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641.
It has been reported that at high pH, the rate of bonded phase packing degradation in methanol/water mobile phases is greater for carbonate and phosphate buffers than for amine buffers. This conclusion was based on buffer pH determined in the aqueous buffer before dilution with methanol. Changes in buffer species pKa, and therefore buffer pH, upon methanol dilution are consistent with the observed degradation results. Measurements of pH in the methanol/water solutions confirm that the carbonate and phosphate buffers were considerably more basic than the amine buffer, even though all the buffers were pH 10 before dilution with methanol. These results demonstrate that it can be misleading to extrapolate aqueous pH data to partially aqueous solutions. Measurements of pH in the mixed solvent provide more reliable predictions of column and sample stability. 相似文献
642.
Anisotomenes, bicyclic irregular diterpenes found in the genus Anisotome(family Apiaceae), are shown to be products of a unique biosynthetic pathway, involving head-to-head coupling of two geranyl diphosphate units. (13)C labelling studies with [1-(13)C]-glucose on plantlets of A. lyallii also revealed that the isoprene subunits were formed via the MEP pathway. The in vitro-cultured plant material also yielded a new irregular, linear diterpene alcohol, that shares the same biosynthetic pathway. 相似文献
643.
Bartholomew GP Rumi M Pond SJ Perry JW Tretiak S Bazan GC 《Journal of the American Chemical Society》2004,126(37):11529-11542
A series of alpha,omega-bis donor substituted oligophenylenevinylene dimers held together by the [2.2]paracyclophane core were synthesized to probe how the number of repeat units and through-space delocalization influence two-photon absorption cross sections. Specifically, the paracyclophane molecules are tetra(4,7,12,15)-(4'-dihexylaminostyryl)[2.2]paracyclophane (3R(D)), tetra(4,7,12,15)-(4' '-(4'-dihexylaminostyryl)styryl)[2.2]paracyclophane (5R(D)), and tetra(4,7,12,15)-(4' "-(4' '-(4'-dihexylaminostyryl)styryl)styryl)[2.2]paracyclophane (7R(D)). The compounds bis(1,4)-(4'-dihexylaminostyryl)benzene (3R) and bis(1,4)-(4' '-(4'-dihexylaminostyryl)styryl)benzene (5R) were also synthesized to reveal the properties of the "monomeric" counterparts. The two-photon absorption cross sections were determined by the two-photon induced fluorescence method using both femtosecond and nanosecond pulsed lasers as excitation sources. While there is a red shift in the linear absorption spectra when going from the "monomer" chromophore to the paracyclophane "dimer" (i.e., 3R --> 3R(D), 5R --> 5R(D)), there is no shift in the two-photon absorption maxima. A theoretical treatment of these trends and the dependence of transition dipole moments on molecular structure rely on calculations that interfaced time-dependent density functional theory (TDDFT) techniques with the collective electronic oscillator (CEO) program. These theoretical and experimental results indicate that intermolecular interactions can strongly affect B(u) states but weakly perturb A(g) states, due to the small dipole-dipole coupling between A(g) states on the chromophores in the dimer. 相似文献
644.
Shirtcliffe NJ McHale G Newton MI Perry CC 《Langmuir : the ACS journal of surfaces and colloids》2005,21(3):937-943
Rough and patterned copper surfaces were produced using etching and, separately, using electrodeposition. In both of these approaches the roughness can be varied in a controlled manner and, when hydrophobized, these surfaces show contact angles that increase with increasing roughness to above 160 degrees . We show transitions from a Wenzel mode, whereby the liquid follows the contours of the copper surface, to a Cassie-Baxter mode, whereby the liquid bridges between features on the surface. Measured contact angles on etched samples could be modeled quantitatively to within a few degrees by the Wenzel and Cassie-Baxter equations. The contact angle hysteresis on these surfaces initially increased and then decreased as the contact angle increased. The maximum occurred at a surface area where the equilibrium contact angle would suggest that a substantial proportion of the surface area was bridged. 相似文献
645.
Catherine J. Carpenter Petra A. M. van Koppen Paul R. Kemper John E. Bushnell Patrick Weis Jason K. Perry Michael T. Bowers 《International journal of mass spectrometry》2003,230(2-3):161
Gas-phase clustering reactions of CoCp+ with H2 and with CH4 were investigated using temperature-dependent equilibrium experiments. In both systems, the CoCp+ ion was found to form strong interactions with two ligands. The first and second H2 groups cluster to CoCp+ with bond energies of 16.2 and 16.8 kcal/mol, respectively, while the first and second CH4 groups cluster to CoCp+ with bond energies of 24.1 and 12.1 kcal/mol, respectively. These bond energies are in good agreement with those determined by density functional theory (DFT). Molecular geometries for the four clusters determined with DFT are also presented. Weak experimental bond energies of 0.9 kcal/mol for the third H2 and 2.2 kcal/mol for the third CH4 clustering to CoCp+ suggest these ligands occupy the second solvation shell of the ion. In addition to clustering in the methane system, H2-elimination from CoCp(CH4)2+ was observed. The mechanism for this reaction was investigated by collision-induced dissociation experiments and DFT, which suggest the predominate H2-elimination product is (c-C5H6)Co+---C2H5. Theory indicates that dehydrogenation requires the active participation of the Cp ring in the mechanism. Transfer of H and CH3 groups to the C5-ring ligand allows the metal center to avoid the high-energy Co(IV) oxidation state required when it forms two covalent bonds in addition to its interaction with a C5-ring ligand. 相似文献
646.
647.
Charles S. Owen Miriam L. Wahl Dennis B. Leeper Heather D. Perry Suzanne B. Bobyock Matthew Russell Wendy Woodward 《Journal of fluorescence》1995,5(4):329-335
Fluorescent measurements of intracellular H+ and Na+ are improved by using whole spectra of the fluorescent indicators BCECF and SBFI, respectively. The extra data in whole spectra enable both an accurate calibration and a ready detection of artifacts which are not possible to identify using a more conventional data analysis that relies upon only two wavelength windows in the fluorescence spectra. The whole-spectrum technique is applicable to cell suspensions in a conventional fluorimeter (as is reported here with SBFI), as well as to attached cells using a fluorimeter combined with an inverted epifluorescence microscope. The spectral method was highly reproducible in that pairs of successive pH measurements differed, on average, by only 0.01±0.02 U. Random uncertainty from sample to sample was estimated numerically from the standard deviation of measurements on ionophore-treated cells. When full-spectrum analysis was employed, this scatter showed a two-fold improvement over results obtained using the two-wavelength ratio method. Because SBFI has a relatively narrow dynamic range, whole-spectrum analysis has been applied to improve the accuracy of sodium determinations. The calibrated system measured [Na+]i with excellent linearity over the range 2–150 mM and with an accuracy of approximately 5 mM. 相似文献
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