured signal is dependent upon Ca2+ concentration (rather than activity), making ionic strength adjustment unnecessary. The concentration dependence is induced by application of voltage pulses greater than ~ 15 mV in amplitude. Selectivities of the potentiometric and conductometric methods are shown to be comparable for a variety of interfering monovalent and divalent cations. The conductometric method yields a fast i.s.e. response because of induced migration of Ca2+ into the membrane. Response time decreases as the pulse height increases. Pulses greater than 2 V in magnitude yield response times limited by the solution mixing time rather than by the electrode. 相似文献
35.
Timothy D. Shaffer Virgil Percec 《Journal of polymer science. Part A, Polymer chemistry》1987,25(10):2755-2779
Thermotropic liquid crystalline (LC) polyethers and copolyethers have been synthesized from 4,4′-dihydroxy-α-methylstilbene (HMS) and α,ω-dichlorooligo(oxyethylene)s having between 2 and 5 as well as 8.7 oxyethylene units. Copolyethers were prepared from a 1:1 mol/mol ratio of two dissimilar spacers. These polymers have been prepared by a phase transfer catalyzed (PTC) polyetherification of bisphenols with these electrophiles by utilizing 50 mol% tetrabutylammonium hydrogen sulfate per phenol group. Kinetic experiments with either 5 or 50 mol% catalyst vs phenol groups in the polyetherification of 4,4%-isopropylidenediphenol with 2-chloroethyl ether have shown that a change in catalyst primarily affects the rate of reaction, with 50 mol % being faster. The prepared polyethers and copolyethers are soluble in common organic solvents. Both polyethers and copolyethers are crystalline. Polymers prepared to contain tetraoxyethylene spacers exhibit monotropic LC behavior. Copolymers prepared to contain tri- and tetraoxyethylene spacers (1 : 1 mol/mol) [PE34] were the only polymers exhibiting enantiotropic LC behavior. Longer spacers tend to destabilize the phase transitions, as suggested by the dependence of thermal transition temperatures upon the differential scanning calorimeter rate. All prepared polymers act as podants in solution, measured by picrate extraction experiments. Solid state complexes have been prepared from the polymer with a pentaoxyethylene spacer [PE5] and PE34 with LiCF3SO3. PE5 can dissolve LiCF3SO3 in the range of 0.21–2.2 mol salt/mol polymer (m.r.u.) [S/P] without the observation of free salt. PE5 complexes of/or below S/P of 0.43, upon annealing at room temperature, exhibited the two melting transitons observed in the polymer alone. PE5 complexes of/or above S/P of 0.77 only exhibited a Tg. The Tg of PE5 complexes were found to change nonlinearly with S/P, while Tm1 changed linearly. Tm2 was independent of S/P. Only one complex with PE34 gave two transitions (Tm2,Ti) in dynamic DSC experiments. Other PE34 complexes followed a behavior similar to PE5 complexes. 相似文献
36.
Gary H. Posner Timothy P. Kogan Stephen R. Haines Leah L. Frye 《Tetrahedron letters》1984,25(25):2627-2630
A short, reliable, and practical synthesis of ()-(+)-2-(-tolylsulfinyl)-2-buten-4-olide has been developed, and the utility of this Michael acceptor for highly enantiocontrolled synthesis of 3-substituted 4-butanolides has been demonstrated. 相似文献
37.
Competitive ion-dipole, ion-water, and water-water interactions were investigated at the molecular level in M+ (CH3CN)n(H2O)m cluster ions for M = Na and K. Different [n,m] combinations for two different n + m cluster sizes were characterized with infrared predissociation spectroscopy in the O-H stretch region and MP2 calculations. In all cases, no differences were observed between the two alkali metal ions. The results showed that at the n + m = 4 cluster size, the solvent molecules interact only with the ion, and that the interaction between the ion and the large dipole moment of CH3CN decreases the ion-water electrostatic interactions. At the n + m = 5 cluster size, at least two different hydrogen-bonded structures were identified. In these structures, the ion-dipole interaction weakens the ability of the ion to polarize the hydrogen bonds and thus decreases the strength of the water-water interactions in the immediate vicinity of the alkali metal ion. 相似文献
38.
The elucidation of the structure of the cytotoxic marine sponge alkaloid pyrinodemin A by synthesis is described. Based on the 13C NMR spectra of three double bond positional isomers and the natural product, it is concluded the C14′-C15′ isomer best represents the true structure of pyrinodemin A. In addition, the structural assignment of pyrinodemin C is evaluated. 相似文献
39.
John W. Ledbetter John M. Hanckel Timothy J. Cornish 《Photochemistry and photobiology》1981,34(1):115-118
Abstract— The lifetime of the p -quinoid intermediate of transamination was determined through photoinduction in the metal-ion model system. From the dependence of the lifetime on temperature, the Arrhenius activation energy for proton transfer was 4.2–8.4 kJ/mol (1-2 kcal/mol). The activation energy correlated well with the R group of the amino acid and demonstrated that native substrates have a very labile proton. 相似文献
40.
Briggs TF Winemiller MD Collum DB Parsons RL Davulcu AH Harris GD Fortunak JM Confalone PN 《Journal of the American Chemical Society》2004,126(17):5427-5435
The 1,2-addition of lithium phenylacetylide (PhCCLi) to quinazolinones was investigated using a combination of structural and rate studies. (6)Li, (13)C, and (19)F NMR spectroscopies show that deprotonation of quinazolinones and phenylacetylene in THF/pentane solutions with lithium hexamethyldisilazide affords a mixture of lithium quinazolinide/PhCCLi mixed dimer and mixed tetramer along with PhCCLi dimer. Although the mixed tetramer dominates at high mixed aggregate concentrations and low temperatures used for the structural studies, the mixed dimer is the dominant form at the low total mixed aggregate concentrations, high THF concentrations, and ambient temperatures used to investigate the 1,2-addition. Monitoring the reaction rates using (19)F NMR spectroscopy revealed a first-order dependence on mixed dimer, a zeroth-order dependence on THF, and a half-order dependence on the PhCCLi concentration. The rate law is consistent with the addition of a disolvated PhCCLi monomer to the mixed dimer. Investigation of the 1,2-addition of PhCCLi to an O-protected quinazolinone implicates reaction via trisolvated PhCCLi monomers. 相似文献