全文获取类型
收费全文 | 4079篇 |
免费 | 115篇 |
国内免费 | 22篇 |
专业分类
化学 | 2532篇 |
晶体学 | 29篇 |
力学 | 122篇 |
数学 | 602篇 |
物理学 | 931篇 |
出版年
2023年 | 22篇 |
2022年 | 39篇 |
2021年 | 82篇 |
2020年 | 66篇 |
2019年 | 54篇 |
2018年 | 52篇 |
2017年 | 49篇 |
2016年 | 129篇 |
2015年 | 113篇 |
2014年 | 102篇 |
2013年 | 221篇 |
2012年 | 225篇 |
2011年 | 260篇 |
2010年 | 161篇 |
2009年 | 165篇 |
2008年 | 239篇 |
2007年 | 229篇 |
2006年 | 239篇 |
2005年 | 221篇 |
2004年 | 200篇 |
2003年 | 159篇 |
2002年 | 146篇 |
2001年 | 80篇 |
2000年 | 67篇 |
1999年 | 52篇 |
1998年 | 46篇 |
1997年 | 41篇 |
1996年 | 54篇 |
1995年 | 33篇 |
1994年 | 34篇 |
1993年 | 33篇 |
1992年 | 23篇 |
1991年 | 17篇 |
1990年 | 31篇 |
1989年 | 19篇 |
1988年 | 21篇 |
1987年 | 13篇 |
1986年 | 23篇 |
1985年 | 52篇 |
1984年 | 47篇 |
1983年 | 26篇 |
1982年 | 38篇 |
1981年 | 29篇 |
1980年 | 37篇 |
1979年 | 40篇 |
1978年 | 15篇 |
1977年 | 22篇 |
1976年 | 20篇 |
1974年 | 14篇 |
1973年 | 16篇 |
排序方式: 共有4216条查询结果,搜索用时 109 毫秒
41.
42.
Pau Bernadó Renato Barbieri Esteve Padrós Claudio Luchinat Miquel Pons 《Journal of the American Chemical Society》2002,124(3):374-375
Addition of Tb3+ to purple membrane (PM) suspensions changes the orientation of the menbrane normal from parallel to perpendicular with respect to the magnetic field. Residual dipolar couplings measured in protein L in the presence of PM are scaled by a factor of -1/2. NMR line broadening and cross-correlation effects induced by the addition of PM are partially reversed by Tb3+ but not by Tm3+ that has no effect on the orientation of PM. This is interpreted as the result of anisotropically restricted motion of protein L in the transiently PM-bound form. 相似文献
43.
Minoia C Gatti A Aprea C Ronchi A Sciarra G Turci R Bettinelli M 《Rapid communications in mass spectrometry : RCM》2002,16(13):1313-1319
A method was developed for the determination of molybdenum (Mo) in human urine by direct dilution of the sample in doubly distilled water with 1% HNO3 (v/v) and inductively coupled mass spectrometry (ICP-MS). In and Y were used as internal standards. Since (98)Mo provides a higher sensitivity, it was chosen as the reference isotope. The influence of different factors, such as sample dilution, HNO3 concentration and the stability of the analyte were evaluated. The detection limit (LOD) was assessed at 0.2 microg/L Mo, while the lower limit of quantification (LOQ) was 0.6 microg/L. Recoveries ranged between 97.2 and 100.7% from solutions containing from 10 to 50 microg/L Mo. Linear calibration curves were generated from 2.1 and 52.1 microg/L with coefficients of variation (CV ) ranging from 1.62 to 3.56%. In order to establish reference values (RV) for molybdenum, the procedure presented here was used to determine Mo in the urine of a population group living in Tuscany, Italy. 相似文献
44.
Roubin P Chandesris D Rossi G Lecante J Desjonquères MC Tréglia G 《Physical review letters》1986,56(12):1272-1275
45.
Polo F Antonello S Formaggio F Toniolo C Maran F 《Journal of the American Chemical Society》2005,127(2):492-493
The rate constant of intramolecular electron transfer through oligopeptides based on the alpha-aminoisobutyric acid residue was determined as a function of the peptide length and found to depend weakly on the donor-acceptor separation. By measuring the electron-transfer activation energy and estimating the energy gap between donor and bridge, we were able to discard the electron hopping mechanism. 相似文献
46.
Carra C Iordanova N Hammes-Schiffer S 《Journal of the American Chemical Society》2003,125(34):10429-10436
Theoretical calculations of a model for tyrosine oxidation in photosystem II are presented. In this model system, an electron is transferred to ruthenium from tyrosine, which is concurrently deprotonated. This investigation is motivated by experimental measurements of the dependence of the rates on pH and temperature (Sj?din et al. J. Am. Chem. Soc. 2000, 122, 3932). The mechanism is proton-coupled electron transfer (PCET) at pH < 10 when the tyrosine is initially protonated and is single electron transfer (ET) for pH > 10 when the tyrosine is initially deprotonated. The PCET rate increases monotonically with pH, whereas the single ET rate is independent of pH and is 2 orders of magnitude faster than the PCET rate. The calculations reproduce these experimentally observed trends. The pH dependence for the PCET reaction arises from the decrease in the reaction free energies with pH. The calculations indicate that the larger rate for single ET arises from a combination of factors, including the smaller solvent reorganization energy for ET and the averaging of the coupling for PCET over the reactant and product hydrogen vibrational wave functions (i.e., a vibrational overlap factor in the PCET rate expression). The temperature dependence of the rates, the solvent reorganization energies, and the deuterium kinetic isotope effects determined from the calculations are also consistent with the experimental results. 相似文献
47.
