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431.
Jorgensen WL Jensen KP Alexandrova AN 《Journal of chemical theory and computation》2007,3(6):1987-1992
Variations in hydrogen-bond strengths are investigated for complexes of nine para-substituted phenols (XPhOH) with a water molecule and chloride ion. Results from ab initio HF/6-311+G(d, p) and MP2/6-311+G(d, p)//HF/6-311+G(d, p) calculations are compared with those from the OPLS-AA and OPLS/CM1A force fields. In the OPLS-AA model, the partial charges on the hydroxyl group of phenol are not affected by the choice of para substituent, while the use of CM1A charges in the OPLS/CM1A approach does provide charge redistribution. The ab initio calculations reveal a 2.0-kcal/mol range in hydrogen-bond strengths for the XPhOH?OH(2) complexes in the order X = NO(2) > CN > CF(3) > Cl > F > H >OH >CH(3) > NH(2). The pattern is not well-reproduced with OPLS-AA, which also compresses the variation to 0.7 kcal/mol. However, the OPLS/CM1A results are in good accord with the ab initio findings for both the ordering and range, 2.3 kcal/mol. The hydrogen bonding is, of course, weaker with XPhOH as acceptor, the order for X is largely inverted, and the range is reduced to ca. 1.0 kcal/mol. The substituent effects are found to be much greater for the chloride ion complexes with a range of 11 kcal/mol. For quantitative treatment of such strong ion-molecule interactions the need for fully polarizable force fields is demonstrated. 相似文献
432.
Jang SS Jang YH Kim YH Goddard WA Choi JW Heath JR Laursen BW Flood AH Stoddart JF Nørgaard K Bjørnholm T 《Journal of the American Chemical Society》2005,127(42):14804-14816
Bistable [2]rotaxanes display controllable switching properties in solution, on surfaces, and in devices. These phenomena are based on the electrochemically and electrically driven mechanical shuttling motion of the ring-shaped component, cyclobis(paraquat-p-phenylene) (CBPQT(4+)), between a monopyrrolotetrathiafulvalene (mpTTF) unit and a 1,5-dioxynaphthalene (DNP) unit located along a dumbbell component. The most stable state of the rotaxane (CBPQT(4+)@mpTTF) is that in which the CBPQT(4+) ring encircles the mpTTF unit, but a second less favored metastable co-conformation with the CBPQT(4+) ring surrounding the DNP (CBPQT(4+)@DNP) can be formed experimentally. For both co-conformations of an amphiphilic bistable [2]rotaxane, we report here the structure and surface pressure-area isotherm of a Langmuir monolayer (LM) on a water subphase as a function of the area per molecule. These results from atomistic molecular dynamics (MD) studies are validated by comparing with experiments based on similar amphiphilic rotaxanes. For both co-conformations, we found that as the area per molecule increases the thickness of the LM decreases while the molecular tilt increases. Both co-conformations led to similar LM thicknesses at the same packing area. From the simulated LM systems, we calculated the electron density profiles of the monolayer as a function of area per molecule, which show good agreement with experimental analyses from synchrotron X-ray reflectivity measurements of related systems. Decomposing the overall electron density profiles into component contributions, we found distinct differences in molecular packing in the film depending upon the co-conformation. Thus we find that the necessity of allowing the tetracationic ring to become solvated by water leads to differences in the structures for the two co-conformations in the LM. At the same packing area, the value of the overall tilt angle does not seem to be sensitive to whether the CBPQT(4+) ring is encircling the mpTTF or the DNP unit. However, the conformation of the dumbbell does depend on the location of the CBPQT(4+) ring, which is reflected in the segmental tilt angles of the mpTTF and DNP units. Using the Kirkwood-Buff formula in conjunction with MD calculations, we find the surface pressure-area isotherms for each co-conformation in which the CBPQT(4+)@mpTTF form has smaller surface tension and therefore larger surface pressure than the CBPQT(4+)@DNP at the same packing area, differences that decreases with increasing area per molecule, which is verified experimentally. 相似文献
433.
Several membrane phase separators have been designed and tested for use in a flow-injection extraction manifold. The membrane is sandwiched between two pieces of perspex with grooves facing the membrane. A PTFE membrane with polyethylene backing proved to be most suitable. With this type of phase separator the total dispersion in the extraction system is less than that obtained with the conventional T-piece separator. Alcohols, alkanes, chlorinated hydrocarbons and aromatic solvents pumped at a flow rate of 0.5–1.0 ml min-1 can be segmented with aqueous phase and later separated from it with a recovery of up to 95%. The organic phase passing through the detector flow cell is not contaminated by the aqueous phase to any measurable extent. 相似文献
434.
