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111.
Giuliano Alagona Caterina Ghio Peter A. Kollman 《Journal of Molecular Structure》1997,390(1-3):217-223
There is chemical and kinetic evidence that the mechanism of action of mandelate racemase occurs via two acid/base catalysts, one to abstract a proton from the -carbon and the second to deliver it back to the opposite face of the chiral center. Since the mechanistic details are not known, a few hypotheses have been put forward. Therefore we examine the viability of both concerted and sequential mechanisms on proton abstraction from the -carbon of mandelate, either without taking into account environmental effects or with the addition to the model system of several functional groups of the residues belonging to the active site, in order to restrain the partners to particular positions without actually imposing arbitrary constraints. The importance of the environment in screening the most highly charged groups in the substrate and in facilitating the -carbon-bound proton abstraction is evident. From the perusal of the geometries of the system along the reaction energy profile, it seems that there is no tendency toward a concerted mechanism: the proton delivery from mandelate to Lys occurs and subsequently the extra proton on protonated histidine is in turn delivered to mandelate. 相似文献
112.
Fabrea salina is a marine ciliate that shows photomotile responses such as positive phototaxis and a step-down photophobic reaction. We found that preilluminated F. salina cells show a phototactic response significantly greater than that of dark-adapted cells when exposed to the same phototactic light stimulus. In particular, positive phototaxis is strongly enhanced by preillumination. This enhancement effect depends on the preillumination light irradiance, on the total preillumination dose, and on the duration of the dark interval between preillumination and the phototaxis measurement. Our results show that the determining factor is the total preillumination dose given to the sample. The enhancement effect shows an asymptotic behavior over a certain range of energy values (10-200 W/m2). Further, the effect is transient; after 120 s in the dark, the cells lose any memory of the preillumination, independent of the preillumination energy received. These results are tentatively discussed in terms of light-driven membrane potential or membrane channel conductances. 相似文献
113.
The intramolecular mechanism we earlier proposed [Alagona, G.; Desmeules, P.; Ghio, C.; Kollman, P. A. J Am Chem Soc 1984, 106, 3623] for the second proton transfer of the reaction catalyzed by triosephosphate isomerase (TIM) is examined ab initio at the HF and MP2/6-31+G** levels in vacuo for two conformers of the enediolate phosphate (ENEP), with the ethereal oxygen of the phosphate group either syn (X), as in the crystal structure, or anti (Y) with respect to the enediolate carbonyl O. The barrier height for the intramolecular proton transfer occurring in enediolate is very sensitive to electron correlation corrections. The MP2 internal energy barrier is much lower than the HF one, while the free energy (FE) barrier is even more favorable, indicating that the enzyme presence is not requested to speed up that step. An investigation of the dynamical aspects of the mechanism, along the pathway from ENEP A (with H on O(1)) to TS and from TS to ENEP B (with H on O(2)), was, however, carried out in the presence of the enzyme field while using a neutral His-95 with its proton on Ndelta. To perform the FE simulations, it was necessary to parametrize in the AMBER force-field the ENEP A, TS and B species, whose partial charges have been determined with the RESP procedure, with the X and Y arrangements of the phosphate head. Actually, the FE/QM approach produced a low barrier and a substantial balance between A and B, especially at the MP2 level. The trajectories, analyzed paying a particular attention to the positions assumed by His-95 and by the other active site residues, put forward somewhat different H-bond patterns around the X or Y enediolate phosphate. 相似文献
114.
Roberto Marangoni Nicola Messina Domenico Gioffr Giuliano Colombetti 《Photochemistry and photobiology》2004,80(1):78-83
Purpose of this work was to study the effect of UV irradiation on a microecosystem consisting of several interacting species. The system chosen was of a hypersaline type, where all the species present live at high salt concentration; it comprises different bacteria; a producer, the photosynthetic green alga Dunaliella salina; and a consumer, the ciliated protozoan Fabrea salina, which form a complete food chain. We were able to establish the initial conditions that give rise to a selfsustaining microecosystem, stable for at least 3 weeks. We then determined the effect of UV irradiation on this microecosystem under laboratory‐controlled conditions, in particular by measuring the critical UV exposure for the two main components of the microecosystem (algae and protozoa) under UV‐B irradiances comparable to those of solar irradiation. In our experiments, we varied irradiance, total dose and spectral composition of the actinic light. The critical doses at irradiances of the order of 56 kJ/m2 (typical average daily irradiance in a sunny summer day in Pisa), measured for each main component of the microecosystem (algae and ciliates), turned out to be around 70 kJ/m2 for ciliates and 50 kJ/m2 for D. salina. By exposing microecosystems to daily UV‐B irradiances of the order of 8 kJ/m2 (typical average daily irradiance in a sunny winter day in Pisa), we found no effect at total doses of the order of the critical doses at high irradiances, showing that the reciprocity law does not hold. We have also measured a preliminary spectral‐sensitive curve of the UV effects, which shows an exponential decay with wavelength. 相似文献
115.
