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931.
The use of time-dependent density functional calculations for the optimization of excited-state structures and the subsequent calculation of resonance Raman intensities within the transform-theory framework is compared to calculations of Hartree-Fock/configuration interaction singles-type (CIS). The transform theory of resonance Raman scattering is based on Kramers-Kronig relations between polarizability tensor components and the optical absorption. Stationary points for the two lowest excited singlet states of uracil are optimized and characterized by means of numerical differentiation of analytical excited-state gradients. It is shown that the effect of electron correlation leads to substantial modifications of the relative intensities. Calculations of vibrational frequencies for ground and excited states are carried out, which show that the neglect of Duschinsky mixing and the assumption of equal wave numbers for ground and excited state are not in all cases good approximations. We also compare the transform-theory resonance Raman intensities with those obtained within a simple approximation from excited-state gradients at the ground-state equilibrium position, and find that they are in qualitative agreement in the case of CIS, but show some important differences in calculations based on density functional theory. Since the results from CIS calculations are in better agreement with experiment, we also present approximate resonance Raman spectra obtained using excited-state gradients from multireference perturbation theory calculations, which confirm the CIS gradients. 相似文献
932.
Bernd Mayer Giancarlo Marconi Christian Klein Gottfried Köhler Peter Wolschann 《Journal of inclusion phenomena and macrocyclic chemistry》1997,29(1):79-93
Complexation trajectories and the variation ofinduced circular dichroism are calculated for thedocking of phenol and 2,4,6-trimethyl-phenol with-cyclodextrin. The results are compared toexperimental chirality data to elucidate themechanism of nonspecific molecular recognitionprocesses in aqueous solution. Large geometricalchanges along nearly isoenergetic Dynamic Monte Carlotrajectories show the conformational flexibility ofsuch host–guest systems. This proves diffuseintermolecular interactions, van der Waals orelectrostatic in nature, as the main contributions to thebinding energy. The number and position of the methylsubstituents of the guest reduces the complexityof the conformational space as the guests positionbecomes fixed by steric constraints.The solvation free energy is calculated from thesolvent accessible surface area weighted byrespective atomic solvation parameters. Consideringthe solvation term in the dynamic simulationsrestricts the conformational flexibility of themacromolecular system. The relative importance ofvarious contributions to the solvation energy isdiscussed and it is shown that those terms arisingfrom the interaction of hydrophobic groups with theaqueous environment are essential for thedetermination of the complex structure. Consideringthese terms in the dynamic simulation model, the signand strength of the calculated rotatory strength isin perfect agreement with induced circular dichroismobtained from experimentally determined averagedspectra. The results demonstrate the accuracy of thegeometrical properties of host–guest systems obtainedfrom these simulations. 相似文献
933.
Helge Klare Sebastian Hanft Dr. Jörg M. Neudörfl Dr. Nils E. Schlörer Prof. Dr. Axel Griesbeck Prof. Dr. Bernd Goldfuss 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(37):11847-11855
Modular cyclodiphosph(V)azanes are synthesised and their affinity for chloride and actetate anions were compared to those of a bisaryl urea derivative ( 1 ). The diamidocyclodiphosph(V)azanes cis‐[{ArNHP(O)(μ‐tBu)}2] [Ar=Ph ( 2 ) and Ar=m‐(CF3)2Ph ( 3 )] were synthesised by reaction of [{ClP(μ‐NtBu)}2] ( 4 ) with the respective anilines and subsequent oxidation with H2O2. Phosphazanes 2 and 3 were obtained as the cis isomers and were characterised by multinuclear NMR spectroscopy, FTIR spectroscopy, HRMS and single‐crystal X‐ray diffraction. The cyclodiphosphazanes 2 and 3 readily co‐crystallise with donor solvents such as MeOH, EtOH and DMSO through bidentate hydrogen bonding, as shown in the X‐ray analyses. Cyclodiphosphazane 3 showed a remarkably high affinity (log[K]=5.42) for chloride compared with the bisaryl urea derivative 1 (log[K]=4.25). The affinities for acetate (AcO?) are in the same range ( 3 : log[K]=6.72, 1 : log[K]=6.91). Cyclodiphosphazane 2 , which does not contain CF3 groups, exhibits weaker binding to chloride (log[K]=3.95) and acetate (log[K]=4.49). DFT computations and X‐ray analyses indicate that a squaramide‐like hydrogen‐bond directionality and Cα?H interactions account for the efficiency of 3 as an anion receptor. The Cα?H groups stabilise the Z,Z‐ 3 conformation, which is necessary for bidentate hydrogen bonding, as well as coordinating with the anion. 相似文献
934.
