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111.
A perturbative approach is employed to solve the Bloch-Torrey equations in the presence of distant-dipole fields in nuclear magnetic resonance. The procedure, although only carried out to first order in the perturbation parameter a=1/k2Dtaud, could, in principle, be generalized to higher orders. Here D is the diffusivity, taud the dipolar demagnetization time, and k is the wave vector of the spatial modulation of magnetization produced by the magnetic field gradient. The results are especially interesting for dilute binary mixtures consisting of molecular species with different diffusivities. In this case the calculated two-dimensional correlation spectroscopy revamped by asymmetric Z-gradient echo detection spectra are shown to be free from some inadequacies resulting from a simplistic application of standard approximations. 相似文献
112.
The palladium-catalyzed amidation of electron-deficient aryl chlorides proceeds readily in the presence of low CO pressures and a slight excess of an iodide salt. The rates of amidation are accelerated over those without added salt, and iodide is preferred over bromide or chloride. More electron-rich aryl chlorides were not effectively amidated, either with or without added iodide. We postulate that an intermediate anionic palladium(0) iodide complex is responsible for the enhanced reactivity. 相似文献
113.
In an earlier binding study conducted in our laboratory using Thermobifida fusca cellulases Cel6B, Cel9A, and Cel5A (formally Thermomonospora fusca E3, E4, and E5), it was observed that binding capacities for these three cellulases were 18–30 times higher on BMCC than on Avicel. These
results stimulated an interest in how the difference in accessibility between the two cellulosic substrates would affect synergism
observed with cellulase mixtures. To explore the impact of substrate, accessibility on the extent of conversion and synergism,
three binary T. fusca cellulase mixtures were tested over a range of cellulase ratios and total molar cellulase concentrations on Avicel and BMCC.
Higher extents of conversion were observed for BMCC due to the higher enzyme to substrate ratio resulting from the higher
binding The processive endoglucanase, Cel9A, had four times the extent of conversion of the end endocellulase Cel5A, while
the exocellulase Cel6B had three times the extent of conversion of Cel5A. Approximately 500 nmol/g of the cel9A+Cel6B mixture
was needed to obtain 80% conversion, while the Cel6B+Cel5A and Cel9A+Cel5A mixtures required 1500 and 1250 nmol/g, respectively,
to obtain 80% conversion. Thus, it appears that the more accessible structure of BMCC, as reflected by its binding capacity,
results in relative higher processive activity. 相似文献
114.
B. Montanari B. Civalleri C. M. Zicovich‐Wilson R. Dovesi 《International journal of quantum chemistry》2006,106(7):1703-1714
The equilibrium structural parameters, high‐ and low‐frequency dielectric tensors, Born effective charges, and Γ‐point vibrational frequencies of bulk Al2O3 corundum are calculated by using the periodic, ab initio program CRYSTAL, which adopts an all‐electron Gaussian‐type basis set. The effect of basis set and the performance of three different functionals, i.e., LDA, PW91, and B3LYP, are discussed. The mean absolute deviation from the measured frequencies is as small as 7 cm?1 for both the LDA and B3LYP functionals, indicating that these functionals perform extremely well in this case. The mean absolute deviation increases to 18 cm?1 when the PW91 functional is used. All three functionals reproduce the equilibrium geometry of corundum to a high level of accuracy, with LDA and B3LYP outperforming PW91 slightly. The comparison of the current all‐electron calculations with previous plane‐wave, pseudo‐potential calculations shows an overall similar performance. The results of isotopic substitution for both Al and O are also presented. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 相似文献
115.
The yellowing of white and pastel coloured textiles and garments has been a problem for many years in the textile industry. In the last 15 years much evidence has been published which suggests that a large proportion of storage yellowing is attributable not to the yellowing of fibre substrate or textile finish but to the yellowing of phenolic antioxidants which migrate onto the textiles from polyethylene and polypropylene packaging. These mobile phenolic compounds interact with atmospheric pollutants, particularly oxides of nitrogen, to give yellow compounds. A brief outline of the problems of yellowing is given, together with the identification of the yellow substances produced by BHT (2,6-di-tert-butyl-4-methyl phenol) and possible mechanisms of their formation. Prophylactic measures that can be taken by the garment manufacturer and finisher to prevent or minimise yellowing are also discussed. 相似文献
116.
