全文获取类型
收费全文 | 485篇 |
免费 | 10篇 |
专业分类
化学 | 409篇 |
晶体学 | 2篇 |
力学 | 1篇 |
数学 | 18篇 |
物理学 | 65篇 |
出版年
2023年 | 2篇 |
2022年 | 37篇 |
2021年 | 35篇 |
2020年 | 15篇 |
2019年 | 17篇 |
2018年 | 7篇 |
2017年 | 9篇 |
2016年 | 18篇 |
2015年 | 16篇 |
2014年 | 13篇 |
2013年 | 24篇 |
2012年 | 25篇 |
2011年 | 33篇 |
2010年 | 22篇 |
2009年 | 20篇 |
2008年 | 20篇 |
2007年 | 22篇 |
2006年 | 29篇 |
2005年 | 16篇 |
2004年 | 13篇 |
2003年 | 15篇 |
2002年 | 8篇 |
2001年 | 5篇 |
2000年 | 4篇 |
1999年 | 5篇 |
1998年 | 3篇 |
1997年 | 2篇 |
1996年 | 1篇 |
1995年 | 5篇 |
1992年 | 2篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1989年 | 4篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 7篇 |
1980年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 5篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1965年 | 1篇 |
排序方式: 共有495条查询结果,搜索用时 0 毫秒
101.
102.
Monolayers of phosphatidylethanolamine (PE), cholesterol (Ch), phosphatidylcholine (PC), and binary mixtures (PC-PE or PE-Ch) were investigated at the air/water interface. The surface tension values of pure and mixed monolayers were used to calculate π -A isotherms. The surface tension measurements were carried out at 20°C using an improved Teflon trough and a Nima 9000 tensiometer. The Teflon trough was filled with a subphase of triple-distilled water. Known amounts of lipid dissolved in 1-chloropropane were placed at the surface using a syringe. The interactions between phosphatidylethanolamine and cholesterol as well as phosphatidylcholine and phosphatidylethanolamine result in significant deviations from the additivity rule. An equilibrium theory to describe the behavior of monolayer components at the air/water interface was developed in order to obtain the stability constants of PC-PE and PE-Ch complexes. We considered the equilibrium between the individual components and the complex and established that phosphatidylethanolamine and cholesterol as well as phosphatidylcholine and phosphatidylethanolamine formed highly stable 1:1 complexes. 相似文献
103.
Acevska J Dimitrovska A Stefkov G Brezovska K Karapandzova M Kulevanova S 《Journal of AOAC International》2012,95(2):399-405
An HPLC method for the separation of six target alkaloids from Papaver somniferum L. (morphine, codeine, oripavine, thebaine, papaverine, and noscapine) was developed, optimized, and validated. The chromatographic behavior of these alkaloids was investigated using a reversed-phase chromatography at acidic and alkaline pH. The effects of ion-pairing agents, pH value of the mobile phase, concentration of the buffer components, mobile phase organic modifier, and column temperature were studied. Regardless of the large differences in their pKa values, all alkaloids were separated within a close retention window, and good peak shape was achieved for each of the six alkaloids. The proposed method has adequate selectivity, linearity, accuracy, precision, and reproducibility and is applicable for poppy straw. 相似文献
104.
Modeling toxicity by using supervised kohonen neural networks 总被引:2,自引:0,他引:2
Mazzatorta P Vracko M Jezierska A Benfenati E 《Journal of chemical information and computer sciences》2003,43(2):485-492
Counterprogation neural network is shown to be a powerful and suitable tool for the investigation of toxicity. This study mined a data set of 568 chemicals. Two hundred eighty-two objects were used as the training set and 286 as the test set. The final model developed presents high performances on the data set R(2) = 0.83 (R(2) = 0.97 on the training set, R(2) = 0.59 on the test set). This technique distinguishes itself also for the ability to give to the expert two-dimensional maps suitable for the study of the distribution/clustering of the data and the identification of outliers. 相似文献
105.
