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931.
Carreño MC García I Núñez I Merino E Ribagorda M Pieraccini S Spada GP 《Journal of the American Chemical Society》2007,129(22):7089-7100
Two series of enantiopure azobenzenes with a p-tolylsulfoxide at the ortho or meta position with respect to the azo group, have been regioselectively synthesized. Both can act as enantiopure molecular switches showing different structural features owing to the presence of the stereogenic sulfur. The photoisomerization process, studied by UV-vis, circular dichroism (CD), NMR, and chiral HPLC evidenced a double role of the sulfoxide. A transfer of chirality from the sulfoxide to the azo system was observed by CD in both cis and trans-isomers of the meta sulfinyl derivatives 3, whereas this perturbation was evident for the ortho sulfinyl series 7 only in the cis isomer. The NMR study evidenced that the s-cis rigid conformation of the bisaromatic sulfoxide was fixing a different orientation of the overall system in each series both in the trans and cis isomers, by forcing a final U-shaped structure in cis-3 and an S-shaped structure in cis-7. Very different values of specific optical rotations were measured in both trans and cis isomers, also reflecting the existence of distinct chiral entities in the photostationary states. The easy and reversible changes occurring between different conformational states could find applications in the photocontrol of several molecular switches. 相似文献
932.
Aluminosilicate surfaces as promoters for peptide bond formation: an assessment of Bernal's hypothesis by ab initio methods 总被引:1,自引:0,他引:1
The role in prebiotic chemistry that Br?nsted and Lewis sites, both present at the surface of common aluminosilicates, may have played in favoring the peptide bond formation has been addressed by ab initio methods within a cluster approach. B3LYP/6-31+G(d,p) free energy potential energy surfaces have been fully characterized for the model reaction glycine + NH3 --> 2-NH2 acetamide (mimicking the true 2 Gly --> GlyGly one) occurring on (i) a Lewis site, (ii) a Br?nsted site, and (iii) a combined action of Lewis/Br?nsted sites. Compared to the gas-phase (gp) activation free energy of 50 kcal/mol, the Lewis site alone reduces the gp barrier to 41 kcal/mol, whereas the activation by the Br?nsted site dramatically reduces the barrier to about 18 kcal/mol. Nevertheless, formation of the prereactant complex in this latter case will rarely occur, since water will easily displace the glycine molecule interacting with the Br?nsted site. However, if a realistic feldspar surface with neighboring Br?nsted and Lewis sites is considered, the proper prereactant complex is highly stabilized by a simultaneous interaction with the Lewis and the Br?nsted sites, in such a way that the Lewis site strongly attaches the glycine molecule to the surface whereas the Br?nsted site efficiently catalyzes the condensation reaction, showing that the interplay between Lewis/Br?nsted sites is an important issue. The free energy barrier computed for the realistic feldspar surface model is 26 kcal/mol. The role of dispersive interactions on the free energy barrier and the stabilization of the final product, not accounted for by the B3LYP functional, have been estimated and shown to be substantial. Speculations about further elongation of the formed dipeptide have been put forward on the basis of the relatively strong interaction energy of the formed GlyGly dipeptide with the aluminosilicate surface. 相似文献
933.
The Crooks equation [Eq. (10) in J. Stat. Phys. 90, 1481 (1998)] relates the work done on a system during a nonequilibrium transformation to the free energy difference between the final and the initial state of the transformation. Recently, the authors have derived the Crooks equation for systems in the canonical ensemble thermostatted by the Nose-Hoover or Nose-Hoover chain method [P. Procacci et al., J. Chem. Phys. 125, 164101 (2006)]. That proof is essentially based on the fluctuation theorem by Evans and Searles [Adv. Phys. 51, 1529 (2002)] and on the equations of motion. Following an analogous approach, the authors derive here the Crooks equation in the context of molecular dynamics simulations of systems in the isothermal-isobaric (NPT) ensemble, whose dynamics is regulated by the Martyna-Tobias-Klein algorithm [J. Chem. Phys. 101, 4177 (1994)]. Their present derivation of the Crooks equation correlates to the demonstration of the Jarzynski identity for NPT systems recently proposed by Cuendet [J. Chem. Phys. 125, 144109 (2006)]. 相似文献
934.
Puertas AM De Michele C Sciortino F Tartaglia P Zaccarelli E 《The Journal of chemical physics》2007,127(14):144906
We report a numerical investigation of the viscoelastic behavior in models for steric repulsive and short-ranged attractive colloidal suspensions, along different paths in the attraction strength vs packing fraction plane. More specifically, we study the behavior of the viscosity (and its frequency dependence) on approaching the repulsive glass, the attractive glass, and in the reentrant region where viscosity shows a nonmonotonic behavior on increasing attraction strength. On approaching the glass lines, the increase of the viscosity is consistent with a power-law divergence with the same exponent and critical packing fraction previously obtained for the divergence of the density fluctuations. Based on mode-coupling calculations, we associate the increase of the viscosity with specific contributions from different length scales. We also show that the results are independent of the microscopic dynamics by comparing Newtonian and Brownian simulations for the same model. Finally, we evaluate the Stokes-Einstein relation approaching both glass transitions, finding a clear breakdown which is particularly strong for the case of the attractive glass. 相似文献
935.
