全文获取类型
收费全文 | 3896篇 |
免费 | 119篇 |
国内免费 | 16篇 |
专业分类
化学 | 2423篇 |
晶体学 | 20篇 |
力学 | 217篇 |
数学 | 764篇 |
物理学 | 607篇 |
出版年
2023年 | 17篇 |
2022年 | 33篇 |
2021年 | 106篇 |
2020年 | 78篇 |
2019年 | 59篇 |
2018年 | 70篇 |
2017年 | 63篇 |
2016年 | 124篇 |
2015年 | 121篇 |
2014年 | 91篇 |
2013年 | 207篇 |
2012年 | 223篇 |
2011年 | 275篇 |
2010年 | 158篇 |
2009年 | 136篇 |
2008年 | 239篇 |
2007年 | 244篇 |
2006年 | 182篇 |
2005年 | 167篇 |
2004年 | 162篇 |
2003年 | 126篇 |
2002年 | 104篇 |
2001年 | 48篇 |
2000年 | 30篇 |
1999年 | 46篇 |
1998年 | 43篇 |
1997年 | 39篇 |
1996年 | 42篇 |
1995年 | 35篇 |
1994年 | 37篇 |
1993年 | 57篇 |
1992年 | 23篇 |
1991年 | 29篇 |
1990年 | 33篇 |
1989年 | 22篇 |
1988年 | 29篇 |
1987年 | 24篇 |
1986年 | 25篇 |
1985年 | 56篇 |
1984年 | 46篇 |
1983年 | 26篇 |
1982年 | 31篇 |
1981年 | 30篇 |
1980年 | 27篇 |
1979年 | 27篇 |
1978年 | 27篇 |
1977年 | 23篇 |
1976年 | 22篇 |
1975年 | 15篇 |
1974年 | 21篇 |
排序方式: 共有4031条查询结果,搜索用时 15 毫秒
911.
912.
Pieraccini S Ferrarini A Gottarelli G Lena S Masiero S Spada GP 《The Journal of organic chemistry》2005,70(20):8009-8016
[Chemical reaction: See text] The circular dichroism spectra and the twisting ability of a series of 2-aryl-4,5-dimethyl-1,3-dioxolanes used as dopants in nematic solvents have been related to their absolute configuration. Whereas the circular dichroism (CD) spectra are deeply affected by the substituents present in the aromatic ring, which in several cases cause sign inversion, the helical twisting power beta is only marginally influenced. The values of beta also seem not very sensitive to the rotamer population around the aromatic ring; this indicates the predominant importance of the chiral dioxolane ring in determining the cholesteric induction. These facts can be explained by the different nature of the two observables: in CD, the chirality is read by the absorbing chromophore and is deeply influenced even by small changes of this group. In cholesteric induction we are dealing instead with chiral solute-solvent interactions that determine a twist in the solvent. In light of the present and previous results, this process seems predominantly determined by short-range interactions, which are modulated by the molecular shape. From a practical point of view, a configurational correlation using CD for the present series of compounds seems problematic, while the values of beta are nicely correlated to the absolute configurations. Calculations with the surface chirality method predict well the sign and order of magnitude of beta and their limited sensitivity to the phenyl substituents and rotamer population. 相似文献
913.
Vinje J Intini FP Natile G Sletten E 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(14):3569-3578
The reactions of the anticancer complex trans-[PtCl(2)[(E)-HN==C(OMe)Me](2)] (trans-EE) with both single-stranded and double-stranded deoxyribonucleotides have been studied by HPLC and 2D [(1)H,(15)N] HMQC NMR spectroscopy and compared with those of cis-[PtCl(2)(NH(3))(2)] (cis-DDP). Reactions of trans-EE with the single-stranded oligonucleotides d(CCTCGCTCTC) and d(CCTGGTCC) proceed rapidly through solvolysis of the starting substrate and subsequent formation of G-N7/monochloro trans-EE adducts. The rate of reaction is comparable to that of formation of an adduct from trans-EE and the dinuclotide d(ApG). Quite unexpectedly, the double-helical duplexes, d(TATGGTACCATA)(2) and d(TATGGCCATA)(2), with no terminal G residues, are practically inert towards trans-EE, and only minor species (< 5 % as estimated from HPLC traces) appear during 24 h reaction time. However, the duplexes d[(CCTCGCTCTC). (GAGAGCGAGG)] and d(GATAGGCCTATC)(2), which contain both terminal and central G residues, undergo platination only at the terminal, solvent-exposed, G residues, thereby confirming that the interior of the duplex is not accessible to trans-EE due to steric hindrance. In contrast, cis-DDP was found to bind exclusively to the central GG pair in d(GATAGGCCTATC)(2). 相似文献
914.
