全文获取类型
收费全文 | 1139篇 |
免费 | 67篇 |
国内免费 | 2篇 |
专业分类
化学 | 875篇 |
晶体学 | 2篇 |
力学 | 17篇 |
数学 | 156篇 |
物理学 | 158篇 |
出版年
2023年 | 10篇 |
2022年 | 6篇 |
2021年 | 15篇 |
2020年 | 21篇 |
2019年 | 14篇 |
2018年 | 23篇 |
2017年 | 13篇 |
2016年 | 44篇 |
2015年 | 46篇 |
2014年 | 46篇 |
2013年 | 71篇 |
2012年 | 100篇 |
2011年 | 84篇 |
2010年 | 51篇 |
2009年 | 58篇 |
2008年 | 68篇 |
2007年 | 84篇 |
2006年 | 74篇 |
2005年 | 74篇 |
2004年 | 57篇 |
2003年 | 42篇 |
2002年 | 29篇 |
2001年 | 15篇 |
2000年 | 13篇 |
1999年 | 15篇 |
1998年 | 13篇 |
1997年 | 9篇 |
1996年 | 9篇 |
1995年 | 8篇 |
1994年 | 9篇 |
1993年 | 6篇 |
1992年 | 7篇 |
1991年 | 8篇 |
1990年 | 7篇 |
1989年 | 2篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1986年 | 6篇 |
1985年 | 5篇 |
1984年 | 10篇 |
1982年 | 6篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1940年 | 1篇 |
1938年 | 1篇 |
1934年 | 1篇 |
1929年 | 1篇 |
1925年 | 1篇 |
1920年 | 1篇 |
排序方式: 共有1208条查询结果,搜索用时 15 毫秒
11.
12.
Luis JM Torrent-Sucarrat M Solà M Bishop DM Kirtman B 《The Journal of chemical physics》2005,122(18):184104
Our new simple method for calculating accurate Franck-Condon factors including nondiagonal (i.e., mode-mode) anharmonic coupling is used to simulate the C2H4+X2B3u<--C2H4X1A(g) band in the photoelectron spectrum. An improved vibrational basis set truncation algorithm, which permits very efficient computations, is employed. Because the torsional mode is highly anharmonic it is separated from the other modes and treated exactly. All other modes are treated through the second-order perturbation theory. The perturbation-theory corrections are significant and lead to a good agreement with experiment, although the separability assumption for torsion causes the C2D4 results to be not as good as those for C2H4. A variational formulation to overcome this circumstance, and deal with large anharmonicities in general, is suggested. 相似文献
13.
Clément JL Ferré N Siri D Karoui H Rockenbauer A Tordo P 《The Journal of organic chemistry》2005,70(4):1198-1203
[structure: see text] Spin trapping consists of using a nitrone or a nitroso compound to "trap" an unstable free radical as a long-lived nitroxide that can be characterized by electron paramagnetic resonance (EPR) spectroscopy. The formation of DMPO-OOH, the spin adduct resulting from trapping superoxide (O(2)(*)(-)) with 5,5-dimethyl-1-pyrroline N-oxide (DMPO), has been exploited to detect the generation of superoxide in a wide variety of biological and chemical systems. The 12-line EPR spectrum of DMPO-OOH has been either reported or mentioned in more than a thousand papers. It has been interpreted as resulting from the following couplings: A(N) approximately 1.42 mT, A(H)beta approximately 1.134 mT, and A(H)gamma(1H) approximately 0.125 mT. However, the DMPO-OOH EPR spectrum has an asymmetry that cannot be reproduced when the spectrum is calculated considering a single species. Recently, it was proposed that the 0.125 mT splitting was misassigned and actually results from the superimposition of two individual EPR spectra associated with different conformers of DMPO-OOH. We have prepared 5,5-dimethyl-[3,3-(2)H(2)]-1-pyrroline N-oxide (DMPO-d(2)), and we showed that the EPR spectrum of the corresponding superoxide spin adduct is composed of only six lines, in agreement with the assignment of the 0.125 mT splitting to a gamma-splitting from a hydrogen atom bonded to carbon 3 of DMPO. This result was supported by DFT calculations including water solvation, and the asymmetry of the DMPO-OOH EPR spectrum was nicely reproduced assuming a chemical exchange between two conformers. 相似文献
14.
Martí MP Pino J Boqué R Busto O Guasch J 《Analytical and bioanalytical chemistry》2005,382(2):440-443
The aromatic composition of sugar cane spirits and, in general, of alcoholic beverages, is mainly influenced by the ageing process in wood barrels. There are several factors that affect the quality of the final aged product, but the time of the storage in the barrel is perhaps the most important one. Ageing time must therefore be controlled in order to detect counterfeits; however, this parameter is very difficult to control and, at present, there is no analytical method available to determine it. We propose a quantitative method for determining the ageing time of sugar cane spirits in oak barrels by using an electronic nose based on coupling directly a headspace sampler to a mass spectrometer (HS-MS), and multivariate calibration. The method developed is simple and provides, in 5 min, the ageing time of spirits with an accuracy of about 1 month. 相似文献
15.
