全文获取类型
收费全文 | 1487篇 |
免费 | 51篇 |
国内免费 | 10篇 |
专业分类
化学 | 1208篇 |
晶体学 | 28篇 |
力学 | 6篇 |
数学 | 138篇 |
物理学 | 168篇 |
出版年
2023年 | 12篇 |
2022年 | 20篇 |
2021年 | 28篇 |
2020年 | 23篇 |
2019年 | 20篇 |
2018年 | 11篇 |
2017年 | 12篇 |
2016年 | 51篇 |
2015年 | 31篇 |
2014年 | 36篇 |
2013年 | 87篇 |
2012年 | 95篇 |
2011年 | 126篇 |
2010年 | 65篇 |
2009年 | 53篇 |
2008年 | 77篇 |
2007年 | 96篇 |
2006年 | 88篇 |
2005年 | 89篇 |
2004年 | 93篇 |
2003年 | 83篇 |
2002年 | 58篇 |
2001年 | 33篇 |
2000年 | 12篇 |
1999年 | 11篇 |
1998年 | 16篇 |
1997年 | 8篇 |
1996年 | 19篇 |
1995年 | 9篇 |
1994年 | 6篇 |
1993年 | 6篇 |
1992年 | 14篇 |
1991年 | 11篇 |
1990年 | 5篇 |
1989年 | 11篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 9篇 |
1985年 | 16篇 |
1984年 | 8篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1978年 | 7篇 |
1977年 | 7篇 |
1976年 | 8篇 |
1975年 | 6篇 |
1968年 | 4篇 |
1935年 | 4篇 |
排序方式: 共有1548条查询结果,搜索用时 15 毫秒
991.
Structure of Nano‐sized CeO2 Materials: Combined Scattering and Spectroscopic Investigations 下载免费PDF全文
Huw R. Marchbank Adam H. Clark Dr. Timothy I. Hyde Dr. Helen Y. Playford Dr. Matthew G. Tucker Dr. David Thompsett Dr. Janet M. Fisher Dr. Karena W. Chapman Kevin A. Beyer Dr. Manuel Monte Dr. Alessandro Longo Prof. Gopinathan Sankar 《Chemphyschem》2016,17(21):3494-3503
The structure of several nano‐sized ceria, CeO2, systems was investigated using neutron and X‐ray diffraction and X‐ray absorption spectroscopy. Whilst both diffraction and total pair distribution functions (PDFs) revealed that in all of the samples the occupancy of both Ce4+ and O2? are very close to the ideal stoichiometry, the analysis using Reverse Monte Carlo technique revealed significant disorder around oxygen atoms in the nano‐sized ceria samples in comparison to the highly crystalline NIST standard. In addition, the analysis revealed that the main differences observed in the pair correlations from various X‐ray and neutron diffraction techniques were attributable to the particle size of the CeO2 prepared by the reported three methods. Furthermore, detailed analysis of the Ce L3‐ and K‐edge EXAFS data support this finding; in particular the decrease in higher shell coordination numbers with respect to the NIST standard, is attributed to differences in particle size. 相似文献
992.
Rhys C. Griffiths Dr. Frances R. Smith Dr. Jed E. Long Dr. Huw E. L. Williams Prof. Robert Layfield Dr. Nicholas J. Mitchell 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(52):23867-23875
The development of site-selective chemistry targeting the canonical amino acids enables the controlled installation of desired functionalities into native peptides and proteins. Such techniques facilitate the development of polypeptide conjugates to advance therapeutics, diagnostics, and fundamental science. We report a versatile and selective method to functionalize peptides and proteins through free-radical-mediated dechalcogenation. By exploiting phosphine-induced homolysis of the C−Se and C−S bonds of selenocysteine and cysteine, respectively, we demonstrate the site-selective installation of groups appended to a persistent radical trap. The reaction is rapid, operationally simple, and chemoselective. The resulting aminooxy linker is stable under a variety of conditions and selectively cleavable in the presence of a low-oxidation-state transition metal. We have explored the full scope of this reaction using complex peptide systems and a recombinantly expressed protein. 相似文献
993.
This article provides necessary and sufficient conditions for each group of order 32 to be realizable as a Galois group over an arbitrary field. These conditions, given in terms of the number of square classes of the field and the triviality of specific elements in related Brauer groups, are used to derive a variety of automatic realizability results. 相似文献
994.
Athanasios Gagatsis Helen Demetriadou 《International Journal of Mathematical Education in Science & Technology》2013,44(1):105-125
An analysis of written solutions to geometry exercises in the last year of Greek Lyceum (15–18 years old) showed relatively low performance on vector methods, which is justified on the one hand by pupils' false preconceptions with regards to the concept of vector, and on the other hand by the strong influence of classical geometry teaching in the previous years. 相似文献
995.