Jan Epsztajn Adam Bieniek Justyna A. Kowalska Jacek Ścianowski 《Monatshefte für Chemie / Chemical Monthly》1992,123(12):1125-1134
Summary The lithiation (Bu
nLi/THF) of 2-chloro- (1), 3-chloro- (2) and 4-chlorobenzanilides (3) and the subsequent reactions of the corresponding bis-lithiated anilides4–6 with electrophiles (MeI, CH2=CH-CH2Br,Me
3SiCl,MeCHO,o-MeOC6H4CHO,p-MeOC6H4CHO,Me
2NCHO andp-MeOC6H4CONMe
2) towards the synthesis of theortho substituted chlorobenzoesic acids derivatives12–14 have been described. The effect of the chlorine substituent upon the generation and stability of the bis-lithiated chloro-anilides4–6 has been studied. It has been found that the bis-lithiated chloro-anilide5 derived fromm-chloro-benzanilide (2) at a temperature above –30°C converts into the corresponding benzyne9. The anilide moiety (masking group) of the formedortho-substituted chlorobenzanilides appeared to be effectively removable on acid-driven hydrolysis.
Anwendungen von Organolithium und verwandten Reagenzien in organischen Synthesen, 10. Mitt. Metallierung und nachfolgende elektrophile Substitution sekundärer Chlorbenzamide
Zusammenfassung Die Lithiierung (Bu nLi/THF) der 2-Chlor-(1), 3-Chlor- (2) and 4-Chlorbenzanilide (3) und nachfolgende Reaktion der entsprechenden doppellithiierten Anilide4–6 mit elektrophilen Reagenzien (MeI, CH2=CH-CH2Br,Me 3SiCl,MeCHO,o-MeOC6H4CHO,p-MeOC6H4CHO,Me 2NCHO undp-MeOC6H4CONMe 2) zur Synthese vonortho-substituierten Benzoesäurederivaten12–14 wird beschrieben. Der Einfluß des Chlorsubstituierten auf die Bildung und Stabilität der doppellithiierten Chloranilide4–6 wurde untersucht. Es wurde festgestellt, daß das doppellithiierte, vomm-Chlorbenzanilid erhaltene Chloranilid5 bei Temperaturen über –30°C das entsprechende Arin9 bildet. Der Anilidrest (eine maskierende Gruppe) der gebildetenortho-substituierten Chloranilide kann durch saure Hydrolyse abgespalten werden.相似文献
106.
Hisham Ben Hamidane Aleksey Vorobyev Maud Larregola Aneta Lukaszuk Dirk Tourwé Dr. Solange Lavielle Philippe Karoyan Dr. Yury O. Tsybin Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(15):4612-4622
We report on the characteristics of the radical‐ion‐driven dissociation of a diverse array of β‐amino acids incorporated into α‐peptides, as probed by tandem electron‐capture and electron‐transfer dissociation (ECD/ETD) mass spectrometry. The reported results demonstrate a stronger ECD/ETD dependence on the nature of the amino acid side chain for β‐amino acids than for their α‐form counterparts. In particular, only aromatic (e.g., β‐Phe), and to a substantially lower extent, carbonyl‐containing (e.g., β‐Glu and β‐Gln) amino acid side chains, lead to N? Cβ bond cleavage in the corresponding β‐amino acids. We conclude that radical stabilization must be provided by the side chain to enable the radical‐driven fragmentation from the nearby backbone carbonyl carbon to proceed. In contrast with the cleavage of backbones derived from α‐amino acids, ECD of peptides composed mainly of β‐amino acids reveals a shift in cleavage priority from the N? Cβ to the Cα? C bond. The incorporation of CH2 groups into the peptide backbone may thus drastically influence the backbone charge solvation preference. The characteristics of radical‐driven β‐amino acid dissociation described herein are of particular importance to methods development, applications in peptide sequencing, and peptide and protein modification (e.g., deamidation and isomerization) analysis in life science research. 相似文献
107.
Bacterial adhesion to epithelial cells is a key step in infections, allowing subsequent colonization, invasion and internalization of pathogens into tissues. Anti-adhesive agents are therefore potential prophylactic tools against bacterial infections. The range of anti-adhesive compounds is largely confined to carbohydrate analogues. Tannins are generously recognized as potent antimicrobials, but little data exist on their anti-adherence potency. Using a model for mucosal pathogenesis with labeled group A-streptococci (GAS) and human laryngeal HEp-2 cells, a series of flavan-3-ols (epicatechin, epigallocatechin, epigallocatechin-3-O-gallate) and highly purified and chemically characterized proanthocyanidin samples including procyanidins based on epicatechin, catechin or 'mixed' constituent flavanyl units, prodelphinidins made up of (epi)gallocatechin monomeric unts as well as oligomers possessing A-type units in their molecules was evaluated for anti-adhesive effects. Reduced microbial adherence was observed exclusively for prodelphinidins, suggesting that pyrogallol-type elements, i.e., (epi)gallocatechin units are important structural features. This is the first report on structure-activity relationships regarding the anti-adhesive potency of proanthocyanidins. In addition, the structures of the first chemically defined proanthocyanidins from Pelargonium sidoides are disclosed. 相似文献
108.
