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61.
Deficiencies in energetics obtained using the common semiempirical methods, AM1, PM3, and MNDO, may partly be traced to the use of pseudoatomic equivalents for conversion of molecular energies to heats of formation at 298 K. We present an alternative scheme based on the use of bond and group equivalents. Values for the 61 bond and group equivalents necessary for treatment of molecules containing the common organic elements, hydrogen, carbon, nitrogen, and oxygen have been derived. For a set of 583 neutral, closed-shell molecules mean absolute errors in AM1, PM3, and MNDO heats of formation are reduced from 6.6, 4.2, and 8.2 kcal/mol to 2.3, 2.2, and 3.0 kcal/mol, respectively. Several systematic problems are overcome in the present scheme including relative stabilities of branched hydrocarbons, energetics of conjugated systems, heats of formation of long chain hydrocarbons, and enthalpies of molecules containing multiple heteroatoms. Although the approach is restricted to molecules with well-defined functional groups, the equivalents are easy to incorporate and are chemically relevant. This revised procedure allows semiempirical methods to be used for far more reliable evaluations of heats of reactions. Estimates are made of the errors inherent in these semiempirical formalisms, arising from integral approximations and the neglect of explicit treatment of electron correlation effects, while excluding those from inadequate parameterization. 相似文献
62.
Chandrasekhar Roychoudhuri 《Foundations of Physics》1978,8(11-12):845-849
According to the Rayleigh criterion of classical optics, the finite resolving power of a microscope is due to the width of the central peak of the Fraunhofer diffraction pattern produced by the microscope's finite lens aperture. During the last few decades, theories and techniques for superresolution beyond the Rayleigh criterion have been developed in classical optics. Thus, Heisenberg's microscope could also in principle be made to give superresolution and thereby appear to violate the uncertainty relation. We believe that this paradox is due to the inappropriate use of a definition, based purely on experimental convenience, to support a quantum mechanical theorem. 相似文献
63.
Michael R.?Douglas Suresh?Govindarajan T.?Jayaraman Alessandro?TomasielloEmail author 《Communications in Mathematical Physics》2004,248(1):85-118
We show how to compute terms in an expansion of the world-volume superpotential for fairly general D-branes on the quintic Calabi-Yau using linear sigma model techniques, and show in examples that this superpotential captures the geometry and obstruction theory of bundles and sheaves on this Calabi-Yau.Louis Michel Professor 相似文献
64.
D.?A.?Dikin V.?Chandrasekhar V.?R.?Misko V.?M.?Fomin J.?T.?DevreeseEmail author 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,34(2):231-235
We study the phase transition of a star-shaped superconductor, which covers smoothly the range from zero to two dimensions with respect to the superconducting coherence length
. Detailed measurements and numerical calculations show that the nucleation of superconductivity in this device is very inhomogeneous: the superconducting order parameter is strongly enhanced and mostly robust in regions close to multiple boundaries. The strong inhomogeneity of the order parameter results in a rich structure of the superconducting transition as a function of temperature and magnetic field.Received: 26 February 2003, Published online: 4 August 2003PACS:
74.78.-w Superconducting films and low-dimensional structures -
74.78.Na Mesoscopic and nanoscale systems -
74.20.De Phenomenological theories
D.A. Dikin: On leave from: Institute for Low Temperature Physics and Engineering, 61164 Kharkov, UkraineV.R. Misko: On leave from: Institute of Applied Physics, 2028 Kishinev, Republic of MoldovaV.M. Fomin: On leave from: State University of Moldova, 2009 Kishinev, Republic of Moldova; Also at: Technische Universiteit Eindhoven, 5600 MB Eindhoven, The NetherlandsJ.T. Devreese: Also at: Universiteit Antwerpen (RUCA), B-2020 Antwerpen, Belgium and Technische Universiteit Eindhoven, 5600 MB Eindhoven, The Netherlands 相似文献
65.
Govindasamy Rajarajan Natesan Jayachandramani Subramaniyan Manivarman Jayaraman Jayabharathi Venugopal Thanikachalam 《Transition Metal Chemistry》2008,33(3):393-397
The kinetics of oxidation of aldonitrones by 4-(dimethylamino)pyridinium chlorochromate (DMAPCC) has been studied in aqueous
N,N-dimethylformamide in the presence of perchloric acid. The reaction follows first-order kinetics with respect to each of DMAPCC
and nitrone. The order with respect to [H+] was found to be close to zero. The rate of oxidation was unaltered by the variation of [NaClO4] but addition of MnSO4 decreased the rate. The reaction rate was found to decrease with a decrease in dielectric constant of the medium. Electron-releasing
and electron-withdrawing groups perturb the reaction rate with a good Hammett correlation. The oxidation products were found
to be benzaldehyde and nitrosobenzene. The reaction was carried out at four different temperatures and the activation parameters
have been calculated. On the basis of the experimental findings, a suitable mechanism has been proposed. 相似文献
66.
