全文获取类型
收费全文 | 194篇 |
免费 | 6篇 |
国内免费 | 1篇 |
专业分类
化学 | 132篇 |
晶体学 | 3篇 |
力学 | 1篇 |
数学 | 8篇 |
物理学 | 57篇 |
出版年
2022年 | 4篇 |
2021年 | 6篇 |
2020年 | 4篇 |
2019年 | 5篇 |
2017年 | 2篇 |
2016年 | 4篇 |
2015年 | 7篇 |
2014年 | 7篇 |
2013年 | 21篇 |
2012年 | 8篇 |
2011年 | 12篇 |
2010年 | 11篇 |
2009年 | 6篇 |
2008年 | 13篇 |
2007年 | 9篇 |
2006年 | 8篇 |
2005年 | 7篇 |
2004年 | 2篇 |
2003年 | 2篇 |
2001年 | 2篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1995年 | 3篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 6篇 |
1981年 | 4篇 |
1979年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1967年 | 2篇 |
1966年 | 2篇 |
1965年 | 2篇 |
1964年 | 3篇 |
1959年 | 1篇 |
1958年 | 2篇 |
1957年 | 2篇 |
1956年 | 1篇 |
排序方式: 共有201条查询结果,搜索用时 15 毫秒
181.
Khanduri D Collins S Kumar A Adhikary A Sevilla MD 《The journal of physical chemistry. B》2008,112(7):2168-2178
In previous work, we have shown that photoexcitation of guanine cation radical (G*+) in frozen aqueous solutions of DNA and its model compounds at 143 K results in the formation of neutral sugar radicals with substantial yield. In this report, we present electron spin resonance (ESR) and theoretical (DFT) evidence regarding the formation of sugar radicals after photoexcitation of guanine cation radical (G*+) in frozen aqueous solutions of one-electron-oxidized RNA model compounds (nucleosides, nucleotides and oligomers) at 143 K. Specific sugar radicals C5'*, C3'* and C1'* were identified employing derivatives of Guo deuterated at specific sites in the sugar moiety, namely, C1'-, C2'-, C3'- and C5'-. These results suggest C2'* is not formed upon photoexcitation of G*+ in one-electron-oxidized Guo and deuterated Guo derivatives. Phosphate substitution at C5'- (i.e., in 5-GMP) hinders formation of C5'* via photoexcitation at 143 K but not at 77 K. For the RNA-oligomers studied, we observe on photoexcitation of oligomer-G*+ the formation of mainly C1'* and an unidentified radical with a ca. 28 G doublet. The hyperfine coupling constants of each of the possible sugar radicals were calculated employing the DFT B3LYP/6-31G* approach for comparison to experiment. This work shows that formation of specific neutral sugar radicals occurs via photoexcitation of guanine cation radical (G*+) in RNA systems but not by photoexcitation of its N1 deprotonated species (G(-H)*). Thus, our mechanism regarding neutral sugar formation via photoexcitation of base cation radicals in DNA appears to be valid for RNA systems as well. 相似文献
182.
Ahmad R Clymer B Vikram DS Deng Y Hirata H Zweier JL Kuppusamy P 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2007,184(2):246-257
The broad spectrum of spin probes used for electron paramagnetic resonance imaging (EPRI) result in poor spatial resolution of the reconstructed images. Conventional deconvolution procedures can enhance the resolution to some extent but obtaining high resolution EPR images is still a challenge. In this work, we have implemented and analyzed the performance of a postacquisition deblurring technique to enhance the spatial resolution of the EPR images. The technique consists of two steps; noniterative deconvolution followed by iterative deconvolution of the acquired projections which are then projected back using filtered backprojection (FBP) to reconstruct a high resolution image. Further, we have proposed an analogous technique for iterative reconstruction algorithms such as multiplicative simultaneous iterative reconstruction technique (MSIRT) which can be a method of choice for many applications. The performance of the suggested deblurring approach is evaluated using computer simulations and EPRI experiments. Results suggest that the proposed procedure is superior to the standard FBP and standard iterative reconstruction algorithms in terms of mean-square-error (MSE), spatial resolution, and visual judgment. Although the procedure is described for 2D imaging, it can be readily extended to 3D imaging. 相似文献
183.
184.
Combination therapy with acyclovir and zidovudine is used for the treatment of herpes-infected immunocompromised patients. In the view of the optimal drug concentrations (minimum effective concentrations) for viral suppression and avoidance of drug toxicity, monitoring of drug levels has been considered essential to determine drug concentrations in plasma after administration of a dose of acyclovir and zidovudine. A simple, precise, and rapid RP-HPLC method has been developed for this purpose. Chromatographic separation was performed using methanol-water (50 + 50, v/v), pH 2.5 adjusted with orthophosphoric acid, as an isocratic mobile phase at a flow rate of 0.8 mL/min with an Inertsil ODS (C18) column (5 microm particle size, 250 x 4.60 mm id). Detection was carried out using a UV photo diode array detector at 258 nm. The plasma samples were prepared by a protein precipitation method. The retention time for acyclovir and zidovudine was 3.5 +/- 0.2 and 6.2 +/- 0.3 min, respectively. The method was linear in the range of 200-1800 and 400-3600 ng/mL with LOQ of 200 ng (SD = +/-1.4) and 400 ng (SD = +/-0.9) for zidovudine and acyclovir, respectively, in plasma. The mean accuracy was 98.0 and 96.4%, with average extraction recovery of 64.8 +/- 2.1 and 77.5 +/- 1.7% for lower nominal concentrations of acyclovir and zidovudine, respectively. 相似文献
185.
