全文获取类型
收费全文 | 35743篇 |
免费 | 1287篇 |
国内免费 | 269篇 |
专业分类
化学 | 25243篇 |
晶体学 | 299篇 |
力学 | 700篇 |
综合类 | 1篇 |
数学 | 6065篇 |
物理学 | 4991篇 |
出版年
2023年 | 173篇 |
2022年 | 335篇 |
2021年 | 436篇 |
2020年 | 621篇 |
2019年 | 607篇 |
2018年 | 424篇 |
2017年 | 395篇 |
2016年 | 956篇 |
2015年 | 847篇 |
2014年 | 1021篇 |
2013年 | 1734篇 |
2012年 | 2217篇 |
2011年 | 2491篇 |
2010年 | 1346篇 |
2009年 | 1199篇 |
2008年 | 2292篇 |
2007年 | 2127篇 |
2006年 | 2144篇 |
2005年 | 1997篇 |
2004年 | 1717篇 |
2003年 | 1412篇 |
2002年 | 1348篇 |
2001年 | 504篇 |
2000年 | 494篇 |
1999年 | 429篇 |
1998年 | 410篇 |
1997年 | 434篇 |
1996年 | 467篇 |
1995年 | 325篇 |
1994年 | 381篇 |
1993年 | 347篇 |
1992年 | 315篇 |
1991年 | 293篇 |
1990年 | 258篇 |
1989年 | 223篇 |
1988年 | 221篇 |
1987年 | 227篇 |
1986年 | 192篇 |
1985年 | 386篇 |
1984年 | 341篇 |
1983年 | 290篇 |
1982年 | 348篇 |
1981年 | 286篇 |
1980年 | 293篇 |
1979年 | 249篇 |
1978年 | 240篇 |
1977年 | 225篇 |
1976年 | 216篇 |
1975年 | 195篇 |
1974年 | 169篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A series of experiments in the UO(2)(CH(3)CO(2))(2).2H(2)O/H(2)SO(4)/1-(2-aminoethyl)piperazine/H(2)O system were conducted to determine the effects of variation in initial reactant concentrations on the reaction products. Several reaction gels were produced, in which the composition varied from 16:80:4:500 UO(2)(CH(3)CO(2))(2).2H(2)O/H(2)SO(4)/1-(2-aminoethyl)piperazine/H(2)O to 4:92:4:500 UO(2)(CH(3)CO(2))(2).2H(2)O/H(2)SO(4)/1-(2-aminoethyl)piperazine/H(2)O. Single crystals of two new organically templated uranium sulfates, [N(3)C(6)H(18)](2)[(UO(2))(5)(H(2)O)(SO(4))(8)].5H(2)O and [N(3)C(6)H(18)][(UO(2))(2)(H(2)O)(SO(4))(3)(HSO(4))].4.5H(2)O, were isolated. Both compounds exhibit structures in which the inorganic frameworks are two-dimensional and the protonated amines reside between layers, participating in extensive hydrogen bonding. The composition and structure of each compound is dependent on the nature of the starting concentrations. Crystal data: for [N(3)C(6)H(18)](2)[(UO(2))(5)(H(2)O)(SO(4))(8)].5H(2)O, monoclinic, space group P2(1)/n (No. 14), a = 21.5597(3) A, b = 10.2901(2) A, c = 22.8403(3) A, beta = 96.7436(7) degrees, and Z = 4; for [N(3)C(6)H(18)][(UO(2))(2)(H(2)O)(SO(4))(3)(HSO(4))].4.5H(2)O, monoclinic, space group P2(1)/a (No. 14), a = 15.7673(4) A, b = 10.5813(3) A, c = 16.7710(5) A, beta = 99.9216(9) degrees, and Z = 4. 相似文献
992.
Michael JP 《Natural product reports》2002,19(6):742-760
This review covers the isolation, structure determination, synthesis and biological activity of quinoline, quinazoline and acridone alkaloids from plant, microbial and animal sources. The literature from July 2000 to June 2001 is reviewed, and 119 references are cited. 相似文献
993.
Fluoro- und aminofluoro-silanes react with the lithium salt of N,O-bis(trimethylsilyl)hydroxylamine under LiF elimination and substitution. Alkyl- and amino-fluorosilanes give O-fluorosilyl-N,N-bis(trimethylsilyl)hydroxylamines, arylfluorosilanes give N-fluorosilyl-N,O-bis(trimethylsilyl)hydroxylamines. By the further reaction of O-difluorosilyl-N,N-bis(trimethylsilyl)hydroxylamine with the lithiated hydroxylamine, O,O′-fluoromethylsilyldi[N,N-bis(trimethylsilyl)hydroxylamine] is formed. On heating N-difluorophenylsilyl-N,O-bis(trimethylsilyl)hydroxylamine di[fluorophenylsilyl(methyl)amino]pentamethylsiloxane is formed by methyl group migration. The NMR and mass spectra of the compounds are reported. 相似文献
994.
