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排序方式: 共有197条查询结果,搜索用时 15 毫秒
101.
102.
A detailed comparison between inelastic α and p scattering in the giant resonance region of 24, 26Mg, 28Si and 40Ca shows that there is no evidence for ΔT = 1, E1 excitation in the (p, p′) spectra. This is consistent with DWBA calculations using a recently obtained isovector interaction potential. 相似文献
103.
104.
B. Drevillon S. Borenstein B. Chaurand J.M. Gago R.A. Salmeron R. Bartloutaud A. Borg M. Spiro C. Wohl K. Paler C. Comber S.N. Tovey T.P. Shah 《Nuclear Physics B》1975,97(3):392-402
Results are presented of a bubble chamber experiment on K?p elastic scattering at 14.3 GeV/c, in four-momentum transfer range 0.04 < |t| < 2.74 GeV2 using an initial set of 40 000 events. The total elastic cross section is (2.96 ± 0.10) mb. The results are compared with K+p elastic scattering data at 13.8 GeV/c, and the effective Regge trajectory is calculated using K?p data from 5 to 100 GeV/c. 相似文献
105.
The reduction chemistry of (mu-bridge)[Fe(CO)3]2 [bridge = propane-1,3-dithiolate (1) and ethane-1,2-dithiolate (2)] is punctuated by the formation of distinct products, resulting in a marked difference in CO inhibition of electrocatalytic proton reduction. The products formed following reduction of 2 have been examined by a range of electrochemical, spectroelectrochemical, and spectroscopic approaches. Density functional theory has allowed assessment of the relative energies of the structures proposed for the reduction products and agreement between the calculated spectra (IR and NMR) and bond distances with the experimental spectra and EXAFS-derived structural parameters. For 1 and 2, one-electron reduction is accompanied by dimerization, but the structure, stability, and reaction with CO of the dimer is different in the two cases, and this is responsible for the different CO inhibition response for electrocatalytic proton reduction. Calculations of the alternate structures of the two-electron, one-proton reduced forms of 2 show that the isomers with terminally bound hydrides are unlikely to play a significant role in the chemistry of these species. The hydride-transfer chemistry of the 1B species is more reasonably attributed to a hydride-bridged form. The combination of experimental and computational results provides a solid foundation for the interpretation of the reduction chemistry of dithiolate-bridged diiron compounds, and this will underpin translation of the diiron subsite of the [FeFe] hydrogenase H cluster into an abiological context. 相似文献
106.
Best SP Borg SJ White JM Razavet M Pickett CJ 《Chemical communications (Cambridge, England)》2007,(42):4348-4350
We show that a dinuclear assembly apparently providing the first example of a synthetic molecule exhibiting key features of the diiron subsite of [FeFe] hydrogenase, viz. CO-bridging of a coordinatively unsaturated, dithiolate-bridged mixed-valence diiron centre, is in fact a diamagnetic tetranuclear complex. 相似文献
107.
The conversion of the plant photoreceptor phytochrome from an inactive (Pr) to an active form (Pfr) is accomplished by a red-light induced Z --> E photoisomerization of its phytochromobilin chromophore. In recent years, the question whether the photoactivation involves a change in chromophore protonation state has been the subject of many experimental studies. Here, we have used quantum chemical methods to calculate relative ground and excited-state pKa values of the different pyrrole moieties of phytochromobilin in a protein-like environment. Assuming (based on experimental data) a Pr ZaZsZa chromophore and considering isomerizations at C15 and C5, it is found that moieties B and C are the strongest acids both in the ground state and in the bright first singlet excited state, which is rationalized in simple geometric and electronic terms. It is also shown that neither light absorption nor isomerization increases the acidity of phytochromobilin relative to the reference Pr state with all pyrrolenic nitrogens protonated. Hence, provided that the subset of chromophore geometries under investigation is biologically relevant, there appears to be no intrinsic driving force for a proton-transfer event. In a series of benchmark calculations, the performance of ab initio and time-dependent density functional theory methods for excited-state studies of phytochromobilin is evaluated in light of available experimental data. 相似文献
108.
Tommi H. Nyrönen Reijo Suontamo Ilkka Pitkänen 《Theoretical chemistry accounts》1999,101(1-3):209-214
A theoretical study of the structure, charge distribution, rotational barrier and fundamental vibrations of anhydrous betaine
(CH3)3NCH2COO (trimethylglycine) was carried out and compared with available experimental data. Calculations were carried out at HF,
MP2 and B3LYP levels using a 6-31+G(d,p) basis set. The calculated rotational barrier of the betaine carboxylic group is 40.5 kJ/mol at the MP4(SDQ)/6-311G(d,p)//HF/6-31+G(d,p) level of theory. The rotation of the carboxylic group changes the molecule from a highly symmetric (C
s
) conformation into a twisted conformation resulting in shortening of the molecule by about 50 pm. Natural population analysis
(NPA) indicates intramolecular interaction between the carboxylic oxygen and the nearest methyl hydrogens resulting in internal
hydrogen bonding. MP4(SDQ)/6-311G(d,p) single-point NPA calculations on a betaine monohydrate model taken from the X-ray geometry show an expected weakening in
the internal hydrogen bond. Calculations explain why betaine preferentially crystallizes in high local C
s
symmetry.
Received: 24 March 1998 / Accepted: 3 September 1998 / Published online: 7 December 1998 相似文献
109.
110.
Mesoscale calculations have been conducted in order to gain further insight into the dynamic compaction characteristics of granular ceramics. The primary goals of this work are to numerically determine the shock response of granular tungsten carbide and to assess the feasibility of using these results to construct the bulk material Hugoniot. Secondary goals include describing the averaged compaction wave behavior as well as characterizing wave front behavior such as the strain rate versus stress relationship and statistically describing the laterally induced velocity distribution. The mesoscale calculations were able to accurately reproduce the experimentally determined Hugoniot slope but under predicted the zero pressure shock speed by 12%. The averaged compaction wave demonstrated an initial transient stress followed by asymptotic behavior as a function of grain bed distance. The wave front dynamics demonstrate non-Gaussian compaction dynamics in the lateral velocity distribution and a power-law strain rate–stress relationship. 相似文献