Cinellu MA Minghetti G Stoccoro S Zucca A Manassero M 《Chemical communications (Cambridge, England)》2004,(14):1618-1619
Gold alkene complexes [Au(bipyR)(eta2-alkene)][PF6] (bipyR = 6-alkyl-2,2'-bipyridine) have been obtained by reaction of gold(III) oxo complexes [Au2(bipyR)2(mu-O)2][PF6]2 with alkenes. The crystal structure of the styrene adduct [Au(bipy(ip))(eta2-CH2=CHPh)][PF6] (bipy(ip) = 6-isopropyl-2,2'-bipyridine) has been solved by X-ray analysis. 相似文献
915.
Substituted isoxazolidines formed by 1,3-dipolar cycloaddition of nitrones to alkenes undergo ring-opening elimination to α,β-enones when treated with trimethyl phosphate. The reaction involves initial alkylation giving the isoxazolidinium intermediate which collapses to the α,β-enone by a Hofmann-like elimination having an orientation controlled by electronic factors, the first step being rate-determining. 相似文献
916.
Evaluation of fourier transform of two-center charge distribution for arbitrary slater-type orbitals
Carla Guidotti Giovanni P. Arrighini Francis Marinelli 《Theoretical chemistry accounts》1979,53(2):165-173
Fourier transform of two-center charge distributions corresponding to arbitrary Slater-type orbitals are evaluated by a Gaussian quadrature procedure without any preliminary series expansion of the integrand. Convergence and accuracy of the method are discussed and illustrated. 相似文献
917.
A versatile and concise strategy has been developed (Scheme 1) for the enantioselective synthesis of semperoside A 1 which is endowed with an unusually glucosylated iridane structure. The crucial step was a Hg(II)-mediated electrophilic heteroatom cyclization of beta-glucoside 4 that readily led to the iridane skeleton while installing the C-2 and C-3 stereocenters with complete stereocontrol. This expeditious route is unprecedented among synthetic approaches to iridoid glycosides and smoothly overcomes the hemiacetals glucosidation issue. The present inaugural total synthesis of semperoside A was achieved in 10 steps and 17% overall yield from the enantiomerically pure lactone 8, thus proving the absolute stereochemistry of 1 unequivocally. 相似文献
918.
Barbara Richichi Stefano CicchiUgo Chiacchio Giovanni Romeo Alberto Brandi 《Tetrahedron》2003,59(28):5231-5240
The synthesis of two new bicyclic nucleoside analogues is reported. These compounds are iso-homonucleoside and are synthesised through a 1,3-dipolar cycloaddition of an enantiopure cyclic nitrone to protected allyl acohol and subsequent introduction of thymine by a Mitsunobu reaction. 相似文献
919.
Chillemi G D'Angelo P Pavel NV Sanna N Barone V 《Journal of the American Chemical Society》2002,124(9):1968-1976
In this paper we have developed an effective computational procedure for the structural and dynamical investigation of ions in aqueous solutions. Quantum mechanical potential energy surfaces for the interaction of a transition metal ion with a water molecule have been calculated taking into account the effect of bulk solvent by the polarizable continuum model (PCM). The effective ion-water interactions have been fitted by suitable analytical potentials, and have been utilized in molecular dynamics (MD) simulations to obtain structural and dynamical properties of the ionic aqueous solutions. This procedure has been successfully applied to the Co2+-H2O open-shell system and, for the first time, Co-oxygen and Co-hydrogen pair potential functions have been determined and employed in MD simulations. The reliability of the whole procedure has been assessed by applying it also to the Zn2+ and Ni2+ aqueous solutions, and the structural and dynamical properties of the three systems have been calculated by means of MD simulations and have been found to be in very good agreement with experimental results. The structural parameters of the first solvation shells issuing from the MD simulations provide an effective complement to extended X-ray absorption fine structure (EXAFS) experiments. 相似文献
920.
Giovanni Orzalesi Renato Selleri Rodolfo Landi Vittory Franco Innocenti Giuliano Grandolini 《Journal of heterocyclic chemistry》1977,14(5):733-738
2-Amino-5-chloro-α-phenylbenzylidene hydrazone ( 1 ) or its methyl derivative 2 or acetyl derivative 10 react with diethylmalonic esters to give the corresponding malonyl derivatives 3, 4 and 8 . These esters were hydrolyzed to the acids 5 and 6 . Treating 5 with dehydrating agents the mesoionic compound 7-chloro-9-phenyl-3,3-diethyl-3H-pyrazolo[5,1-b]quinazolin-10-ium-2-olate (14) was obtained, while the methyl derivative 6 afforded the desired 9-chloro-2,3,4,5-tetrahydro-1-methyl-3,3-diethyl-7-phenyl-1H-benzo-1,5,6-triazonine-2,4-dione ( 17 ). Some derivatives of these compounds were also described. The structures of the new compounds were confirmed by an alternative synthesis and by mass and prnr spectral data. 相似文献