全文获取类型
收费全文 | 1612篇 |
免费 | 239篇 |
国内免费 | 104篇 |
专业分类
化学 | 438篇 |
晶体学 | 12篇 |
力学 | 248篇 |
综合类 | 13篇 |
数学 | 318篇 |
物理学 | 926篇 |
出版年
2023年 | 10篇 |
2022年 | 22篇 |
2021年 | 29篇 |
2020年 | 32篇 |
2019年 | 33篇 |
2018年 | 38篇 |
2017年 | 52篇 |
2016年 | 43篇 |
2015年 | 48篇 |
2014年 | 69篇 |
2013年 | 141篇 |
2012年 | 69篇 |
2011年 | 116篇 |
2010年 | 101篇 |
2009年 | 120篇 |
2008年 | 105篇 |
2007年 | 111篇 |
2006年 | 99篇 |
2005年 | 69篇 |
2004年 | 85篇 |
2003年 | 86篇 |
2002年 | 69篇 |
2001年 | 49篇 |
2000年 | 53篇 |
1999年 | 49篇 |
1998年 | 37篇 |
1997年 | 37篇 |
1996年 | 23篇 |
1995年 | 29篇 |
1994年 | 20篇 |
1993年 | 19篇 |
1992年 | 18篇 |
1991年 | 9篇 |
1990年 | 7篇 |
1989年 | 14篇 |
1988年 | 6篇 |
1987年 | 3篇 |
1986年 | 7篇 |
1985年 | 5篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 6篇 |
1980年 | 1篇 |
1977年 | 1篇 |
1976年 | 4篇 |
1974年 | 1篇 |
1971年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有1955条查询结果,搜索用时 15 毫秒
1.
2.
The structure and rotational barrier for the mesityl-silicon bond of 2,2-dimesityl-1,1,1,3,3,3-hexamethyltrisilane have been investigated by 1H- and 13C-variable temperature nuclear magnetic resonance (NMR) as well as by density functional theory structural calculations. The calculations show that the lowest energy structure has C2 symmetry with nonequivalent ortho methyl groups, consistent with the crystal structure and solution NMR. The nonequivalent ortho methyl groups exchange through a Cs transition state with a calculated relative free energy of 11.0 kcal mol−1. The barrier for this rotation found by dynamic NMR is 13.4 ± 0.2 kcal mol−1 at 298 K. 相似文献
3.
Wenbin Guo Dr. Nathalie Daro Dr. Sébastien Pillet Dr. Mathieu Marchivie Dr. El-Eulmi Bendeif Dr. Elodie Tailleur Dr. Kittipong Chainok Dominique Denux Dr. Guillaume Chastanet Prof. Philippe Guionneau 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(57):12927-12930
The current craze for research around the spin crossover phenomenon can be justified to some extent by the mechanical properties due to the decrease of volume associated with the transition of the metal ion from the HS state to the LS state. As demonstrated here, the molecular complex [Fe(PM-pBrA)2(NCS)2] exhibits, on the contrary, an increase of the unit-cell volume from HS to LS. This counter-intuitive and unprecedented behavior that concerns both the thermal and the photoexcited spin conversions is revealed by a combination of single-crystal and powder X-ray diffraction complemented by magnetic measurements. Interestingly, this abnormal volume change appears concomitant with the wide rotation of a phenyl ring which induces a drastic modification, though reversible, of the structural packing within the crystal. In addition, the light-induced HS state obtained through the Light-Induced Excited Spin-State Trapping shows a remarkably high relaxation temperature, namely T(LIESST), of 109 K, one of the highest so far reported. The above set of quite unusual characteristics opens up new fields of possibilities within the development of spin crossover materials. 相似文献
4.
5.
This article deals with the various heat source responses in a transversely isotropic hollow cylinder under the purview of three-phase-lag (TPL) generalized thermoelasticity theory. In presence of magnetic field and due to the rotating behavior of the cylinder, the governing equations are redefined for generalized thermoelasticity with thermal time delay. In order to obtain the stress, displacement and temperature field, the field functions are expressed in terms of modified Bessel functions in Laplace transformed domain. When the outer radius of hollow cylinder tends to infinity, the corresponding results are discussed. Finally an appropriate Laplace transform inversion technique is adopted. 相似文献
6.
Émeline Schmisser 《Stochastic Processes and their Applications》2019,129(12):5364-5405
In this article, we consider a jump diffusion process , with drift function , diffusion coefficient and jump coefficient . This process is observed at discrete times . The sampling interval tends to 0 and the time interval tends to infinity. We assume that is ergodic, strictly stationary and exponentially -mixing. We use a penalized least-square approach to compute adaptive estimators of the functions and . We provide bounds for the risks of the two estimators. 相似文献
7.
Acetyl Methyl Torsion in N‐Ethylacetamide: A Challenge for Microwave Spectroscopy and Quantum Chemistry 下载免费PDF全文
Raphaela Kannengießer Marcel J. Lach Prof. Dr. Wolfgang Stahl Dr. Ha Vinh Lam Nguyen 《Chemphyschem》2015,16(9):1906-1911
The gas‐phase structures and parameters describing acetyl methyl torsion of N‐ethylacetamide are determined with high accuracy, using a combination of molecular beam Fourier‐transform microwave spectroscopy and quantum chemical calculations. Conformational studies at the MP2 level of theory yield four minima on the energy surface. The most energetically favorable conformer, which possesses C1 symmetry, is assigned. Due to the torsional barrier of 73.4782(1) cm?1 of the acetyl methyl group, fine splitting up to 4.9 GHz is found in the spectrum. The conformational structure is not only confirmed by the rotational constants, but also by the orientation of the internal rotor. The 14N quadrupole hyperfine splittings are analyzed and the deduced coupling constants are compared with the calculated values. 相似文献
8.
在大学本科一二年级的物理化学教学中,化学动力学章节的学习涉及到对不同时间尺度的认识。本文利用过渡态理论得到的速率常数表达公式,以乙烷分子碳/碳单键旋转和水分子氢键交换的动力学过程为例,对这2个应用实例的动力学过程所发生的时间尺度做出估算。这些时间尺度的估算对于学生理解不同动力学过程的物理图像至关重要,同时也有助于加强学生对重要公式的理解并能够将这些公式在化学体系中灵活运用。 相似文献
9.
The translational diffusion coefficients of rubrene and tetracene in amorphous polyisobutylene (PIB) were measured using the holographic fluorescence recovery after photobleaching technique. Over the temperature range from 400 to 235 K (T
g
=205 K), tracer diffusion coefficients from 10–7 to 10–14 cm2/s were observed. These diffusion coefficients have essentially the same temperature dependence as the rotational correlation times for these two probes in PIB. Both of these observables have a slightly stronger temperature dependence than does the viscosity. These results contrast strongly with the results of similar experiments on polystyrene and polysulfone. These results are consistent with the hypothesis that local segmental dynamics are more spatially homogeneous in PIB than in polystyrene and polysulfone.Dedicated to Prof. John D. Ferry on the occasion of his 85th birthday. 相似文献
10.
梁板壳的几何大变形--从近似的非线性理论到有限变形理论 总被引:6,自引:0,他引:6
对梁板壳的线性理论、近似几何非线性理论与有限变形理论作了比较,介绍了有限转动理论,指出了应用有限变形理论求解梁板壳的大变形问题的高效率、高精度的巨大优越性。 相似文献