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41.
Pascal Vermeeren Michael T. Doppert F. Matthias Bickelhaupt Trevor A. Hamlin 《Chemical science》2021,12(12):4526
We have studied the activation of dihydrogen by metallylenes using relativistic density functional theory (DFT). Our detailed activation strain and Kohn–Sham molecular orbital analyses have quantified the physical factors behind the decreased reactivity of the metallylene on going down Group 14, from carbenes to stannylenes. Along this series, the reactivity decreases due to a worsening of the back-donation interaction between the filled lone-pair orbital of the metallylene and the σ*-orbital of H2, which, therefore, reduces the metallylene–substrate interaction and increases the reaction barrier. As the metallylene ligand is varied from nitrogen to phosphorus to arsenic a significant rate enhancement is observed for the activation of H2 due to (i) a reduced steric (Pauli) repulsion between the metallylene and the substrate; and (ii) less activation strain, as the metallylene becomes increasingly more predistorted. Using a rationally designed metallylene with an optimal Group 14 atom and ligand combination, we show that a number of small molecules (i.e. HCN, CO2, H2, NH3) may also be readily activated. For the first time, we show the ability of our H2 activated designer metallylenes to hydrogenate unsaturated hydrocarbons. The results presented herein will serve as a guide for the rational design of metallylenes toward the activation of small molecules and subsequent reactions.Quantum chemical analyses reveal how model metallylene catalysts activate H2. This is the first step towards the rational design of metallylenes for the activation of small molecules and subsequent reactions. 相似文献
42.
Two electromeric forms, a and b (a is the ground state in a solvent) exist for the hydroxo-iron complex 1, an intermediate in the rebound mechanism of alkane hydroxylation by cytochrome P450. Results of density functional and model solvent calculations of various species are in agreement with experimental findings, and imply the role of 1 a in the rebound mechanism. 相似文献
43.
The results of Fenske-Hall molecular orbital calculations are reported for the trigonal prismatic complexes Mo(S(2)C(2)H(2))(3) and Mo(S(2)C(6)H(4))(3). Both complexes exhibit a bend of the S-C-C-S ligand plane away from the S-Mo-S plane. A series of calculations which systematically follow the changes in electronic structure as the bend angle alpha is varied between 0 and 30 degrees indicates that the bend can be attributed to a second order Jahn-Teller distortion. The driving force for this distortion, which allows mixing between a set of ligand pi orbitals and the metal d(z)()()2 orbital, should be greatest for d(0) systems. In these systems the bent geometry leads to the stabilization of the doubly occupied HOMO. The driving force for ligand bending should be lower in systems having more or fewer electrons (e.g. Re(S(2)C(2)Ph(2))(3) or V(S(2)C(2)Ph(2))(3), respectively). While the steric bulk of the dithiolate ligands in the latter complexes may also influence the degree of ligand bending, this is probably a secondary effect. 相似文献
44.
T H Huisman F Kutlar A Kutlar J B Wilson H F Harris 《Journal of chromatography. A》1987,388(2):429-439
High-performance liquid chromatographic procedures have been used in the detection and identification of a new gamma chain of human fetal hemoglobin (Hb). This M gamma chain is characterized by a Leu----Met replacement at position gamma 141; no other structural variations have been observed. The M gamma chain has been detected in red cell lysates of subjects with a heterozygosity for one of many types of so-called hereditary persistence of fetal hemoglobin conditions, which are characterized by an increased level of Hb F in adult life, in sickle cell anemia, and in a few cord blood samples. At present it is not possible to definitely identify the genetic cause of this newly discovered heterogeneity; an infidelity in translation or the existence of an unrecognized gamma globin gene should be considered. 相似文献
45.
Detection of single molecules, particles, and rapid redox events is a challenge of electrochemical investigations and requires either an amplification strategy or significant averaging for the electrochemical current to exceed the noise level. We consider the minimum number of electrons required to reach the limit of quantification in these electrochemical measurements. A survey of the literature indicates that the state-of-the-art limit in current detection for different types of measurements (e.g. voltammetry, single-molecule redox cycling, ion channel recordings of single molecules, metal nanoparticle collision, and phase nucleation) is independent of the nature of the measurement and increases linearly with reciprocal response time, Δt?1, over ~5 orders of magnitude (from ~10 to ~106 s?1). We demonstrate that the practical limit of quantification requires cumulative measurement of ~2100 electrons during Δt and is determined by statistics of counting electrons, that is, the shot noise in the current. 相似文献
46.
The pyrolyses of silyl esters of pyruvic acid and phenylglyoxylic acid gave rise to acylsilanes in high yield. An intramolecular rearrangement involving an intermediate siloxycarbene is proposed to account for the reaction. 相似文献
47.
Robin K. Harris Judith A. K. Howard Abdolraouf Samadi-Maybodi Jing Wen Yao Warren Smith 《Journal of solid state chemistry》1995,120(2)
The first crystal structure is reported for a silicate clathrate hydrate involving a triply charged cation [C18H30N3]3+ and an octameric cubic silicate cage. The structure is essentially a host/guest system, with the silicate cages linked into a framework by hydrogen bonding to water molecules. The space group is P
with Z = 2, and the asymmetric unit includes a complete cation and half the anion, plus 21 water molecules (4 of which are in disordered positions). Solid-state (CPMAS) 29Si and 13CNMR spectra are consistent with the diffraction-determined structure and indicate substantial distortion of the anion from cubic symmetry. Solution-state spectra of precursor solutions and of melted material are also presented and discussed. 相似文献
48.
Leslie D. Field Adrian V. George Trevor W. Hambley Peter Turner Brian J. Wallace 《Journal of organometallic chemistry》1997,540(1-2):95-99
Reaction of cis-[FeH2(dmpe)2](1) with diphenylbutadiyne results in an insertion into both of the iron-hydride bonds to form an iron metallocycle. Spectroscopic and crystallographic data of [Fe(PhHCC2CHPh)(dmpe)2] (3) show 1,4-diphenylbutatriene is symmetrically bound to the metal via the central double bond. The reaction to form the metallocyclic complex is greatly accelerated by application of external pressure. A 41% yield of (3) is isolated after two days at atmospheric pressure or after approximately 75 min at 800MPa. 相似文献
49.
Single crystals of zinc blende phase MnS transform at 200°C to single crystals of [NaCl] phase by single diffusive jumps of the cations. Dimensional changes cause random distortion of the crystal. Evidence is also presented for a second structural correspondence in which {111} zinc blende planes become {100} [NaCl] planes. The corresponding deformational mechanism might be favored at low temperatures. 相似文献
50.
The data set of more than 40,000 crystal structures containing the carboxylate group that have been deposited in the CSD has been used to examine the structural changes that occur in the carboxylate C-O bond lengths upon binding to different elemental centres. We report here quantifiable structural changes that are dependent on the elemental centre with which the group is interacting. For the main-group elements the trends are entirely periodic and follow those traditionally associated with covalency; elements exhibiting electronegativity closest to that of oxygen exhibit the largest structural change. In addition, we find the measure is extendable to both the transition metals and the lanthanoids and actinoids. Amongst the transition metals the trends of Pauling neutrality are not only maintained, but are quantifiable. The difference between the two C-O bond lengths increases with oxidation state and decreases with an increase in coordination number. All of the lanthanoids exhibit covalency within error of each other and the bonds to the actinoids are found to be more covalent than those to the lanthanoids. From the data analysis we are able to derive a correlation between the lengths of the two carboxylate arms that allows us to quantify percentage covalent character defined in terms of the resonance contributions to the carboxylate group. 相似文献