全文获取类型
收费全文 | 1541篇 |
免费 | 41篇 |
国内免费 | 9篇 |
专业分类
化学 | 1063篇 |
晶体学 | 16篇 |
力学 | 19篇 |
数学 | 203篇 |
物理学 | 290篇 |
出版年
2023年 | 11篇 |
2022年 | 20篇 |
2021年 | 32篇 |
2020年 | 33篇 |
2019年 | 17篇 |
2018年 | 25篇 |
2017年 | 27篇 |
2016年 | 38篇 |
2015年 | 37篇 |
2014年 | 54篇 |
2013年 | 89篇 |
2012年 | 93篇 |
2011年 | 112篇 |
2010年 | 84篇 |
2009年 | 86篇 |
2008年 | 117篇 |
2007年 | 112篇 |
2006年 | 76篇 |
2005年 | 87篇 |
2004年 | 81篇 |
2003年 | 53篇 |
2002年 | 38篇 |
2001年 | 31篇 |
2000年 | 28篇 |
1999年 | 22篇 |
1998年 | 10篇 |
1997年 | 14篇 |
1996年 | 11篇 |
1995年 | 9篇 |
1994年 | 7篇 |
1993年 | 15篇 |
1992年 | 12篇 |
1991年 | 9篇 |
1990年 | 7篇 |
1989年 | 7篇 |
1988年 | 10篇 |
1987年 | 4篇 |
1985年 | 14篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 9篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1977年 | 6篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1966年 | 3篇 |
1894年 | 1篇 |
排序方式: 共有1591条查询结果,搜索用时 11 毫秒
51.
Erich Hellemann Dr. Rubens R. Teles Prof. Dr. Fernando Hallwass Prof. Dr. W. Barros Jr. Dr. Armando Navarro‐Vázquez Prof. Dr. Roberto R. Gil 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(46):16632-16635
Anisotropic NMR parameters, such as residual dipolar couplings (RDCs), residual chemical shift anisotropies (RCSAs) and residual quadrupolar couplings (RQCs or ΔνQ), appear in solution‐state NMR when the molecules under study are subjected to a degree of order. The tunable alignment by reversible compression/relaxation of gels (PMMA and p‐HEMA) is an easy, user‐friendly, and very affordable method to measure them. When using this method, a fraction of isotropic NMR signals is observed in the NMR spectra, even at a maximum degree of compression. To explain the origin of these isotropic signals we decided to investigate their physical location inside the NMR tube using deuterium 1D imaging and MRI micro‐imaging experiments. It was observed that after a certain degree of compression the gels start to buckle and they generate pockets of isotropic solvent, which are never eliminated. The amount of buckling depends on the amount of cross‐linker and the length of the gel. 相似文献
52.
An expression for the atom-atom poiarizability between the1s orbitals of geminal or vicinal protons has been derived by perturbation theory using thePople-Santry Theory of electron systems. The coupling constants calculated on this basis agree well with these obtained from full molecular orbital calculations. Expressions have been obtained for the variation of vicinal coupling constants with dihedral angle in ethane and the HCC bond angle and C-C bond length in ethylene, and these agree well with corresponding expressions obtained using the VB method.
Zusammenfassung Es wurde ein Ausdruck für die Atom-Atom-Polarisierbarkeit zwischen den1s Orbitalen geminaler bzw. vicinaler Protonen abgeleitet mittels Störungsrechnung unter Benutzung derPople-Santry-Theorie für-Elektronensysteme. Die auf dieser Basis berechneten Koppelungskonstanten stimmen gut mit denen aus einer vollständigen MO Rechnung überein. Es wurden Ausdrücke erhalten für die Änderung der vicinalen Koppelungskonstanten mit dem Diederwinkel in Äthan sowie dem HOC Bindungswinkel und der C-C Bindungsiänge in Äthylen, die gut mit den entsprechenden Ausdrücken aus der VB Methode übereinstimmen.