Leslie C. Damude Philip A.W. Dean Michael D. Sefcik Jacob Schaefer 《Journal of organometallic chemistry》1982,226(2):105-114
The cross-polarization magic angle spinning 13C NMR spectra of Hg(SbF6)2 - 2 Arene (Arene = C6HMe5, 1,2,4,5-C6H2Me4, 1,2,3,4-C6H2Me4, or C6H6) have been measured. The spectra of the complexes of C6HMe5 and 1,2,4,5-C6H2Me4 are consistent with static η1-bonding of the mercury to the arene at an unsubstituted carbon atom, while the spectra of the 1,2,3,4-C6H2Me4 and C6H6 complexes show the arene to have time-averaged Cs or C2, and C6 symmetry respectively, at the temperature of measurement (300 K).The reduced temperature 13C NMR spectra of Hg(Arene)n2+ (n = 1 or 2; Arene = 1,3,5-C6H3R3 (R = Me, i-Pr, or t-Bu)) in SO2 solution are also reported and affirm that in these intramolecularly mobile species the mercury bonds in an η1-manner, with unsubstituted aryl carbon atoms being the strongly preferred point of mercury attachment. This site preference is further demonstrated by the solution 13C NMR spectra of Hg(Arene)n2+ (Arene = 1,2,3,4-C6H2-Me4, n = 1 or 2; Arene = 1,4-C6H4R2, R = Me or t-Bu, n = 1). The spectra of the 1,4-C6H4R2 complexes and Hg(p-C6H4-t-BuMe)2+ provide clear evidence for steric influence of the binding site.Like Hg(C6Me6)22+, but unlike most of the complexes of substituted benzenes which have been studied, Hg(1,3,5-C6H3-i-Pr3)22+ exchanges only slowly with excess free ligand. 相似文献
995.
Zero-field Mössbauer spectra of powder samples of Fe(2,9-di-CH3-phenanthroline) SO4 over the range 1.7 to 300°K show a large (~ 3.6 mm/sec) temperature-independent quadrupole splitting corresponding to an orbital singlet ground term. The chemical isomer shift, δFE=O, is 1.16 mm/sec (source and absorber at 4.2°K) corresponding to six coordinate high-spin iron (II). Below 4.2°K, the compound exhibits magnetic hyperfine splitting suggesting slow relaxation and the possibility of long-range three-dimensional magnetic ordering with a critical temperature Tc such that 3.5°K < Tc < 4.2°K and an internal hyperfine field Hn = 325 kG at 1.69°K. High-field Mössbauer spectra at 300°K indicate that the principal component of the electric field gradient tensor is positive and axial. Similar spectra at 4.2°K show an absence of nuclear Zeeman splitting for applied fields up to 60 kG, and indicate that at 4.2°K the material is rapidly relaxing but with substantial magnetic polarization and a negative internal hyperfine field. The temperature dependence of the magnetic susceptibility confirms antiferromagnetic interactions with a broad maximum χ′M at ~ 11.8°K presumably due to low-dimensionality exchange interactions (possibly one) along MOSOM chains. Least-squares fits of χ′M?1 versus T for T50°K indicate (Curie-Weiss behavior with C = 3.26 emu/mole, θ = -21.95°K, and μeff = 5.11. 相似文献
996.
Michael Chariss Andrea Zickgraf Heike Stenger Elmar Bru Cristelle Desmarquet Martin Drger Silke Gerstmann Dainis Dakternieks James Hook 《Polyhedron》1998,17(25-26)
44 members of thecompound series Ph4−nMRn (M=Si, Ge, Sn, Pb; R=o-, m-, p-Tol; n=0–4) were synthesized (15 newcompounds). The crystal structures of Ph3Sn (o-Tol) and PhSn (o-Tol)3 were determined and compared to 16 known structures. Subject to the distanced (M–C), an interplay between through-space π–π repulsion and π–σ attraction leads to either elongated or compressed tetrahedral geometry. 29 Si-, 119 Sn- and 207 Pb-NMR chemical shifts were determined in solution and in the solid state. 73 Ge chemical shifts were measured only in solution. Anupfield or downfield sagging of the chemical shifts along each series is rationalized in terms of a π–σcharge transfer which is constrained by torsion of the aromatic groups. 相似文献
997.
