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781.
Par Paul Baret Eve-Marie Rivoirard Jean-Louis Pierre 《Journal of heterocyclic chemistry》1980,17(1):201-204
Aziridinium ions were prepared from aziridines bearing a functionality on a ring carbon and as isolated crystalline fluorosulfate salts either by protonation in fluorosulfuric acid solution or by alkylation with methyl fluorosulfate. The structural assignement of these ions is based on nmr spectra and on quenching experiments with sodium carbonate. The configurations of the epimeric ions have been assigned by the nuclear Qverhauser effect and the stereoselectivity in protonation is disucussed. 相似文献
782.
783.
Summary LetX be a smooth, projective variety defined over a number field and let CH2 (X) denote the Chow group of codimension two cycles modulo rational equivalence. We show that if the cohomology groupH
2(X,Ox) vanishes then the torsion subgroup of CH2 (X) is a finite group. This result covers all previous results in this direction. The hypothesisH
2(X,Ox)=0 is used to lift line bundles.
Oblatum 17-IX-1990 相似文献
Oblatum 17-IX-1990 相似文献
784.
785.
Albert M Cretier G Guillarme D Heinisch S Rocca JL 《Journal of separation science》2005,28(14):1803-1811
The beneficial effects of high temperature on separation and detection of basic compounds, the detection being performed by MS via ESI, are investigated. The influence of various parameters on both separation and detection performances is studied. These parameters include the mobile phase pH, the temperature, and the type of stationary phase. Experiments are performed under gradient elution conditions. The results obtained with four different supports, silica-, zirconia-, carbon-, and polymer-based columns, are compared by means of different criteria including the elution composition, the peak asymmetry, and the S/N. High temperature liquid chromatography at high pH with volatile buffers suitable for MS detection was shown to be an interesting choice for solutes with basic sites. 相似文献
786.
Let us say that a partition of the positive integer n represents a, 0 a n, if there is a submultiset of the multiset of the parts whose sum is a. Erd os and Szalay have proved that almost all partitions of n represent all integers a, 0 a n. If
is a finite set of positive integers, let us denote by p~(n,
) the number of partitions of n which represent all integers a, 0 a n, a
, n – a
but do not represent a for a
. For instance, p~(n,) is the number of partitions of n which represent all integers between 0 and n; the result of Erd os and Szalay can be reformulated as p~(n,) p(n), where p(n) is the total number of partitions of n. The aim of this paper is the study of p~(n,
): we shall compare the values of p~(n,
) for small sets
and we shall give a close formula for p~(n,
) when
is the set of the first k integers. 相似文献
787.
788.
Alvarez G Antón R Marfisi S Márquez L Salager JL 《Langmuir : the ACS journal of surfaces and colloids》2004,20(13):5179-5181
Winsor type I equilibrated surfactant-oil-water (SOW) systems produce o/w emulsions upon stirring. However, if the surfactant is initially dissolved in the oil phase, the attained type after inmediate emulsification is usually w/o. If the SOW system is partially equilibrated, it could result in a normal o/w emulsion, as if it were fully equilibrated. The minimum contact time for that to happen, the so-called apparent equilibration time tAPE, was previously shown (Langmuir 2002, 18, 607) to strongly depend on formulation, surfactant molecular weight, and oil viscosity. The present report shows that it depends on alcohol concentration and location in the unequilibrated system. 相似文献
789.
Jean-Louis Verger-Gaugry 《Discrete and Computational Geometry》2005,33(1):143-155
We give an explicit upper bound of the minimal number T,n of balls of radius 1/2 which form a covering of a ball of radius T > 1/2 in n, n \geq 2. The asymptotic estimates of T,n we deduce when n is large
are improved further by recent results of Böröczky, Jr. and Wintsche on the asymptotic estimates of the minimal numberof equal balls of n covering the sphere Sn-1. The optimality of the asymptotic estimates is discussed. 相似文献
790.
In this work we present all-electron fixed-node diffusion Monte Carlo (FN-DMC) calculations of the low-lying electronic states of the copper atom and its cation. The states considered are those which are the most relevant for the organometallic chemistry of copper-containing systems, namely, the (2)S, (2)D, and (2)P electronic states of Cu and the (1)S ground state of Cu(+). We systematically compare our FN-DMC results to CCSD(T) calculations using very large atomic-natural-orbital-type all-electron basis sets. The FN-DMC results presented in this work provide, to the best of our knowledge, the most accurate nonrelativistic all-electron correlation energies for the lowest-lying states of copper and its cation. To compare our results to experimental data we include the relativistic contributions for all states through numerical Dirac-Fock calculations, which for copper (Z=29) provide almost the entire relativistic effects. It is found that the fixed-node errors using Hartree-Fock nodes for the lowest transition energies of copper and the first ionization potential of the atom cancel out within statistical fluctuations. The overall accuracy achieved with quantum Monte Carlo for the nonrelativistic correlation energy (statistical fluctuations of about 1600 cm(-1) and near cancelation of fixed-node errors) is good enough to reproduce the experimental spectrum when relativistic effects are included. These results illustrate that, despite the presence of the large statistical fluctuations associated with core electrons, accurate all-electron FN-DMC calculations for transition metals are nowadays feasible using extensive but accessible computer resources. 相似文献