全文获取类型
收费全文 | 4387篇 |
免费 | 186篇 |
国内免费 | 12篇 |
专业分类
化学 | 3073篇 |
晶体学 | 34篇 |
力学 | 95篇 |
数学 | 462篇 |
物理学 | 921篇 |
出版年
2023年 | 52篇 |
2022年 | 56篇 |
2021年 | 80篇 |
2020年 | 100篇 |
2019年 | 88篇 |
2018年 | 93篇 |
2017年 | 108篇 |
2016年 | 165篇 |
2015年 | 102篇 |
2014年 | 210篇 |
2013年 | 369篇 |
2012年 | 321篇 |
2011年 | 336篇 |
2010年 | 227篇 |
2009年 | 187篇 |
2008年 | 243篇 |
2007年 | 197篇 |
2006年 | 139篇 |
2005年 | 192篇 |
2004年 | 139篇 |
2003年 | 141篇 |
2002年 | 132篇 |
2001年 | 40篇 |
2000年 | 38篇 |
1999年 | 29篇 |
1998年 | 36篇 |
1997年 | 39篇 |
1996年 | 33篇 |
1995年 | 29篇 |
1994年 | 29篇 |
1993年 | 32篇 |
1992年 | 25篇 |
1991年 | 33篇 |
1990年 | 26篇 |
1989年 | 24篇 |
1988年 | 27篇 |
1987年 | 25篇 |
1986年 | 27篇 |
1985年 | 31篇 |
1984年 | 39篇 |
1983年 | 33篇 |
1982年 | 24篇 |
1981年 | 24篇 |
1980年 | 27篇 |
1979年 | 20篇 |
1978年 | 23篇 |
1977年 | 32篇 |
1976年 | 14篇 |
1975年 | 19篇 |
1974年 | 18篇 |
排序方式: 共有4585条查询结果,搜索用时 234 毫秒
111.
Some aspects of resonant vibrations of self-interstitials in the 100-dumbbell configuration are discussed by using a simple defect model incorporating nearest-neighbour interactions, with parameters fitted to the results of computer simulation. Especially we calculate the change of the lattice specific heat due to interstitials. An appreciable increase of the specific heat is obtained yielding a peak at 9–10 K in C/C for Cu. Following a recent suggestion of W. Schilling the diffuse inelastic one-phonon scattering is calculated in the range of the resonance frequencies forq-values away from the phonon-line. In particular, an appreciable diffuse intensity is obtained for theE
g-mode making an experimental observation by neutron scattering feasible. 相似文献
112.
Summary Novel bimetallic alkoxides of copper(II) of the type Cu[M(OPr-i)6]2 (M = Nb or Ta) have been synthesised by the interaction of CuCl2 (1 mol) with K[M(OPr-i)6] (2 mol). These bimetallic isopropoxides undergo facile alcohol interchange with primary alcohols such as MeOH, EtOH,n-PrOH andn-BuOH. However, witht-BuOH, the reaction cannot be driven to completion even under prolonged continuous azeotropic fractionation of the liberatedi-PrOH and the final product corresponded in analysis to Cu[Ta(OPr-i)2(OBu-t)4]2.I.r., electronic and electron spin resonance spectral and magnetic susceptibility measurements indicate a distorted octahedralD
4h geometry for copper(II) in these bimetallic alkoxides. 相似文献
113.
Badorrey R Cativiela C Díaz-de-Villegas MD Díez R Galbiati F Gálvez JA 《The Journal of organic chemistry》2005,70(24):10102-10105
[reaction: see text] Addition of trimethylsilyl cyanide to ketimines derived from (R)-2,2-dimethyl-1,3-dioxolan-4-yl methyl ketone to generate a quaternary stereocenter has been achieved with high yields and excellent diastereoselectivity. The stereoselectivity was found to be temperature and solvent dependent. The beta-hydroxy-alpha-amino nitrile of syn configuration was the major compound in kinetically controlled reactions, whereas the anti stereoisomer was obtained in excess in thermodynamically controlled reactions. Double stereodifferentiation under kinetic control conditions was successful, and the cyanation reaction occurred with complete syn diastereoselectivity using the matched pair. The versatility of the resulting amino nitrile as a synthetic intermediate was tested by performing the synthesis of orthogonally protected (R)-(2-aminomethyl)alanine. 相似文献
114.
Gupta Madan Mohan Srivastava Alpana Tripathi Arvind Kumar Misra Himanshu Verma Ram Kishore 《平面色谱法杂志一现代薄层色谱法》2006,19(4):282-287
JPC – Journal of Planar Chromatography – Modern TLC - High-performance thin-layer chromatography (HPTLC) has been used for normal-phase separation of the components of hexane,... 相似文献
115.
K. Nagaiah D. Sreenu R. Srinivasa Rao G. Vashishta J.S. Yadav 《Tetrahedron letters》2006,47(26):4409-4413
trans-Fused pyrano- and furanotetrahydroquinoline derivatives have been synthesized via a one-pot three-component aza-Diels-Alder reaction of aryl imines formed in situ from aromatic amines and arylaldehydes with cyclic enol ethers,such as 3,4-dihydro-2H-pyran and 2,3-dihydrofuran,in the presence of 1 mol % of phosphomolybdic acid under solvent-free conditions at room temperature in good to excellent yields. 相似文献
116.
[reaction: see text] Optically pure allylic amines have been synthesized from alpha,beta-unsaturated aldehydes via allylboration with (-)-B-allyldiisopinocampheylborane, followed by Overman rearrangement. By incorporating crotyl and alkoxyallylboration, functionalization at delta-position was readily accomplished. By applying this methodology, the synthesis of several chiral alpha-amino acids has been achieved. 相似文献
117.
