全文获取类型
收费全文 | 279篇 |
免费 | 9篇 |
专业分类
化学 | 221篇 |
晶体学 | 1篇 |
力学 | 1篇 |
数学 | 24篇 |
物理学 | 41篇 |
出版年
2023年 | 1篇 |
2022年 | 2篇 |
2021年 | 5篇 |
2020年 | 4篇 |
2019年 | 3篇 |
2018年 | 3篇 |
2017年 | 3篇 |
2016年 | 2篇 |
2015年 | 6篇 |
2014年 | 14篇 |
2013年 | 18篇 |
2012年 | 26篇 |
2011年 | 25篇 |
2010年 | 11篇 |
2009年 | 7篇 |
2008年 | 20篇 |
2007年 | 20篇 |
2006年 | 22篇 |
2005年 | 15篇 |
2004年 | 17篇 |
2003年 | 14篇 |
2002年 | 8篇 |
2001年 | 4篇 |
2000年 | 5篇 |
1999年 | 2篇 |
1998年 | 5篇 |
1997年 | 3篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1983年 | 2篇 |
1981年 | 1篇 |
1977年 | 2篇 |
1965年 | 1篇 |
排序方式: 共有288条查询结果,搜索用时 0 毫秒
121.
Braunschweig H Gross M Kraft M Kristen MO Leusser D 《Journal of the American Chemical Society》2005,127(10):3282-3283
The first [2]borametallocenophanes of Zr and Hf are reported, which were obtained from 1,2-bis(dimethylamino)-1,2-dicyclopentadienyldiborane(4). The constitution of these complexes in solution and, for the Hf complex, in the crystalline state is discussed. The title complexes were studied as Ziegler-Natta-type catalyst precursors for the polymerization of ethylene. Both complexes produced PE of high molecular mass up to 1.6 x 106 g/mol, and very interestingly, the Hf complex proved to be as active as its Zr analogue. 相似文献
122.
Microwave spectra and the metal-hydrogen bond lengths for the C5H5Mo(CO)3H and C5H5W(CO)3H complexes
Tanjaroon C Keck KS Sebonia MM Karunatilaka C Kukolich SG 《The Journal of chemical physics》2004,121(3):1449-1453
The measurements of rotational spectra and metal-hydrogen bond lengths for molybdenum and tungsten hydride complexes were recently completed in our laboratory. The W-H and Mo-H bond lengths were obtained from high resolution rotational spectra of C5H5Mo(CO)3H, C5H5W(CO)3H, C5H5Mo(CO)3D, and C5H5W(CO)3D. Data for five molybdenum and four tungsten isotopomers were obtained for both the normal and deuterium-substituted species. The asymmetric-top rotational parameters A, B, C, DeltaJ, and deltaJ were determined from the least-squares fits and these results indicate that the structures of these complexes are nearly rigid. The hydrogen bond lengths were determined for both complexes using Kraitchman analyses. The molybdenum-hydrogen bond length for the C5H5Mo(CO)3H complex is rMo-H=1.80(1) A. The tungsten-hydrogen bond length for the C5H5W(CO)3H complex is rW-H=1.79(4) A. Density functional theory (DFT) calculations of the structures were performed to obtain the optimized theoretical structures for C5H5Mo(CO)3H and C5H5W(CO)3H. Results obtained from the DFT calculations are in good agreement with the experimental parameters, and the Mo-H value is in good agreement with previously reported Mo-H bond lengths for similar complexes. 相似文献
123.
Vignesh Palani Melecio A. Perea Kristen E. Gardner Richmond Sarpong 《Chemical science》2021,12(4):1528
The synthesis of diverse N-fused heterocycles, including the pyrido[1,2-a]indole scaffold, using an efficient pyrone remodeling strategy is described. The pyrido[1,2-a]indole core was demonstrated to be a versatile scaffold that can be site-selectively functionalized. The utility of this novel annulation strategy was showcased in a concise formal synthesis of three fascaplysin congeners.The synthesis of diverse N-fused heterocycles, including the pyrido[1,2-a]indole scaffold, using an efficient pyrone remodeling strategy is described. 相似文献
124.
