全文获取类型
收费全文 | 1486篇 |
免费 | 76篇 |
国内免费 | 11篇 |
专业分类
化学 | 1063篇 |
晶体学 | 6篇 |
力学 | 49篇 |
数学 | 264篇 |
物理学 | 191篇 |
出版年
2023年 | 13篇 |
2022年 | 27篇 |
2021年 | 27篇 |
2020年 | 21篇 |
2019年 | 31篇 |
2018年 | 22篇 |
2017年 | 16篇 |
2016年 | 39篇 |
2015年 | 54篇 |
2014年 | 59篇 |
2013年 | 70篇 |
2012年 | 94篇 |
2011年 | 119篇 |
2010年 | 57篇 |
2009年 | 57篇 |
2008年 | 95篇 |
2007年 | 98篇 |
2006年 | 99篇 |
2005年 | 73篇 |
2004年 | 77篇 |
2003年 | 67篇 |
2002年 | 60篇 |
2001年 | 16篇 |
2000年 | 20篇 |
1999年 | 24篇 |
1998年 | 13篇 |
1997年 | 15篇 |
1996年 | 18篇 |
1995年 | 13篇 |
1994年 | 15篇 |
1993年 | 6篇 |
1992年 | 10篇 |
1991年 | 9篇 |
1990年 | 13篇 |
1989年 | 5篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 8篇 |
1984年 | 7篇 |
1983年 | 6篇 |
1982年 | 12篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1979年 | 9篇 |
1978年 | 9篇 |
1977年 | 5篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1974年 | 7篇 |
排序方式: 共有1573条查询结果,搜索用时 15 毫秒
31.
Two polyesters containing rigid biphenyl and hydroquinone bisbenzoate groups and presenting flexible thioether moieties with different lengths (14 elements in the flexible group) have been carried out using a reaction of the Michaël type. The properties of these polyesters have been compared to those of polyesters of the same type presenting shorter flexible groups (11 elements). All these polymers present thermotropic properties: the biphenyl ones are smectic and the bisbenzoate ones are nematic. The biphenyl polyesters present two types of dielectric relaxations: α and β. The bisbenzoate ones show three relaxations, α and two β (β and β′). The lengthening of the flexible group increases significantly the flexibility of the molecular chains. 相似文献
32.
This work describes the synthesis of certain nucleoside synthons which are necessary in order to obtain oligoxylonucleotides. The utilization of a non-symmetrically substituted xyloside substituted at the 2-position permits one to obtain, in satisfactory yields, the described nucleoside which is conveniently protected. All of the compounds described have been fully characterized using the usual analytical methods. 相似文献
33.
Tomás Moya-Pizarro Roger Salmon Léopold Fournes Gilles Le Flem Barbara Wanklyn Paul Hagenmuller 《Journal of solid state chemistry》1984,53(3):387-397
Single crystals of the high-temperature form of NaFeP2O7 have been grown by a flux technique. II-NaFeP2O7 crystallizes in the monoclinic space group with lattice parameters: a = 7.298(2) Å, b = 7.874(2) Å, c = 9.536(3) Å, β = 111.85(2)°. The structure refined from 1481 independent reflections leads to R = 0.044 and confirms the work previously published by M. Gabelica-Robert, M. Goreaud, P. Labbe, and B. Raveau (J. Solid State Chem.45, 389, 1982). Magnetic Mössbauer resonance studies have shown the existence of antiferromagnetic ordering with a weak ferromagnetic component below 30 K. The FeO bond is markedly ionic in character due to the highly polarizing power of phosphorus in tetrahedral site. 相似文献
34.
