全文获取类型
收费全文 | 6922篇 |
免费 | 248篇 |
国内免费 | 45篇 |
专业分类
化学 | 4496篇 |
晶体学 | 46篇 |
力学 | 212篇 |
数学 | 1284篇 |
物理学 | 1177篇 |
出版年
2023年 | 34篇 |
2022年 | 72篇 |
2021年 | 120篇 |
2020年 | 123篇 |
2019年 | 137篇 |
2018年 | 134篇 |
2017年 | 136篇 |
2016年 | 210篇 |
2015年 | 217篇 |
2014年 | 213篇 |
2013年 | 422篇 |
2012年 | 490篇 |
2011年 | 502篇 |
2010年 | 318篇 |
2009年 | 284篇 |
2008年 | 381篇 |
2007年 | 381篇 |
2006年 | 374篇 |
2005年 | 340篇 |
2004年 | 279篇 |
2003年 | 248篇 |
2002年 | 197篇 |
2001年 | 115篇 |
2000年 | 96篇 |
1999年 | 75篇 |
1998年 | 60篇 |
1997年 | 54篇 |
1996年 | 59篇 |
1995年 | 41篇 |
1994年 | 53篇 |
1993年 | 31篇 |
1992年 | 41篇 |
1991年 | 55篇 |
1990年 | 52篇 |
1989年 | 50篇 |
1988年 | 45篇 |
1987年 | 38篇 |
1985年 | 45篇 |
1984年 | 67篇 |
1983年 | 33篇 |
1982年 | 43篇 |
1981年 | 36篇 |
1980年 | 40篇 |
1979年 | 31篇 |
1978年 | 33篇 |
1977年 | 44篇 |
1976年 | 31篇 |
1975年 | 30篇 |
1974年 | 31篇 |
1973年 | 28篇 |
排序方式: 共有7215条查询结果,搜索用时 31 毫秒
951.
Klitgaard SK Galsbøl F Weihe H 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2006,63(4):836-839
We present the angular variation of the resonance magnetic fields as well as the angular variation of the linewidths obtained from single-crystal electron paramagnetic resonance spectra of Delta+ [Cr((-)chxn)(3lambda lambda lambda)]3+ (chxn = trans-1,2-cyclohexanediamine) doped into the nitrate salt of the isostructural Rh(III) ion Delta+ [Rh((-)chxn)(3lambda lambda lambda](NO3)3)3.H2O. An analysis of the angular variation of the resonance magnetic fields indicates axial symmetry for the cation. An analysis of the angular variation of the linewidths, on the other hand, reveals that not all the paramagnetic sites have perfect axial symmetry. 相似文献
952.
Moreau L Ziarelli F Grinstaff MW Barthélémy P 《Chemical communications (Cambridge, England)》2006,(15):1661-1663
Hollow microspheres featuring a hybrid lipid-cation multilamellar shell are prepared by hydration of a nucleoside based amphiphile with an aqueous solution containing either actinide or lanthanide salts. The physico-chemical data collected clearly indicate that the formation of these microspheres is a consequence of the following concomitant stabilizing factors: (i) hydrophobic interactions, (ii) nucleobase dimer formation and (iii) phosphate/f-block element salt binding. 相似文献
953.
Lyskawa J Sallé M Balandier JY Le Derf F Levillain E Allain M Viel P Palacin S 《Chemical communications (Cambridge, England)》2006,(21):2233-2235
The formation of the two dimeric species [(TTF)2]+* and (TTF+*)2 can be monitored by complexation of Na+ on a calix[4]arene-TTF assembly. 相似文献
954.
