全文获取类型
收费全文 | 3799篇 |
免费 | 138篇 |
国内免费 | 27篇 |
专业分类
化学 | 2760篇 |
晶体学 | 12篇 |
力学 | 103篇 |
数学 | 564篇 |
物理学 | 525篇 |
出版年
2024年 | 6篇 |
2023年 | 34篇 |
2022年 | 90篇 |
2021年 | 91篇 |
2020年 | 90篇 |
2019年 | 85篇 |
2018年 | 56篇 |
2017年 | 45篇 |
2016年 | 128篇 |
2015年 | 98篇 |
2014年 | 132篇 |
2013年 | 227篇 |
2012年 | 266篇 |
2011年 | 297篇 |
2010年 | 180篇 |
2009年 | 178篇 |
2008年 | 284篇 |
2007年 | 257篇 |
2006年 | 196篇 |
2005年 | 221篇 |
2004年 | 190篇 |
2003年 | 136篇 |
2002年 | 140篇 |
2001年 | 42篇 |
2000年 | 35篇 |
1999年 | 26篇 |
1998年 | 29篇 |
1997年 | 29篇 |
1996年 | 37篇 |
1995年 | 30篇 |
1994年 | 25篇 |
1993年 | 18篇 |
1992年 | 20篇 |
1991年 | 23篇 |
1990年 | 30篇 |
1989年 | 16篇 |
1988年 | 25篇 |
1987年 | 14篇 |
1986年 | 12篇 |
1985年 | 19篇 |
1984年 | 12篇 |
1982年 | 16篇 |
1981年 | 12篇 |
1980年 | 9篇 |
1979年 | 9篇 |
1978年 | 6篇 |
1977年 | 5篇 |
1976年 | 5篇 |
1975年 | 5篇 |
1973年 | 5篇 |
排序方式: 共有3964条查询结果,搜索用时 0 毫秒
61.
Commodari F Khiat A Ibrahimi S Brizius AR Kalkstein N 《Magnetic resonance in chemistry : MRC》2005,43(7):567-572
The NMR-derived solution structure of trans-3,4',5-trihydroxystilbene (resveratrol) was compared with two recent literature crystal x-ray structures, resveratrol in complex with human transthyretin (TTR-RES) from 1DVS.pdb and resveratrol bound to chalcone synthase (CHS-RES) from 1CGZ.pdb. 1H and 13C NMR spectra of resveratrol were acquired in DMSO-d6. Assignments were obtained from an analysis of DQF-COSY, TOCSY, DEPT, HMQC/HSQC, HMBC and INADEQUATE NMR spectra. Past 1H and 13C NMR literature assignments are corrected. The dihedral angle 2-1-1'-2' provides an indication of the relative spatial orientation of the two phenolic rings. Values of 1.62, - 54.10 and 12.6 +/- 1.1 degrees were found for the 1DVS.pdb, 1CGZ.pdb and NMR resveratrol structures, respectively. The 1DVS.pdb resveratrol structure is 'flat' with the two phenolic rings along the same plane. The 1CGZ.pdb structure has these two rings almost orthogonal to each other, and the NMR structure has these two rings much closer to being along the same plane. The angles 1-alpha--alpha' and 1'-alpha'--alpha are along the same trace and of similar magnitude for the 1CGZ.pdb and NMR resveratrol structures. For the 1DVS.pdb resveratrol structure, these angles are about 7-10 degrees greater, with alpha and alpha' being 180 degrees out-of-phase from the other two structures. The alpha rings did not overlap, with the NMR result representing a 'median model' of the two x-ray structures. 相似文献
62.
Karen Wright Fernando FormaggioClaudio Toniolo Roland TörökAntal Péter Michel WakselmanJean-Paul Mazaleyrat 《Tetrahedron letters》2003,44(22):4183-4186
Resolution of trans 3-(9-fluorenylmethyloxycarbonylamino)-1-oxyl-2,2,5,5-tetramethylpyrrolidine-4-carboxylic acid (Fmoc-POAC-OH) was quickly achieved upon esterification with (aR)-1,1′-binaphthyl-2,2′-diol, chromatographic separation of the obtained diastereomers, and facile saponification of the aryl ester function with removal of the chiral auxiliary. 相似文献
63.
