全文获取类型
收费全文 | 4626篇 |
免费 | 131篇 |
国内免费 | 16篇 |
专业分类
化学 | 2924篇 |
晶体学 | 19篇 |
力学 | 246篇 |
数学 | 914篇 |
物理学 | 670篇 |
出版年
2023年 | 28篇 |
2022年 | 83篇 |
2021年 | 124篇 |
2020年 | 93篇 |
2019年 | 77篇 |
2018年 | 82篇 |
2017年 | 76篇 |
2016年 | 153篇 |
2015年 | 135篇 |
2014年 | 109篇 |
2013年 | 262篇 |
2012年 | 266篇 |
2011年 | 320篇 |
2010年 | 188篇 |
2009年 | 162篇 |
2008年 | 261篇 |
2007年 | 277篇 |
2006年 | 223篇 |
2005年 | 203篇 |
2004年 | 178篇 |
2003年 | 160篇 |
2002年 | 140篇 |
2001年 | 49篇 |
2000年 | 34篇 |
1999年 | 44篇 |
1998年 | 43篇 |
1997年 | 41篇 |
1996年 | 47篇 |
1995年 | 33篇 |
1994年 | 38篇 |
1993年 | 52篇 |
1992年 | 26篇 |
1991年 | 34篇 |
1990年 | 33篇 |
1989年 | 29篇 |
1988年 | 33篇 |
1987年 | 28篇 |
1986年 | 27篇 |
1985年 | 58篇 |
1984年 | 54篇 |
1983年 | 31篇 |
1982年 | 33篇 |
1981年 | 38篇 |
1980年 | 35篇 |
1979年 | 32篇 |
1978年 | 29篇 |
1977年 | 30篇 |
1976年 | 31篇 |
1975年 | 24篇 |
1974年 | 27篇 |
排序方式: 共有4773条查询结果,搜索用时 0 毫秒
31.
The cyclic voltammetric technique is used to study hydrogen-bond formation in some polar organic solvents (S) of electroanalytical interest (1,2-dimethoxy, tetrahydrofuran, dimethylformamide, dimethyl sulphoxide and pyridine). The cathodic shift of the proton reduction caused by stepwise addition of the solvent investigated to a solution of anhydrous perchloric acid in acetonitrile is utilized. The theoretical treatment applied produced evidence that HS+ and HS+2 are the only acidic species involved, so that the relevant formation constants can be evaluated. The data obtained mostly compare well with those available in the literature. The features that condition the tendency to hydrogen-bonding and the effect of hydrogen bonding on solvent basicity are discussed. 相似文献
32.
Sergio Bocchini Sandrine Morlat-Thérias Giovanni Camino 《Polymer Degradation and Stability》2007,92(10):1847-1856
Nanocomposites containing pure or organically modified nanoboehmites of different sizes were prepared by melt compounding with polypropylene. The samples were UV light irradiated in artificial accelerated conditions representative of solar irradiation (λ > 300 nm) at 60 °C in air. The chemical modifications resulting from photooxidation were followed by IR and UV-visible spectroscopies. The presence of pristine nanoboehmites was shown to change the rate of oxidation of polypropylene by reducing the oxidation induction period due to the presence of residual processing antioxidant. The differences of the oxidation induction periods between the nanocomposites and the pristine polymer disappear after solvent extraction of the antioxidant. The inefficiency of traditional antioxidant in retarding the photooxidation of polypropylene containing nanodispersed boehmite is proved. Antioxidant migration to the boehmite surface induced by the preferential interaction with the polar filler is proposed as an explanation. The oxidative behaviour of the organically modified boehmites was shown to depend on the type of organic substituent. p-Toluenesulfonate reduces the adsorption of antioxidants while the presence of a long-chain alkyl benzensulfonate increased the oxidation rate by generation of radical initiators. 相似文献
33.