Microwave spectra were obtained for two distinct structural isomers of 1,1'-dimethylferrocene, an eclipsed synperiplanar isomer (phi = 0 degrees, the E0 isomer), with A = 1176.9003(2) MHz, B = 898.3343(2) MHz, C = 668.7469(2) MHz, and an eclipsed synclinal isomer (phi = 72 degrees, the E72 isomer) with A = 1208.7117(14) MHz, B = 806.4101(12) MHz, and C = 718.7179(8) MHz. The b-dipole, asymmetric-top spectra of both structural isomers were measured in the frequency range of 5-12 GHz using a Flygare-Balle type of spectrometer. A very good fit to observed transitions, with small distortion constants, was obtained for the E0 conformer, indicating that this conformer is nearly rigid. The deviations obtained in a similar least-squares fit for the E72 confomer are significantly larger, indicating possible fluxional behavior for this conformer. In addition, 7 out of the 26 transitions observed for the E72 isomer conformer clearly exhibit very small splittings, giving further evidence for internal motion. DFT calculations for the different possible conformations of 1,1'- dimethylferrocene arising from rotation of one methyl cyclopentadienyl ligand relative to the other about the nominal C5 axis by an angle phi (dihedral angle) were performed using the B3PW91 functional. The calculations converged and were optimized for five structures on this torsional potential energy surface corresponding to different dihedral angles phi; three yielded energy minima, and two gave energy maxima, corresponding to transition states. The experimental results are in very good agreement with the results of the DFT calculations. 相似文献
125.
We use laser photoacoustic spectroscopy to obtain overtone spectra at three through six quanta of O-H stretch excitation (3nu(OH)-6nu(OH)) for methyl hydroperoxide (MeOOH). Extending the spectral regions beyond our previous work reveals new features that can be attributed to transitions involving torsion about the O-O bond. Experimental spectral profiles (3nu(OH)-6nu(OH)) and cross sections (3nu(OH)-5nu(OH)) at room temperature show a good agreement with the simulated spectra that we obtain from ab initio calculations employing a vibration-torsion model at 298 K. A Birge-Sponer analysis yields experimental values for the O-H stretch frequency (omega=3773+/-15 cm(-1)) and anharmonicity (omegax=94+/-3 cm(-1)). We also detect OH radicals by laser-induced fluorescence and present photodissociation action spectra of MeOOH in the regions of 4nu(OH) and 5nu(OH). While the spectral profile at 5nu(OH) mimics the photoacoustic spectrum, the peak intensity for transitions to torsionally excited states is relatively more intense in the action spectrum at 4nu(OH), reflecting the fact that the 4nu(OH) excitation energy is below the literature dissociation energy (D0=42.6+/-1 kcal mol(-1)) so that features in the action spectrum come from thermally populated excited states. Finally, we use our calculations to assign contributions to individual peaks in the room-temperature spectra and relate our findings to a recent dynamics study in the literature. 相似文献
126.
Quigley WW Bramanti E Staggemeier BA Miller KE Nabi A Skogerboe KJ Synovec RE 《Analytical and bioanalytical chemistry》2004,378(1):134-143
A drop-based dynamic surface-tension detector (DSTD) has been used to study the dynamic surface tension behavior of proteins denatured in guanidine thiocyanate (GndSCN). The dynamic surface tension at the air–liquid interface is obtained by measuring the internal pressure of drops that grow and detach at a specified rate. In the method the sample of interest is injected and subsequently flows to the DSTD-sensing capillary tip. For this work, a novel DSTD calibration procedure utilizing two distinct mobile phases is applied. Here, the mobile phases are aqueous with different constituents, for example GndSCN and phosphate buffer, either added or omitted. The dual-mobile phase calibration procedure gives the analyst the capability of making protein measurements in a GndSCN–phosphate buffer mobile phase, while measuring a calibration standard in another mobile phase, such as water, in which the surface tension of the calibration standard is readily available. Results are presented with drop volumes of either 2 L (i.e. 2-s drops) or 7 L (i.e. 7-s drops) for proteins varying in molar mass from 12,000 to 330,000 g mol–1. We demonstrate that the DSTD can be used to determine the molar mass of proteins denatured in GndSCN. The method applies a regime where the denatured protein is detected by surface-active properties, and selectivity with regard to molar mass is contained in the dynamic component of the DSTD signal. The dynamic surface pressure signals of the denatured proteins suggest that diffusion plays a large role in the kinetics of the surface activity. The limit of detection for the denatured proteins studied ranged from 3 mg L–1 to 14 mg L–1. The DSTD, coupled with the novel dual-mobile phase calibration procedure, can be used to investigate the fundamental properties of proteins. Insight into the behavior at the air–liquid interface for native and denatured proteins is achieved; this is a novel tool for studying protein denaturation, complementary to other common approaches such as spectroscopy and calorimetry. Furthermore, the reported method could be widely applied to the study of effects on the interfacial properties of proteins after a variety of chemical and physical modifications that are possible with the dual-mobile phase calibration procedure. 相似文献
127.