Helen Schneider Dr. Volker Strauss Dr. Sarah Vogl Prof. Dr. Dr. h.c. Markus Antonietti Dr. Svitlana Filonenko 《Chemphyschem》2023,24(17):e202300180
Under isochoric and solvent-free conditions, the reaction between ammonium formate and citric acid results in a deeply purple reaction product with fluorescent properties. This brings this reaction in the realm of bio-based fluorophores and bottom-up carbon nanodots from citric acid. The reaction conditions are optimized in terms of UV–vis spectroscopic properties and, subsequently, the main reaction product is separated. While the structural analysis does not give any indication for carbon nanodots in a general sense, it points towards the formation of molecular fluorophores that consist of oligomerized citrazinic acid derivatives. Furthermore, EPR spectroscopy reveals the presence of stable free radicals in the product. We hypothesize that such open-shell structures may play a general role in molecular fluorophores from citric acid and are not yet sufficiently explored. Therefore, we believe that analysis of these newly discovered fluorophores may contribute to a better understanding of the properties of fluorophores and CND from citric acid in general. 相似文献
996.
Ingrid Socorro Antonia Neels Helen Stoeckli‐Evans 《Acta Crystallographica. Section C, Structural Chemistry》2004,60(1):m13-m15
Bis[μ‐1‐(3‐pyridyl)ethanone oxime‐κ2N:N′]bis[nitratosilver(I)], [Ag2(NO3)2(C7H8N2O)2], crystallizes as a centrosymmetric binuclear macrocylic complex containing silver(I) ions bridged by the organic 1‐(3‐pyridyl)ethanone oxime ligand. The ligand coordinates via the pyridine and the oxime N atoms. A similar metal–ligand arrangement was found in the copper(I) complex catena‐poly[[bis[μ‐1‐(3‐pyridyl)ethanone oxime‐κ2N:N′]dicopper(I)]‐di‐μ‐iodo], [Cu2I2(C7H8N2O)2]n, but here the centrosymmetric macrocycles are connected by double anion bridges, resulting in the formation of a one‐dimensional coordination polymer. 相似文献
997.
Jolliet P Gianini M von Zelewsky A Bernardinelli G Stoeckli-Evans H 《Inorganic chemistry》1996,35(17):4883-4888
The palladium(II) and platinum(II) bis-homoleptic complexes M(C&arcraise;N)(2), where C&arcraise;N is benzo[h]quinoline (H-bhq), 2-phenylpyridine (H-phpy), 2-(2'-benzothienyl)pyridine (H-bthpy), 2-(2'-thienyl)quinoline (H-thq), and 2-(2'-thienyl)pyridine (H-thpy), were prepared by metal exchange of the lithiated ligands C&arcraise;N with M(Et(2)S)(2)Cl(2). The palladium(II) bis-heteroleptic complexes, Pd(C&arcraise;N)(C'&arcraise;N'), were synthesized from Pd(C&arcraise;N)(2) bis-homoleptic complexes, which were converted by HCl into the dichloro-bridged compounds [Pd(C&arcraise;N)Cl](2). By addition of Et(2)S, the Pd(C&arcraise;N)(Et(2)S)Cl complexes were formed, which were allowed to react with Li(C'&arcraise;N'), yielding M(C&arcraise;N)(C'&arcraise;N') compounds. An alternative way for obtaining the bis-heteroleptic molecules is by ligand exchange, according to the equilibrium M(C&arcraise;N)(2) + M(C'&arcraise;N')(2) = 2M(C&arcraise;N)(C'&arcraise;N'). The crystal structures of Pt(bhq)(2) (1) and Pt(thq)(2) (3) present an important distortion of the square planar (SP-4) geometry toward a two-bladed helix. Bis-homoleptic and some bis-heteroleptic complexes of palladium(II) have also been synthesized. In both cases, the steric interactions between the two ligands cause again a helical distortion rather than yielding trans compounds. For cis-bis(benzo[h]quinoline)platinum(II) (1), in the crystal (monoclinic, space group P2(1)/n, a = 13.728(3) ?, b = 6.9537(15) ?, c = 19.701(5) ?, beta = 106.17(2) degrees, Z = 4, rho(calcd) = 2.028 g.cm(-)(3); diffractometer measurements, block-matrix least-squares refinement, R = 0.043, R(w) = 0.047) the average Pt-N and Pt-C distances are 2.151(6) and 1.988(7) ?, respectively. One benzo[h]quinoline ligand is significantly less planar than the other. For cis-bis[2-(2'-thienyl)quinoline]platinum(II) (3), in the crystal (trigonal, space group P3(2)21, a = b = 9.373(1) ?, c = 20.152(3) ?, Z = 3, rho(calcd) = 2.022 g.cm(-)(3); diffractometer measurements, full-matrix least-squares refinement, R = 0.010, R(w) = 0.010) the molecule has C(2) symmetry and is chiral. The Pt-N and Pt-C bond lengths are 2.156(2) and 1.984(3) ?, respectively. The quinoline moitey is not planar but bent about the fused bond by 6.8 degrees. The thiophene moiety is inclined to the best plane through the quinoline moiety by 24.4 degrees. 相似文献
998.