We review the recent studies of the photoisomerization dynamics of azobenzene and its derivatives by surface hopping simulations based on semiempirical potential energy surfaces. We examine the ability of semiclassical methods to predict the excited state dynamics and to reproduce transient spectroscopic signals that constitute the most direct experimental evidence in this field. We show that the available simulation methods yield a deep insight into the mechanism of photochemical reactions and excited state decay, and a fairly good quantitative agreement with experimental findings. Probably the most important technical improvements we can envisage concern the surface hopping algorithm and the usage of ab initio data in the simulation of transient spectra. Concerning azobenzene, our results show that the isomerization mechanism is torsion of the N=N double bond, both by n → π* and by π → π* excitation. The influence of the solvent and the findings of some recent femtochemistry experiments deserve further work to be fully interpreted. 相似文献
34.
Liu Y Vinje J Pacifico C Natile G Sletten E 《Journal of the American Chemical Society》2002,124(43):12854-12862
The reactions of the anticancer complex trans-[PtCl2{(E)-HN=C(OMe)Me}2] (trans-EE) with a series of ribo and deoxyribodinucleotides have been studied by HPLC and 2D [1H, 15N] HMQC NMR spectroscopy and compared with those of the inactive trans isomer of cisplatin, trans-[PtCl2(NH3)2] (trans-DDP). Reactions of trans-EE with r(ApG) and d(ApG) take place through solvolysis of the starting substrate and subsequent formation of trans G-N7/monochloro and G-N7/monoaqua adducts. Slowly, the monofunctional adducts evolve to a bifunctional adduct forming an unprecedented and unexpected A-N3/G-N7 platinum cross-link spanning two trans positions. For stereochemical reasons, trans platinum complexes cannot form N7/N7 cross-links between adjacent purines in di- or polynucleotides. For the reverse sequence r(GpA), no chelate structure was formed even after a two-week reaction. The reaction of trans-DDP with r(ApG) produces many more products than the analogous reaction with trans-EE. One of these products was identified as the A-N3/G-N7 trans-chelate. 相似文献
35.
Ciminelli C Granucci G Persico M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(9):2327-2341
We have simulated the photoisomerization dynamics of azobenzene, taking into account internal conversion and geometrical relaxation processes, by means of a semiclassical surface hopping approach. Both n-->pi* and pi-->pi* excitations and both cis-->trans and trans-->cis conversions have been considered. We show that in all cases the torsion around the N==N double bond is the preferred mechanism. The quantum yields measured are correctly reproduced and the observed differences are explained as a result of the competition between the inertia of the torsional motion and the premature deactivation of the excited state. Recent time-resolved spectroscopic experiments are interpreted in the light of the simulated dynamics. 相似文献
36.
Zanoni G Porta A Meriggi A Franzini M Vidari G 《The Journal of organic chemistry》2002,67(17):6064-6069
Regioconvergent synthesis of the key lactone 1 from an equimolar mixture of the two olefins 4 and 5 was achieved by unique Pd(II) chemistry. The synthetic versatility of lactone 1 has been demonstrated in the synthesis of iridoids and of the endo-Corey lactone 2, which is a key intermediate for the F(2)-isoprostane synthesis. Upon exposure of the sodium salts of 4 and 5 to a catalytic amount of Pd(OAc)(2) under oxygen, in the presence of AcOH, an isomeric lactone 12 was obtained in addition to the title compound 1. The Pd(II) lactonization was optimized by fine-tuning all the factors participating in the catalytic cycle: solvent, oxidant, co-oxidant, and Pd(II) source. The Hosokawa's heterobimetallic couple emerged as the catalyst of choice. With a Cu(II)-Pd(II) couple, the redox process was transferred to copper, and the formal oxidation state of palladium remained constant during the reaction. By virtue of this new methodology, lactone 1 was obtained in a rewarding 60% yield, along with isomeric lactone 12 in 30% yield. A detailed mechanistic study was carried out in order to elucidate the formation of lactones 1 and 12. Lactone 1 was formed from either olefin 8 or olefin 10; on the other hand, lactone 12 was formed exclusively from olefin 10. An intramolecular 1,2-acyloxypalladiation was invoked for the transformation of 8 into 1, whereas the pi-allyl complexes 13 and 11 were involved in the transformation of olefin 10 into 12 and 1, respectively. 相似文献
37.
