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651.
Correa NM Pires PA Silber JJ El Seoud OA 《The journal of physical chemistry. B》2005,109(44):21209-21219
Noninvasive techniques such as FT-IR and (1)H NMR spectroscopy have been employed to investigate the solubilization of formamide, FA, and its aqueous solution, FA-water, by sodium 1,4-bis(2-ethylhexyl)sulfosuccinate, AOT, in heptane or isooctane reverse micelles, respectively. Partially deuterated FA (FADH) was used in the FT-IR experiments and nu(OD), n(ND) were analyzed. Also, the nu(C=O) band of FA was investigated. For AOT, the changes of the SO(3)(-) group's symmetric, nu(s), and asymmetric, nu(a), bands were also studied. The results are showing that FA is interacting strongly with the Na+ counterions of the surfactant through electrostatic interactions maintaining their hydrogen bond network present in the FA bulk. Accordingly, partially deuterated FA is "frozen" inside the aggregates and it is possible to detect, by FT-IR technique, the cis and trans isomers. Curve fitting of the nu(OD) (in the FA-water mixture) band requires use of two peaks because the band is asymmetric, not because the solubilizate molecules are present in layers of different structure. The chemical shifts of the (1)H bound to N and C of FA were studied by (1)H NMR. The comparison of the chemical shift of AOT in reverse micelles with FA and the FA-water mixture in the polar core of the aggregate shows that there is a strong preferential solvation of Na+ by FA (through electrostatic interaction) and the AOT's sulfonate group by water (through hydrogen bond interaction). 相似文献
652.
Freccero M Gandolfi R Sarzi-Amadè M Rastelli A 《The Journal of organic chemistry》2002,67(24):8519-8527
We studied, with the RB3LYP/6-311+G(d,p) method, the mechanism of peroxyformic acid epoxidation of norbornene, norbornadiene, tetramethylethene, and anti- and syn-sesquinorbornenes. The transition structures (TSs) for the reaction of tetramethylethene and norbornene show a perfect spiro geometry (the peroxy acid plane is perpendicular to the C=C bond axis) with synchronous bond formation. Also three out of the four TSs of the norbornadiene reaction are spiro-like, but the highly asynchronous syn,endo-TS has a planar-like geometry. anti- and syn-sesquinorbornenes are substrates that, because of steric constraints, cannot easily accommodate spiro-like TSs. In fact, we managed to locate only a planar-like TS and a planar TS (the peroxy acid plane contains the C=C bond axis), respectively, for these substrates. These planar TSs are "nonconcerted" since they are strongly unsymmetrical and only one of the C-O bonds of the oxirane ring is significantly formed. IRC analysis, while confirming that formation of one C-O bond fully precedes that of the other, also suggests that all this can take place without formation of intermediates, that is, within a "nonconcerted one-step process". Our theoretical data correctly reproduce the experimental facial syn selectivity of norbornene and norbornadiene epoxidations and compare well with the experimental activation free energies of the peroxy acid epoxidation of all the olefins reported here. This accord validates the method used as adequate to deal with the reactivity of these systems. 相似文献
653.
A simple lab-made apparatus allowing the use of a SPME fiber as sorptive device for isolation of analytes from varied samples using dynamic headspace sampling (DHS) is presented here. The principal parameters for the operation of this device - the extraction time and the flow of purging gas - were studied using aqueous test solutions of volatile and semi-volatile analytes. Compared to conventional HS-SPME, DHS-SPME provided a substantial reduction in the extraction time with linearity, accuracy and precision comparable to the conventional approach. For a complex matrix such as mango pulp, the extraction efficiency of volatiles using the DHS-SPME combination was up to six times higher than that of conventional HS-SPME. 相似文献
654.
Giuseppe Cilento Nelson Durán Klaus Zinner Carmen C. C. Vidigal Olga M. M. Faria Oliveira Marcela Haun Adelaide Fauoni Ohara Augusto Roberto Casadei de Baptista Etelvino J. H. Bechara 《Photochemistry and photobiology》1978,28(4-5):445-450
Abstract— Several hemeprotein-catalyzed reactions generate products of the type expected from the cleavage of a high energy intermediate. For some systems, the formation, in high yield, of a carbonyl compound in its excited triplet state has been firmly established on the basis of (i) equivalence of the chemiluminescence and phosphorescence spectra of the expected products; (ii) energy transfer to sensitizers containing heavy atoms and (iii) occurrence of photoproducts. The excited species appears to be generated within the enzyme and shielded from quenching by oxygen. It may be quenched, however, via long-range triplet-singlet energy transfer.