Laurette Schmitt Gaël Labat Helen Stoeckli‐Evans 《Acta Crystallographica. Section C, Structural Chemistry》2010,66(11):m343-m347
The reaction of 2,2′:6′,2′′‐terpyridine (terpy) with CuCl2 in the presence of sodium sulfite led to the synthesis of the ionic complex aquachlorido(2,2′:6′,2′′‐terpyridyl‐κ3N,N′,N′′)copper(II) chlorido(dithionato‐κO)(2,2′:6′,2′′‐terpyridyl‐κ3N,N′,N′′)cuprate(II) dihydrate, [CuCl(C15H11N3)(H2O)][CuCl(S2O6)(C15H11N3)]·2H2O, (I), and the in situ synthesis of the S2O62− dianion. Compound (I) is composed of a [CuCl(terpy)(H2O)]+ cation, a [Cu(S2O6)(terpy)]− anion and two solvent water molecules. Thermogravimetric analysis indicated the loss of two water molecules at ca 363 K, and at 433 K the weight loss indicated a total loss of 2.5 water molecules. The crystal structure analysis of the resulting pale‐green dried crystals, μ‐dithionato‐κ2O:O′‐bis[chlorido(2,2′:6′,2′′‐terpyridyl‐κ3N,N′,N′′)copper(II)] monohydrate, [Cu2Cl2(S2O6)(C15H11N3)2]·H2O, (II), revealed a net loss of 1.5 water molecules and the formation of a binuclear complex with two [CuCl(terpy)]+ cations bridged by a dithionate dianion. The crystal‐to‐crystal transformation involved an effective reduction in the unit‐cell volume of ca 7.6%. In (I), the ions are linked by O—H...O hydrogen bonds involving the coordinated and solvent water molecules and O atoms of the dithionate unit, to form ribbon‐like polymer chains propagating in [100]. These chains are linked by Cu...Cl interactions [3.2626 (7) Å in the cation and 3.3492 (7) Å in the anion] centred about inversion centres, to form two‐dimensional networks lying in and parallel to (01). In (II), symmetry‐related molecules are linked by O—H...O hydrogen bonds involving the partially occupied disordered water molecule and an O atom of the bridging thiosulfite anion, to form ribbon‐like polymer chains propagating in [100]. These chains are also linked by Cu...Cl interactions [3.3765 (12) Å] centred about inversion centres to form similar two‐dimensional networks to (I) lying in and parallel to (02), crosslinked into three dimensions by C—H...O=S and C—H...O(water) interactions. 相似文献
999.
A series of short block length methoxy poly(ethylene glycol)-block-poly(caprolactone) diblock copolymers was synthesized and characterized in order to assess the potential of these copolymers as a micellar drug-delivery system. Varying the caprolactone:MePEG weight ratio in the reaction mixture allowed the synthesis of diblock copolymers with a MePEG molecular weight of 750 g/mol and PCL block lengths of 2, 5 or 10 repeat units. Phase diagrams of aqueous solutions of the copolymers were constructed which displayed characteristic cloud points and Krafft points. As the degree of polymerization of PCL increased, critical micelle concentration (CMC) values decreased from 6.97 x 10(-1) to 3.38 x 10(-3) g/l, partition equilibrium coefficients (Kv) increased from 1.09 x 10(4) to 22.2 x 10(4),and hydrodynamic diameters increased from 12.2 to 19.5 nm. The micelle morphology was determined to be spherical by transmission electron microscopy. 相似文献
1000.
Helen E. Mason Judith A. K. Howard Hazel A. Sparkes 《Acta Crystallographica. Section C, Structural Chemistry》2020,76(9):927-931
The synthesis and structures of three isoxazole‐containing Schiff bases are reported, namely, (E)‐2‐{[(isoxazol‐3‐yl)imino]methyl}phenol, C10H8N2O2, (E)‐2‐{[(5‐methylisoxazol‐3‐yl)imino]methyl}phenol, C11H10N2O2, and (E)‐2,4‐di‐tert‐butyl‐6‐{[(isoxazol‐3‐yl)imino]methyl}phenol, C18H24N2O2. All three structures contain an intramolecular O—H…N hydrogen bond, alongside weaker intermolecular C—H…N and C—H…O contacts. The C—O(H) and imine C=N bond lengths were consistent with structures existing in the enol rather than the keto form. Despite having dihedral angles <25°, none of the compounds were observed to be strongly thermochromic, unlike their anil counterparts; however, all three compounds showed a visible colour change upon irradiation with UV light. 相似文献