全文获取类型
收费全文 | 3210篇 |
免费 | 384篇 |
国内免费 | 146篇 |
专业分类
化学 | 1110篇 |
晶体学 | 56篇 |
力学 | 932篇 |
综合类 | 6篇 |
数学 | 335篇 |
物理学 | 1301篇 |
出版年
2024年 | 2篇 |
2023年 | 16篇 |
2022年 | 53篇 |
2021年 | 48篇 |
2020年 | 78篇 |
2019年 | 66篇 |
2018年 | 63篇 |
2017年 | 67篇 |
2016年 | 86篇 |
2015年 | 111篇 |
2014年 | 105篇 |
2013年 | 292篇 |
2012年 | 153篇 |
2011年 | 200篇 |
2010年 | 128篇 |
2009年 | 204篇 |
2008年 | 214篇 |
2007年 | 236篇 |
2006年 | 178篇 |
2005年 | 166篇 |
2004年 | 162篇 |
2003年 | 140篇 |
2002年 | 103篇 |
2001年 | 74篇 |
2000年 | 102篇 |
1999年 | 90篇 |
1998年 | 84篇 |
1997年 | 61篇 |
1996年 | 73篇 |
1995年 | 59篇 |
1994年 | 48篇 |
1993年 | 40篇 |
1992年 | 42篇 |
1991年 | 36篇 |
1990年 | 23篇 |
1989年 | 13篇 |
1988年 | 33篇 |
1987年 | 8篇 |
1986年 | 12篇 |
1985年 | 16篇 |
1984年 | 15篇 |
1983年 | 3篇 |
1982年 | 11篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1979年 | 7篇 |
1978年 | 3篇 |
1976年 | 1篇 |
1973年 | 4篇 |
排序方式: 共有3740条查询结果,搜索用时 15 毫秒
61.
用ab initio方法研究NaN_3的几何构型、电子结构和热力学、动力学性质 总被引:2,自引:0,他引:2
用abinitioMO方法,在MP2(ful)/6311G水平下,全优化计算了叠氮化钠(NaN3)分子的线状和环状两种稳定构型及其转化过渡态的几何参数、电荷分布、分子总能量和振动频率,并研究了它们的热力学性质及转化速率常数和平衡常数.结果表明,线状比环状构型稍稳定(能量低6.04kJ/mol);两者相互转化的能垒分别为13.15kJ/mol(线型→环状)和7.11kJ/mol(环状→线型).热力学和动力学计算均表明:NaN3通常主要以线型结构存在(占85%以上),且随温度升高而增多(在1000K大于91%). 相似文献
62.
63.
V. A. Shlyapochnikov L. S. Khaikin O. E. Grikina N. O. Cherskaya L. E. Maksimova N. F. Pyatakov 《Russian Chemical Bulletin》1998,47(11):2173-2176
Vibrational spectra of several nitramines in the long-wave region (50–450 cm−1) were studied. The frequencies of intra- and intermolecular vibrations were separated and a tentative assignment of the frequencies
of self-associative complexes was performed.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2241–2244, November, 1998. 相似文献
64.
65.
本文通过红外光谱(FT-IR)测量数据和预测数据,计算与研究了十九种长链正烷基脂肪酸(简称脂肪酸或碳酸)的碳链骨架振动能量。结果显示出脂肪酸的碳链骨架振动能量随碳数增加呈阶梯状关系,而且脂肪酸的碳链骨架振动能量∑E出现奇偶数更迭规律。此结果可用来解释脂肪酸的熔点或熔化热呈现奇偶数更迭现象。 相似文献
66.
Two New Alkylimido-Methylgallanes with Cage Structure The tetrameric alkylimido-methylgallanes (MeGa? NR)4 (R = CH(CH3)2 ( iPr), C(CH3)3 ( tBu) and Me = CH3) have been prepared by pyrolyses of the dimeric amido compounds (Me2Ga? N(H)R)2 at 250–260°C. The mass, NMR and vibrational spectra are discussed, they prove almost identical structures of the skeletons. The X-Ray structure determination of (MeGa? NtBu)4 shows four heterocubane molecules in the trigonal unit cell (space group P3 c1, a = b = 1061.7(1), c = 3191.8(5) pm) and two disordered benzene solvate molecules. The Ga? N bonds range between 198.4 to 199.9 pm, the Ga? N? Ga and N? Ga? N bond angles lie between 90.6 to 91.4 and 88.6 to 89.2°, respectively. The structure was refined to an R1(R2)-value of 0.049 (0.057). 相似文献
67.