S. Chandrasekhar Mrunal Pendke Chandrashekar Muththe Srirama Murthy Akondi Prathama S. Mainkar 《Tetrahedron letters》2012,53(10):1292-1295
A straight forward entry to α,α-dideutero-β-amino esters starting from the corresponding imines and deuterated acetonitrile has been developed involving a two-step process. 相似文献
67.
Dr. Srivari Chandrasekhar Kakita Veera Mohana Rao Mallikanti Seenaiah Police Naresh Ambure Sharada Devi Dr. Bharatam Jagadeesh 《化学:亚洲杂志》2014,9(2):457-461
Hybrid peptidic oligomers comprising natural and unnatural amino acid residues that can exhibit biomolecular folding and hydrogen‐bonding mimicry have attracted considerable interest in recent years. While a variety of hybrid peptidic helices have been reported in the literature, other secondary structural patterns such as γ‐turns and ribbons have not been well explored so far. The present work reports the design of novel periodic γ‐turns in the oligomers of 1:1 natural‐α/unnatural trans‐β‐norborenene (TNAA) amino acid residues. Through DFT, NMR, and MD studies, it is convincingly shown that, in the mixed conformational pool, the heterogeneous backbone of the hybrid peptides preferentially adopt periodic 8‐membered (pseudo γ‐turn)/7‐membered (inverse γ‐turn) hydrogen bonds in both polar and non‐polar solvent media. It is observed that the stereochemistry and local conformational preference of the β‐amino acid building blocks have a profound influence on accessing the specific secondary fold. These findings may be of significant relevance for the development of molecular scaffolds that facilitate desired positioning of functional side‐chains. 相似文献
68.
A. Kangarlu H.R. Chandrasekhar M. Chandrasekhar F.A. Chambers B.A. Vojak J.M. Meese 《Superlattices and Microstructures》1986,2(6)
We report the effect of hydrostatic pressure on the photoreflectance spectra derived from the Γ, X and L bands of a GaAs---AlxGa1−xAs heterostructure. The pressure dependence of the quantum well transitions and the valence band-offsets are accurately determined. 相似文献
69.
P. Madhavan Bandlamudi Mallikarjuna Rao B. Pravin S. Abhishek P. R. Kumar M. Sreenivasulu K. B. Chandrasekhar 《Chromatographia》2007,66(3-4):243-246
A chiral liquid chromatographic method for enantiomeric resolution of β-amino-β-(3-methoxyphenyl) propionic acid was developed
and validated. The “hybrid” π-electron donor–acceptor based stationary phase (R,R) Whelk-01 was found to be enantiomerically
selective for (R) and (S) enantiomers of β-amino-β-(3-methoxyphenyl) propionic acid with a resolution greater than 2.0. The effects of isopropyl alcohol
and ethanol on enantioselectivity and resolution of enantiomers were evaluated. Calibration curves were linear over the range
of 0.10–1.00, with a regression coefficient (r) of 0.999. The limit of detection (LOD) and limit of quantification (LOQ) were 300 and 1,000 ng mL−1 respectively for 10 μL injection volume. The percentage RSD of the peak area of six replicate injections of (S) enantiomer at LOQ concentration was 2.8. The percentage recovery of (S) enantiomer from (R) enantiomer samples ranged from 92 to 102. The test solution was observed to be stable up to 24 h after the preparation.
The developed normal phase chiral LC method can be used for the enantiomeric purity evaluation of R-β-amino-β-(3-methoxyphenyl) propionic acid. 相似文献
70.
K. Ramulu B. M. Rao P. Madhavan M. Lalitha Devi M. K. Srinivasu K. B. Chandrasekhar 《Chromatographia》2007,65(3-4):249-252
A simple and new isocratic normal phase chiral HPLC method has been developed for the determination of enantiomeric purity
of pemetrexed disodium (l-enantiomer) in bulk drugs with a short run time of about 20 min. Chromatographic separation of l and d-enantiomers of pemetrexed disodium was achieved on an amylose based chiral stationary phase using a mobile phase consists
of hexane, ethanol and trifluoro acetic acid. The resolution between the enantiomers was found to be more than 2.0. The system
precision and method precision were found to be within 5% RSD for the distomer (d-enantiomer) at its specification level (i.e. not more than 1.0% w/w). The limit of detection and limit of quantification of distomer were 1.6 and 5 μg mL−1, respectively for 10 μL injection volume. The percentage recovery of distomer was ranged from 90.6 to 105.7 in bulk drug
samples. The test solution was found to be stable in the diluent for 48 h. The method was found to be specific for the enantiomers
of pemetrexed disodium and can be conveniently used for the quantification of undesired d-enantiomer present in the bulk drug samples of pemetrexed disodium. 相似文献