This work strongly supports our hypothesis that excited electronic states are also formed in biological systems which are not necessarily bioluminescent. One of the functions which peroxidases may thus fulfill might be the utilization of the potential of photochemistry in the absence of light. 相似文献
This work strongly supports our hypothesis that excited electronic states are also formed in biological systems which are not necessarily bioluminescent. One of the functions which peroxidases may thus fulfill might be the utilization of the potential of photochemistry in the absence of light. 相似文献
655.
Cingolani A Effendy Pellei M Pettinari C Santini C Skelton BW White AH 《Inorganic chemistry》2002,41(25):6633-6645
Adducts of triorganophosphine, triphenylarsine, and triphenylstibine with silver(I) nitrite have been synthesized and characterized both in solution ((1)H, (31)P NMR) and in the solid state (IR, single-crystal X-ray structure analysis). In addition aggregates of AgNO(2) and ER(3) (E = P, As, Sb) have been identified in solution by electrospray ionization mass spectrometry (ESI-MS). The topology of the structures in the solid state was found to depend on the nature of ER(3) and on the stoichiometric ratio AgNO(2):ER(3). The adducts AgNO(2):EPh(3) (1:1) (E = P or Sb) are one-dimensional polymers, the role of NO(2)(-) being to bridge successive metal atoms by coordination of the two oxygens to one silver atom and the nitrogen lone pair to a successive Ag. The adduct AgNO(2):P(o-tolyl)(3) (1:1) is mononuclear, due to steric hindrance of the phosphine, the nitrite being O,O'-bidentate, a rare example of a quasi-linear P-Ag-X array. AgNO(2):P(p-F-C(6)H(4))(3) (1:1) is a dimer, the nitrite being coordinated through both oxygens, the first unidentate, the second bridging bidentate. P(o-tolyl)(3) and Pcy(3) form 1:2 adducts, also mononuclear, the nitrite still an O,O'-chelate. In contrast, the adduct AgNO(2):AsPh(3) (1:2) is a centrosymmetric dimer, essentially an aggregate of a pair of [Ag(O(2)N)(AsPh(3))(2)] arrays with one nitrite oxygen being the bridging atom. The adducts AgNO(2):EPh(3) (1:3) (E = As, Sb) are mononuclear, the nitrite behaving as a consistently strong O,O'-chelate. The E = As adduct is a triclinic solvated form, whereas the unsolvated E = Sb species is monoclinic. ESI-MS spectra of acetonitrile solutions of these complexes show the existence of [Ag(ER(3))](+), [Ag(CH(3)CN)](+), [Ag(CH(3)CN)(2)](+), [AgCl(2)](-), [Ag(NO(2))(2)](-), [Ag(ER(3))(CH(3)CN)](+), and [Ag(ER(3))(2)](+) as well as higher aggregates [Ag(2)(NO(2))(ER(3))(2)](+), [Ag(2)(NO(2))(3)](-) and [Ag(2)Cl(2)(NO(2))](-), which are less prevalent. 相似文献
656.
Shimizu S Pires PA El Seoud OA 《Langmuir : the ACS journal of surfaces and colloids》2004,20(22):9551-9559
The enthalpies of micellization of the surfactant series benzyl(2-acylaminoethyl)dimethylammonium chlorides, RABzMe(2)Cl, have been determined by calorimetry and conductivity measurements in the temperature range 15-75 degrees C. Here R stands for an acyl group containing 10-16 carbon atoms and A, Bz, and Me stand for NH(CH(2))(2)N(+), benzyl, and methyl groups, respectively. The enthalpy of micellization, DeltaH(mic) degrees , and the critical micelle concentration, cmc, were calculated directly from calorimetric data. The free energy of micellization, DeltaG(mic) degrees , was obtained from the cmc and the conductance-based degree of counterion dissociation. There is an excellent agreement between DeltaG(mic) degrees calculated from the data of both techniques, but the DeltaH(mic) degrees , the entropy of micellization, values differ. The dependence of the thermodynamic parameters of micellization on the chain length of the hydrophobic group and on the temperature has been analyzed by considering the delicate balance between the factors that contribute to micelle formation, including transfer of the surfactant hydrocarbon chain from the aqueous environment to the micelle, with concomitant release of the solvating water molecules, and the effect of temperature on the structure of water. DeltaG(mic) degrees is more negative, that is, more favorable for RABzMe(2)Cl than for the structurally related alkylbenzyldimethylammonium chlorides. This is attributed to direct and water-mediated H bonding between the amide groups of molecules of the former series. 相似文献
657.