Dmitry A. Loginov Zoya A. Starikova Piero Zanello Fulvio Rossi Alexander R. Kudinov 《Journal of organometallic chemistry》2007,692(26):5777-5787
The reaction of (η5-C9H2Me5)Rh(1,5-C8H12) (1) with I2 gives the iodide complex [(η5-C9H2Me5)RhI2]2 (2). The solvate complex [(η5- C9H2Me5)Rh(MeNO2)3]2+ (generated in situ by treatment of 2 with Ag+ in nitromethane) reacts with benzene and its derivatives giving the dicationic arene complexes [(η5-9H2Me5)Rh(arene)]2+ [arene = C6H6 (3a), C6Me6 (3b), C6H5OMe (3c)]. Similar reaction with the borole sandwich compound CpRh(η5-C4H4BPh) results in the arene-type complex [CpRh(μ-η5:η6-C4H4BPh)Rh(η5-C9H2Me5)]2+ (4). Treatment of 2 with CpTl in acetonitrile affords cation [(η5-C9H2Me5)RhCp]+ (5). The structure of [3c](BF4)2 was determined by X-ray diffraction. The electrochemical behaviour of complexes prepared was studied. The rhodium-benzene bonding in series of the related complexes [(ring)Rh(C6H6)]2+ (ring = Cp, Cp∗, C9H7, C9H2Me5) was analyzed using energy and charge decomposition schemes. 相似文献
936.
Joana Domingues Nicole Bonelli Rodorico Giorgi Piero Baglioni 《Applied Physics A: Materials Science & Processing》2014,114(3):705-710
Semi-interpenetrating (IPN) poly (2-hydroxyethyl methacrylate)/polyvinylpyrrolidone hydrogels were synthesized and used for the removal of adhesives from the back of canvas paintings. The high water retention capability and the specific mechanical properties of these gels allow the safe cleaning of water-sensitive artifacts using water-based detergent systems. The cleaning action is limited to the contact area and layer-by-layer removal is achieved while avoiding water spreading and absorption within water-sensitive substrates, which could lead, for example, to paint detachment. The use of these chemical gels also avoids leaving residues over the treated surface because the gel network is formed by covalent bonds that provide high mechanical strength. In this contribution, the physicochemical characterization of semi-IPN chemical hydrogels is reported. The successful application of an o/w microemulsion confined in the hydrogel for the removal of adhesives from linen canvas is also illustrated. 相似文献
937.
Weingart O Altoè P Stenta M Bottoni A Orlandi G Garavelli M 《Physical chemistry chemical physics : PCCP》2011,13(9):3645-3648
The photochemical cis-trans isomerization of retinal in rhodopsin is investigated by structure sampling and excited state QM/MM trajectories with surface hopping. The calculations uncover the motions responsible for photoproduct formation and elucidate the reasons behind the efficient photoisomerization in the primary event of visual transduction. 相似文献
938.
Maschio L Civalleri B Ugliengo P Gavezzotti A 《The journal of physical chemistry. A》2011,115(41):11179-11186
A comparative analysis of the intermolecular energy for a data set including 60 molecular crystals with a large variety of functional groups has been carried out using three different computational approaches: (i) a method based on a physically meaningful empirical partition of the interaction energy (PIXEL), (ii) density functional methods with a posteriori empirical correction for the dispersion interactions (DFT-D), and (iii) a full periodic ab initio quantum mechanical method based on M?ller-Plesset perturbation theory for the electron correlation using localized crystal orbitals (LMP2). Due to the large computational cost, LMP2 calculations have been restricted to a subset of seven molecular crystal comprising benzene, formic acid, formamide, succinic anhydride, urea, oxalic acid, and nitroguanidine, and the results compared with PIXEL and DFT-D data as well as with the experimental data show excellent agreement among all adopted methods. This shows that both DFT-D and PIXEL approaches are robust predictive tools for studying molecular crystals. A detailed analysis shows a very similar dispersion contribution of the two methods across the 60 considered molecular crystals. The study also confirms that pure DFT shows serious deficiencies in properly handling molecular crystals in which the dispersive contribution is large. Due to the negligible requested computational resources, PIXEL is the method of choice in screening of a large number of molecular crystals, an essential step to predict crystal polymorphism or to study crystal growth processes. DFT-D can then be used to refine the ranking emerged from PIXEL calculations due to its general applicability and robustness in properly handling short-range interactions. 相似文献
939.
Bennett TD Simoncic P Moggach SA Gozzo F Macchi P Keen DA Tan JC Cheetham AK 《Chemical communications (Cambridge, England)》2011,47(28):7983-7985
We report the reversible pressure-induced amorphization of a zeolitic imidazolate framework (ZIF-4, [Zn(Im)(2)]). This occurs irrespective of pore occupancy and takes place via a novel high pressure phase (ZIF-4-I) when solvent molecules are present in the pores. A significant reduction in bulk modulus upon framework evacuation is also observed for both ZIF-4 and ZIF-4-I. 相似文献
940.
We study the existence of solutions u:R3→R2 for the semilinear elliptic systems where W:R2→R is a double well symmetric potential. We use variational methods to show, under generic non-degenerate properties of the set of one dimensional heteroclinic connections between the two minima a± of W, that (0.1) has infinitely many geometrically distinct solutions u∈C2(R3,R2) which satisfy u(x,y,z)→a± as x→±∞ uniformly with respect to (y,z)∈R2 and which exhibit dihedral symmetries with respect to the variables y and z . We also characterize the asymptotic behavior of these solutions as |(y,z)|→+∞. 相似文献
equation(0.1)
−Δu(x,y,z)+∇W(u(x,y,z))=0,