Garavelli M Ruggeri F Ogliaro F Bearpark MJ Bernardi F Olivucci M Robb MA 《Journal of computational chemistry》2003,24(11):1357-1363
MMVB is a QM/MM hybrid method, consisting of a molecular mechanics force field coupled to a valence bond Heisenberg Hamiltonian parametrized from ab initio CASSCF calculations on several prototype molecules. The Heisenberg Hamiltonian matrix elements Q(ij) and K(ij), whose expressions are partitioned here into a primary contribution and second-order correction terms, are calculated analytically in MMVB. When the original MMVB force field fails to produce potential energy surfaces accurate enough for dynamics calculations, we show that significant improvements can be made by refitting the second-order correction terms for the particular molecule(s) being studied. This "local" reparametrization is based on values of K(ij) extracted (using effective Hamiltonian techniques) from CASSCF calculations on the same molecule(s). The method is demonstrated for the photoisomerization of s-cis butadiene, and we explain how the correction terms that enabled a successful MMVB dynamics study [Garavelli, M.; Bernardi, F.; Olivucci, M.; Bearpark, M. J.; Klein, S.; Robb, M. A. J Phys Chem A 2001, 105, 11496] were refitted. 相似文献
64.
Yolanda Prieto Marcelo Muoz Vernica Arancibia Mauricio Valderrama Fernando J. Lahoz M. Luisa Martín 《Polyhedron》2007,26(18):5527-5532
The reaction of the dimer complex [{Ru(CO)3Cl2}2] with the ligands 4,6-dichloroquinoline-5,8-dione and 6-methoxybenzo[g]quinoline-5,10-dione in ethanol solution led to the neutral mononuclear complexes of general formula [Ru(CO)2Cl2(κ2-quinolinedione-N,O)]. The complexes were characterized by elemental analysis, IR and RMN spectroscopy, and the molecular structure of [Ru(CO)2Cl2(6-methoxybenzo[g]quinoline-5,10-dione)] was determined by single-crystal X-ray diffraction. The redox chemistry of ligands and complexes was investigated by cyclic voltammetry, and their potential antitumor activity was also evaluated. 相似文献
65.
66.
Daniel Carmona Roberto Medrano Isabel T. Dobrinovich Fernando J. Lahoz Joaquina Ferrer Luis A. Oro 《Journal of organometallic chemistry》2006,691(26):5560-5566
Half-sandwich complexes of formula [(ηn-ring)MClL]PF6 [L = (S)-2-[(Sp)-2-(diphenylphosphino)ferrocenyl]-4-isopropyloxazoline; (ηn-ring)M = (η5-C5Me5)Rh; (η5-C5Me5)Ir; (η6-p-MeC6H4iPr)Ru; (η6-p-MeC6H4iPr)Os] have been prepared and spectroscopically characterised. The molecular structures of the rhodium and iridium compounds have been determined by X-ray crystallography. The related solvate complexes [(η5-C5Me5)ML(Me2CO)]2+ (M = Rh, Ir) are active catalysts for the Diels-Alder reaction between methacrolein and cyclopentadiene. 相似文献
67.