Falcaro P Costacurta S Mattei G Amenitsch H Marcelli A Guidi MC Piccinini M Nucara A Malfatti L Kidchob T Innocenzi P 《Journal of the American Chemical Society》2005,127(11):3838-3846
One-pot self-assembled hybrid films were synthesized by the cohydrolysis of methyltriethoxysilane and tetraethoxysilane and deposited via dip-coating. The films show a high "defect-free" mesophase organization that extends throughout the film thickness and for domains of a micrometer scale, as shown by scanning transmission electron microscopy. We have defined these films defect-free to describe the high degree of order that is achieved without defects in the pore organization, such as dislocations of pores or stacking faults. A novel mesophase, which is tetragonal I4/mmm (space group), is observed in the films. This phase evolves but retains the same symmetry throughout a wide range of temperatures of calcination. The thermal stability and the structural changes as a function of the calcination temperature have been studied by small-angle X-ray scattering, scanning transmission electron microscopy, and Fourier transform infrared spectroscopy. In situ Fourier transform infrared spectroscopy employing synchrotron radiation has been used to study the kinetics of film formation during the deposition. The experiments have shown that the slower kinetics of silica species can explain the high degree of organization of the mesostructure. 相似文献
658.
Izquierdo RA Barros CM Santana-Marques MG Correia AJ Silva AM Tomé AC Silva A Neves MG Cavaleiro JA 《Rapid communications in mass spectrometry : RCM》2004,18(22):2601-2611
Neutral and cationic pyrrolidine-fused chlorins and isobacteriochlorins derived from meso-tetrakis(pentafluorophenyl)porphyrin undergo cycloreversion reactions in the gas phase, either when desorbed from a liquid matrix by ion bombardment or when electrosprayed. Cycloreversion occurs through loss of either neutral or charged moieties, with and without hydrogen and methyl radical migration, and both as high- and low-energy collision processes. For the doubly charged isobacteriochlorin, one-electron reduction with methyl loss occurs under ion bombardment and electrospray, through hypervalent pyrrolidinium radical formation. 相似文献
659.
Milton Rogério Pereira Pedro Augusto Arroyo Maria Angélica Simões Dornellas de Barros Viviane Monteiro Sanches Edson Antonio da Silva Isabel Maria Fonseca Rafael García Lovera 《Adsorption》2006,12(2):155-162
In this work, Cr(III) adsorption on activated carbon obtained from olive stones in an upflow fixed-bed column at 30∘C was studied. The flow rate influence on the breakthrough curves at a feed concentration of 0.87 meq/L was investigated in
an attempt to minimize the diffusional resistances. Breakthrough curves for a flow range of 2–8 mL/min were obtained at 10.5
cm bed height and inlet diameter of 0.9 cm. The mass transfer parameters indicated that the bed minimal resistance was attained
at 2 mL/min. Therefore, the data equilibrium was carried out until the bed was saturated at 2 mL/min. The dynamic system generated
a favorable isotherm with a maximum chromium uptake of 0.45 meq/g. A column sorption mathematical model was created considering
the axial dispersion in the column and the intraparticle diffusion rate-controlling steps. The isotherm was successfully modeled
by the Langmuir equation and the mathematical model described the experimental dynamic data adequately for feed concentrations
from 0.26 to 3.29 meq/L. 相似文献
660.
Highly porous solid-phase microextraction fiber coating based on poly(ethylene glycol)-modified ormosils synthesized by sol-gel technology 总被引:2,自引:0,他引:2
The preparation and characteristics of solid-phase microextraction (SPME) fibers coated with Carbowax 20M ormosil (organically modified silica) are described here. Raw fused silica fibers were coated with Carbowax 20M-modified silica using sol-gel process. Scanning electron micrographs of fibers revealed a highly porous, sponge-like coating with an average thickness of (8 +/- 1) microm. The sol-gel Carbowax fibers were compared to commercial fibers coated with 100 microm polydimethylsiloxane (PDMS) and 65 microm Carbowax-divinylbenzene (DVB). Shorter equilibrium times were possible with the sol-gel Carbowax fiber: for headspace extraction of the test analytes, they ranged from less than 3 min for benzene to 15 min for o-xylene. Extraction efficiencies of the sol-gel Carbowax fiber were superior to those of conventional fibers: for o-xylene, the extracted masses were 230 and 540% of that obtained with 100 microm PDMS and 65 microm Carbowax-DVB fibers, respectively. 相似文献