Résumé A l'aide de la théorie dePople etSantry sur les systèmes d'électrons, nous dérivons de la théorie des perturbations une expression pour la polarisabiiité atome-atome entre les orbitales1s des protons géminaux ou vicinaux. Les constantes de couplage ici calculées s'accordent bien à celles obtenues des calculs OM complets. Les expressions obtenues pour la variation du couplage vicinal avec l'angle dihédrique dans l'éthane, et l'angle HCC et la distance C-C dans l'éthylène, s'accordent bien aux formules correspondantes de la méthode de la mésomérie.相似文献
53.
A novel method for cobalt preconcentration by cloud point extraction with on-line phase separation in a PTFE knotted reactor and further determination by electrothermal atomic absorption spectrometry (ETAAS) is proposed. The cloud point system was formed in the presence of non-ionic micelles of polyethyleneglycolmono-p-nonylphenylether (PONPE 7.5) and it was retained on the inner walls of a knotted reactor (KR). The surfactant rich-phase was removed from the knotted reactor with 75 microL of methanol acidified with 0.8 mol L(-1) nitric acid, directly into the dosing hole of the L'Vov graphite tube. An enrichment factor of 15 was obtained with a preconcentration time of 60 s, with respect to the direct determination of cobalt by ETAAS in aqueous solutions. The value of the detection limit for the preconcentration of 5 mL of sample solution was 10 ng L(-1). The precision, expressed as the relative standard deviation (R.S.D.), for 10 replicate determinations at 0.5 microg L(-1) Co level was 4.5%. Verification of the accuracy was carried out by analysis of a standard reference material (NIST SRM 1640e "Trace elements in natural water"). The method was successfully applied to the determination of cobalt in drinking water samples. 相似文献
54.
Dicobalt hexacarbonyl complexes of alkynyl imines were allowed to react with ketenes via Staudinger reaction. Sequential [2 + 2] cycloaddition/Pauson-Khand reaction led to structurally new fused-tricyclic beta-lactams and fused-azabicyclic cyclopentenones. Chemoselectivity, scope, and limitation of the process were investigated. 相似文献
55.
He W Hatcher E Balaeff A Beratan DN Gil RR Madrid M Achim C 《Journal of the American Chemical Society》2008,130(40):13264-13273
This paper describes the results of a 1D and 2D NMR spectroscopy study of a palindromic 8-base pair PNA duplex GGCATGCC in H2O and H2O-D2O solutions. The (1)H NMR peaks have been assigned for most of the protons of the six central base pairs, as well as for several amide protons of the backbone. The resulting 36 interbase and base-backbone distance restraints were used together with Watson-Crick restraints to generate the PNA duplex structure in the course of 10 independent simulated annealing runs followed by restrained molecular dynamics (MD) simulations in explicit water. The resulting PNA structures correspond to a P-type helix with helical parameters close to those observed in the crystal structures of PNA. Based on the current limited number of restraints obtained from NMR spectra, alternative structures obtained by MD from starting PNA models based on DNA cannot be ruled out and are also discussed. 相似文献
56.
A. A. Lamberov E. V. Dement’eva Kh. Kh. Gil’manov S. R. Egorova N. V. Romanova 《Russian Journal of Applied Chemistry》2006,79(9):1453-1457
The effect of the paste molding pressure on the physicomechanical and texture characteristics of catalysts was studied. An indirect criterion is suggested to evaluate the molding pressure in industrial extruders, whose optimal value ensures good physicomechanical properties of the catalysts and kinetic control over the catalyzed reaction. The results obtained were verified in paste molding on various industrial extruders, and this enabled the optimal choice of the molding equipment. 相似文献
57.
Vanessa C. Gonçalves Marystela Ferreira Clarissa A. Olivati Marcos R. Cardoso Cleber R. Mendonça Debora T. Balogh 《Colloid and polymer science》2008,286(12):1395-1401
The conductivity and luminescence of conjugated polymers may be combined with the photoisomerization capability of azobenzene
materials to achieve unique properties for a variety of applications, particularly if conjugated polymers with azobenzene
side chains are processed as nanostructured films. In this study, we report on Langmuir–Blodgett (LB) films of a polythiophene-bearing
azobenzene moieties, which displayed photoluminescence, thermochromism, electroactivity, and photoinduced birefringence. The
latter three properties were enhanced in the LB films, as compared to spin-coated films of the polyazothiophene, and this
has been attributed to differences in film morphology that could be probed with atomic force microscopy. 相似文献
58.
