全文获取类型
收费全文 | 3828篇 |
免费 | 497篇 |
国内免费 | 477篇 |
专业分类
化学 | 4091篇 |
晶体学 | 67篇 |
力学 | 22篇 |
综合类 | 11篇 |
数学 | 5篇 |
物理学 | 606篇 |
出版年
2024年 | 10篇 |
2023年 | 65篇 |
2022年 | 98篇 |
2021年 | 97篇 |
2020年 | 157篇 |
2019年 | 133篇 |
2018年 | 122篇 |
2017年 | 111篇 |
2016年 | 231篇 |
2015年 | 186篇 |
2014年 | 197篇 |
2013年 | 307篇 |
2012年 | 268篇 |
2011年 | 202篇 |
2010年 | 217篇 |
2009年 | 223篇 |
2008年 | 227篇 |
2007年 | 234篇 |
2006年 | 216篇 |
2005年 | 216篇 |
2004年 | 184篇 |
2003年 | 163篇 |
2002年 | 147篇 |
2001年 | 87篇 |
2000年 | 68篇 |
1999年 | 78篇 |
1998年 | 54篇 |
1997年 | 76篇 |
1996年 | 70篇 |
1995年 | 76篇 |
1994年 | 42篇 |
1993年 | 48篇 |
1992年 | 42篇 |
1991年 | 25篇 |
1990年 | 23篇 |
1989年 | 28篇 |
1988年 | 20篇 |
1987年 | 3篇 |
1986年 | 7篇 |
1985年 | 5篇 |
1984年 | 7篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1974年 | 5篇 |
1973年 | 4篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有4802条查询结果,搜索用时 15 毫秒
71.
Otto Vogl Jan Bartus Joseph R. Murdoch 《Monatshefte für Chemie / Chemical Monthly》1990,121(4):311-316
Summary We report techniques for measuring optical rotation of suspended solids, including dissymmetric molecular crystals (e.g. sodium chlorate and bromate), synthetic polymers, polypeptides and biopolymers. Examples include poly(triphenylmethyl methacrylate), poly-S-lactide, poly-L-hydroxyproline, hemocyanine, cellulose acetate andRNA. The results indicate that optical rotation measurements are sensitive to differences in secondary structure and can serve as a probe for structural differences between solid-state and solution.
Messung der optischen Rotation von Makromolekülen im Festzustand unter Verwendung von Pulversuspensionen
Zusammenfassung Es wird über eine Methode zur Messung der optischen Rotation von Festkörpern in Suspensionen berichtet, wobei dissymetrische Kristalle (z. B. Natriumchlorat und -bromat), synthetische Polymere, Polypeptide und Biopolymere behandelt werden. Die Beispiele umfassen Poly(triphenylmethylmethacrylat), Poly-S-lactid, Poly-L-hydroxyprolin, Hemocyanin, Celluloseacetat undRNA. Die Ergebnisse zeigen, daß die optischen Rotationen bezüglich Unterschiede in der Sekundärstruktur empfindlich sind und daß diese Messungen als ein Maß für Strukturunterschiede zwischen Festzustand und Lösung herangezogen werden können.相似文献
72.
Several Li+- and Na+-acetonitrile models were derived from ab initio calculations at the counterpoise-corrected MP2/TZV++(d,p) level for distorted ion-(MeCN)n clusters with n=1, 4 and 6. Two different many-body ion-acetonitrile models were constructed: an effective three-body potential for use with the six-site effective pair model of Böhm et al., and an effective polarizable many-body model. The polarizable acetonitrile model used in the latter model is a new empirical model which was also derived in the present paper. Mainly for comparative purposes, two ion-acetonitrile pair potentials were also constructed from the ab initio cluster calculations: one pure pair potential and one effective pair potential. Using all these potential models, MD simulations in the NPT ensemble were performed for the pure acetonitrile liquid and for Li+(MeCN) and Na+(MeCN) solutions with 1 ion in 512 solvent molecules and with a simulation time of at least 120 ps per system. Thermodynamic properties, solvation-shell structure and the self-diffusion coefficient of the ions and of the solvent molecules were calculated and compared between the different models and with experimental data, where available. The Li+ ion is found to be four-coordinated when the new many-body potentials are used, in contrast to the six-coordinated structure obtained for the pure pair and effective pair potentials. The coordination number of Na+ is close to six for all the models derived here, although the coordination number becomes slightly smaller with the many-body potentials. For both ions, the solvent molecules in the first shell point their nitrogen ends towards the cation, while in the second shell the opposite orientation is the most common. 相似文献
73.
