全文获取类型
收费全文 | 998篇 |
免费 | 37篇 |
国内免费 | 2篇 |
专业分类
化学 | 780篇 |
晶体学 | 17篇 |
力学 | 13篇 |
数学 | 54篇 |
物理学 | 173篇 |
出版年
2022年 | 5篇 |
2021年 | 6篇 |
2020年 | 9篇 |
2019年 | 9篇 |
2018年 | 12篇 |
2017年 | 7篇 |
2016年 | 14篇 |
2015年 | 21篇 |
2014年 | 15篇 |
2013年 | 53篇 |
2012年 | 38篇 |
2011年 | 58篇 |
2010年 | 37篇 |
2009年 | 27篇 |
2008年 | 71篇 |
2007年 | 54篇 |
2006年 | 53篇 |
2005年 | 57篇 |
2004年 | 43篇 |
2003年 | 39篇 |
2002年 | 36篇 |
2001年 | 29篇 |
2000年 | 22篇 |
1999年 | 13篇 |
1998年 | 10篇 |
1996年 | 9篇 |
1995年 | 10篇 |
1994年 | 5篇 |
1993年 | 12篇 |
1992年 | 16篇 |
1991年 | 7篇 |
1990年 | 8篇 |
1989年 | 6篇 |
1988年 | 8篇 |
1987年 | 10篇 |
1986年 | 14篇 |
1985年 | 18篇 |
1984年 | 12篇 |
1983年 | 13篇 |
1982年 | 14篇 |
1981年 | 14篇 |
1980年 | 15篇 |
1979年 | 9篇 |
1978年 | 17篇 |
1977年 | 25篇 |
1976年 | 11篇 |
1975年 | 10篇 |
1974年 | 9篇 |
1973年 | 8篇 |
1962年 | 3篇 |
排序方式: 共有1037条查询结果,搜索用时 15 毫秒
951.
952.
The first example to couple aldehydes and 3-bromo-2-halopropenes in a catalytic asymmetric manner is reported. The coupling reaction is effected by the use of a chiral sulfonamide-Cr complex (prepared in situ from 1d, CrBr3, Fe(III) or from Co(II), Et3N, and Mn), TMSCl, and 2,6-lutidine. The method reported here is operationally simple and scalable, furnishing 3-halohomoallylic alcohols with a synthetically useful level of enantiomeric excess. 相似文献
953.
Hasegawa M Aliu E Andringa S Aoki S Argyriades J Asakura K Ashie R Berns H Bhang H Blondel A Borghi S Bouchez J Burguet-Castell J Casper D Cavata C Cervera A Chen SM Cho KO Choi JH Dore U Espinal X Fechner M Fernandez E Fukuda Y Gomez-Cadenas J Gran R Hara T Hasegawa T Hayashi K Hayato Y Helmer RL Hill J Hiraide K Hosaka J Ichikawa AK Iinuma M Ikeda A Inagaki T Ishida T Ishihara K Ishii T Ishitsuka M Itow Y Iwashita T Jang HI Jeon EJ Jeong IS Joo KK Jover G Jung CK Kajita T Kameda J Kaneyuki K 《Physical review letters》2005,95(25):252301
We report the result from a search for charged-current coherent pion production induced by muon neutrinos with a mean energy of 1.3 GeV. The data are collected with a fully active scintillator detector in the K2K long-baseline neutrino oscillation experiment. No evidence for coherent pion production is observed, and an upper limit of is set on the cross section ratio of coherent pion production to the total charged-current interaction at 90% confidence level. This is the first experimental limit for coherent charged pion production in the energy region of a few GeV. 相似文献
954.
We report results of a study on the incident energy and the surface-temperature dependence of the steric effects in the dissociative adsorption of CH3Cl on a Si{100} surface. Data presented here show that the initial sticking probability for the Cl-end collision is larger at an incident energy of 120 meV than that in the CH3-end collision. Furthermore, this steric preference is quite sensitive to the kinetic energy and the rotational state of CH3Cl and the surface temperature. This study shows that the nonequilibrium surface trapping plays a key role in the initial step of the decomposition of CH3Cl on Si{100}. 相似文献
955.
956.
