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41.
Inside Back Cover: A Terminal,Fluxional η4‐Benzene Complex with a Thermally Accessible Triplet State is the Primary Photoproduct in the Intercyclobutadiene Haptotropism of (CpCo)phenylenes (Chem. Eur. J. 12/2015) 下载免费PDF全文
42.
Experimentally, tri-trans-[12]annulene and tris(cyclohexeno)[12]annulene exhibit differing reactivities. Whereas the former, after isomerizing to its di-trans isomer, undergoes sequential electrocyclizations, the latter follows a Diels-Alder pathway after initial electrocyclization. B3PW91/6-31+G*//B3LYP/6-31G* calculations indicate that cyclohexenofusion simultaneously hinders the second electrocyclization and facilitates Diels-Alder reaction, primarily by inducing greater puckering in the intermediate eight-membered ring. 相似文献
43.
Vysotsky YB Belyaeva EA Vollhardt D Aksenenko EV Miller R 《Journal of colloid and interface science》2008,326(2):339-346
A simplified method is proposed to estimate the thermodynamic parameters of clusterization at the air/water interface for various classes of amphiphilic compounds with a single alkyl chain. The method is based on the calculation of thermodynamic characteristics only for one of the homologous series of dimers (n=6-16) governing the formation of infinite clusters. The method is used to calculate the thermodynamic parameters of clusterization for alcohols, thioalcohols, carboxylic acids and amines, and the dependencies of the Gibbs energy of clusterization on the alkyl chain length are evaluated. It is shown that the alkyl chain length, at which the spontaneous clusterization begins, as calculated using the proposed simplified method, is in fact the same as that calculated using the additive scheme developed earlier. The simplified method proposed was verified using alkylnitriles as example. In contrast to alcohols, thioalcohols and amines, infinite 'rhombic' clusters are formed rather than 'rectangular' clusters for this class of compounds. Spontaneous clusterization of nitriles is shown to start for alkyl chains containing 18-19 carbon atoms. This value agrees with that obtained from experimental data with 17-18 carbon atoms. The proposed simplified method introduces an exact and suitable tool for the estimation of thermodynamic parameters of the clusterization of amphiphilic compounds. 相似文献
44.
Nandi N Thirumoorthy K Vollhardt D 《Langmuir : the ACS journal of surfaces and colloids》2008,24(17):9489-9494
Chiral discrimination in Langmuir monolayers of amphiphilic 1-stearylamine-glycerol is studied using the hybrid quantum mechanical/molecular mechanical method. Using the experimental information about the lattice structure [D. Vollhardt, U. Gehlert, J. Phys. Chem. B. 2002, 106, 4419], the intermolecular interaction profiles for enantiomeric and racemic pair are studied as a function of mutual tilt and azimuth for different values of intermolecular separation. The present study reveals that, at shorter separation, the interaction profile of the racemic pair has deeper minima than the enantiomeric pair, whereas at larger separation the minimum of the enantiomeric pair is deeper. Thus, the theoretical studies reveal an interesting crossover from heterochiral preference to homochiral preference in 1-stearylamine-glycerol monolayers, with the increase in the intermolecular separation corresponding to a larger area per molecule in the monolayer. This predicts that, with gradual compression, the interactions between racemic pair dominate the experimental features, whereas, under nonequilibrium conditions at the beginning of the formation of the condensed phase, the experimental characteristics of homochirality are observable. The study conclusively shows that the chiral structure of the molecule and the lattice packing drive the chiral preference at the mesoscopic level. 相似文献
45.
Thirumoorthy K Nandi N Vollhardt D 《Langmuir : the ACS journal of surfaces and colloids》2007,23(13):6991-6996
The role of dipolar interactions in determining the lipid domain shapes at the air-water interface with a change in the chemical structure of the head groups of lipids is theoretically studied. The phospholipids considered are dipalmitoylphosphatidylcholine (D,L-DPPC) and dipalmitoylphosphatidylethanolamine (DPPE). Despite closely similar chemical structures, the domains of the two lipids are strikingly different. The DPPC domains exhibit elongated arms, while the DPPE domains are nearly round-shaped. To compare the dipolar repulsions in the domains of the two phospholipids, different energy-minimized conformers of DPPC and DPPE are studied using the semiempirical quantum chemical method (PM3). It is found that the dipole moment of DPPC is significantly larger than that of DPPE. The in-plane and out-of-plane components of the dipole moments are calculated using grazing incidence X-ray diffraction data at different surface pressure values, as used in the experiment. The result indicates that the magnitude of the dipolar interaction is significantly larger in DPPC than that in DPPE over the surface pressure range considered. The enhanced dipolar repulsion corroborates well with the difference in the domain shapes in the two phospholipid monolayers. The larger dipolar repulsion in DPPC leads to development of elongated domain arms, while relatively less dipolar repulsion allows a closed shape of the condensed-phase DPPE domains. 相似文献
46.
