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排序方式: 共有1043条查询结果,搜索用时 31 毫秒
961.
Brothers EN Izmaylov AF Rusakov AA Scuseria GE 《The journal of physical chemistry. B》2007,111(50):13869-13872
Calculating the enthalpy of formation of a polymer with ab initio methods requires two choices. The first decision is whether to use oligomeric extrapolation or periodic boundary conditions to model the extended system, and the second choice is between formation reactions to be modeled, for example, formation from atoms, formation from standard states, or formation from some set of molecular systems. Utilizing trans-polyacetylene and polyethylene as examples, the oligomeric and periodic techniques are contrasted, leading to a discussion of the larger than minimal unit cell required when frequency calculations only include in-phase vibrations, that is, only the k = 0 frequencies, in an enthalpy of formation calculation. The accuracy of calculating the enthalpy of formation, in light of density functional theory's increased error with larger systems and with respect to various reference states, is also discussed. The calculation of the enthalpy of formation for a polymer is most accurate when the reference states are chosen carefully and most efficient when using periodic boundary conditions. 相似文献
962.
Crespo-Quesada M Yarulin A Jin M Xia Y Kiwi-Minsker L 《Journal of the American Chemical Society》2011,133(32):12787-12794
The activity and selectivity of structure-sensitive reactions are strongly correlated with the shape and size of the nanocrystals present in a catalyst. This correlation can be exploited for rational catalyst design, especially if each type of surface atom displays a different behavior, to attain the highest activity and selectivity. In this work, uniform Pd nanocrystals with cubic (in two different sizes), octahedral, and cuboctahedral shapes were synthesized through a solution-phase method with poly(vinyl pyrrolidone) (PVP) serving as a stabilizer and then tested in the hydrogenation of 2-methyl-3-butyn-2-ol (MBY). The observed activity and selectivity suggested that two types of active sites were involved in the catalysis--those on the planes and at edges--which differ in their coordination numbers. Specifically, semihydrogenation of MBY to 2-methyl-3-buten-2-ol (MBE) occurred preferentially at the plane sites regardless of their crystallographic orientation, Pd(111) and/or Pd(100), whereas overhydrogenation occurred mainly at the edge sites. The experimental data can be fit with a kinetic modeling based on a two-site Langmuir-Hinshelwood mechanism. By considering surface statistics for nanocrystals with different shapes and sizes, the optimal catalyst in terms of productivity of the target product MBE was predicted to be cubes of roughly 3-5 nm in edge length. This study is an attempt to close the material and pressure gaps between model single-crystal surfaces tested under ultra-high-vacuum conditions and real catalytic systems, providing a powerful tool for rational catalyst design. 相似文献
963.
Cywinski PJ Moro AJ Ritschel T Hildebrandt N Löhmannsröben HG 《Analytical and bioanalytical chemistry》2011,399(3):1215-1222
Novel fluorescent nanosensors, based on a naphthyridine receptor, have been developed for the detection of guanosine nucleotides,
and both their sensitivity and selectivity to various nucleotides were evaluated. The nanosensors were constructed from polystyrene
nanoparticles functionalized by (N-(7-((3-aminophenyl)ethynyl)-1,8-naphthyridin-2-yl)acetamide) via carbodiimide ester activation. We show that this naphthyridine
nanosensor binds guanosine nucleotides preferentially over adenine, cytosine, and thymidine nucleotides. Upon interaction
with nucleotides, the fluorescence of the nanosensor is gradually quenched yielding Stern–Volmer constants in the range of
2.1 to 35.9 mM−1. For all the studied quenchers, limits of detection (LOD) and tolerance levels for the nanosensors were also determined.
The lowest (3σ) LOD was found for guanosine 3’,5’-cyclic monophosphate (cGMP) and it was as low as 150 ng/ml. In addition,
we demonstrated that the spatial arrangement of bound analytes on the nanosensors’ surfaces is what is responsible for their
selectivity to different guanosine nucleotides. We found a correlation between the changes of the fluorescence signal and
the number of phosphate groups of a nucleotide. Results of molecular modeling and ζ-potential measurements confirm that the
arrangement of analytes on the surface provides for the selectivity of the nanosensors. These fluorescent nanosensors have
the potential to be applied in multi-analyte, array-based detection platforms, as well as in multiplexed microfluidic systems. 相似文献
964.
965.
We prove that the set of all complex series which are nonabsolutely convergent is c-algebrable. We establish a similar result for the set of all divergent complex series with bounded partial sums. 相似文献
966.
We prove exponential contraction of renormalization along hybrid classes of infinitely renormalizable unimodal maps (with
arbitrary combinatorics), in any even degree d. We then conclude that orbits of renormalization are asymptotic to the full renormalization horseshoe, which we construct.
Our argument for exponential contraction is based on a precompactness property of the renormalization operator (“beau bounds”),
which is leveraged in the abstract analysis of holomorphic iteration. Besides greater generality, it yields a unified approach
to all combinatorics and degrees: there is no need to account for the varied geometric details of the dynamics, which were
the typical source of contraction in previous restricted proofs. 相似文献
967.
Reliability of calculated (1)H and (13)C NMR chemical shifts for various classes of organic compounds obtained with gauge-invariant atomic orbital (GIAO) approach has been studied at the PBE/3ζ level (as implemented in PRIRODA code) using linear regression analysis with experimental data. Empirical corrections for the calculated chemical shifts δ(H,calc) = δ(PBE/3ζ) - 0.08 ppm (RMS 0.18 ppm, MAD 0.66 ppm) and δ(C,calc) = δ(PBE/) (3) (ζ) - 6.35 ppm (RMS 3.09 ppm, MAD 9.42 ppm) have been developed using the sets of 263 and 308 experimental values for (1)H and (13)C chemical shifts, respectively. The confidence intervals of NMR chemical shifts at 95% confidence probability are δ(H,calc) ± 0.35 ppm for (1)H and δC,calc) ± 6.05 ppm for (13)C. 相似文献
968.
Raquel S. G. R. Seixas Artur M. S. Silva Ibon Alkorta José Elguero 《Monatshefte für Chemie / Chemical Monthly》2011,18(5):731-742
Abstract
We report the synthesis and structural study of eight compounds, either quinolin-4(1H)-ones or quinolines. Tautomerism as well as (E) → (Z) and rotational isomerism were studied both experimentally (1H and 13C NMR) and theoretically [B3LYP/6-311++G(d,p)]. 相似文献969.
Pinto SM Lourenço MA Calvete MJ Abreu AR Rosado MT Burrows HD Pereira MM 《Inorganic chemistry》2011,50(17):7916-7918
Zinc and manganese complexes of porphyrin triads have been synthesized and are shown to be efficient as highly sensitive and selective tripod optical sensors for amines at the picomolar level. 相似文献
970.
Garbacz JK Furmaniak S Terzyk AP Grabiec M 《Journal of colloid and interface science》2011,359(2):512-519
Using the thermodynamic idea of complementary systems, and based on fundamental concepts of the theory of volume filling of micropores, we derived a new universal model describing adsorption from solutions with limited and unlimited miscibility of components. The model takes into account the differences in collision diameters of adsorbed molecules as well as the competitive nature of adsorption from solutions. The applicability of this new approach is tested against experimental data. 相似文献