A simple strategy to compose a new all-atom protein force field (named as the SAAP force field), which utilizes the single amino acid potential (SAAP) functions obtained in various solvents by ab initio molecular orbital calculation applying the isodensity polarizable continuum model (IPCM), is presented. We considered that the total energy function of a protein force field (E(TOTAL)) is divided into three components; a single amino acid potential term (E(SAAP)), an interamino acid nonbonded interaction term (E(INTER)), and a miscellaneous term (E(OTHERS)), which is ignored (or considered to be constant) at the current version of the force field. The E(INTER) term consists of electrostatic interactions (E(ES')) and van der Waals interactions (E(LJ')). Despite simplicity, the SAAP force field implicitly involves the correlation among individual terms of the Lifson's potential function within a single amino acid unit and can treat solvent effects unambiguously by choosing the SAAP function in an appropriate solvent and the dielectric constant (D) of medium. Application of the SAAP force field to the Monte Carlo simulation of For-Ala(2)-NH(2) in vacuo reasonably reproduced the results of the extensive conformational search by ab initio molecular orbital calculation. In addition, the preliminary Monte Carlo simulations for For-Gly(10)-NH(2) and For-Ala(10)-NH(2) showed reversible transitions from the extended to the pseudosecondary structures in water (D = 78.39) as well as in ether (D = 4.335). The result suggested that the new approach is efficient for fast modeling of protein structures in various environments. Decomposition analysis of the total energy function (E(TOTAL)) by using the SAAP force field suggested that conformational propensities of single amino acids (i.e., the E(SAAP) term) may play definitive roles on the topologies of protein secondary structures. 相似文献
957.
The first example of ammoximation of bulky ketones over highly hydrophobic titanium incorporated ethane bridged hybrid mesoporous silsesquioxane with high selectivity of oximes is presented. 相似文献
958.
[reaction: see text] Enantiomerically enriched beta-(diphenylphosphatoxy)nitroalkanes undergo radical ionic fragmentation, induced by tributyltin hydride and AIBN in benzene at reflux, to give alkene radical cations in contact radical ion pairs. These contact ion pairs are trapped intramolecularly by amines to give pyrrolidines and piperidines with significant enantioselectivity ( approximately 60% ee), indicative of cyclization competing effectively with equilibration within the ion pairs. Use of an intramolecular N-propargylamine as a nucleophile provides an enantiomerically enriched pyrrolizidine skeleton via a tandem polar/radical crossover sequence. 相似文献
959.
The hydrogenation of ethyl phenylglyoxylate catalyzed by bis(dimethylglyoximato)cobalt(II)-quinine gave a novel reductive dimerization product, diethyl diphenyltartrate, with a slight optical activity. The crystals obtained from its benzene-hexane solution were investigated by X-ray diffraction method. The crystal is triclinic, space group p1, a = 10.078(3), b = 10.283(2), c = 9.781(3) Å, α = 109.80(2), β = 102.35(3), γ = 78.50(2)°, Z = 2, V = 922.7 Å3, Dc = 1.29 g cm?3,μ = 7.46 cm?1 for Cu Kα. The structure was solved by the direct method, and refined by a block-diagonal least-squares procedure: R = 0.059 for 2568 reflections with | Fo | >3σ(| Fo | ). The relative configuration around the two chiral carbons indicated that the molecule is in the threo-form, and the Newman projection showed that the molecule has the staggered conformation. There were two relatively strong intramolecular hydrogen bonds between the two hydroxyl groups and the two carbonyl oxygen atoms of ethoxycarbonyl groups. 相似文献
960.
Michio Takami 《Journal of Molecular Spectroscopy》1980,80(2):301-306
About 140 a- and b-type millimeter-wave transitions of propynal-d1, DCCCHO, were measured in the ground vibrational state. The accurate rotational and centrifugal distortion constants were determined from the observed frequencies including the previous microwave measurements. Seven microwave transitions observed by infrared-microwave double resonance were also included in the analysis. The determined constants are A = 66778.016(12), B = 4463.8489(7), C = 4177.7950(7), ΔJ = 0.0015919(5), ΔJK = −0.139214(13), ΔK = 9.4328(18), δJ = 0.0002885(4), δK = 0.03069(4), HJK = −0.817(13) × 10−6, HKJ = −9.62(4) × 10−6, HK = 0.00255(8), hJ = 0.0047(3) × 10−6, in MHz. 相似文献