全文获取类型
收费全文 | 365篇 |
免费 | 4篇 |
专业分类
化学 | 229篇 |
晶体学 | 2篇 |
力学 | 6篇 |
数学 | 30篇 |
物理学 | 102篇 |
出版年
2020年 | 5篇 |
2019年 | 6篇 |
2016年 | 5篇 |
2013年 | 33篇 |
2012年 | 20篇 |
2011年 | 23篇 |
2010年 | 10篇 |
2009年 | 8篇 |
2008年 | 17篇 |
2007年 | 13篇 |
2006年 | 18篇 |
2005年 | 17篇 |
2004年 | 11篇 |
2003年 | 5篇 |
2002年 | 8篇 |
2001年 | 6篇 |
2000年 | 7篇 |
1998年 | 3篇 |
1996年 | 4篇 |
1995年 | 8篇 |
1994年 | 8篇 |
1993年 | 5篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1987年 | 5篇 |
1985年 | 4篇 |
1984年 | 7篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 3篇 |
1980年 | 5篇 |
1979年 | 2篇 |
1978年 | 11篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1974年 | 6篇 |
1973年 | 5篇 |
1972年 | 5篇 |
1971年 | 2篇 |
1970年 | 5篇 |
1969年 | 6篇 |
1968年 | 3篇 |
1966年 | 3篇 |
1965年 | 2篇 |
1959年 | 3篇 |
1939年 | 2篇 |
1932年 | 2篇 |
排序方式: 共有369条查询结果,搜索用时 0 毫秒
101.
Ray V.H. Jones Greg C. Paddon-Jones Peter N. Preston 《Journal of organometallic chemistry》2006,691(11):2378-2385
Homogeneous catalysis by palladium complexes with phosphorus(III) ligands of the carbonylation of o-xylylene dihalides in the presence of water to form 3-isochromanone has been studied. Triphenylphosphine was found to provide the most effective catalyst, and by-products and intermediates of systems containing this ligand have been investigated. 2-Indanone is one by-product but is unstable to decomposition under catalytic conditions. Excess PPh3 is necessary to prolong activity of the catalyst but is also transformed to bis-phosphonium compound [o-C6H4(CH2PPh3)2]X2 (X = Cl or Br); this quaternization has been investigated and the structure of the bromide salt determined by X-ray diffraction. An unstable oxidative addition product of Pd(PPh3)4 was detected as a probable intermediate and related to the previously reported but catalytically-inactive complex trans-Pd(o-CH2C6H4CH2Cl)Cl(PMe3)2, which has been structurally characterized by X-ray diffraction in this work. 相似文献
102.
Douglas J. Morrison Karen Taylor Tom Preston 《Rapid communications in mass spectrometry : RCM》2010,24(12):1755-1762
The introduction of liquid chromatography coupled with isotope ratio mass spectrometry (LC/IRMS) as an analytical tool for the measurement of isotope ratios in non‐volatile analytes has somewhat simplified the analytical cycle from sample collection to analysis mainly due to the avoidance of the extensive sample processing and derivatisation that were necessary for gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). Here we test the performance of coupling strong anion exchange to IRMS using only the second commercially available interface; the Liquiface. The system was modified from installation specification to improve peak resolution in the interface and maintain peak separation from the column to the mass spectrometer. The system performance was assessed by the determination of sensitivity, accuracy and precision attained from carbohydrate separations. The system performed satisfactorily after modifications, resulting in maintenance of peak resolution from column to mass spectrometer. The sensitivity achieved suggested that ~150 ng carbon could be analysed with acceptable precision (<0.3‰). Accuracy was maintained in the interface as determined by correlation with offline techniques, resulting in regression coefficient of r2 = 0.98 and a slope of 0.99. The average precision achieved for the separation of seven monosaccharides was 0.36‰. The integration of a carbonate removal device limited the effect of background carbon perturbations in the mass spectrometer associated with eluent gradients, and the coupling of strong anion‐exchange chromatography with IRMS was successfully achieved using the Liquiface. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
103.
Emilie Combet Tom Preston Kenneth E. L. McColl 《Rapid communications in mass spectrometry : RCM》2010,24(5):529-534
Nitrite has long been considered a potential pre‐carcinogen for gastric cancer. Acidification of salivary nitrite, derived from dietary nitrate, produces nitrosative species such as NOSCN, NO+ and N2O3, which can form potentially carcinogenic N‐nitroso compounds. Ascorbic acid inhibits nitrosation by converting the nitrosative species into nitric oxide (NO). However, NO diffuses rapidly to adjacent lipids, where it reacts with oxygen to reform nitrosative species. Nitrosation has been studied in vitro in aqueous systems and less frequently in organic systems; however, there is a need to investigate acid‐catalysed nitrosation in a system combining aqueous and lipid environments, hence providing a physiologically relevant model. Here, we describe a two‐phase system, which can be used as a tool to understand acid‐catalysed nitrosation. Using gas chromatography/ion trap tandem mass spectrometry, we investigated the nitrosation of secondary amines as a function of the lipid phase composition and reaction mixing. An increased interface surface area was a driver for nitrosation, while incorporation of unsaturated fatty acids affected morpholine and piperidine nitrosation differently. Linoleic acid methyl esters did not affect morpholine nitrosation and only had a limited effect on N‐nitrosopiperidine formation, while incorporation of free linoleic acid to the lipid phase significantly reduced N‐nitrosopiperidine formation, but increased N‐nitrosomorpholine formation at low levels. The mechanisms driving these effects are thought to involve amine partitioning, polarity and unsaturated fatty acids acting as scavengers of nitrosating species, findings relevant to the nitrosative chemistry occurring in the stomach, where the gastric acid meets a range of dietary fats which are emulsified during digestion. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
104.
