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881.
Carl Henrik Grbitz 《Acta Crystallographica. Section C, Structural Chemistry》2004,60(5):o371-o373
The structure of the title compound, C15H22N2O3·2H2O, was derived from data collected on a very thin twinned needle. The peptide molecule is in a rare conformation normally associated with hydrophobic dipeptides that form nanotubes. Nevertheless, the present structure is divided into hydrophobic and hydrophilic layers. 相似文献
882.
Haukur Jhannesson Sven Macholl Jan Henrik Ardenkjaer-Larsen 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2009,197(2):167-175
Dynamic Nuclear Polarization (DNP) of the 13C nucleus has been investigated for [1-13C]pyruvic acid, doped with the trityl radical OX063Me, at 4.64 T and 1.15 K. The dependence of the polarization on microwave frequency, radical concentration and electron saturation was studied. For optimized conditions, a 13C polarization equal to 64 ± 5% was obtained, an increase by more than a factor of two compared with earlier results at 3.35 T of the same system. It was furthermore observed that the addition of gadolinium, which resulted in a twofold polarization increase at 3.35 T, only resulted in a minor improvement at 4.64 T. The dependence of the electron saturation on microwave frequency and microwave power was quantified by first moment measurements which were obtained by nucleus–electron double resonance (NEDOR) experiments. Complete electron saturation was observed for a microwave frequency close to the centre frequency of the ESR line, and by using maximum power of the microwave source. The DNP build-up time at 4.64 T (3000 s) was prolonged by approximately a factor three over the build-up time at 3.35 T (1200 s). However, after approximately 20 min of microwave irradiation the polarization at 4.64 T exceeded the polarization at 3.35 T. 相似文献
883.
In this paper we study front tracking for a model of one dimensional, immiscible flow of several isentropic gases, each governed
by a gamma-law. The model consists of the p-system with variable gamma representing the different gases. The main result is the convergence of a front tracking algorithm
to a weak solution, thereby giving existence as well. This convergence holds for general initial data with a total variation
satisfying a specific bound. The result is illustrated by numerical examples. 相似文献
884.
Henrik Levämäki Liyun Tian Kalevi Kokko Levente Vitos 《The European Physical Journal B - Condensed Matter and Complex Systems》2018,91(6):128
We use three gradient level and two nonlocal density functional approximations to study the thermodynamic properties of Cu-Au compounds. It is found that a well-designed gradient level approximation (quasi non-uniform approximation, QNA) reproduces the experimental equilibrium volumes and the formation energies of L12 and L10 phases. On the other hand, QNA predicts a non-existent β2 phase, which can be remedied only when employing the nonlocal hybrid-level Heyd-Scuseria-Ernzerhof (HSE06) or Perdew-Burke-Ernzerhof (PBE0) approximations. Gradient-level approximations lead to similar electronic structures for the Cu-Au compounds whereas hybrids shift the d-band towards negative energies and account for the complex d-d hybridization more accurately. 相似文献
885.
886.
Sangeeta Yadav Ouissam ElBakouri Kjell Jorner Hui Tong Christian Dahlstrand Miquel Sol Henrik Ottosson 《化学:亚洲杂志》2019,14(10):1870-1878
Due to the reversal in electron counts for aromaticity and antiaromaticity in the closed‐shell singlet state (normally ground state, S0) and lowest ππ* triplet state (T1 or T0), as given by Hückel's and Baird's rules, respectively, fulvenes are influenced by their substituents in the opposite manner in the T1 and S0 states. This effect is caused by a reversal in the dipole moment when going from S0 to T1 as fulvenes adapt to the difference in electron counts for aromaticity in various states; they are aromatic chameleons. Thus, a substituent pattern that enhances (reduces) fulvene aromaticity in S0 reduces (enhances) aromaticity in T1, allowing for rationalizations of the triplet state energies (ET) of substituted fulvenes. Through quantum chemical calculations, we now assess which substituents and which positions on the pentafulvene core are the most powerful for designing compounds with low or inverted ET. As a means to increase the π‐electron withdrawing capacity of cyano groups, we found that protonation at the cyano N atoms of 6,6‐dicyanopentafulvenes can be a route to on‐demand formation of a fulvenium dication with a triplet ground state (T0). The five‐membered ring of this species is markedly Baird‐aromatic, although less than the cyclopentadienyl cation known to have a Baird‐aromatic T0 state. 相似文献
887.
888.
A new implementation of the approximate coupled cluster singles and doubles CC2 linear response model using Cholesky decomposition of the two-electron integrals is presented. Significantly reducing storage demands and computational effort without sacrificing accuracy compared to the conventional model, the algorithm is well suited for large-scale applications. Extensive basis set convergence studies are presented for the static and frequency-dependent electric dipole polarizability of benzene and C60, and for the optical rotation of CNOFH2 and (-)-trans-cyclooctene (TCO). The origin-dependence of the optical rotation is calculated and shown to persist for CC2 even at basis set convergence. 相似文献
889.
An N-protection protocol employing the 4,4'-dimethoxytrityl (Dmt) group in combination with borane reduction of resin-bound polyamides was shown to be an efficient methodology that enables synthesis of novel analogues of natural acylpolyamine toxins. Thus, three philanthotoxins containing polyamine chains with piperidyl and cyclohexyl structural elements, which introduce conformational rigidity, increased lipophilicity, and altered proteolytic properties, were obtained in 39-44% overall yield. 相似文献
890.
Filinchuk YE Birkedal H Cerný R Hostettler M Yanson TI Bodak OI Yvon K 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(12):2972-2976
An X-ray study of single crystals extracted from an arc-melted Yb-Fe-Ga alloy showed that the diffraction pattern can be modeled by an intergrown crystal that has three sorts of domains: one hexagonal (1, LuFe(9.5) type) and two rhombohedral (2 a and 2 b, PrFe(7) type), the last two twinned by reticular merohedry. Crystals 1 and 2 are essentially polytypes with maximum degree of order (MDO polytypes), built up of nearly identical slabs that are stacked along [001] in ABAB em leader (1). and ABCABC em leader (2). sequences. Structure refinement was performed by a newly developed program that allowed us to refine several structures on a single data set. We found that the hexagonal and rhombohedral domains differ in chemical composition: while 1 shows a higher rate of Yb substitution by Fe(2) dumbbells, 2 shows partial substitution of Fe by Ga. Our observation of the nanoscale phase segregation is supported by latest finding of nonrandom distribution of stacking faults in a similar 2:17 alloy. An unequal distribution of chemical substitutions in 1 and 2 apparently compensates the inherent mismatch of basal plane dimensions of the individual MDO polytypes and thus constrains their cell parameters within the syntaxy. According to our knowledge this is the first example of two chemically distinct polytypes constituting a single crystal, refined on a single set of diffraction data. 相似文献