Surface tensions (sigma) for [1,3-dioxolane+methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, and 1-pentanol] and excess molar volumes (v(E)) for [1,3-dioxolane+methanol, ethanol, 1-propanol, 2-propanol, and 2-butanol] at the temperature 298.15 K and normal atmospheric pressure have been determined as a function of mole fractions. The magnitude of these experimental quantities is discussed in terms of the nature and type of intermolecular interactions in binary mixtures. In order to analyze the surface tension behavior, the extended Langmuir (EL) model was used and the results obtained for the systems containing 1,3-dioxolane were compared with those of other formerly published series: [1,4-dioxane+alkanes] and [1,4-dioxane+alcohols]. 相似文献
118.
Rajendra Singh?Ghadwal Ram?C.?Mehrotra Anirudh?SinghEmail author 《Transition Metal Chemistry》2005,30(7):836-844
An entirely new class of heterobimetallic homoleptic glycolate complexes of the type Nb(OGO)3{Ta(OGO)2} [where G=CMe2CH2CH2CMe2 (G1) (3); CMe2CH2 CHMe(G2) (4); CHMeCHMe (G3) (5); CH2CMe2CH2 (G4) (6); CMe2CMe2(G5) (7); CH2CHMeCH2 (G6) (8); CH2CEt2CH2 (G7) (9); CH2CMe(Prn)CH2 (G8) (10)] have been prepared by the reactions of Nb(OGO)2(OGOH) [G=G1 (1a); G2 (1b); G3 (1c); G4 (1d); G5 (1e); G6 (1f); G7 (1g); G8 (1h)] with Ta(OGO)2 (OPri) (G=G1 (2a); G2 (2b); G3 (2c); G4 (2d); G5 (2e) G6 (2f); G7 (2g); G8 (2h). In addition to the novel derivatives (2)–(10), our earlier investigations on heterobimetallic glycolate-alkoxide derivatives have been extended to derivatives of the type
Nb(OGO)
[where M=A1 n=3, G=G3 (11);G4 (12); G6 (13) G7 (14); Gs (15); G9=CH2CH2CH2 (16) and M=Ti (n=4, G=G4) (17), Zr(n=4,G=G4) (18)], which are conveniently prepared by the reactions of metalloligands Nb(OGO)2(OGOH) [G=G3 (1c); G4 (1d); G6 (1f); G7 (1g); G8 (1h); G9 (1i)] with different metal alkoxides. All of these new complexes have been characterized by elemental analyses, molecular weight
determinations, and spectroscopic (I.r. and 1H, 27Al-n.m.r.) studies. Structural features of the new derivatives have been elucidated on the basis of molecular weight and spectroscopic
data. 相似文献
119.
11 and 12 molar reactions of dioxouranium(VI) acetate dihydrate with the monobasic bidentateSchiff bases,o-HOC6H4CH=NR oro-HOC10H6CH=NR (R=C2H5,n-C3H7,n-C4H9 or C6H5) and bibasic tridentateSchiff bases,o-HOC6H4CH=NR(OH) oro-HOC10H6CH=NR(OH) (R=–CH2CH(CH3)- or —CH2CH2CH2–) have been studied and derivates of the type UO2(OAc)2(SBH), UO2(OAc)2(SBH)2, UO2(OAc)2(SBH
2) and UO2(OAc)2(SBH
2)2 (whereSBH andSBH
2 represent monobasic bidentate and bibasic tridentateSchiff base molecules respectively) have been isolated. These have been characterized by elemental analysis, conductance measurements and IR spectral studies.
UO2 2+-Komplexe von Schiff-Basen. VII. Uranylacetat-Komplexe mit monobasischen zweizähnigen und bibasischen dreizähnigen Schiff-Basen
Zusammenfassung Es wurden in 1:1- und 1:2-molaren Reaktionen von UO2(OAc)2·2H2O mitSchiff-Basen (L) Komplexe des Typs UO2(OAc)2 L bzw. UO2(OAc)2 L 2 isoliert. Die Komplexe wurden mittels Elementaranalyse, Leitfähigkeitsmessungen und IR-Spektren untersucht.相似文献
120.
Bromophenol blue indicator was used in UV-visible spectrophotometric measurements to study ion association constants of alkali metal fluorides. The equilibrium constants for the ion pair formation of the alkali metal fluorides were determined as a function of ionic strength at one atmosphere pressure and 25°C. The effect of pressure on these association constants was measured at a constant total ionic strength of 1.0 mol-kg–1 over a pressure range of 1 to 2000 atmospheres at 25°C. The pressure dependences of the stoichiometric association constants of the alkali metal fluorides are given by: lnK
LiF
*
=0.77–2.47×10–4P–2.12×10–8P2; lnK
NaF
*
=0.53–1.08×10–4P–1.66×10–8P2; lnK
KF
*
=0.24–4.41×10–5P–7.15×10–8P2; lnK
RbF
*
=–0.17–8.65×10–5P–4.51×10–8P2; and lnK
CsF
*
= –0.37–1.14×10–4P–6.82×10–8P2, where P is the pressure in atmospheres. The stoichiometric molar volume and compressibility changes for ion pair formation of the alkali metal fluorides were evaluated from the pressure dependence of K
MF
*
data. The thermodynamic association constants were also calculated making use of activity coefficient data from the Pitzer equations. The partial molal volume and compressibility changes for ion pair formation of each alkali metal fluoride are reported. 相似文献