Kovalchuk NM Vollhardt D Fainerman VB Aksenenko EV 《The journal of physical chemistry. B》2007,111(28):8283-8289
Progress in the understanding of interfacial molecular recognition kinetics is obtained by use of the sweeping technique for experimental studies of the reaction kinetics between a host monolayer and a non-surface-active species dissolved in the aqueous subphase. The experimental results show that the interfacial recognition reaction between a 2C(11)H(23)-melamine (2,4-di(n-undecylamino)-6-amino-1,3,5-triazine) monolayer and dissolved barbituric acid is reversible when the 2C(11)H(23)-melamine/barbituric acid monolayer is transferred back onto a pure water subphase. The kinetics of the recognition and dissociation reaction is experimentally and theoretically investigated. The approximate additive theoretical model developed recently is extended to consider the dissociation kinetics of the interfacial supramolecular complex. The kinetic constants for the recognition and dissociation reactions in the mixed monolayer consisting of 2C(11)H(23)-melamine and 2C(11)H(23)-melamine/barbituric acid complex are determined. It is shown that the kinetic constant of the recognition reaction is nearly independent of temperature, whereas that of the dissociation reaction increases with increasing temperature. 相似文献
47.
48.
K. Byczuk W. Hofstetter U. Yu D. Vollhardt 《The European physical journal. Special topics》2009,180(1):135-151
Several new aspects of the subtle interplay between electronic correlations and disorder are reviewed. First, the dynamical
mean-field theory (DMFT) together with the geometrically averaged (“typical”) local density of states is employed to compute
the ground state phase diagram of the Anderson-Hubbard model at half-filling. This non-perturbative approach is sensitive
to Anderson localization on the one-particle level and hence can detect correlated metallic, Mott insulating and Anderson
insulating phases and can also describe the competition between Anderson localization and antiferromagnetism. Second, we investigate
the effect of binary alloy disorder on ferromagnetism in materials with f-electrons described by the periodic Anderson model.
A drastic enhancement of the Curie temperature Tc caused by an increase of the local f-moments in the presence of disordered conduction electrons is discovered and explained. 相似文献
49.
Gandon V Agenet N Vollhardt KP Malacria M Aubert C 《Journal of the American Chemical Society》2006,128(26):8509-8520
The mechanism of the cobalt-mediated [2 + 2 + 2] cycloaddition of two alkynes to one alkene to give CpCo-complexed 1,3-cyclohexadienes (cyclic oligomerization) has been studied by means of DFT computations. In contrast to the mechanism of alkyne cyclotrimerization, in which final alkyne inclusion into the common cobaltacyclopentadiene features a direct "collapse" pathway to the complexed arene, alkene incorporation proceeds via insertion into a Co-C sigma-bond rather than inter- or intramolecular [4 + 2] cycloaddition. The resulting seven-membered metallacycle 7 is a key intermediate which leads to either CpCo-complexed cyclohexadiene 5 or hexatriene 13. The latter transformation, particularly favorable for ethene, accounts, in part, for the linear oligomerization observed occasionally in these reactions. With aromatic double bonds, a C-H activation mechanism by the cobaltacyclopentadiene seems more advantageous in hexatriene product formation. Detailed investigations of high- and low-spin potential energy surfaces are presented. The reactivity of triplet cobalt species was found kinetically disfavored over that of their singlet counterparts. Moreover, it could not account for the formation of CpCo-complexed hexatrienes. However, triplet cobalt complexes cannot be ruled out since all unsaturated species appearing in this study were found to exhibit triplet ground states. Consequently, a reaction pathway that involves a mixing of both spin-state energy surfaces is also described (two-state reactivity). Support for such a pathway comes from the location of several low-lying minimum-energy crossing points (MECPs) of the two surfaces. 相似文献
50.
Kovalchuk NM Pimienta V Tadmouri R Miller R Vollhardt D 《Langmuir : the ACS journal of surfaces and colloids》2012,28(17):6893-6901
A system far from equilibrium, where the surfactant transfer from a small drop located in the aqueous bulk to the air-water interface results in spontaneous nonlinear oscillations of surface tension, is theoretically and experimentally considered. The oscillations in this system are the result of periodically arising and terminating Marangoni instability. The surfactant under consideration is octanoic acid, the dissociated form of which is much less surface-active than the protonated form. Numerical simulations show how the system behavior can be controlled by changes in pH and ionic strength of the aqueous phase. The results of numerical simulations are in good agreement with experimental data. 相似文献