全文获取类型
收费全文 | 1720篇 |
免费 | 53篇 |
国内免费 | 13篇 |
专业分类
化学 | 1300篇 |
晶体学 | 10篇 |
力学 | 41篇 |
综合类 | 1篇 |
数学 | 147篇 |
物理学 | 287篇 |
出版年
2022年 | 20篇 |
2021年 | 19篇 |
2020年 | 32篇 |
2019年 | 24篇 |
2018年 | 27篇 |
2017年 | 24篇 |
2016年 | 56篇 |
2015年 | 46篇 |
2014年 | 43篇 |
2013年 | 81篇 |
2012年 | 106篇 |
2011年 | 88篇 |
2010年 | 68篇 |
2009年 | 57篇 |
2008年 | 111篇 |
2007年 | 112篇 |
2006年 | 107篇 |
2005年 | 114篇 |
2004年 | 88篇 |
2003年 | 68篇 |
2002年 | 64篇 |
2001年 | 25篇 |
2000年 | 22篇 |
1999年 | 22篇 |
1998年 | 21篇 |
1997年 | 12篇 |
1996年 | 20篇 |
1995年 | 18篇 |
1993年 | 10篇 |
1991年 | 10篇 |
1988年 | 12篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 18篇 |
1984年 | 19篇 |
1982年 | 21篇 |
1981年 | 15篇 |
1980年 | 12篇 |
1979年 | 25篇 |
1978年 | 13篇 |
1977年 | 18篇 |
1976年 | 15篇 |
1975年 | 12篇 |
1974年 | 7篇 |
1972年 | 7篇 |
1970年 | 8篇 |
1969年 | 9篇 |
1968年 | 5篇 |
1967年 | 7篇 |
1966年 | 6篇 |
排序方式: 共有1786条查询结果,搜索用时 9 毫秒
31.
Yuji Shinohara Tsuyoshi Nakajima Satoshi Suzuki 《Journal of Molecular Structure》1999,460(1-3):231-244
Using a semi-empirical molecular orbital method, PM3, and 2-propanol as an example, the dehydration and the dehydrogenation processes of alcohol on oxide catalysts were studied. The catalysts addressed here were four kinds of oxides (Al2O3, SiO2, ZnO, CdO) whose reaction selectivities had been experimentally determined. The usual models consisting of a surface metal ion, several oxide ions and an isopropoxy group were used in calculations. For the dehydration, heats of formation of the models were calculated at each point of the process where the distance between a β-hydrogen of the group and a basic site (i.e. oxygen of the group or a surface oxide ion) or a metal ion was gradually shortened, or where the length of the C–O bond of the group was gradually increased. A reasonable dehydration mechanism was estimated by comparing activation energies calculated from the transitions of the heats of formation. The most probable dehydrogenation mechanism was also estimated in a similar way by gradually making an -hydrogen close to a surface oxide ion, the metal ion or a surface proton. It was concluded that the dehydration proceeds by scission of the C–O bond of the group after its oxygen was attacked by some electrophile on the surface and that the dehydrogenation proceeds by a mechanism in which an -hydrogen of the group was extracted by the metal ion.
Based on the dehydration mechanism thus deduced, alkoxy groups generated by adsorption of the primary, secondary and tertiary alcohols on SiO2 were calculated in order to estimate the activation energies of their decompositions. In result, the order of the energies was found to be in good agreement with that of the decomposition rates experimentally determined by Kitahara. This agreement gives support to the validity of the mechanism deduced for the dehydration of alcohol. 相似文献
32.
A significant acceleration effect of a beta-silicon atom was observed on the electrocyclic ring-opening reaction of benzocyclobutene derivatives to form o-quinodimethanes. This acceleration effect could be attributed to an electronic sigma-donating nature of the C(alpha)-Si(beta) bond, associated with alpha-anion driven pericyclic reactions. 相似文献
33.
Photooxygenations of 4-2H-anisole () and o-xylene () by 3-methylpyridazine 2-oxide () or pyridine 1-oxide () were studied in a variety of solvents at varying irradiation temperatures. Remarkable solvent effect on NIH-shift coupled with their hydroxylation processes was observed. 相似文献
34.
Efficient monocyclic 1,2-diazepine formation via a tandem electrocyclization reaction of cyclobutenones with lithiodiazoacetate is demonstrated. The reaction proceeds through an oxy anion-accelerated 4π-ring opening of cyclobutene followed by an 8π-ring closure of the resultant oxy anion-substituted diazo-diene under mild conditions to furnish a 1,2-diazepine via formal diazomethylene insertion into the C-C bond of cyclobutenone. 相似文献
35.
Hidemasa Yamaguchi Yuji Minoura 《Journal of polymer science. Part A, Polymer chemistry》1970,8(6):1467-1479
Optically active N-bornylmaleimide (NBMI) was copolymerized with styrene, methyl methacrylate, and vinylidene chloride with a free-radical catalyst to obtain optically active copolymers. The monomer reactivity ratios for the radical copolymerization of NBMI (M2) with styrene, methyl methacrylate, and vinylidene chloride were: ST-NBMI, r1 = 0.13, r2 = 0.05; MMA-NBMI, r1 = 2.02, r2 = 0.16; VCl2-NBMI, r1 = 1.15, r2 = 0.47. The Q-e values for NBMI were Q2 = 0.48 and e2 = +1.47. The specific rotation and optical rotatory dispersion of these copolymers were measured. The correlation between the specific rotation and composition of these copolymers was not linear. The value of λc for each copolymer was independent of the copolymer composition and the comonomer, being 260 mμ for the St-NBMI system, 262 mμ for the MMA-system, and 260 mμ for the VCl2-NBMI system. The effects of solvents and temperature on the specific rotation of these copolymers were investigated. 相似文献
36.
Yoshitaka Ogiwara Hitoshi Kubota Yuji Kimura 《Journal of polymer science. Part A, Polymer chemistry》1978,16(11):2865-2873
The influence of ferric acetylacetonate [Fe(acac)3] on the photodegradation of model compounds of polyethylene (PE) was examined. By studying electron-spin-resonance (ESR) spectra of photoirradiated compounds such as 1-octene and 1,7-octadiene, which contain carbon double bonds, Fe(acac)3 was found to accelerate the formation of allyl radical by contact with them. On the other hand, Fe(acac)3 suppressed the radical formation based on Norrish type I reaction in a carbonyl group of compounds such as 2-octanone and 3-octanone. Based on the study of ultraviolet (UV) spectra as well as ESR spectra on photoirradiated samples, the influence of Fe(acac)3 on the photodecomposition of model compounds of PE was discussed. Finally, the discussion turned to the mechanism of photodecomposition of PE in the presence of ferric salt. 相似文献
37.
Hidemasa Yamaguchi Hiroshi Ueno Yuji Minoura 《Journal of polymer science. Part A, Polymer chemistry》1971,9(4):887-895
The melt and solution condensation copolymerization of nylon salts which were prepared from d-camphoric acid and adipic acid with hexamethylenediamine were carried out, and optically active copolyamides were obtained. The copolyamides obtained had a positive specific rotation. The specific rotations for the copolyamides increased with increasing content of d-camphoryl units in the copolymers. The optical rotatory dispersion of the copolyamides had positive curves and were found to fit the simple Drude equation. The λc values of the polymers obtained by the melt and solution condensation polymerization were 241 mμ and 245 mμ, respectively. 相似文献
38.
39.
40.