Marc Gongoll Bernd H. Müller Stephan Peitz Bhaskar R. Aluri Normen Peulecke Anke Spannenberg 《Phosphorus, sulfur, and silicon and the related elements》2013,188(12):1845-1854
Abstract On the basis of the known aminodiphosphinoamine ligand Ph2PN(i-Pr)P(Ph) N(i-Pr)-H (3a), differently substituted aminodiphosphinoamine PNPN-H ligands (3) were prepared. By using different synthetic methods, the N-substituted ligands Ph2PN (i-Pr)P(Ph)N(c-Hex)-H (3b), Ph2PN(c-Hex)P(Ph)N(i-Pr)-H (3g), and Ph2PN(i-Pr)P(Ph) N[(CH2)3Si(OEt)3]-H (3c), in addition to the formerly described Ph2PN(n-Hex)P (Ph)N (i-Pr)-H (3h), Ph2PN(i-Pr)P(Ph)N(Et)-H (3d), Ph2PN(i-Pr)P(Ph)N(Me)-H (3e), and Ph2PN(c-Hex)P(Ph)N(c-Hex)-H (3f), were obtained. In addition, Ph2PN(i-Pr)P(Me)N(i-Pr)-H (3i), (cyclopentyl)2PN(i-Pr)P(Ph)N(i-Pr)-H (3j), (-O-CH2-CH2-O-)PN(i-Pr)P(Ph)N(i-Pr)-H (3k), and (1-Ad)2PN(i-Pr)P(Ph)N(i-Pr)-H (3l) were prepared with different P-substitutions. All compounds were characterized and the molecular structures of the intermediates Ph2PN(i-Pr)P(Ph)Cl (1a) and (cyclopentyl)2PN(i-Pr)P(Ph)Cl (1e) and the ligand (1-Ad)2PN(i-Pr)P(Ph)N(i-Pr)-H (3l) were investigated by single-crystal X-ray diffraction. Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file. 相似文献
935.
JPC – Journal of Planar Chromatography – Modern TLC - 相似文献
936.
Jan Holec Dr. Jiří Rybáček Dr. Jaroslav Vacek Dr. Manfred Karras Dr. Lucie Bednárová Dr. Miloš Buděšínský Michaela Slušná Dr. Petr Holý Prof. Bernd Schmidt Dr. Irena G. Stará Dr. Ivo Starý 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(49):11494-11502
Racemic and highly enantioenriched 3-methoxycarbonyl, 3-carboxy, and 3-hydroxymethyl derivatives of dibenzo[6]helicene were prepared. The Langmuir layers of these helicenes were formed at the air–water interface and transferred onto solid substrates to afford Langmuir–Blodgett films, which were then studied by ambient atomic force microscopy and (chir)optical spectroscopy. Significant differences were found in the behaviour of the Langmuir layers as well as in the morphology, UV/Vis, electronic circular dichroism (ECD), and fluorescence spectra of the Langmuir–Blodgett thin films depending on the molecular chirality and nature of the polar group. The experimental results were supported by molecular dynamics simulations. 相似文献
937.
938.
939.
Luís M.N.B.F. Santos Luís M. Spencer S. Lima Carlos F.R.A.C. Lima Fernão D. Magalhães Manuel C. Torres Bernd Schröder Manuel A.V. Ribeiro da Silva 《The Journal of chemical thermodynamics》2011,43(6):834-843
A new Knudsen effusion apparatus, enabling simultaneous gravimetric and quartz crystal microbalance mass loss detection, is described. This device allows the measurement of vapour pressures of small sample mass (50 to 100) mg over a wide temperature range (350 to 650) K using very short effusion time intervals. The performance of the apparatus was checked by measuring the vapour pressures of anthracene, benzanthrone, and 1,3,5-triphenylbenzene, between (0.1 and 1) Pa, over temperature intervals of 20 K. The derived standard molar enthalpies of sublimation and vapour pressures are in excellent agreement with the mean of the available literature values and with the recommended values. The new working methodology and design of this apparatus allows the measurement of high quality vapour pressure data due to: accurate temperature measurement and control; improvement in vacuum thermal contact between the effusion cell and the oven metal block; optimisation of the quartz crystal sensor head microbalance position; efficient temperature control of the quartz crystal microbalance head; accurate measurement of the resonance crystal frequency using an impedance circuit analyser methodology. 相似文献