全文获取类型
收费全文 | 19124篇 |
免费 | 743篇 |
国内免费 | 99篇 |
专业分类
化学 | 13099篇 |
晶体学 | 120篇 |
力学 | 420篇 |
数学 | 2912篇 |
物理学 | 3415篇 |
出版年
2023年 | 123篇 |
2022年 | 172篇 |
2021年 | 269篇 |
2020年 | 378篇 |
2019年 | 397篇 |
2018年 | 235篇 |
2017年 | 236篇 |
2016年 | 601篇 |
2015年 | 556篇 |
2014年 | 644篇 |
2013年 | 1026篇 |
2012年 | 1116篇 |
2011年 | 1248篇 |
2010年 | 746篇 |
2009年 | 659篇 |
2008年 | 1036篇 |
2007年 | 1021篇 |
2006年 | 950篇 |
2005年 | 882篇 |
2004年 | 773篇 |
2003年 | 638篇 |
2002年 | 606篇 |
2001年 | 312篇 |
2000年 | 291篇 |
1999年 | 274篇 |
1998年 | 243篇 |
1997年 | 233篇 |
1996年 | 267篇 |
1995年 | 247篇 |
1994年 | 217篇 |
1993年 | 214篇 |
1992年 | 200篇 |
1991年 | 175篇 |
1990年 | 176篇 |
1989年 | 174篇 |
1988年 | 149篇 |
1987年 | 126篇 |
1986年 | 114篇 |
1985年 | 168篇 |
1984年 | 161篇 |
1983年 | 127篇 |
1982年 | 156篇 |
1981年 | 157篇 |
1980年 | 133篇 |
1979年 | 140篇 |
1978年 | 159篇 |
1977年 | 115篇 |
1976年 | 101篇 |
1975年 | 118篇 |
1974年 | 90篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
Wing W. Lam Gary E. Martin Vincent M. Lynch Stanley H. Simonsen Charles M. Lindsay Keith Smith 《Journal of heterocyclic chemistry》1986,23(3):785-791
The synthesis of 2-azathianthrene ([1,4]benzodithiino[2,3-c]pyridine), the only remaining monoazathianthrene yet to be reported, is described. Attempts at the direct condensation of disubstituted pyridines with the dianion of 1,2-dimercaptobenzene were generally unsuccessful requiring that the alternative condensation of the dianion with disubstituted pyridine 1-oxides be employed. The title compound was characterized by physical means including 13C-nmr spectroscopy. One analog, 4-nitro-2-azathianthrene was also studied by X-ray crystallographic means; the molecule crystallized with two molecules in the asymmetric unit P21/n, a = 20.712(3), b = 7.8109(13), c = 13.720(2)Å, β = 107.880(11)°, Z = 8, the data refined to a final R = 0.051 for 3061 reflections. Dihedral angles between the planes of the phenyl rings were 135.00(13) and 132.52(13)° for the two independent molecules contained in the crystal. Close non? bonded S ?O intramolecular contacts were observed in both molecules between the sulfur and nitro-group oxygens. Both nitro groups are twisted out of the plane of the pyridine ring and are oriented at angles of 28.75 and 38.82° respectively. 相似文献
994.
Michael J. Quast Gary E. Martin Vincent M. Lynch Stanley H. Simonsen John G. Stuart Marvin L. Tedjamulia Raymond N. Castle Milton L. Lee 《Journal of heterocyclic chemistry》1986,23(4):1115-1118
The crystal structure of 9-methylphenanthro[4,3-a]dibenzothiophene, C25H16S, Mr = 348.47, has been determined. Monoclinic, P21/c, a = 11.364(3), b, = 14.257(3), c = 11.575(2)Å, β = 116.26(2)°, V = 1681.9(7)Å3, Z = 4, Dx = 1.38 g/cc, MoKα radiation λ = .71069 Å, F(000) = 728, T = 163K, R = .0458 for 2330 reflections. The structure compares favorably with that of hexahelicene and methylated derivatives. The thiophene moiety increases the helical core radius and decreases the pitch with respect to hexahelicene and its derivatives. 相似文献
995.