128.
Jianqing Li Daniel Smith Subramaniam Krishnananthan Dauh-Rurng Wu Dawn Sun Peng Li Kristen Ryan Min Hu Wenge Cui Jennifer Naginskaya Shuang Liu Paul C. Lobben Alicia T. Ng Richard Olson Arvind Mathur 《Tetrahedron: Asymmetry》2017,28(1):196-202
BMS-911278 was identified as a potent triple reuptake inhibitor potentially useful for the treatment of depression. The original racemic synthesis suffered from tedious and low recovery resolution and HPLC separation, as well as low-yielding hazardous N-demethylation at the API step. To support further preclinical studies, a scalable enantioselective synthesis was developed. Herein, we report an efficient asymmetric synthesis of BMS-911278 featuring two key steps: an enantioselective Miyaura reaction and an intramolecular regioselective cyclization. 相似文献
129.
Kristen A. Pace Dr. Vladislav V. Klepov Travis K. Deason Dr. Mark D. Smith Gyanendra B. Ayer Dr. David P. Diprete Dr. Jake W. Amoroso Prof. Hans-Conrad zur Loye 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(57):12941-12944
NanMAn6F30 is an extremely versatile framework structure for incorporating tetravalent actinides (An) and cerium along with divalent or trivalent d-metals (M); moreover, the structure exhibits a high resistance to harsh chemical conditions. This extreme robustness can potentially be exploited for the sequestration of plutonium in a stable matrix; however, no NanMPu6F30 compounds have been reported so far. Herein, we present four new plutonium fluorides that have been prepared as single crystals by mild hydrothermal synthesis methods. Structural characterizations revealed their compositions to be Na3AlPu6F30, Na3FePu6F30, Na3CoPu6F30, and Na2.4Mn1.6Pu6F30. Surprisingly, in the plutonium series, it was found that Co2+ and Mn2+ precursors oxidized to form Na3CoIIIPu6F30 and Na2.4MnII/III1.6Pu6F30, whereas the analogous reactions for cerium result in reduction of the transition metal, even when beginning with a M3+ precursor. While cerium is often used as a surrogate for plutonium, this work serves as an example that deviations between their chemistries do occur. 相似文献
130.
F. Stevens H. Vrielinck F. Callens E. Pauwels V. Van Speybroeck M. Waroquier 《International journal of quantum chemistry》2005,102(4):409-414
Calculations using density functional theory are performed to study the electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) properties of S and Se impurities in alkali halide lattices. Cluster in vacuo models are used to describe the defect and the lattice surroundings. The trivacancy defect model proposed in the literature is able to reproduce both the experimental principal values and directions of the g tensor for S and Se defects doped in alkali halides. The alternative monovacancy model gives rise to important discrepancies with experiment and can be discarded. For the KCl lattice, the hyperfine tensors of the S and Se molecular ions also agree well with the available experimental data, giving further evidence to the trivacancy model. In addition, for NaCl:S and KCl:S computational results for the 23Na and 35Cl superhyperfine and quadrupole tensors are compared with experimental ENDOR parameters. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 相似文献