The reaction of dimethylzinc and tri(tert‐butyl)silylphosphane in toluene yielded dimeric methylzinc tri(tert‐butyl)silylphosphanide ( 1 ) which crystallized tetrameric. Compound 1 was deprotonated with sodium in DME and the solvent‐separated dimeric ion pair [(dme)3Na]+ [(dme)Na(MeZn)2(μ‐PSitBu3)2]? ( 2 ) was isolated. The reaction of 1 in THF with two equivalents of potassium and one equivalent of tri(tert‐butyl)silylphosphane gave dimeric [{tBu3Si(H)P}{(thf)2K}2(MeZn)(PSitBu3)]2 ( 3 ). Both of these phosphanylzincates contain Zn2P2 cycles with Zn‐P bond lengths of approximately 237 pm, whereas in 1 larger Zn‐P bond lengths of 248.5 pm were found due to the larger coordination numbers of the phosphorus and zinc atoms. 相似文献
74.
Snezana S.?MiticEmail author Gordana Z.?Miletic Aleksandra N.?Pavlovic Snezana B.?Tosic 《Monatshefte für Chemie / Chemical Monthly》2004,135(8):927-936
Summary. A new selective, sensitive, and simple kinetic method is developed for the determination of trace amounts of chromium (VI). The method is based on the catalytic effect of Cr(VI) on the reaction of sodium pyrogallol-5-sulphonate (PS) with hydrogen peroxide. The reaction is followed spectrophotometrically by tracing the oxidation product at 437nm within 1min after addition of H2O2. The optimum reaction conditions are PS (1.32·10–3mol·dm–3), H2O2 (0.32mol·dm–3), HClO4 (2.6·10–3mol·dm–3) at 25°C. Following this procedure, chromium (VI) can be determined with a linear calibration graph up to 0.25ng·cm–3 and a detection limit of 0.024ng·cm–3, based on the 3 criterion. The interference effect of several species was also investigated and it was found that the most common cations and anions do not interfere with the determination. The developed procedure was successfully applied to the determination of Cr(VI) and total Cr in river waters and total Cr in herbal samples. 相似文献
75.
The preconcentration and voltammetric behavior of BiIII on a sodium humate modified carbon paste electrode was studied by means of cyclic voltammetry (CV) and differential pulse stripping voltammetry (DPSV). The proposed measurement involves an initial nonelectrolytic preconcentration step in which BiIII is complexed by the surface modifier in a solution of 0.05 M KNO3-0.0106 M HNO3 (pH 2.0) and a subsequent electrochemical scan step in which the preconcentrated BiIII was reduced and then oxidized promptly in supporting electrolyte of 0.5 M HNO3. The resulting DPSV anodic current was proportional to the concentration of BiIII ion over the range of 4.78 × 10−8–1.44 × 10−5 M. The detection limit was 4.78 × 10−8 M. The proposed method was used to determine bismuth in various samples. Various factors affecting the electrode behavior were also investigated at the same time. 相似文献
76.
Cs[Na(NH3)6][B10H10]·NH3 was synthesised from cesium and disodium‐decahydro‐closo‐decaborate Na2B10H10 in liquid ammonia, from which it crystallized in form of temperature sensitive colorless plates (triclinic, P1¯, a = 8.4787(7) Å, b = 13.272(1) Å, c = 17.139(2) Å, α = 88.564(1)°, β = 89.773(1)°, γ = 81.630(1)°, V = 1907.5(3) Å3, Z = 4). The compound is the first example of an alkali metal boranate with two different types of cations. The decahydro‐closo‐decaborate dianions [B10H10]2— and the cesium cations form a equation/tex2gif-stack-1.gif[Cs2(B10H10)2]2— layer parallel to the ac plane. These layers are separated by N—H···N‐hydrogen bonded hexamminesodium cations. 相似文献
77.
Manfred Schlosser 《Angewandte Chemie (International ed. in English)》1974,13(11):701-706
Merits and drawbacks of known carbon-carbon linking procedures are outlined. Two novel methods are discussed in some detail: the copper-catalyzed alkylation of Grignard reagents and reactions with allylpotassium compounds. Both methods provide a very efficient access to saturated, unsaturated, as well as functionally substituted hydrocarbons and moreover permit an astonishing degree of regio- and stereoselective control of olefin synthesis. 相似文献
78.
79.
Taterao M. Potewar 《Tetrahedron letters》2007,48(10):1721-1724
An efficient synthesis leading directly to 1-substituted-1H-1,2,3,4-tetrazoles from easily available amines and sodium azide in stoichiometric proportions using a room-temperature ionic liquid, namely, 1-n-butylimidazolium tetrafluoroborate in excellent yields is described. The inherent Brønsted acidity and high polarity of the IL results in a significant enhancement in the reaction rate. 相似文献
80.
Farajollah?MohanazadehEmail author Mostafa?Hosini Mahmoud?Tajbakhsh 《Monatshefte für Chemie / Chemical Monthly》2005,136(12):2041-2043
Summary. Reduction of aldehydes and ketones to the corresponding alcohols was achieved in a simple procedure using the system sodium
borohydride and ammonium carbonate. This system needs lower excesses of reducing agent and leads to significantly shortened
reaction times. 相似文献