全文获取类型
收费全文 | 1777篇 |
免费 | 280篇 |
国内免费 | 154篇 |
专业分类
化学 | 883篇 |
晶体学 | 4篇 |
力学 | 4篇 |
综合类 | 10篇 |
数学 | 25篇 |
物理学 | 1285篇 |
出版年
2023年 | 11篇 |
2022年 | 20篇 |
2021年 | 33篇 |
2020年 | 51篇 |
2019年 | 40篇 |
2018年 | 46篇 |
2017年 | 42篇 |
2016年 | 42篇 |
2015年 | 59篇 |
2014年 | 57篇 |
2013年 | 87篇 |
2012年 | 68篇 |
2011年 | 66篇 |
2010年 | 58篇 |
2009年 | 86篇 |
2008年 | 108篇 |
2007年 | 193篇 |
2006年 | 111篇 |
2005年 | 69篇 |
2004年 | 54篇 |
2003年 | 74篇 |
2002年 | 109篇 |
2001年 | 95篇 |
2000年 | 79篇 |
1999年 | 94篇 |
1998年 | 78篇 |
1997年 | 38篇 |
1996年 | 25篇 |
1995年 | 36篇 |
1994年 | 25篇 |
1993年 | 26篇 |
1992年 | 31篇 |
1991年 | 19篇 |
1990年 | 19篇 |
1989年 | 20篇 |
1988年 | 17篇 |
1987年 | 9篇 |
1986年 | 14篇 |
1985年 | 18篇 |
1984年 | 15篇 |
1983年 | 15篇 |
1982年 | 12篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1975年 | 16篇 |
1974年 | 2篇 |
1973年 | 2篇 |
排序方式: 共有2211条查询结果,搜索用时 421 毫秒
51.
Br2分子里德堡态的高分辨转动谱研究 总被引:2,自引:2,他引:0
用窄线宽(0.08cm^-^1)脉冲可调谐紫外激光和(2+1)多光子电离方法测量了Br2的里德堡(Rydberg)态光谱,在70000-71500cm^-^1范围内,获得了溴分子[П3/2]4d振动系列,测量(v',v”)=(1,0),(2,0)的高分辨振转谱,得到其转动常数B’79-81分别为0.08832和0.08805cm^-^1,并提出此系列的角动量量子数Ω应为1。 相似文献
52.
1 INTRODUCTION The bimetallic nanoclusters are of standing inte- rest since they can exhibit catalytic, electronic and optical properties distinct from those of corre- sponding pure nanoclusters[1~4]. Palladium and pla- tinum, well known for their catalytic properties, are often used as the catalyst in different fuel cells[5~8]. Several experimental results illustrate that for the oxygen reduction reaction (ORR), which is one of the primary reactions taking place in many fuel cells and… 相似文献
53.
Four density functionals — including that recently introduced by Perdew ((1986) Phys Rev B33: 8822)—are tested for first-row atoms, hydrides and dimers. Calculated contributions of the correlation energy to the ionization potentials and electron affinities of atoms and to the dissociation energies of molecules are compared with empirical values which were reevaluated for this purpose. An improvement over Hartree-Fock is found in all cases if the self-interaction or the gradient correction are included in the density functional, although there is a rather large variation in the accuracy of the predictions. 相似文献
54.
V. Lj. Marković M. M. Pejović Z. Lj. Petrović 《Plasma Chemistry and Plasma Processing》1996,16(2):195-208
The late afterglow in nitrogen with iron electrode is studied by the breakdown time delay method, i.e., by measuring the breakdown time delay td as a function of the afterglow time . It is proposed that the cause of the secondary electrons initiating the breakdown is the energy of the surface recombination of nitrogen atoms on the iron electrode. The gas-phase and macrokinetic diffusive models are used to describe the experimental breakdown time delay data. By fitting the theoretical curve to the experimental data: (1) it has been confirmed that the recombination on the molybdenum glass is of the second order and the value of the surface recombination coefficient is determined at 4 mbar; (2) it has been shown that the surface recombination on the iron electrode is of the second order, and the effective recombination coefficients are determined; (3) the analytical form of the recombination coefficient as a function of the adsorption characteristics of surfaces and the pressure of the parent gas has been derived. In addition, the orders of surface recombination on the molybdenum-, aluminum-, and gold-plated electrode were determined by the same method. 相似文献
55.
The mononuclear vanadium(Ⅴ) complex with 2-hydroxyl-1-naphthaldehyde isoncotinoyl hydrazone was synthesized and characterized by single-crystal X-ray diffraction.The crystal is of monoclinic,space group P2 1/n with a = 10.3153(15),b = 7.4024(11 ),c = 24.413(4) (A),β= 101.911(2)°,V= 1824.0(5) (A)3,Z = 4,Mr = 427.28,Dc = 1.556 g/cm3,F(000) = 880,μ(MoKα) =0.591,the final R = 0.0372 and wR = 0.0926.The vanadium(Ⅴ) atom adopts a distorted square-pyramidal geometry coordinated by the tridentate ligand and two oxygen atoms.The hydrogen bonds and strong π…π stacking interactions link the complex molecules into a 3-D network structure. 相似文献
56.
