全文获取类型
收费全文 | 3946篇 |
免费 | 445篇 |
国内免费 | 311篇 |
专业分类
化学 | 2586篇 |
晶体学 | 27篇 |
力学 | 197篇 |
综合类 | 20篇 |
数学 | 488篇 |
物理学 | 1384篇 |
出版年
2023年 | 50篇 |
2022年 | 119篇 |
2021年 | 98篇 |
2020年 | 133篇 |
2019年 | 98篇 |
2018年 | 101篇 |
2017年 | 94篇 |
2016年 | 130篇 |
2015年 | 138篇 |
2014年 | 165篇 |
2013年 | 236篇 |
2012年 | 322篇 |
2011年 | 290篇 |
2010年 | 216篇 |
2009年 | 192篇 |
2008年 | 255篇 |
2007年 | 210篇 |
2006年 | 193篇 |
2005年 | 161篇 |
2004年 | 128篇 |
2003年 | 113篇 |
2002年 | 102篇 |
2001年 | 89篇 |
2000年 | 102篇 |
1999年 | 72篇 |
1998年 | 66篇 |
1997年 | 55篇 |
1996年 | 75篇 |
1995年 | 56篇 |
1994年 | 60篇 |
1993年 | 52篇 |
1992年 | 54篇 |
1991年 | 36篇 |
1990年 | 28篇 |
1989年 | 41篇 |
1988年 | 37篇 |
1987年 | 31篇 |
1986年 | 41篇 |
1985年 | 30篇 |
1984年 | 35篇 |
1983年 | 26篇 |
1982年 | 20篇 |
1981年 | 14篇 |
1980年 | 22篇 |
1979年 | 12篇 |
1978年 | 11篇 |
1977年 | 15篇 |
1976年 | 17篇 |
1975年 | 17篇 |
1973年 | 9篇 |
排序方式: 共有4702条查询结果,搜索用时 31 毫秒
101.
A comparison between the construction of symmetry-correlation diagrams and the perturbation method for studying chemical reactions is carried out. The perturbation method consists of decomposing the system Hamiltonian H into a sum, H = H0 + H′. Various symmetry correlation schemes appearing in the literature may be explained by the nonuniqueness of the decomposition scheme. All symmetry selection rules may be viewed as the varieties. By examining the symmetry-correlation diagrams, processes under investigation may be called “forbidden” or “allowed,” depending on the topological feature. Of particular importance is the topology associated with the “avoided crossing.” By making the comparison, we can establish the correspondence of the two methods and conclude that the perturbation order furnishes the origin of the “forbiddenness” of a process. 相似文献
102.
An attempt to classify various types of avoided surface crossings originally discussed by Salem and co-workers is presented. From the present scheme, an order-of-forbiddenness criterion regarding reactivity is established from a consideration of electron and orbital counting. Since the new classification is qualitatively related to the energy gap created in the avoided crossing, the classification appears to be more systematic and informative. Energy transfer processes involved in electronically excited states may also be rationalized by such an application. Three major mechanistic types are distinguished: concerted two-electron process, concerted one-electron process, and stepwise one-electron process involving an ionic-pair intermediate. 相似文献
103.
Daniel T. W. Chu Robert E. Maleczka Carl W. Nordeen 《Journal of heterocyclic chemistry》1988,25(3):927-930
The synthesis and antibacterial activity of 1-substituted amino-2-fluoro-4,12-dihydro-4-oxoquino[1,8a,8-a,b]quinoxaline-5-carboxylic acid derivatives are described. The synthetic route includes a carbon homologation and two intramolecular nucleophilic displacement cyclization reactions leading to the novel 4,12-dihydro-4-oxoquino[1,8a,8-a,b]quinoxaline-5-carboxylic acid heterocycle. 相似文献
104.
Qichen Huang Zheming Wang Qianli Chu Shizheng Zhu 《Journal of inclusion phenomena and macrocyclic chemistry》2006,54(3-4):177-180
X-ray analysis has revealed that N,N,N′,N′-tetramethyl-ethylenediamine 1 form donor–acceptor complex 3 with 1-bromo-2-iodo-tetrafluoroethane 2, in which the N X (X = Br, I) distances are longer than the average covalent bond length between X and N, but are also definitively shorter than the sum of the corresponding van der Waals radii of X and N, thus that indicating weak interactions between the nitrogen and bromine or iodine atoms. In our experimental section, a valuable method for recrystallization and collect X-ray data from crystals that easily exhibit decay and can be cracked is reported. 相似文献
105.
We report the synthesis and radioligand binding analysis of a series of naphthalenic melatonin receptor ligands, N-[2-(7-alkoxy-2-methoxy-1-naphthyl)ethyl]propionamide. This series of ligands exhibits subpicomolar binding affinity to both MT1 and MT2 melatonin receptors expressed in chinese hamster ovary (CHO) cells. 相似文献
106.