Effect of Dosimetric and Physiological Factors on the Lethal Photosensitization of Porphyromonas gingivalis in vitro 总被引:4,自引:0,他引:4
Manpreet Bhatti Alexander MacRobert Sajeda Meghji Brian Henderson Michael Wilson 《Photochemistry and photobiology》1997,65(6):1026-1031
The aims of this study were to (1) determine the effect of dosimetric and physiological factors on the lethal photosensitization of Porphyromonas gingivalis using tolui-dine blue O (TBO) and light from a helium/neon (HeNe) laser; (2) determine the influence of sensitizer concentration, preirradiation time, serum and growth phase on sensitizer uptake by P. gingivalis. The dosimetric factors studied were concentration of TBO, light dose and preirradiation time. The physiological factors were presence of serum, pH and bacterial growth phase. Sensitizer uptake by P. gingivalis under various conditions was determined using tritiated TBO (3H-TBO). In the presence of TBO, a light dose-dependent increase in kill was attained (100% kill at 4.4 J). There was no significant effect on the numbers killed when TBO was increased from 12.5 to 50 µg/mL. An increase in preirradiation time gave slightly increased kills. High kills were achieved at all three pH (6.8–8.0). Although kills were substantial in the presence of serum, they were significantly less than those obtained in the presence of saline. Cells in all three growth phases were susceptible to lethal photosensitization, although stationary phase cells were slightly less susceptible. Maximum uptake of TBO occurred within 60 s and uptake in serum was less than in saline. The uptake by the log phase cells was greater at lower concentrations of sensitizer (50 µg/mL), compared to the other two phases. 相似文献
998.
Theoretical and synthetic studies of the tricyclic 10pi-electron hydrocarbon cyclobuta[1,2:3,4]dicyclopentene (1), a nominally aromatic structure that has never been synthesized, are described. Geometry optimization by density-functional-theory calculations (B3LYP/6-31G(d,p)) predict that 1 is a D(2h) symmetric structure with nonalternant C-C single and double bonds. The calculations also predict that 1 is 4.7 kcal/mol higher in energy than the isomeric hydrocarbon 1,6-didehydro[10]annulene (2), a molecule known to isomerize to 1,5-didehydronaphthalene (4) above -50 degrees C. Calculated enthalpic changes of homodesmotic reactions support the notion that 1 is an aromatic molecule with a resonance stabilization energy (RSE) about half to two-thirds that of benzene on a per-molecule basis. Investigations of potential synthetic pathways to 1 initially utilized as starting material the tricyclic carbonate 11, the product of an intramolecular [2 + 2]-photocyclization reaction. In these studies, 11 was transformed in several steps to the distannane 12, which upon treatment with boron fluoride ethyl etherate is believed to have formed the unstable hydrocarbon bicyclopentadienylidene (13). In an effort to avoid cleavage of the central, four-membered ring of unsaturated tricyclo[5.3.0.0(2,6)]decane intermediates (perhaps the result of 10-electron electrocyclic ring opening of the tetraene 8), synthetic approaches to 1 employing cobalt-cyclobutadiene complexes 18 and 19 were pursued. Treatment of 18 with excess methyllithium led to the novel cobaltacyclic product 30, and dehydration of 19 in the presence of pyridine produced the ring-opening cobaltacyclic product 35. It is proposed that both processes may occur by a 10-electron electrocyclic ring-opening reaction of eta(2)-organocobalt intermediates. These processes may be related to the hypothetical transformation of tetraene 8 to bicyclopentadienylidene (13). 相似文献
999.
[reaction: see text] The tricyclic benzothiazine 15 was prepared in a straightforward fashion via a completely stereoselective intramolecular Friedel-Crafts alkylation. This compound represents a potential precursor to the antitubercular agent, pseudopteroxazole. Its synthesis proceeded via a completely selective, intramolecular addition of a sulfoximine-stabilized carbanion to an alpha,beta-unsaturated ester, followed by functional group manipulations. 相似文献
1000.
Wilson MH Ledwaba LP Field JS McMillin DR 《Dalton transactions (Cambridge, England : 2003)》2005,(16):2754-2759
As part of an effort to develop new lumaphors involving late transition metal ions, this report describes the synthesis and characterization of the first platinum(II) derivatives containing 2,2':6',2'-terpyridine (trpy) and cyanide as co-ligands. According to existing models, including cyanide in the coordination sphere should raise the energies and minimize the influence of short-lived d-d excited states that otherwise compromise the excited-state lifetime. Both [Pt(trpy)(CN)]+ and the 4'-cyano-2,2':6',2'-terpyridine analogue [Pt(CN-T)(CN)]+ are emissive in dichloromethane solution, but the signals are weak. Part of the problem is that the d-pi* charge-transfer excited states also rise in energy, so that the emission actually originates from a (3)pi-pi* state with a relatively low radiative rate constant. However, another member of the series, the 4'-dimethylamino-2,2':6',2'-terpyridine (dma-T) derivative [Pt(dma-T)(CN)]+, proves to be a very promising platform with an emission quantum yield of phi= 0.26 and an excited-state lifetime of tau = 22 micros in room-temperature, deoxygenated dichloromethane solution. In the dma-T complex the electron-rich dimethylamino substituent provides the basis for an emissive, but largely ligand-based, charge-transfer excited state. The orbital parentage is such that the photoluminescence persists in donating solvents like dimethylformamide, which ordinarily quenches d-pi* excited states in complexes of this type. 相似文献