A new method for the analysis of three ecological insecticides, namely azadyrachtin, spinosad (sum of spinosyn A and spinosyn
D) and rotenone, in produce and soil samples is presented. Investigated compounds are one of the most significant insecticides
authorized for organic farming crop protection in many countries. Extraction of the pesticides from plant and soil matrices
was performed by using a modified quick, easy, cheap, effective, rugged, and safe (QuEChERS) method. The method entailed a
single extraction of the investigated compounds with acidified acetonitrile followed by a dispersive solid-phase extraction
cleanup step prior to the final determination by reverse-phase ultra-performance liquid chromatography/tandem quadrupole mass
spectrometry (UPLC-MS/MS). Validation studies were carried out on cabbage, tomato and soil samples. Recoveries of the spiked
samples were in the range between 67% and 108%, depending on the matrix and the spiking level. Relative standard deviations
for all matrix–compound combinations did not exceed 12%. The limits of quantification were ≤0.01 mg kg−1 in all cases, except for azadirachtin. The developed method was applied to the analysis of real samples originating from
organic farming production. 相似文献
109.
E. Kowalska J. Radomska P. Konarski R. Diduszko J. Oszczudłowski T. Opalińska M. Więch Z. Duszyc 《Journal of Thermal Analysis and Calorimetry》2006,86(1):137-140
Two components
of electronic wastes (sample A – a mixture of three types of printed
circuit boards, sample B – a mixture of electronic junctions with metal
wires) were investigated using thermogravimetric analysis (TG). Thermogravimetric
and derivative thermogravimetric data (TG and DTG) give information on the
thermal stability of A and B samples and allows finding the correct conditions
for their degradation using pyrolysis in an experimental system, built on
the laboratory scale for utilization of hazardous wastes. X-ray fluorescence
measurements prove that brominated flame retardant is present in sample A,
whilst chlorinated flame retardant is a probable component of sample B. Preliminary
liquid chromatography of oil products obtained as a result of thermal waste
degradation shows that the hydrocarbons released during pyrolysis could be
used as a fuel. 相似文献
110.
Panuszko A Gojło E Zielkiewicz J Smiechowski M Krakowiak J Stangret J 《The journal of physical chemistry. B》2008,112(8):2483-2493
The hydration of formamide (F), N-methylformamide (NMF), N,N-dimethylformamide (DMF), acetamide (A), N-methylacetamide (NMA), and N,N-dimethylacetamide (DMA) has been studied in aqueous solutions by means of FTIR spectra of HDO isotopically diluted in H2O. The difference spectra procedure has been applied to remove the contribution of bulk water and thus to separate the spectra of solute-affected HDO. To facilitate the interpretation of obtained spectral results, DFT calculations of aqueous amide clusters were performed. Molecular dynamics (MD) simulation for the cis and trans forms of NMA was also carried out for the SPC model of water. Infrared spectra reveal that only two to three water molecules from the surrounding of the amides are statistically affected, from among ca. 30 molecules present in the first hydration sphere. The structural-energetic characteristic of these solute-affected water molecules differs only slightly from that in the bulk and corresponds to the clathrate-like hydrogen-bonded cage typical for hydrophobic hydration, with the possible exception of F. MD simulations confirm such organization of water molecules in the first hydration sphere of NMA and indicate a practical lack of orientation and energetic effects beyond this sphere. The geometry of hydrogen-bonded water molecules in the first hydration sphere is very similar to that in the bulk phase, but MD simulations have affirmed subtle differences recognized by the spectral method and enabled their understanding. The spectral data and simulations results are highly compatible. In the case of F, NMF, and A, there is a visible spectral effect of water interactions with N-H groups, which have destabilizing influence on the amides hydration shell. There is no spectral sign of such interaction for NMA as the solute. The energetic stability of water H-bonds in the amide hydration sphere and in the bulk fulfills the order: NMA > DMA > A > NMF > bulk > DMF > F. Microscopic parameters of water organization around the amides obtained from the spectra, which have been used in the hydration model based on volumetric data, confirm the more hydrophobic character of the first three amides in this sequence. The increased stability of the hydration sphere of NMA relative to DMA and of NMF relative to DMF seems to have its origin in different geometries, and so the stability, of water cages containing the amides. 相似文献