Bobby L. Wilson Rudolph R. Schwarzer Callista O. Chukwuenye 《Microchemical Journal》1982,27(4):558-563
This paper reports the development of a field electrode method for the determination of total sulfide in water. The method involves the use of preweighed sodium sulfide crystals in the standardization process. Sodium sulfide crystals were weighed and sealed in air-tight plastic volumetric flasks. Standards were prepared in the field by adding a sulfide antioxidant buffer to the flasks containing the sulfide crystals and diluting it to mark with deionized deaerated water. Standards of lower concentration were prepared by serial dilutions of first standards.The results of the reproducibility determination revealed that sulfide concentrations as low as 6 ppb could be measured with a reproducibility of better than ±10%.Water samples were collected from a series of lakes in Fort Bend County, Texas (near Houston) and analyzed for sulfide content. The sulfide ion concentrations of these samples were determined directly in the field by use of an Orion Model 407A/F specific meter equipped with a silver/sulfide ion selective electrode in conjunction with a double junction reference electrode. 相似文献
117.
Budarin VL Clark JH Tavener SJ Wilson K 《Chemical communications (Cambridge, England)》2004,(23):2736-2737
The reaction of localised C[double bond, length as m-dash]C bonds on the surface of activated carbons has been shown to be an effective method of chemical modification especially using microwave-assisted reactions. 相似文献
118.
Coen M Lindon JC Gavaghan C Holmes E Humpfer E Wilson ID Stanley PD Nicholson JK 《The Analyst》2001,126(5):548-550
High resolution 1H magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra have been obtained on typical C18 bonded silicas used in chromatographic solid-phase extraction separations. It has been shown for the first time that water molecules distributed in distinct physico-chemical environments within the chromatographic system can be detected directly using a simple 1H MAS NMR measurement. The resonances assigned to water protons in differing physico-chemical environments have distinct chemical shifts, line widths, relaxation times (T1 and T2) and also exhibit temperature dependent coalescence behaviour. This novel MAS approach may lead to a better understanding of the environments of other analytes in mixtures during such separations. 相似文献
119.
Three theories which have been proposed to explain the observed shortening of a single bond when it is adjacent to a double bond are discussed. Possible predictions from these theories are examined, especially for comparison with various quantities measurable by microwave spectroscopy. It is concluded that the steric theory is probably untenable, that some conjugation appears necessary to explain observed barries to internal rotation, and that it is difficult to find testable predictions from the hybridization theory. 相似文献
120.
Li AC Alton D Bryant MS Shou WZ 《Rapid communications in mass spectrometry : RCM》2005,19(14):1943-1950
Bioanalytical support of plasma pharmacokinetic (PK) studies for drug discovery programs primarily involves the quantitative analysis of dosed compounds using liquid chromatography/atmospheric pressure ionization tandem mass spectrometry (LC/MS/MS) operated in selected reaction monitoring (SRM) mode. However, there is a growing need for information on the metabolism of new chemical entities (NCEs), in addition to the time-concentration profiles from these studies. In this paper, we present a novel approach to not only quantify parent drugs with SRM, but also simultaneously screen for metabolites using a hybrid triple quadrupole/linear ion trap (QqQ(LIT)) instrument. This was achieved by incorporating both the conventional SRM-only acquisition of parent compounds and the SRM-triggered information-dependent acquisition (IDA) of potential metabolites within the same scan cycle during the same LC/MS/MS run. Two test compounds were used to demonstrate the applicability of this approach. Plasma samples from PK studies were processed by simple protein precipitation and the supernatant was diluted with water before injection. The fast scanning capability of the linear ion trap allowed for the information-dependent acquisition of metabolite MS/MS spectra (<1 s/scan), in addition to the collection of adequate data points for SRM-only channels. The MS/MS spectra obtained from potential metabolites in post-dose samples correlated well with the spectra of the parent compounds studied, therefore providing additional confirmatory structure information without the need for repetitive analyses. Relative quantitative time-concentration profiles of identified metabolites were also obtained. Furthermore, this articulated SRM+SRM-IDA approach generated equivalent quantitative results for parent compounds to those obtained by conventional SRM-only analysis. This approach has been successfully used to support discovery PK screening programs. 相似文献