Emil Damgaard-Møller Dr. Lennard Krause Kasper Tolborg Dr. Giovanni Macetti Dr. Alessandro Genoni Prof. Jacob Overgaard 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(47):21389-21395
Reported here is an entirely new application of experimental electron density (EED) in the study of magnetic anisotropy of single-molecule magnets (SMMs). Among those SMMs based on one single transition metal, tetrahedral CoII-complexes are prominent, and their large zero-field splitting arises exclusively from coupling between the d and dxy orbitals. Using very low temperature single-crystal synchrotron X-ray diffraction data, an accurate electron density (ED) was obtained for a prototypical SMM, and the experimental d-orbital populations were used to quantify the dxy-d coupling, which simultaneously provides the composition of the ground-state Kramers doublet wave function. Based on this experimentally determined wave function, an energy barrier for magnetic relaxation in the range 193–268 cm−1 was calculated, and is in full accordance with the previously published value of 230 cm−1 obtained from near-infrared spectroscopy. These results provide the first clear and direct link between ED and molecular magnetic properties. 相似文献
435.
V. R. Gavrilov V. D. Ivashchuk U. Kasper V. N. Melnikov 《General Relativity and Gravitation》1997,29(5):599-612
The multidimensional cosmological model describing the evolution of n Einstein spaces in the presence of multicomponent perfect fluid is considered. We define vectors related to the equations of state of the components. If they are orthogonal with respect to the minisuperspace metric, the Einstein equations are integrable and a Kasner-like form of the solutions is presented. For special sets of parameters the cosmological model is reduced to the Euclidean Toda-like system connected with some Lie algebra. The integrable vacuum (1+5+5)-model with two 5-dimensional Einstein spaces and non-zero Ricci tensors is obtained. Its reduction to a (1+5+3+2)-solution is given. For a special choice of the integration constant and one of the spaces (M1 = S5) a non-singular solution with the topology
is obtained. 相似文献
436.
U. Kasper 《International Journal of Theoretical Physics》1992,31(6):1007-1014
Thermodynamic quantities and those describing the source of gravitational fields need not be identified. The standard approach is to identify them. If one does not follow this way, one opens the possibility of describing, on the level of phenomenology, the average of the quantum physical process behind the creation of particles and entropy in a comoving volume. Different approaches are briefly discussed. A variational principle with constraints modeling particle and entropy creation is formulated and leads to higher-order gravity field equations. They suggest on the level of phenomenology that the Minkowski space-time could be unstable against vacuum fluctuations. 相似文献
437.
438.
D. Vázquez M. A. Lage J. C. Parajó J. L. Alonso 《Applied biochemistry and biotechnology》1992,37(2):123-139
Samples ofEucalyptus globulus wood were delignified in HC1-catalyzed acetic acid medium under selected conditions and extracted with alkaline solutions
to improve their susceptibility to enzymatic hydrolysis. The effects of three independent variables defining the operational
conditions of the alkaline extraction stage on five dependent variables measuring both the chemical composition and the susceptibility
of the solid residues to enzymatic hydrolysis were assessed using empirical models deduced from experimental data. The enzymatic
conversion depended mainly on the NaOH concentration used in the alkaline extraction. Hydrolysis yields up to 76% were predicted
for operational conditions within the range studied for the independent variables. 相似文献
439.
Frabetti PL Cheung HW Cumalat JP Dallapiccola C Ginkel JF Greene SV Johns WE Nehring MS Butler JN Cihangir S Gaines I Garbincius PH Garren L Gourlay SA Harding DJ Kasper P Kreymer A Lebrun P Shukla S Vittone M Bianco S Fabbri FL Sarwar S Zallo A Culbertson R Gardner RW Greene R Wiss J Alimonti G Bellini G Caccianiga B Cinquini L Di Corato M Giammarchi M Inzani P Leveraro F Malvezzi S Menasce D Meroni E Moroni L Pedrini D Perasso L Sala A Sala S Torretta D Buchholz D Claes D Gobbi B O'Reilly B 《Physical review letters》1994,72(3):324-327
440.
Naples D Akchurin N Birmingham P Breuer H Chang CC Cihangir S Corcoran MD Davis WL Gustafson HR Holmgren H Kasper P Lincoln D Longo MJ Marraffino J McPherson J Miettinen HE Morrow G Mutchler GS Onel Y Thomas GP Traynor MM Waters JW Webster MS Xu JP Zhu Q 《Physical review letters》1994,72(15):2341-2344