Marina Brustolon Carlo Corvaja Giovanni Giacometti 《Theoretical chemistry accounts》1971,22(1):90-104
The proton hyperfine splitting constants of a large number of positive, negative, and neutral radicals, have been examined in term of the Heller-McConnell relation a
H=Bc cos2
whose validity is discussed. B is taken as a function of the energy of the singly occupied orbital and values are calculated by first order perturbation theory for the cases of a methyl, methylene, and dimethylene group attached to the system. Substantial agreement is found between theory and experiment indicating the correctness of the postulated cause of the B behaviour.
Zusammenfassung Die Konstante der Hyperfeinaufspaltung für -Protonen einer großen Anzahl positiver, negativer und neutraler -Radikale wurde mit der Beziehung von Heller und McConnell als Grundlage untersucht; die Gültigkeit dieser Beziehung wird diskutiert. B wird als von der Energie des einfach besetzten Orbitals abhängig angenommen und seine Werte mit Hilfe der Störungstheorie erster Ordnung für die Fälle von Methyl-, Methylen- und Dimethylengruppe am -System berechnet. Aus der guten Übereinstimmung zwischen Theorie und Experiment wird auf die Richtigkeit der postulierten Ursachen für das Verhalten von B geschlossen.相似文献
38.
An automated procedure for correcting the interference due to mixing the reagents in the determination of ATP by a bioluminescent
technique is described. The measurements were performed by an in-house interfaced PC photometer. An automated procedure based
on a robust regression method and outliers detection was applied to exclude the time of the mixing of the reagents from the
global time of reaction. In fact the mixing of the reagents is responsible of the poor analytical precision (RSD%>25) and
low sensitivity (limit of detection≈2×10-8 mol/l). By this procedure, the precision (RSD%<7) and sensitivity (limit of detection≈8×10-10 mol/l) were improved. The method was applied to the determination of ATP in marine sediments and good recovery within 90–108%
was obtained. For non-polluted samples the measurement can be carried out by direct comparison with standard solutions. The
standard addition procedure is recommended for polluted samples.
Received: 25 April 1996/Revised: 28 June 1996/Accepted: 3 July 1996 相似文献
39.
Mondello L Shellie R Casilli A Quinto Tranchida P Marriott P Dugo G 《Journal of separation science》2004,27(9):699-702
A 5 m x 50 microm capillary column with 0.05 microm stationary phase film thickness, with a calculated efficiency of almost 20,000 plates per metre (under optimum conditions), was used for very fasthigh resolution GC analysis of lime essential oil. The total analysis time of this volatile essential oil was less than 90 s. Fast GC is shown to be appropriate for essential oil quality assurance analysis, and quantitative results of key components are comparable with those obtained by using conventional GC analysis. The fast GC analysis is approximately 33 times faster than the conventional GC method. 相似文献
40.
Mirella Ferrugia Maria Luisa Bajardi Leopoldo Ceraulo Salvatore Plescia 《Journal of heterocyclic chemistry》1992,29(2):565-568
The fragmentation reactions induced by electron impact of eighteen title compounds has been investigated with the aid of low beam energy spectra (14 eV, nom. value), metastable ion detection, high resolution measurements and labelling experiments. The loss of the 4-carbonyl oxygen together with the 3-substituent, which constitutes a characteristic fragmentation route of 3-aryl and 3-heteroaromatic substituted-4(3H)-quinazolinones, is again observed, but the presence of a carboxyethyl group at the 4′-position of the pyrazole ring is responsible of an anomalous loss of 47 daltons from the molecular ion. Lastly, a comparison with the previously described behaviour of 3-(5′-isoxazolyl) derivatives was carried out. 相似文献