105.
Angel E. Lozano Javier De Abajo Jos G. De La Campa Jack Preston 《Journal of polymer science. Part A, Polymer chemistry》1995,33(12):1987-1994
A series of polyisophthalamides having pendent oligomeric benzamide groups were prepared by the Yamazaki reaction from common aromatic diamines and 5-(4-benzoylamino-1-benzoylamino)isophthalic acid. The latter was synthesized from 5-aminoisophthalic acid in a three-step synthesis by successive incorporation of benzamido groups. The new polymers were characterized by NMR, DSC, TGA, and WAXD and the properties were compared to those of corresponding unsubstituted polyisophthalamides. All of the polymers were essentially amorphous and their Tgs were about 20°C higher than the reference polymers. Initial thermal decomposition temperatures ranged from 375 to 420°C. All of the polymers were soluble in aprotic polar solvents without added salts. Properties of particular note were: the water uptake, which was particularly high, ranging from 7.5 to 18.2%, and the temporary insolubilization in concentrated sulfuric acid of films of the polymers heated for a short time to ≥ 200°C. © 1995 John Wiley & Sons, Inc. 相似文献
106.
Leitch MJ Boissevain J Carey TA Jansen DM Jeppesen RG Kapustinsky JS Lane DW Lillberg JW McGaughey PL Moss JM Peng JC Isenhower LD Sadler ME Schnathorst R Gidal G Ho PM Kowitt MS Luk KB Pripstein D Lederman LM Schub MH Brown CN Cooper WE Glass HD Gounder KN Mishra CS Kaplan DM Luebke WR Martin VM Preston RS Sa J Tanikella V Childers R Darden CW Wilson JR Chen YC Kiang GC Teng PK 《Physical review letters》1994,72(16):2542-2545
107.
108.
The d5 species Cr(CO)4+ has been detected by EPR spectroscopy in samples of Cr(CO)6 in krypton γ-irradiated at 77 K. The spectra of samples enriched in 13C proved the chromiun atom to be tetrahedrally bound to the four CO ligands. The ground state is 6A1 in Td symmetry. 相似文献
109.
Joyce J. Kaufman Herbert E. Popkie Harry J. T. Preston 《International journal of quantum chemistry》1977,11(6):1005-1015
We have explored two areas of approximately rigorous calculations for computing nonempirical wave functions for heavy and/or large molecules orders of magnitude faster than with conventional ab-initio methods but with the same chemical accuracy. First, we have developed and used a series of programs (starting from our new fast sets of ab-initio Gaussian SCF and SCF -CI programs) incorporating ab-initio effective core model potentials (MOD -POT ) which allow one to treat only the valence electrons explicitly, plus a charge conserving integral prescreening, which cuts down significantly on the number of integrals that have to be calculated, stored, or processed for a large molecule. We have named this latter procedure VRDDO (variable retention of diatomic differential overlap). With these MODPOT and MODPOT /VRDDO methods we have explored a variety of small, medium, and large systems ranging from electron affinities of atoms through to molecules of biological interest and large boron hydrides. The results compared to ab-initio SCF or SCF /CI calcuations are very good, usually within 0.001 to 0.002 a.u. for orbital energies and gross atomic populations (GAPS ) and even better along potential energy curves. Secondly, we have explored the use of the MS -Xα method for less conventional molecules and properties than those for which it is customarily employed. 相似文献
110.
Preston MA Kornbrekke R White LR 《Langmuir : the ACS journal of surfaces and colloids》2005,21(22):9832-9842
The standard equations developed to describe the electrophoretic motion of a charged particle immersed in an electrolyte subjected to an oscillating electric field are solved numerically with a new technique suitable for stiff systems. The focus of this work is to use this solution to determine the dynamic particle mobility, one of several quantities that can be extracted from these equations. This solution is valid from low frequencies to indefinitely high frequencies and has no restriction on zeta potential, double-layer thickness, or electrolyte composition. The solution has been used to calculate the dynamic electrophoretic mobility of a particle for a wide range of double-layer thicknesses and zeta potentials. The solution agrees with analytic approximations obtained previously by other authors under the conditions of a thin double layer and low zeta potential. The results are also consistent with calculations valid at frequencies where the ion diffusion length extends a significant distance beyond the double layer as obtained by another numerical technique. 相似文献