全文获取类型
收费全文 | 1233390篇 |
免费 | 29793篇 |
国内免费 | 9191篇 |
专业分类
化学 | 599773篇 |
晶体学 | 13584篇 |
力学 | 73240篇 |
综合类 | 171篇 |
数学 | 235696篇 |
物理学 | 349910篇 |
出版年
2021年 | 12582篇 |
2020年 | 15853篇 |
2019年 | 15977篇 |
2016年 | 26880篇 |
2015年 | 20463篇 |
2014年 | 30225篇 |
2013年 | 74017篇 |
2012年 | 33892篇 |
2011年 | 28664篇 |
2010年 | 35083篇 |
2009年 | 38023篇 |
2008年 | 28107篇 |
2007年 | 22858篇 |
2006年 | 31450篇 |
2005年 | 22940篇 |
2004年 | 26240篇 |
2003年 | 24461篇 |
2002年 | 22798篇 |
2001年 | 22502篇 |
2000年 | 19817篇 |
1999年 | 18916篇 |
1998年 | 18349篇 |
1997年 | 18997篇 |
1996年 | 19016篇 |
1995年 | 18449篇 |
1994年 | 17951篇 |
1993年 | 17669篇 |
1992年 | 17222篇 |
1991年 | 17430篇 |
1990年 | 16711篇 |
1989年 | 16782篇 |
1988年 | 16163篇 |
1987年 | 16185篇 |
1986年 | 15053篇 |
1985年 | 21635篇 |
1984年 | 23091篇 |
1983年 | 19279篇 |
1982年 | 21080篇 |
1981年 | 20316篇 |
1980年 | 19619篇 |
1979年 | 19726篇 |
1978年 | 21177篇 |
1977年 | 20772篇 |
1976年 | 20458篇 |
1975年 | 19122篇 |
1974年 | 18741篇 |
1973年 | 19284篇 |
1972年 | 13815篇 |
1968年 | 11857篇 |
1967年 | 12251篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
991.
Seema Yadav Sriparna Ray Ajeet Singh Shaikh M. Mobin Tapta Kanchan Roy Chandrakanta Dash 《应用有机金属化学》2020,34(11):e5942
Dinuclear gold(I)-N-heterocyclic carbene complexes were developed for the hydrohydrazidation of terminal alkynes. The gold(I)-N-heterocyclic carbene complexes 2a-2b were synthesized in good yields from silver complexes synthesized in situ, which in turn were obtained from the corresponding imidazolium salts with Ag2O in dichloromethane as a solvent. The new air-stable gold(I)-NHC complexes, 2a - 2b, were characterized using NMR spectroscopy, elemental analysis, infrared, and mass spectroscopy studies. The gold(I) complex 2a was characterized using X-ray crystallography. Bis-N-heterocyclic carbene–based gold(I) complexes 2a - 2b exhibited excellent catalytic activities for hydrohydrazidation of terminal alkynes yielding acylhydrazone derivatives. The working catalytic system can be used in gram-scale synthesis. In addition, the catalytic reaction mechanism of the hydrohydrazidation of terminal alkynes by gold(I)-NHC complex was studied in detail using density functional theory. 相似文献
992.
Ali El-Dissouky Eslam S. Abu-Elsoud Afaf Abdel Razik Mohamed K. Awad Hammed H.A.M. Hassan Amel F. Elhusseiny 《应用有机金属化学》2020,34(11):e5953
A series of nanometer-sized spherical sulfonamide imine ligands HL 1 -HL 5 and their copper and zinc complexes were synthesized and fully characterized based on elemental analyses, spectroscopic (UV/vis, FT-IR, NMR, EPR, SEM) studies, molar conductance and thermal analyses. Furthermore, computational studies of HL 1 -HL 5 were carried out by the DFT/B3LYP method. TD-DFT, HOMO and LUMO energy values, chemical hardness, electronegativity, electrophilic index, softness, and other parameters were calculated. Screening against several pathogenic microorganisms indicated that HL 1 exhibited high activity against the tested Gram-negative bacteria relative to other analogues and the inhibition activity is greater than the standard Gentamicin. Analogously, HL 2 exhibited high potent activity against the tested Gram-positive bacteria. Copper complexes exhibited a higher potent activity than zinc analogues. Noteworthy, inhibition activity of [Cu ( L 3 )(OAc)] complex is higher than that of the standard Ampicillin. [Cu ( L 2 )(OAc)] complex displayed a similar activity of the standard bactericides and fungicides in use. The complexes showed appreciated values of MIC against bacterial strains: B. subtilis (MIC = 0.4 μg / mL), E. coli and S. pneumonia (MIC = 1.95 μg / mL) and P. aeruginosa (MIC = 7.81 μg / mL). in vitro cytotoxic activities study proved that [Cu ( L 3 )(OAc)] complex exhibited appreciable activity versus (HEPG-2); IC50 = 4.8 μg/ml, while [Cu( L 2 )(OAc)] complex showed a high activity against (MCF-7); IC50 = 6.2 μg/ml. These results could be considered as new findings of promising antitumor candidates for experimental chemotherapy. 相似文献
993.