Ging-Ho Hsiue Wen-Kuei Huang Hsiao-Lung Chu 《Journal of polymer science. Part A, Polymer chemistry》1989,27(13):4397-4402
Differential scanning calorimetry was used to trace the heat of decomposition of the peracid groups (ΔHd) in the oxidized EVA-g-AA resin. From the correlation between ΔHd and the oxidation capacity measured by iodometry, it was found that 35 ± 5 cal of energy evolved per miliequivalent of peracid group decomposed. The ΔHd values are also useful in finding the optimum condition for oxidation of acid groups and can be used to investigate the distribution of active peracid groups across the matrix. The stability and the activation energy of decomposition of grafted type peracid resin were also studied, where the activation energy of decomposition was calculated to be 13.4 kcal/mol. 相似文献
107.
ClSO was produced as an intermediate upon irradiating a flowing mixture of Cl2SO and Ar with a KrF excimer laser at 248 nm. A step-scan Fourier-transform infrared spectrometer coupled with a small multipass absorption cell was employed to detect time-resolved absorption spectrum of ClSO. A transient spectrum in the region 1120-1200 cm(-1), which diminished on prolonged reaction, is assigned to the S-O stretching (nu1) mode of ClSO. A spectrum with a resolution of 0.3 cm(-1) partially reveals rotational structure with the Q-branch at 1162.9 cm(-1). Calculations with density-functional theory (B3LYP/aug-cc-pVTZ) predict the geometry, vibrational, and rotational parameters of ClSO. An IR absorption spectrum of ClSO simulated based on predicted rotational parameters agrees satisfactorily with experimental results. ClSO produced from photolysis of Cl2SO at 248 nm is internally hot. 相似文献
108.
Jue Xiao CAI Zheng Hong ZHOU Kang Ying LI Chi Hung YEUNG Chu Chi TANG* State Key Laboratory of Elemento-Organic Chemistry Institute of Elemento-Organic Chemistry Nankai University Tianjin Open Laboratory of Chirotechnology D 《中国化学快报》2002,13(7)
Optically active 1,1-binaphthalene-2, 2-diol has become a quite important chiral source in different fields of chirotechnology, especially in asymmetric synthesis1. Its synthesis and resolution has been extensively studied and various resolution methods have been reported2. Among the reported resolution methods, the following three, namely, via the formation of phosphoric acid derivatives3, boric acid derivatives4 and inclusion complexes5, are the most important. OHOH+_( )-1(-)-(S)-1(+)-(R)… 相似文献
109.
Zhang B Harb JN Davis RC Kim JW Chu SH Choi S Miller T Watt GD 《Inorganic chemistry》2005,44(10):3738-3745
Horse spleen ferritin (HoSF) containing 800-1500 cobalt or 250-1200 manganese atoms as Co(O)OH and Mn(O)OH mineral cores within the HoSF interior (Co-HoSF and Mn-HoSF) was synthesized, and the chemical reactivity, kinetics of reduction, and the reduction potentials were measured. Microcoulometric and chemical reduction of HoSF containing the M(O)OH mineral core (M = Co or Mn) was rapid and quantitative with a reduction stoichiometry of 1.05 +/- 0.10 e/M forming a stable M(OH)(2) mineral core. At pH 9.0, ascorbic acid (AH(2)), a two-electron reductant, effectively reduced the mineral cores; however, the reaction was incomplete and rapidly reached equilibrium. The addition of excess AH(2) shifted the reaction to completion with a M(3+)/AH(2) stoichiometry of 1.9-2.1, consistent with a single electron per metal atom reduction. The rate of reaction between M(O)OH and excess AH(2) was measured by monitoring the decrease in mineral core absorbance with time. The reaction was first order in each reactant with second-order rate constants of 0.53 and 4.74 M(-1) min(-1), respectively, for Co- and Mn-HoSF at pH 9.0. From the variation of absorbance with increasing AH(2) concentration, equilibrium constants at pH 9.0 of 5.0 +/- 1.9 for Co-HoSF and 2.9 +/- 0.9 for Mn-HoSF were calculated for 2M(O)OH + AH(2) = 2M(OH)(2) + D, where AH(2) and D are ascorbic acid and dehydroascorbic acid, respectively. Consistent with these equilibrium constants, the standard potential for the reduction of Co(III)-HoSF is 42 mV more positive than that of the ascorbic acid reaction, while the standard potential of Mn(III)-HoSF is 27 mV positive relative to AH(2). Fe(2+) in solution with Co- and Mn-HoSF under anaerobic conditions was oxidized to form Fe(O)OH within the HoSF interior, resulting in partial displacement of the Co or Mn by iron. 相似文献
110.
Studies of difference spectra have revealed a dynamic interaction of bilirubin and hemoglobin in the alkaline Jendrassik-Grof diazo blank reagent. The similarity obtained for difference spectra with respect to shape and peak maxima and peak minima of various ferriprotoporphyrin-containing biomolecules, such as hemin, methemoglobin, cyanmethemoglobin, and catalase implicated ferriprotoporphyrin in this interactive process. Further time studies of these mixtures seem to suggest an oxidation-reduction type of reaction, resulting in the formation of ferroprotoporphyrin and presumably the concomitant oxidation of bilirubin. 相似文献