全文获取类型
收费全文 | 3320篇 |
免费 | 274篇 |
国内免费 | 218篇 |
专业分类
化学 | 1931篇 |
晶体学 | 34篇 |
力学 | 66篇 |
综合类 | 38篇 |
数学 | 673篇 |
物理学 | 1070篇 |
出版年
2023年 | 12篇 |
2022年 | 18篇 |
2021年 | 17篇 |
2020年 | 49篇 |
2019年 | 52篇 |
2018年 | 56篇 |
2017年 | 57篇 |
2016年 | 88篇 |
2015年 | 69篇 |
2014年 | 90篇 |
2013年 | 276篇 |
2012年 | 245篇 |
2011年 | 142篇 |
2010年 | 115篇 |
2009年 | 184篇 |
2008年 | 246篇 |
2007年 | 215篇 |
2006年 | 187篇 |
2005年 | 176篇 |
2004年 | 164篇 |
2003年 | 131篇 |
2002年 | 115篇 |
2001年 | 117篇 |
2000年 | 105篇 |
1999年 | 110篇 |
1998年 | 94篇 |
1997年 | 63篇 |
1996年 | 63篇 |
1995年 | 55篇 |
1994年 | 63篇 |
1993年 | 40篇 |
1992年 | 58篇 |
1991年 | 41篇 |
1990年 | 33篇 |
1989年 | 23篇 |
1988年 | 33篇 |
1987年 | 28篇 |
1986年 | 23篇 |
1985年 | 15篇 |
1984年 | 20篇 |
1983年 | 7篇 |
1982年 | 15篇 |
1981年 | 17篇 |
1980年 | 19篇 |
1979年 | 20篇 |
1978年 | 6篇 |
1977年 | 13篇 |
1976年 | 8篇 |
1973年 | 7篇 |
1972年 | 4篇 |
排序方式: 共有3812条查询结果,搜索用时 31 毫秒
991.
M. Gaber S. S. Al-Shihry A. A. El-Bindary 《Journal of Thermal Analysis and Calorimetry》2005,82(1):63-68
Summary Proton-ligand dissociation constant of 2-mercapto-5-(2-hydroxynaphthylideamino)-1,3,4-thiadiazole (MHT) and the stepwise stability
constants of its metal complexes were determined potentiometrically in 40 mass/mass% ethanol-water mixture containing 0.1
M KCl. The stabilities of the complexes follow the order: Cu2+>Ni2+>Co2+>Mn2+. The dissociation constant (pKH) of MHT and the stability constants (logK) of its metal complexes were determined at different temperatures and the corresponding thermodynamic parameters were calculated
and discussed. The proton dissociation process is non-spontaneous, endothermic and entropically unfavoured. The formation
of the metal complexes was found to be spontaneous, endothermic and entropically favoured. 相似文献
992.
Complexation of maltol (MH) with Fe3+ ions in aqueous solutions was studied. The compositions of [FeMa]2+, [FeMa2]+, and [FeMa3] complexes were determined by the method of isomolar series, and their instability constants were calculated. The values of the latter were confirmed by the method of apparent deviation from the Bouger—Lambert—Beer law. An increase in the Ma : Fe3+ ratio from 1 to 3 decreases the instability constants of the complexes. The [FeMa3] complex can be considered as a basis for the antianemic drug with a prolonged effect. 相似文献
993.
Rabindra N. Roy Julie A. Carlsten Jodi Niederschmidt William S. Good J. Michelle Rook Christina Brewe Amanda J. Kilker Lakshmi N. Roy Kathleen M. Kuhler 《Journal of solution chemistry》1997,26(3):309-317
ThepK
2
values for 3-[(l,l-Dimethyl-2-hydroxymethyl)amino]-2-hydroxypropanesulfonic acid (AMPSO), and 3-[N,N-Bis(-hydroxyethyl)amino]-2-hydroxypropanesulfonic
acid (DIPSO) have been determined at 12 temperatures over the range 5 to 55‡C by measurements of the emf of cells without
liquid junction containing hydrogen and silver-silver chloride electrodes. The values of pK
2 for AMPSO and DIPSO were found to be 9.138 and 7.576, respectively, at 25‡C. The thermodynamic quantities, δG‡, δH‡, δS‡,
and δC
p
‡
were calculated from the change of the equilibrium constants with temperature. These buffer substances are useful as secondary
pH standards in the physiological region of pH 7 to 9.
Camille and Henry Dreyfus Fellow, 1994–1996. 相似文献
994.
《Electroanalysis》2004,16(8):612-626
A mathematical conversion of data coming from nonequilibrium and dynamic voltammetric techniques (a direct current sampled (DC) and differential pulse (DP) polarography) into potentiometric sensor type of data is described and tested on a dynamic metal‐ligand system. A combined experiment involving DCP, DPP and glass electrode potentiometry (GEP) was performed on a single solution sample containing a fixed [LT] : [MT] ratio (acid‐base titration). Dedicated potentiometric software ESTA was successfully employed in the refinement operations performed on virtual potentiometric (VP) data obtained from DC and DP polarography. It was possible to refine stability constants either separately, from VP‐DC or VP‐DP, or simultaneously from any combination of VP‐DC, VP‐DP and GEP. The concept of VP‐DC or VP‐DP is reported for the first time and numerous documented and possible advantages are discussed. The proposed procedure can be easily utilized also by nonelectrochemists who are interested in, e.g., the ligand design strategies. 相似文献
995.