Flavone Glycoside Based Validated RP-LC Method for Quality Evaluation of Prishniparni (Uraria picta) 总被引:1,自引:0,他引:1
Akhilesh K. Yadav Deepti Yadav Karuna Shanker Ram K. Verma Ajit K. Saxena Madan M. Gupta 《Chromatographia》2009,69(7-8):653-658
An LC method is developed for the quantitation of rhoifolin in Uraria picta, a plant of high usage frequency in all Asian traditional systems of medicine. An isocratic RP-LC method using C18 column, UV detection 265 nm and specificity with PDA and MS is speeding up, reliable and comprehensive analysis of rhoifolin in U. picta. Good linearity was obtained in the working range (0.02–0.10 mg mL?1), with correlation coefficients >0.99. LOD and LOQ were 2.33 and 7.69 ng, respectively. The method was validated following international guidelines. The described method can be utilized for assays and stability tests of U. picta extracts as well as Ayurvedic drugs based on Prishniparni. 相似文献
186.
Adhikary A Kumar A Munafo SA Khanduri D Sevilla MD 《Physical chemistry chemical physics : PCCP》2010,12(20):5353-5368
By use of ESR and UV-vis spectral studies, this work identifies the protonation states of one-electron oxidized G:C (viz. G˙+:C, G(N1–H)˙:C(+H+), G(N1–H)˙:C, and G(N2-H)˙:C) in a DNA oligomer d[TGCGCGCA]2. Benchmark ESR and UV-vis spectra from one electron oxidized 1-Me-dGuo are employed to analyze the spectral data obtained in one-electron oxidized d[TGCGCGCA]2 at various pHs. At pH ≥7, the initial site of deprotonation of one-electron oxidized d[TGCGCGCA]2 to the surrounding solvent is found to be at N1 forming G(N1–H)˙:C at 155 K. However, upon annealing to 175 K, the site of deprotonation to the solvent shifts to an equilibrium mixture of G(N1–H)˙:C and G(N2–H)˙:C. For the first time, the presence of G(N2–H)˙:C in a ds DNA-oligomer is shown to be easily distinguished from the other prototropic forms, owing to its readily observable nitrogen hyperfine coupling (Azz(N2) = 16 G). In addition, for the oligomer in H2O, an additional 8 G N2–H proton HFCC is found. This ESR identification is supported by a UV-vis absorption at 630 nm which is characteristic for G(N2–H)˙ in model compounds and oligomers. We find that the extent of photo-conversion to the C1′ sugar radical (C1′˙) in the one-electron oxidized d[TGCGCGCA]2 allows for a clear distinction among the various G:C protonation states which can not be easily distinguished by ESR or UV-vis spectroscopies with this order for the extent of photo-conversion: G˙+:C > G(N1–H)˙:C(+H+) ? G(N1–H)˙:C. We propose that it is the G˙+:C form that undergoes deprotonation at the sugar and this requires reprotonation of G within the lifetime of exited state 相似文献
187.
188.
The composite Anderson-Newns model and triangular weighted density of states are used to study the electron density of states,
magnetic moment and charge transfer of adatoms for a chemisorbed system. The model has been applied to hydrogen chemisorbed
on nickel. It has been observed that with the increase in coverage, number of B-AB states as well as bond strength increases,
whereas the magnetic moment and adatom charge decrease with coverage. These results match with the experimental data. 相似文献
189.
Integral enthalpies of solution of some sugars and polyols in water at low concentrations have been determined calorimetrically at 25 and 35°C. These data have been used to derive heat capacities of solution C°p at 30°C. Partial molal heat capacities C°p,2 have been obtained by combining C°p with C
p,2
*
, the heat capacity of pure solid compounds. Apparent molal volumes have been obtained from density data. The sugars as well as polyols show significantly high positive C°p and C°p,2 values. The results have been explained in terms of a specific hydration model. The effect of substitution of-OH by glycosidic-OCH3 and of-CHOH by deoxy-CH2 are also discussed. 相似文献
190.
The excess volume V
E
of binary mixtures of octanol, nonanol and dodecanol in ethylbenzene have been calculated from the densities measured with a vibrating tube densimeter at temperatures from 50 to 100 °C and at pressures from 0.1 to 7.5 MPa. The values of V
E
are positive for all the three mixtures in the complete temperature, pressure and mole fraction ranges studied. The maxima in V
E
is observed at 0.4 mole fraction of alkanol. The results are discussed in terms of specific interactions present in the binary mixtures. The second order thermodynamic quantities (V
E
/T)p,(V
E
/P)T and (V
E
/P)T which have been derived from the effect of temperature and pressure on V
E
, indicate an overall net creation of order in the binary mixtures of ethylbenzene with higher homologues of alkanols. 相似文献