Nonlinear Dynamics - Delay-coordinate maps have been widely used recently to study nonlinear dynamical systems, where there is only access to the time series of one of their variables. Here, we... 相似文献
994.
Vinay S. Sharma Anuj S. Sharma Upendra H. Jadeja Deepak Suthar 《Molecular Crystals and Liquid Crystals》2018,665(1):64-81
In this study, the synthesis, structural characterization and mesomorphic properties of newly thirteen calamitic shaped compounds derived from allylidene amino chalcone and 4-n-alkoxy benzoyloxy benzoic acid. Comp.H1 to H3 exhibited nonliquid crystalline nature, while comp.H4 to comp.H18 displayed enantiotropical smectic C phase. Phase transition temperatures of present synthesised compounds were determined by optical polarising microscopy (POM), differential scanning calorimetric (DSC) and powder X-ray diffraction (PXRD) techniques. Thermal stabilities of smectic to isotropic phase are 150.0 °C and temperature range of mesophase is in decreasing order from comp.H3 to comp.H18 respectively. The presences of SmC phase are the type of broken fan and needle type in present synthesized series. It is also shown that presence of chalcone amino allylidene central linking group favors a calamitic liquid crystalline behaviour in molecules with lower member to higher member aliphatic side chain in alkoxy group (-OR). 相似文献
995.
996.
Haixing Guan Qianwen Zhang Prof. Dr. Patrick J. Walsh Prof. Dr. Jianyou Mao 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(13):5210-5215
A unique nickel/organic photoredox co-catalyzed asymmetric reductive cross-coupling between α-chloro esters and aryl iodides is developed. This cross-electrophile coupling reaction employs an organic reductant (Hantzsch ester), whereas most reductive cross-coupling reactions use stoichiometric metals. A diverse array of valuable α-aryl esters is formed under these conditions with high enantioselectivities (up to 94 %) and good yields (up to 88 %). α-Aryl esters represent an important family of nonsteroidal anti-inflammatory drugs. This novel synergistic strategy expands the scope of Ni-catalyzed reductive asymmetric cross-coupling reactions. 相似文献
997.
Bowen Yang Feng Cai Shuai Huang Prof. Dr. Haifeng Yu 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(10):4064-4071
Imprinting nanopatterns on flexible substrates has diverse applications in advanced fabrication. However, the traditional thermal nanoimprint lithography (T-NIL) often causes shrinkage upon cooling. Here, a simple yet versatile method is introduced to fabricate multiple nanopatterns on a flexible substrate coated with an azopolymer by combining athermal nanoimprint lithography (AT-NIL) and photolithography. The azopolymer has various mechanical properties upon photoirradiation: 1) phototunable glass-transition temperatures (Tg) and concomitantly photoinduced switch from glassy plastic to viscoplastic polymer; 2) prominent modulation of viscoplasticity under light illumination at different wavelengths. Regionally selective multiple nanopatterns are conveniently fabricated, presenting angle-dependent structural color images on poly(ethylene terephthalate) (PET) substrates. The flexible, athermal and multiple nanopatterning method has the potential for on-demand fabrication of complex nanopatterns. 相似文献
998.
Bin Rao Prof. Dr. Rei Kinjo 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(8):3171-3174
Boragermene 3 featuring a double bond between the Ge and dicoordinate B atoms has been synthesized for the first time by reacting the cyclic (alkyl)(boryl)germylene–PMe3 adduct 1 with Cl2BN(SiMe3)2 followed by reductive dehalogenation with KC8. Addition of a Lewis base (MeNHC) to 3 leads to the formation of the corresponding adduct 4 , which shows double bond character between the Ge and tricoordinate B atoms. Compound 3 undergoes hydrogenation with H2 concomitant with a complete scission of the Ge=B bond. 相似文献
999.
Crystallography Reports - The chemical composition of Bryansk hoard enamels (late II?III century) has been investigated by scanning electron microscopy and electron probe microanalysis. Raw... 相似文献
1000.
Zhigalina V. G. Kumskov A. S. Vasiliev A. L. Falaleev N. S. Verbitskiy I. I. Presniakov M. Yu. 《Crystallography Reports》2018,63(5):815-818
The structures of one-dimensional (1D) RbI, AgI, and RbAg4I5 crystals inside single-walled carbon nanotube (SWCNT) channels of 1D crystal@SWCNT nanocomposites formed by the capillary technique have been studied by high-resolution (scanning) transmission electron microscopy and computer modeling. 1D RbI crystals form a cubic lattice in a limited space, while 1D AgI crystals form a hexagonal lattice, as in their ternary compounds. The 1D RbAg4I5 structure differs from known bulk analogs and can be described by a distorted cubic lattice formed in two different directions.
相似文献