M.D. Gil García F. Cañada Cañada M.J. Culzoni L. Vera-Candioti G.G. Siano H.C. Goicoechea M. Martínez Galera 《Journal of chromatography. A》2009,1216(29):5489-5496
An analytical method for the simultaneous determination of seven non-steroidal anti-inflammatory drugs (naproxen, ketoprofen, diclofenac, piroxicam, indomethacin, sulindac and diflunisal) and the anticonvulsant carbamazepine in river and wastewater is reported. The method involves pre-concentration and clean-up by solid-phase microextraction using polydimethylsiloxane/divinylbenzene fibers, followed by liquid chromatography with diode array detection analysis. Owing to the fact that river water samples did not contain interferences and no sensitivity changes due to sample matrix were observed, external calibration was implemented. Standardization was also applied in order to carry out the prediction step by preparing only two diluted standards that were subjected to the pre-concentration step and a set of standards prepared in solvent. For the analysis of wastewater samples, in contrast, it was necessary to implement standard addition calibration in combination with the multivariate curve resolution-alternating least squares (MCR-ALS) algorithm, which allowed us to overcome matrix effect and exploit the second order advantage. Recoveries ranging from 72% to 125% for all pharmaceuticals proved the accuracy of the proposed method in river water samples. On the other hand, wastewater sample recoveries ranged from 83% to 140% for all pharmaceuticals, showing an acceptable performance – considering this sample contains no modeled interferences. 相似文献
59.
Chen-jie Zeng Meng Zhou Chakicherla Gayathri Roberto R. Gil Matthew Y. Sfeir Rongchao Jin 《化学物理学报(中文版)》2018,31(4):555-562
Gold(I) thiolate compounds (i.e. AuI-SR) are important precursors for the synthesis of atomically precise Aun(SR)m nanoclusters. However, the nature of the AuI-SR precursor remains elusive. Here, we report that the Au10(TBBT)10 complex is a universal precursor for the synthesis of Aun(TBBT)m nanoclusters (where TBBT=4-tertbutylbenzenethiol/thiolate). Interestingly, the Au10(TBBT)10 complex is also found to be re-generated through extended etching of the Aun(SR)m nanoclusters with excess of TBBT thiol and O2. The formation of well-defined Au10(TBBT)10 complex, instead of polymeric AuI-SR, is attributed to the bulkiness of the TBBT thiol. Through 1D and 2D NMR characterization, the structure of Au10(TBBT)10 is correlated with the previously reported X-ray structure, which contains two inter-penetrated Au5(TBBT)5 rings. The photophysical property of Au10(TBBT)10 complex is further probed by femtosecond transient absorption spectroscopy. The accessibility of the precise Au10(TBBT)10 precursor improves the efficiency of the synthesis of the Aun(TBBT)m nanoclusters and is expected to further facilitate excellent control and understanding of the reaction mechanisms of nanocluster synthesis. 相似文献
60.
Luciano N. Batista Rosane A. S. San Gil Luciana B. R. Lopes Vera L. P. Soares 《Journal of Thermal Analysis and Calorimetry》2011,106(3):697-702
This work describes thermal analysis evaluation of a glycerol derived compound (fatty acid esters of (2,2-dimethyl-1,3-dioxolan-4-yl)
methanol) developed to work as a biofuel. Mixtures of these ketal–glyceryl esters with fatty acid methyl esters typical of
soybean biodiesel were prepared and evaluated in relation to biodiesel critical thermal properties such as temperature of
crystallization, thermal stability and volatilization measured by differential scanning calorimetry and thermogravimetric
analysis. The volatility of the products containing fatty acid methyl esters and (2,2-dimethyl-1,3-dioxolan-4-yl) methyl esters
could be predicted by thermogravimetric analyses conducted in nitrogen that avoided time consuming distillation and greatly
reduced material expenditure. 相似文献