Bianchini C Giambastiani G Laschi F Mariani P Vacca A Vizza F Zanello P 《Organic & biomolecular chemistry》2003,1(5):879-886
A new potentially hexadentate tetraazamacrocycle based on the cyclen skeleton has been synthesized and fully characterized. The macrocycle 4,10-dimethyl-1,4,7,10-tetraazacyclododecane-1,7-bis(methanephosphonic acid monoethyl ester) dipotassium salt (Me2DO2PME) contains mutually trans monoethyl ester phosphonate acid substituents on two nitrogen atoms, and trans methyl substituents on the other two nitrogen atoms. The protonation constants of this macrocycle and the stability constants of its complexes with Cu2+, Zn2+, Gd3+ and Ca2+ ions have been determined by pH potentiometric titrations. The protonation sequence of the macrocycle has been studied by 1H, 31P[1H] and 13C[1H] NMR spectroscopy: the first and second protonation steps take place at the methyl-substituted nitrogen atoms, while the third protonation involves one oxygen from a phosphonate group. Upon protonation, all the CH2 ring protons become magnetically inequivalent on the NMR time scale due to a slow conformational rearrangement, most likely occasioned by the formation of multiple hydrogen bonds within the macrocyclic ring. Me2DOPM forms neutral, mononuclear complexes with all the metals investigated. The presence of hydroxo complexes was observed for Ca2+ and Zn2+ at high pH values. Structural information on the neutral complex [Cu(Me2DO2PME)] has been obtained by a solution X-Band EPR study. It is proposed that Me2DO2PME binds Cu2+ in a distorted octahedral structure using all of its donor atoms, i.e. the four nitrogen atoms and the two phosphonate oxygen atoms. The redox chemistry of [Cu(Me2DO2PME)] in dimethyl sulfoxide and water has been studied by electrochemical measurements. Cyclic voltammetry in DMSO shows the complex to undergo a quasireversible one-electron reduction step leading to an unstable CuI species. 相似文献
48.
Tozzi C Anfossi L Baggiani C Giovannoli C Giraudi G 《Analytical and bioanalytical chemistry》2003,375(8):994-999
In our work we performed a combinatorial solid-phase synthesis in aqueous medium to prepare peptide libraries from which to select an amino acid sequence with binding properties towards aflatoxins.We used polystyrene beads, functionalised with carboxylic groups as solid support and eight amino acids as monomers. During the first step 64 different sequences of two amino acids were prepared by exploiting the principles of combinatorial chemistry; then the binding properties of all sequences towards aflatoxin B(1 )were checked. We determined binding constants towards aflatoxin B(1) and towards aflatoxins B(2), G(1) and G(2). Results were promising, so we prepared a new library by using the selected dipeptide as the starting solid phase. After selecting the best tetrapeptide sequences, we determined binding constants towards the quoted aflatoxins. We obtained binding constants ( K>10(4) M(-1)) similar to those shown by human serum albumin for similar compounds. Preliminary studies on an extraction column were promising for the development of an SPE system and for its application in food matrices. 相似文献
49.
Mauri P Minoggio M Simonetti P Gardana C Pietta P 《Rapid communications in mass spectrometry : RCM》2002,16(8):743-748
Saccharides in foods play important roles, as they are essential substrates for fermentation processes. In brewing, the concentration of maltooligosaccharides influences the characteristics of beers and therefore their determination is of great practical interest. Electrospray ionization mass spectrometry (ESI-MS) was applied to identify and characterise maltooligosaccharides in beer samples. The effects due to different cation concentrations and dilution of samples were studied. Furthermore, quantitative analyses of maltooligosaccharides by means of flow-injection ESI-MS (FI/ESI-MS) of 1-microL beer samples (diluted 1000-fold) are described. 相似文献
50.
Stefano Vecchio Alessia Catalani Vanessa Rossi Mauro Tomassetti 《Thermochimica Acta》2004,420(1-2):99-104
The thermal behaviour of acetanilide (Ac) and two of its analogues, namely the para-ethoxyacetanilide (p-Eto Ac) and the para-bromoacetanilide (p-Br Ac), which are used as analgesics in the pharmaceutical industry was studied with a simultaneous TG/DSC unit. The examined analgesics showed two endothermic DSC peaks due to melting and vaporization. By combining the experimental TG data with the corresponding reference vapour pressure data obtained with the Antoine equation the plot of P versus v was derived. From the slope of this equation the constant k-value was determined for Ac. Then, using the same k-value the vapour pressures of p-Eto Ac and p-Br Ac were determined in the same temperature range. The vaporization enthalpies for all the studied compounds were obtained from different methods and a very good agreement was found. Vaporization follows a zero-order kinetics. The activation energy of vaporization (Evap) was calculated from the dynamic TG experiments, using the Arrhenius equation. 相似文献