Claudio Bianchini Giuliano Giambastiani Giuseppe Mantovani Andrea Meli David Mimeau 《Journal of organometallic chemistry》2004,689(8):1356-1361
On activation by MAO, 2-(imino)pyridine cobalt dichlorides bearing a benzo[b]thiophen-2-yl substituent in the 6-position of the pyridine ring oligomerise ethylene to α-olefins with turn-over-frequencies as high as 1.5 × 106 mol of C2H4 converted (mol of Co × h)−1 and productivities as high as 3769 kg of oligomers (mol of Co × h × bar)−1. Aldimine precursors are more active than ketimine analogues, yet ketimines give higher molecular weight oligomers. 相似文献
116.
117.
Charles Brown Brian T. Heaton Anthony D.C. Towl Paolo Chini Alessandro Fumagalli Giuliano Longoni 《Journal of organometallic chemistry》1979,181(1):233-254
Platinum-195 spectra are reported for [Ptn(CO)2n]2- (n = 3, 6, 9, 12 and 15) and carbon-13 spectra are reported for n = 6, 9 and 12 over a range of temperatures. The spectra provide evidence for (a) intramolecular rotation of the Pt3-triangles about the principal three-fold axis, (b) inter-exchange of Pt3-triangles, (c) lack of terminal/edge carbonyl exchange within the Pt3(CO)3(μ-CO)3 group. Evidence is also presented for the formation of [Ni3Pt3(CO)12]2- on mixing [Pt6(CO)12]2- [Ni6(CO)12]2-. 相似文献
118.
Commercial monostearates of glycerol, generally used as antistatic agents for thermoplastic polymers, consist of a complex mixture of mono-, di- and trisubstituted glycerides and the corresponding fatty acids. Thus far, the glycerides and the fatty acids have been analyzed separately by high-performance liquid chromatography (HPLC). In fact, the simultaneous analysis of glycerides and fatty acids requires esterification of the acids in order to avoid overlapping of the chromatographic peaks. This paper describes an HPLC method which allows the direct separation and identification, simultaneously and in a single run, of the variously substituted glycerides, and also the corresponding saturated fatty acids that are found as by-products in commercial glycerol monostearates. The procedure is based on the use of a ternary gradient HPLC instrument equipped with an evaporative light-scattering detector; the stationary phase was a reversed-phase RP-8 end-capped (Merck) column; the mobile phase was two consecutive binary gradients consisting of acetonitile-water plus acetic acid (0.1%, v/v) and acetonitrile-methylene chloride. The method is fast and shows high sensitivity and selectivity, being able to separate also the positional isomers of mono- and digycerides in addition to the mixed di- and triglycerides. 相似文献
119.
The reaction of ring opening of epoxides under the action of a halogenic acid has been investigated using as model the system C2H4O + HF → CH2OHCH2F. The physical conditions chosen for the model more directly correspond to the gas phase reaction and under these conditions the preferred mechanism leads to the formation of a halohydrine having the same configuration at the C atom as the reagent. Parallel investigations have been performed on other mechanisms which postulate the preliminary formation of the conjugate acid of the oxirane (C2H4OH+) and proceed via the well known mechanismsA 1 orA 2. In this case the best mechanism corresponds to the so-called “borderlineA 2” mechanism. This last type of mechanism probably is the dominant one in protic solutions, but by coupling the present calculations with experimental conductometric measurements in anhydrous aprotic media one could consider the first concerted mechanism as a possible candidate also for the reaction in “inert” media. A qualitative analysis of the transition state indicates, in addition, that the propension for the retention path, is probably emphasized by the use of HF as reactant, and that with other acids, like HC1, or even by assuming the presence of dimers like HF·HF, the inversion path could be preferred. The investigations have been done by determining the geometry of the transition state and the reaction coordinate withab initio SCF STO-3G calculations on the whole nuclear configuration space (21 dimensions). These calculations have been supplemented by a few CI calculations on the same basis set and by a few SCF calculations with a larger basis set. 相似文献
120.
Giuliano Freddi Patrizia Monti Masanobu Nagura Yohko Gotoh Masuhiro Tsukada 《Journal of Polymer Science.Polymer Physics》1997,35(5):841-847
Structural changes of tussah (Antheraea pernyi) silk fibroin films induced by heat treatment were studied as a function of the treatment temperature in the range 200–250°C. The DSC curve of tussah films with α-helix molecular conformation displayed characteristic endo and exo peaks at 216 and 226°C, respectively. These peaks first weakened and then completely disappeared after heating at 230°C. Accordingly, the TMA thermal shrinkage at 206°C disappeared when the films were heated at 230°C. The onset of weight loss was monitored at 210°C by means of TG measurements. X-ray diffraction profiles gradually changed from α-helix to β-sheet crystalline structure as the treatment temperature increased from 200 to 250°C. On raising the heating temperature above 200°C, the intensity of IR and Raman bands characteristic of β-sheet conformation increased in the whole ranges of amide and skeletal modes. The sample treated at 200°C showed a spectral pattern intermediate between α-helix and β-sheet molecular conformation. The IR marker band for random coil structure, still detectable at 200°C, disappeared at higher treatment temperatures. Spectral changes attributable to the onset of thermal degradation appeared at 230°C. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 841–847, 1997 相似文献