Correia I Costa Pessoa J Duarte MT Henriques RT Piedade MF Veiros LF Jakusch T Kiss T Dörnyei A Castro MM Geraldes CF Avecilla F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(9):2301-2317
The Schiff base N,N'-ethylenebis(pyridoxylideneiminato) (H(2)pyr(2)en, 1) was synthesized by reaction of pyridoxal with ethylenediamine; reduction of H(2)pyr(2)en with NaBH(4) yielded the reduced Schiff base N,N'-ethylenebis(pyridoxylaminato) (H(2)Rpyr(2)en, 2); their crystal structures were determined by X-ray diffraction. The totally protonated forms of 1 and 2 correspond to H(6)L(4+), and all protonation constants were determined by pH-potentiometric and (1)H NMR titrations. Several vanadium(IV) and vanadium(V) complexes of these and other related ligands were prepared and characterized in solution and in the solid state. The X-ray crystal structure of [V(V)O(2)(HRpyr(2)en)] shows the metal in a distorted octahedral geometry, with the ligand coordinated through the N-amine and O-phenolato moieties, with one of the pyridine-N atoms protonated. Crystals of [(V(V)O(2))(2)(pyren)(2)].2 H(2)O were obtained from solutions containing H(2)pyr(2)en and oxovanadium(IV), where Hpyren is the "half" Schiff base of pyridoxal and ethylenediamine. The complexation of V(IV)O(2+) and V(V)O(2) (+) with H(2)pyr(2)en, H(2)Rpyr(2)en and pyridoxamine in aqueous solution were studied by pH-potentiometry, UV/Vis absorption spectrophotometry, as well as by EPR spectroscopy for the V(IV)O systems and (1)H and (51)V NMR spectroscopy for the V(V)O(2) systems. Very significant differences in the metal-binding abilities of the ligands were found. Both 1 and 2 act as tetradentate ligands. H(2)Rpyr(2)en is stable to hydrolysis and several isomers form in solution, namely cis-trans type complexes with V(IV)O, and alpha-cis- and beta-cis-type complexes with V(V)O(2). The pyridinium-N atoms of the pyridoxal rings do not take part in the coordination but are involved in acid-base reactions that affect the number, type, and relative amount of the isomers of the V(IV)O-H(2)Rpyr(2)en and V(V)O(2)-H(2)Rpyr(2)en complexes present in solution. DFT calculations were carried out and support the formation and identification of the isomers detected by EPR or NMR spectroscopy, and the strong equatorial and axial binding of the O-phenolato in V(IV)O and V(V)O(2) complexes. Moreover, the DFT calculations done for the [V(IV)O(H(2)Rpyr(2)en)] system indicate that for almost all complexes the presence of a sixth equatorial or axial H(2)O ligand leads to much more stable compounds. 相似文献
68.
Frank G. Riddell Eric S. Turner Alan R. Katritzky Ranjan C. Patel Fernando M.S. Brito-Palma 《Tetrahedron》1979,35(11):1391-1398
The synthesis and variable temperature 1H and 13C NMR spectra of three tetrahydro-1,2,4-oxadiazines are reported. The N(4)-Me inversion barriers are 6.8–7.0 (ax→ts) and 7.4–7.9 kcal mol?1 (eq→ts) with ΔG° 0.6–0.9 kcal mol?1. The N(2)-Me inversion barriers are 10.4–11.4 (ax→ts) and 11.6–13.1 kcal mol?1 (eq→ts) with ΔGδ 1.2–1.7 kcal mol?1. The barrier to ring inversion is ca. 12.7 kcal mol?1. “R value” analysis shows the ring to have a 56.5±2δ dihedral angle about the C(5)-(6) bond, indicative of the expected chair conformation. 相似文献
69.
Lancelot N Elbayed K Raya J Piotto M Briand JP Formaggio F Toniolo C Bianco A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(6):1317-1323
A tetra- and a hepta-homopeptide from the C(alpha)-tetrasubstituted Aib (alpha-aminoisobutyric acid) residue were covalently linked to the POEPOP resin by the fragment-condensation approach. The conformational preferences of the two model peptides were determined for the first time on a solid support by means of high-resolution magic angle spinning NMR spectroscopy. The results obtained indicate that the Aib homopeptides adopt a regular 3(10)-helical structure even when they are covalently bound to a polymeric matrix, and thus confirm the remarkable conformational stability of the peptides rich in this amino acid. An ATR-FTIR spectroscopic investigation, performed in parallel, also confirmed that these polymer-bound peptides do indeed adopt a helical conformation. The results of this study open the possibility to exploit the peptide-resin conjugates based on C(alpha)-tetrasubstituted alpha-amino acids as helpful, structurally organized templates in molecular recognition studies or as catalysts in asymmetric synthesis. 相似文献
70.
Josep Castells Elisabet Duñach Fernando Geijo Francisco López-Calahorra Margarita Prats Olga Sanahuja Laura Villanova 《Tetrahedron letters》1980,21(23):2291-2294
Triethylamine catalyzed Michael additions of benzoins to chalcone can prevail over the expected Michael-Stetter additions when certain thiazolium ion conjugate bases - prepared from the pertinent thiazolium salts and triethylamine- are used as catalysts. 相似文献