John G. Stuart Michael J. Quast Gary E. Martin Vincent M. Lynch Stanley H. Simonsen Milton L. Lee Raymond N. Castle Jerry L. Dallas Boban K. John LeRoy F. Johnson 《Journal of heterocyclic chemistry》1986,23(4):1215-1234
Syntheses of benzo[3,4]phenanthro[1,2-b]thiophene, benzo[3,4]phenanthro[2,1-b]thiophene and their 1-methyl analogs are reported as potential constituents of solvent refined coal liquids and for mutagenicity testing. The attempted synthesis of the 13-methyl analogs which gave the 11-methyl isomers is also described. Total assignments of the 1H- and 13C-nmr spectra based on long range optimized heteronuclear protoncarbon two-dimensional chemical shift correlation are reported. Carbon assignments obtained for benzo[3,4]-phenanthro[1,2-b]thiophene using this approach were confirmed with a 125 MHz 13C–13C INADEQUATE spectrum. X-Ray crystal structures were determined for benzo[3,4]phenanthro[1,2-b]thiophene and 1-methyl-benzo[3,4]phenanthro[2,1-b]thiophene. Both molecules were helically distorted from planarity. Close intramolecular contacts between the bay region H1–H13 and ClMe-H13 of 2.03 and 2.28 Å, respectively, were responsible for the distortions. There were no close intermolecular contacts of <3.5Å. both molecules refined to an R value of <0.05. 相似文献
996.
Erich Metzger Roland Aeschimann Martin Egli Gaby Suter Ren Dohner Daniel Ammann Max Dobler Wilhelm Simon 《Helvetica chimica acta》1986,69(8):1821-1828
Lipophilic neutral carriers were synthesized which show Li+/Na+ selectivities of up to ca. 80 in highly lipophilic liquid membranes. The sensor membranes exhibit improved response times and increased lifetimes as compared to systems described earlier. They allow reliable measurements of Li+ in blood serum within the clinical concentration range. A 1:1 Li+/ionophore complex of one representative (N,N,N′,N′-tetracyclohexyl-5,5-dimethyl-3,7-dioxaazelaamide) has been prepared, and its structure was elucidated by X-ray analysis. 相似文献
997.
The structure of P,P-bis(diisopropylamino)-4-phenyl-1,3,2lambda(5)-diazaphosphete, 1a, has been determined by a single-crystal X-ray diffraction study (C(19)H(33)N(4)P, monoclinic system, space group P2(1), a = 9.482(1) ?, b = 11.374(3) ?, c = 9.668(2) ?, beta = 97.16(1) degrees, Z = 2). According to quantum chemical calculations at an RHF level of optimization utilizing the 6-31g(d,p) basis set, 1a has a zwitterionic structure with the negative charge delocalized on the NCN allylic fragment and the positive charge localized at the phosphorus. Heterocycle 1a reacts with water and benzaldehyde affording N-phosphoranylbenzamidine 3 (95% yield) and the expected aza-Wittig adduct 4 (85% yield), respectively. Addition of 1 equiv of methyl trifluoromethanesulfonate and of 2 equiv of BH(3).THF to 1a affords cyclic phosphonium salt 5 (94% yield) and the bis(borane) adduct 6a (90% yield), respectively. Dimethyl acetylenedicarboxylate slowly reacts with 1a giving rise to 1,3,4lambda(5)-diazaphosphinine, 9, in 70% yield. The X-ray crystal structures of products 2,3, and 6a are reported (2: C(26)H(38)N(5)P, monoclinic system, space group C2/c, a = 16.337(8) ?, b = 19.810(2) ?, c = 8.800(2) ?, beta = 117.68(2) degrees, Z = 4. 3: C(19)H(35)N(4)OP, orthorhombic system, space group P2(1)2(1)2(1), a = 9.090(1) ?, b = 12.955(2) ?, c = 17.860(3) ?, Z = 4. 6a: C(19)H(39)B(2)N(4)P, orthorhombic system, space group P2(1)2(1)2(1), a = 10.340(1) ?, b = 13.247(1) ?, c = 16.996(1) ?, Z = 4). 相似文献
998.
999.
Christian Bartels Michael Schaefer Martin Karplus 《Theoretical chemistry accounts》1999,101(1-3):62-66
Adaptive umbrella sampling of the potential energy is used as a search method to determine the structures and thermodynamics
of peptides in solution. It leads to uniform sampling of the potential energy, so as to combine sampling of low-energy conformations
that dominate the properties of the system at room temperature with sampling of high-energy conformations that are important
for transitions between different minima. A modification of the procedure for updating the umbrella potential is introduced
to increase the number of transitions between folded and unfolded conformations. The method does not depend on assumptions
about the geometry of the native state. Two peptides with 12 and 13 residues, respectively, are studied using the CHARMM polar-hydrogen
energy function and the analytical continuum solvent potential for treatment of solvation. In the original adaptive umbrella
sampling simulations of the two peptides, two and six transitions occur between folded and unfolded conformations, respectively,
over a simulation time of 10 ns. The modification increases the number of transitions to 6 and 12, respectively, in the same
simulation time. The precision of estimates of the average effective energy of the system as a function of temperature and
of the contributions to the average effective energy of folded conformations obtained with the adaptive methods is discussed.
Received: 11 July 1998 / Accepted: 22 September 1998 / Published online: 17 December 1998 相似文献
1000.