Robert Koeber R. Niessner Josep Maria Bayona 《Analytical and bioanalytical chemistry》1997,359(3):267-273
The performance of two liquid chromatography-mass spectrometry (LC/MS) interfacing techniques, thermospray (TSP) and atmospheric
pressure chemical ionization (APCI), for the analysis of benzo[a]pyrene (BaP) metabolites (hydroxy, epoxy and quinone derivatives) was compared. Interface and detection parameters such as
source temperature, eluent composition or flow rate were optimized using negative ion mode. In TSP, the main ions are mostly
[M]-, [M−H2O]- or [M+CH3COO]-, whereas APCI gives mainly the [M]- and [M−H]- ions. Quantification was carried out by flow injection. Calibration graphs were linear in the range of 10 ng to 1000 ng in
TSP and 0.1 ng to 10 ng in APCI. Detection limits were in the range of 1 ng to 20 ng in TSP and 0.002 ng to 0.2 ng in APCI.
The presence of BaP-1,6-dione, BaP-3,6-dione, and BaP-6,12-dione was confirmed in environmental samples of air particulate
matter.
Received: 6 January 1997/Accepted: 18 April 1997 相似文献
16.
Krishna Bisetty Francesc J. Corcho Josep Canto Hendrik G. Kruger Juan J. Perez 《Journal of Molecular Structure》2006,770(1-3):221-228
In this paper, we report on the conformational profile of the pentacyclo-undecane (PCU) cage tripeptide carried out by molecular dynamics (MD) simulation using water as an explicit solvent. The MD solution phase studies carried on the model peptide analogues (A)=Ac–Ala–Ala–Ala–NHMe; (B)=Ac–Cage–Cage–Cage–NHMe; (C)=Ac–Ala–Cage–Ala–NHMe and (D)=Ac–Ala–Pro–Ala–NHMe, are used as a complimentary technique to the corresponding gas phase simulated annealing (SA) study previously carried out in our laboratory. No significant structural changes were observed over the MD trajectories. However, the results reported here provide further evidence that the (PCU) cage amino acid exhibits C7eq, C7aq, R and L conformations, and the theoretical results suggest that the PCU cage amino acid is a strong β-turn inducer. These results support the prediction that when the PCU cage residues are in the (i) and (i+2) positions, the β-turn can be extended in either direction to form anti-parallel β-pleated sheets, thereby forming the basis of the mechanism for the folding back of the chain in a cross-β-turn structure. 相似文献
17.
The link between the Absar-Coleman and the Valdemoro reduced Hamiltonian has been established. Further, a study of the size-consistency of approximations based on these Hamiltonians has been carried out. It is found that the energies obtained with reduced Hamiltonians that are defined in the full configuration interaction model spaces are not size consistent. 相似文献
18.
Nicolas?Ferré Alessandro?Cembran Marco?Garavelli Massimo?OlivucciEmail author 《Theoretical chemistry accounts》2004,112(4):335-341
A special hybrid quantum mechanics/molecular mechanics forcefield is defined, parameterized and validated for studying the photoisomerization path of the retinal chromophore in the rhodopsin protein. It couples a multireference ab initio Hamiltonian (CASSCF and second-order multireference many-body perturbation theory using a CASSCF reference) to describe the chromophore while the rest of the protein is approximated with the Amber forcefield. The frontier has been carefully parameterized in order to reproduce full quantum mechanics torsional energy profiles, for both the ground state and the first excited state. It is also shown that replacing the chromophore counterion with point charges is a valid approximation. This result is interpreted in terms of a cancellation effect for which a possible explanation is given. 相似文献
19.
Villanueva J Villegas V Querol E Avilés FX Serrano L 《Journal of mass spectrometry : JMS》2002,37(9):974-984
In the post-genomic era, several projects focused on the massive experimental resolution of the three-dimensional structures of all the proteins of different organisms have been initiated. Simultaneously, significant progress has been made in the ab initio prediction of protein three-dimensional structure. One of the keys to the success of such a prediction is the use of local information (i.e. secondary structure). Here we describe a new limited proteolysis methodology, based on the use of unspecific exoproteases coupled with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), to map quickly secondary structure elements of a protein from both ends, the N- and C-termini. We show that the proteolytic patterns (mass spectra series) obtained can be interpreted in the light of the conformation and local stability of the analyzed proteins, a direct correlation being observed between the predicted and the experimentally derived protein secondary structure. Further, this methodology can be easily applied to check rapidly the folding state of a protein and characterize mutational effects on protein conformation and stability. Moreover, given global stability information, this methodology allows one to locate the protein regions of increased or decreased conformational stability. All of this can be done with a small fraction of the amount of protein required by most of the other methods for conformational analysis. Thus limited exoproteolysis, together with MALDI-TOF MS, can be a useful tool to achieve quickly the elucidation of protein structure and stability. 相似文献
20.
Ignés-Mullol J Claret J Albalat R Crusats J Reigada R Romero MT Sagués F 《Langmuir : the ACS journal of surfaces and colloids》2005,21(7):2948-2955
Preparation of Langmuir monolayers of a mixture of trans- and cis-isomers of an azobenzene derivative, 4-[4-[(4-octylphenyl)azo]phenoxy]butanoic acid, results in the segregation of birefringent trans-isomer domains embedded in an isotropic medium of cis-isomers. Brewster angle microscopy observations allow us to identify different textures inside the domains depending on surface pressure, temperature, and domain size. The evolution of the monolayer in the dark, from initial droplets formed after spreading to a stable stripe texture, is described. The dynamics of domain coalescence and some morphological transitions induced by temperature and surface pressure changes are also discussed. A simple theoretical model is included to supplement some of these experimental observations. 相似文献