Messaoud T Duplâtre G Waton G Michels B 《Physical chemistry chemical physics : PCCP》2005,7(22):3839-3844
The new method based on positron annihilation lifetime spectroscopy (PALS) to determine both the mean core radius, R(core), and aggregation number, N(ag), of micelles is applied to the study of aqueous solutions of the triblock Pluronic P84 copolymer as a function of temperature (T), beyond the gelification point (334 K). Two long-lived components appear in the PALS spectra, ascribed to triplet positronium in the water bulk (o-Ps(aq)) and in the organic core of the micelles (o-Ps(org)). Of the various fitting parameters, only the lifetime of the latter species, tau4, and the micellar parameters, R(core) and N(ag), disclose the occurrence of gelification by first increasing up to 334 K, then decreasing. By contrast to what is known in case of phase transition, none of the parameters shows any abrupt change at 334 K, whereas the macroscopic viscosity of the solutions suffers a drastic increase. This is attributed to the fact that positronium is sensitive to the microviscosity of the solutions. At the transition point, the properties of the polyoxipropylene aggregates forming the organic core of the P84 micelles are not greatly affected. Furthermore, the fact that the experimental N(ag) values coincide with those calculated for spheres, from the R(core) values, indicates that the shape of the P84 cores does not change significantly after gelification. The onset of gelification results from a decrease in the hydrogen bonding interactions in the solution with an ensuing relative increase in the interactions between the polyoxipropylene (PPO) groups, initially forming the corona of the P84 micelles, in an intermicellar mode. This increased solicitation of the PPO groups outside their initial location would result in depletion in the number of surfactant molecules forming the micelles, viz. a decrease in both R(core) and N(ag) above 334 K. From the data, additional information can be gained regarding the local viscosity and surface tension in the micellar cores. 相似文献
35.
Argouarch G Stones G Gibson CL Kennedy AR Sherrington DC 《Organic & biomolecular chemistry》2003,1(24):4408-4417
Three chiral 2,6-disubstituted tri-N-methyl azamacrocycles have been prepared by modular methods. These macrocycles were accessed from three chiral 1,4,7-triazaheptanes intermediates that were prepared by two independent routes. The first of these routes involved the benzylamine opening of chiral tosyl aziridines followed by debenzylation but was problematic on solubility grounds. A second, more effective, route was developed which avoided debenzylation by using ammonia in the nucleophilic opening of chiral tosyl aziridines. 相似文献
36.
Linear trigermoxanes, R1 RGeOGe(R3)2OGeRR1, a new series of organogermanium compounds, were synthesized by reaction of a lithium organogermanolate with a suitable organogermanium dihalide. With alkyl or phenyl substituents, these trigermoxanes are structurally unstable viscous oils, due to redistribution reactions. When R3 substituents are bulky groups, such as mesityl, trigermoxanes are thermally and structurally stable oils; depending on the R1 and R2 substituents their viscosities lie in the range 240 to more than 1500 cPo (mPa s) at 20°C. When both terminal germanium atoms are substituted with two mesityl groups, trigermoxanes are stable glassy solids. 相似文献
37.
Hideki Moriishi Osamu Kikuchi Keizo Suzuki Gilles Klopman 《Theoretical chemistry accounts》1984,64(5):319-338
Reaction potential maps (RPM) have been introduced as a new tool for the study of molecular reactivity. The equipotential energy maps, which are created on given planes around a molecule, define reaction contours towards specific counter-reagent models and are evaluated by perturbation theory. Since the calculated interaction energy involves electrostatic, polarization, exchange, and charge transfer energies, the RPM's can be used to predict site selectivity in a variety of chemical reactions. We found that the calculated RPM's of the SCN– anion explained well the experimental observations that it reacts at the S atom with soft electrophiles and at the N atom with hard electrophiles. The difference in reactivity between SCN– and OCN– was clearly shown by the RPM's of these anions. The ambident nucleophilic nature of the NO
2
–
and the CH2CHO– anions was also well represented by their RPM's. 相似文献
38.
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
39.
Yockot D Moreau V Demailly G Djedaïni-Pilard F 《Organic & biomolecular chemistry》2003,1(10):1810-1818
The synthesis of branched beta-cyclodextrins substituted with mannosyl mimetic derivatives at one primary hydroxy group is described. It was shown that the self-inclusion phenomenon observed for the target compounds in water did not preclude the inclusion properties of the cyclodextrin moiety. 相似文献
40.
Monique Nanot Francine Queyroux Jean-Claude Gilles 《Journal of solid state chemistry》1979,28(2):137-147
The homologous phases corresponding to the formula AnBnO3n+2 observed in the systems: La2Ti2O7CaTiO3, Nd2Ti2O7CaTiO3 and Ca2Nb2O7CaTiO3, have a structure derived from the structure of perovskite by periodic crystallographic shears, which delimitate n octahedra thick sheets. In the [0 1 0]1 direction, the sheets are following each other with two types of arrangement.Crystals of terms n = 4,5, 5 and 6 were investigated by X-ray diffraction. According to the type of arrangement of the sheets, the phases studied have either an orthorhombic symmetry or a monoclinic symmetry, this last one giving rise to merohedral twins. 相似文献