Sodium phosphate tellurite glasses in the system (NaPO(3))(x)(TeO(2))(1-) (x) were prepared and structurally characterized by thermal analysis, vibrational spectroscopy, X-ray photoelectron spectroscopy (XPS) and a variety of complementary solid-state nuclear magnetic resonance (NMR) techniques. Unlike the situation in other mixed-network-former glasses, the interaction between the two network formers tellurium oxide and phosphorus oxide produces no new structural units, and no sharing of the network modifier Na(2)O takes place. The glass structure can be regarded as a network of interlinked metaphosphate-type P(2) tetrahedral and TeO(4/2) antiprismatic units. The combined interpretation of the O 1s XPS data and the (31)P solid-state NMR spectra presents clear quantitative evidence for a nonstatistical connectivity distribution. Rather, the formation of homoatomic P--O--P and Te--O--Te linkages is favored over mixed P--O--Te connectivities. As a consequence of this chemical segregation effect, the spatial sodium distribution is not random, as also indicated by a detailed analysis of (31)P/(23)Na rotational echo double-resonance (REDOR) experiments. 相似文献
68.
Alberto Girlando Cristina Sissa Francesca Terenziani Anna Painelli Anna Chwialkowska Geoffrey J Ashwell 《Chemphyschem》2007,8(15):2195-2201
We report visible, Raman, and infrared spectra of self-assembled monolayers (SAMs) formed by the donor-(pi-bridge)-acceptor chromophore, Z-beta-[N-(omega-acetylthioalkyl)-4-quinolinium]-alpha-cyano-4-styryldicyanomethanide (CH3CO-S-CnH2n-Q3CNQ where n=8, 10), on gold-coated substrates. The data are compared with the spectra collected for the same compound in solution and in the solid state, and with those obtained for a Langmuir-Blodgett (LB) monolayer of C16H33-Q3CNQ deposited on gold. Spectral analysis confirms that in solution, in the solid state and in the LB film the chromophore has a zwitterionic (D+-pi-A-) ground state. At variance with this well-known result, our data show that in SAMs deposited on gold the chromophore has a more neutral, quinoid ground state. We relate this difference to the different packing of the molecules in the two different films: in SAMs in fact the chromophores stand almost vertical with respect to the substrate, whereas in LB films they make an angle of about 45 degrees. The Q3CNQ molecule is a well-known molecular rectifier, and for SAMs we were able to check the direction of electron flow at forward bias on the same samples that have been characterized spectroscopically, shedding light on the rectification mechanism. 相似文献
69.
On the Preparation of Di(i-propyl)thiosulfonium Salts [1] The preparation of the mercaptosulfonium salts (i-C3H7)2SSX+SbCl6? (X = H, D) and of the chlorothiosulfonium salts (i-C3H7)2SSCl+MF6? (M = As, Sb) and (i-C3H7)2SSSCl+SbCl6? is reported. They are formed by reaction of chlorinated or protonated (i-C3H7)2S with excess H2S or SCl2 and S2Cl2. The thiosulfonium compounds are characterized by vibrational and NMR spectroscopic methods. 相似文献
70.
Magorzata Bancewicz 《International journal of quantum chemistry》2002,88(6):742-749
The expressions for moments of spectral density distribution of a Hamiltonian representing a system of N coupled Morse oscillators are reduced to the large‐N limit. The effect of the values of coupling constants on the spectral density distribution is analyzed. For the values of coupling constants within the pertinent range for real molecules, the relations between moments appear to be close to those for a Gaussian distribution. Outside this range, the deviations from the normal distribution are substantial. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 相似文献