Galano A Cruz-Torres A Alvarez-Idaboy JR 《The journal of physical chemistry. A》2006,110(5):1917-1924
A theoretical study of the mechanism and kinetics of the OH hydrogen abstraction from isopropylcyclopropane (IPCP) is presented. Optimum geometries, frequencies and gradients have been computed at the BHandHLYP/6-311++G(d,p) level of theory for all stationary points, as well as for additional points along the minimum energy path (MEP). Energies have been improved by single-point calculations at the above geometries using CCSD(T)/6-311++G(d,p) to produce the potential energy surface. The rate coefficients are calculated for the temperature range 260-350 K by using canonical variational theory (CVT) with small-curvature tunneling (SCT) corrections. Our analysis suggests a stepwise mechanism involving the formation of a reactant complex in the entrance channel and a product complex in the exit channel, for all the modeled paths. The reactant complexes are examined in detail, because they exhibit alkene-like structure. The excellent agreement between the overall calculated and experimental rate coefficients at 298 K supports the reliability of the parameters obtained for the temperature dependence and branching ratios of the IPCP + OH reaction, proposed here for the fist time. The expression that best describes the studied reaction is k(overall) = 6.15 x 10(-13)e1747/RT cm3 x molecule(-1) x s(-1). The predicted activation energy is -0.89 kcal/mol. 相似文献
955.
The IR spectra of H2O+NO, HDO+NO, and D2O+NO, isolated in solid neon at low temperature have been investigated. Concentration effects and detailed vibrational analysis of deuterated and partially deuterated species allowed identification of three 1:1 HDO-NO species, two 1:1 D2O-NO species, and only one 1:1 H2O-NO complex. From comparison between the experimental spectra and the results of DFT calculations, it appeared that two different types of weakly bound complexes between water and nitric oxide can be formed in a neon matrix. The first species is a 1:1 complex where bonding occurs between water hydrogen and nitric oxide nitrogen, in which OH-N and OD-N intermolecular bonds are engaged. For this complex only DOD-NO, HOD-NO, and DOH-NO isotopic species have been experimentally detected and no IR bands of HOH-NO were observed. This result could be explained by the fact that the dissociation energy of HOH-NO is lower than those of DOD-NO, HOD-NO and DOH-NO. For the second detected 1:1 H2O-NO complex and its isotopic variants, the H2O-NO potential surface was explored systematically at the B3LYP level, but no stable species corresponding to the complex could be calculated. The structure of the second observed 1:1 H2O-NO complex results from columbic attractions between water and nitric oxide and could be stabilized only in matrix, probably by interaction between NO, water and (Ne)n. 相似文献
956.
957.
Teixeira FC Ramos H Antunes IF Curto MJ Duarte MT Bento I 《Molecules (Basel, Switzerland)》2006,11(11):867-889
A series of indazoles substituted at the N-1 and N-2 positions with ester-containing side chains -(CH2)(n)CO2R of different lengths (n = 0-6, 9, 10) are described.Nucleophilic substitution reactions on halo esters (X(CH2)(n)CO2R) by 1H-indazole inalkaline solution lead to mixtures of N-1 and N-2 isomers, in which the N-1 isomer predominates. Basic hydrolysis of the ester derivatives allowed the synthesis of the corresponding indazole carboxylic acids. All compounds were fully characterised by multinuclear NMR and IR spectroscopies, MS spectrometry and elemental analysis; the NMR spectroscopic data were used for structural assignment of the N-1 and N-2 isomers.The molecular structure of indazol-2-yl-acetic acid (5b) was determined by X-ray diffraction, which shows a supramolecular architecture involving O2-H...N1 intermolecular hydrogen bonds. 相似文献
958.
959.
960.
Joël Merker 《Acta Appl Math》2006,92(2):125-207
We treat the problem of linearizability of a system of second order ordinary differential equations. The criterion we provide has applications to nonlinear Newtonian mechanics, especially in three-dimensional space. Let or , let , let , let and let
be a collection of m analytic second order ordinary differential equations, in general nonlinear. We obtain a new and applicable necessary and sufficient condition in order that this system is equivalent, under a point transformation
to the Newtonian free particle system .Strikingly, the explicit differential system that we obtain is of first order in the case , whereas according to a classical result due to Lie, it is of second order the case of a single equation . 相似文献