Pina Romaniello 《Journal of fluorine chemistry》2004,125(2):145-149
The effect of fluorine atoms on the second-order optical nonlinear response of 4-nitro-4′-methoxy-trans-stilbene and polyene derivatives containing up to three double bonds (n=1, 2, 3) has been investigated within a semiempirical context (PM3 Hamiltonian). Experimental data reported in literature indicate a βvec(SHG) value of 34, 47 and 76 (×10−30 esu) for n=1, 2, 3, respectively, at 0.65 eV in CHCl3 solutions. Our calculations show that fluorinating the nitrophenyl group the βvec(SHG) is doubled. Further increase in the second-order nonlinear response can be obtained fluorinating the CC bond linkers between the aromatic moieties.Besides, the effect of different donor-acceptor pairs has been studied and the results interpreted in the two-state model context.Some results concerning the third-harmonic generation (THG), γ (THG), are presented, and a possible interpretation proposed. 相似文献
57.
The evidence for the stabilizing nature of the H–H bonding in planar biphenyl is succinctly reviewed. The stabilizing nature
of the H–H bonding is revealed through a comparison of the atomic energy of every atom in planar biphenyl with the same atom
in the twisted equilibrium structure. It is shown that the barrier to rotation via the planar transition state is the net
resultant of a stabilisation of the four ortho-hydrogen atoms (by 8 kcal/mol each), a stabilisation of the two para-carbon atoms (by 3 kcal/mol each) and by the dominant destabilisation of the two carbon atoms joining the two rings—the two
junction carbon atoms—(by 22 kcal/mol each). The energetic stabilisation of the four ortho-hydrogen atoms is further shown to be in large proportion due to the formation of the hydrogen–hydrogen interatomic surface.
Furthermore, neither the “bond order” between the two junction carbon atoms nor the total electron delocalisation between
the two rings exhibit a significant change in going from the planar to the twisted equilibrium geometry. These findings are
in contrast with the classical view of a balance between “steric non-bonded repulsion” and better electron delocalisation
as a function of the twist dihedral angle. Similar conclusions have been recently reached by Pacios and Gómez through a study
of the electrostatic potential at the position of the hydrogen nuclei.
We dedicate this article to Professor TM Krygowski on the occasion of his 70th birthday wishing him a long and productive
life. 相似文献
58.
HUANG Xi-He SHENG Tian-Lu XIANG Sheng-Chang FU Rui-Biao HU Sheng-Min LI Ya-Min WU Xin-Tao ② 《结构化学》2007,26(3):333-337
The hydrothermal reaction of 1,2-dicyanobenzene with NaN3 in the presence of Cd(NO3)2 affords a novel 2D cadmium tetrazolyl-benzoate,{Cd(tzbz)(H2O)}n (H2tzbz is 2-(5-tetrazolyl)-benzoate). The tzbz ligand is generated in situ through the 2+3 Sharpless cycloaddition reaction and hydrolyzation. Its crystal structure was determined by single-crystal X-ray diffraction method. The crystal crystallizes in the orthorhombic system,space group Pbca with a=9.6659(19),b=7.6366(15),c=25.964(5) ,V=1916.5(7) 3,Z=8,Mr=318.57,Dc= 2.208 g/cm3,F(000)=1232 and μ=2.276 mm-1. The Cd(Ⅱ) atom is coordinated by four tzbz ligands and one water molecule to form a severely distorted pentangle bipyramid. While each tzbz ligand connects to four Cd(Ⅱ) atoms in a μ4-η1,η2,η1,η1,η1 coordination mode to construct a 2D architecture of the title complex. Additionally,the title complex exhibits strong fluorescence at room temperature in the solid state. 相似文献
59.
Using a very simple trial function and unperturbed electron densities calculated by a new procedure, the frequency-dependent dipole polarizability () of Ne, Ar, Kr and Xe has been calculated in the range 0 0.45 a.u., by a Karplus-Kolker-type variation-perturbation method. Results progressively worsen for larger systems so that, for Xe, (0) is only 75% of the experimental value. Probable reasons for this are discussed. 相似文献
60.
The results of non empirical quantum-chemical calculations using the RHF/6-31G(d) and MP2/6-31G(d) methods do not agree with
proposals for the axial position of the H atom on the N atom in the piperidine molecule. According to RHF/6-31G(d) calculations
for the N-methylpiperidine molecule and its chloro-substituted derivatives an equatorially placed methyl group is energetically
more favored than an axial. The axial C-Cl and C-H bonds in these molecules are longer than the equatorial. The 35 Cl NQR frequencies for the axial Cl atoms are lower than the equatorial. The 35 Cl NQR frequency of the axial chlorine atom in 2-chloro-1-methylpiperidine is anomalously low. This is chiefly due to the
high population density of its p σ-orbital and this is a result of the polarization of the C-Cl bond via the N atom unshared
electron pair directly through the field. The effect of a similar unshared electron pair on the parameters of the C-Cl bond
in the ClCH2NH2 molecule has been studied by the RHF/6-31(g) method for different angles of rotation of the ClCH2 group around the C-N bond.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 1044–1052, July, 2005. 相似文献