Molybdenum-95 NMR spectra of a series of phosphine and phosphite substituted molybdenum carbonyls Mo(CO)6-n
L
n
[L=P(OCH3)3
n=1, 2, 3, 4, 5,L=P(OC2H5)3
n=1, 2, 3,L=P(C6H5)3
n=1] including isomers (cis,trans,fac,mer) are reported, A large range of chemical shifts is found for the title compounds. The coupling constants1
J(95Mo-31P) are derived either from95Mo-NMR spectra or31P-NMR spectra. Syntheses of the measured compounds were performed by thermal or photochemical ligand substitution.
Molybdän-95 NMR einer Reihe von Phosphin- und Phosphit-substituierten Molybdäncarbonylen Mo(CO)6–n(L) n (n=1, 2, 3, 4, 5)
Zusammenfassung Es werden die95Mo-NMR-Spektren der im Titel genannten Verbindungen mitL=P(OCH3)3 n=1–5,L=P(OC2H5)3 n=1–3 undL=P(C6H5)3 n=1, einschließlich von Isomeren (cis, trans, fac, mer) angegeben. Für die chemischen Verschiebungen wurde ein sehr weiter Bereich beobachtet. Die Kopplungskonstanten1 J(95Mo-31P) wurden entweder von den95Mo- oder31P-NMR-Spektren ermittelt. Die Synthese der Verbindungen erfolgte mittels thermischem oder photochemischem Ligandenaustausch.相似文献
996.
We performed an ab initio molecular dynamics simulation of the paramagnetic transition metal ion Cr3+ in aqueous solution. Isotropic hyperfine coupling constants between the electron spin of the chromium ion and nuclear spins
of all water molecules have been determined for instantaneous snapshots extracted from the trajectory. The coupling constant
of first sphere oxygen, A
iso(17OI)=1.9±0.3 MHz, is independent on Cr–OI distance but increases with the tilt angle for the water molecule approaching 180°. First sphere hydrogen spins have A
iso(1 HI)=2.1±0.2 MHz which decreases with increasing tilt angle and shows a Cr–HI distance dependence. The hyperfine coupling constants for second sphere 17O is negative and an order of magnitude smaller (−0.20±0.02 MHz) compared to first sphere. 相似文献
997.
Constantinos G. Panayiotou 《Journal of solution chemistry》1991,20(1):97-114
The LFAS (Lattice-Fluid Associated Solution) model, which has been applied to alkanol + alkane and to alkanol + alkanol mixtures is now extended to mixtures consisting of one self-associated and one active or weakly self-associated component. The types of association complexes considered are AnBm and AnB with a single A-B bond each. The model is subsequently applied to binary alkanol + amine mixtures with an emphasis on vapor-liquid equilibria. Self-association constants for n-alkyl amines and dialkyl amines are presented along with the pure component lattice-fluid scaling constants. These parameters are used for correlating pure component data on vapor pressures, heats of vaporization, and orthobaric densities as well as mixing properties of amine + alkane mixtures.Communicated at the Festsymposium celebrating Dr. Henry V. Kehiaian's 60th birthday, Clermont-Ferrand, France, 17–18 May 1990. 相似文献
998.
The optical constants (real and imaginary refractive indices) of hexafluorobenzene were determined at 25 °C via transmission measurements. Experimental absorbance spectra measured on a Nicolet Impact 410 FTIR were converted to imaginary refractive indices using methods described in the literature. The real refractive indices were obtained by Kramers–Kronig transformation of the imaginary refractive indices. From the complex refractive indices, the molar absorption coefficient (Em) and complex molar polarizability
spectra were calculated. The integrated intensities for the E1u fundamentals were obtained from the areas under the bands in the
spectrum. These integrated intensities are compared to those for benzene and benzene-d6 in the literature. 相似文献
999.
Equilibrium studies of the mixed-ligand complexes of the copper(II) ion with pentamethyldiethylenetriamine (N,N,N′,N″,N″-pentamethyl-[bis(2-aminoethyl)amine], Me5dien) as a primary ligand and methioninehydroxamic acid (2-amino-4-(methylthio)butanehydroxamic acid, Metha) or histidinehydroxamic acid (2-amino-3-(4′-imidazolyl)propanehydroxamicacid, Hisha) as a secondary ligand L were performed by potentiometric titration, UV–Vis and EPR spectroscopy. The results show that in these ternary systems the dinuclear [Cu2(Me5dien)L2H−1]+ mixed-ligand species is formed as a predominant one in the basic solution. The monouclear [Cu(Me5dien)L]+ species is formed in low concentration. Our spectroscopic results indicate that the geometry of these mixed-ligand five-coordinate complexes is strongly distorted towards trigonal-bipyramidal. 相似文献
1000.
Tamara J. Haverlock Peter V. Bonnesen Richard A. Sachleben Bruce A. Moyer 《Journal of inclusion phenomena and macrocyclic chemistry》2000,36(1):21-37
The extraction of CsNO3 by the highly lipophilic calixarene-crown ether calix[4]arene-bis(t-octylbenzo-crown-6) (CABOBC6) in 1,2-dichloroethane (1,2-DCE) at 25 °C has been shown to be consistent with the formation of both 1 : 1 and 2 : 1 metal : ligand species. Variation of the aqueous-phase CsNO concentration up to 1.0 M and variation of the organic-phase calixarene concentration up to 0.10M was modeled by the program SXLSQI. Formation of the organic-phase species CsBNO3 (B = calixarene) was confirmed as well as the organic-phase dissociation products CsB+ and NO3
-. Good evidence for the 2 : 1 metal : ligand organic-phase species Cs2B(NO3)2 was also found, although the dissociation of nitrate from this complex was not observed. Binding of the second Cs+ cation by the ligand is approximately two orders of magnitude weaker than binding of the first Cs+ cation. The logarithm of the apparent partition ratio (log PB) of the calixarene between water and 1,2-DCE was found by 1H-NMR techniques to be > 5.1. 相似文献