全文获取类型
收费全文 | 3103篇 |
免费 | 93篇 |
国内免费 | 6篇 |
专业分类
化学 | 2171篇 |
晶体学 | 24篇 |
力学 | 79篇 |
数学 | 457篇 |
物理学 | 471篇 |
出版年
2023年 | 33篇 |
2022年 | 65篇 |
2021年 | 81篇 |
2020年 | 81篇 |
2019年 | 72篇 |
2018年 | 47篇 |
2017年 | 35篇 |
2016年 | 106篇 |
2015年 | 91篇 |
2014年 | 95篇 |
2013年 | 199篇 |
2012年 | 211篇 |
2011年 | 254篇 |
2010年 | 126篇 |
2009年 | 115篇 |
2008年 | 152篇 |
2007年 | 156篇 |
2006年 | 162篇 |
2005年 | 127篇 |
2004年 | 83篇 |
2003年 | 98篇 |
2002年 | 85篇 |
2001年 | 29篇 |
2000年 | 25篇 |
1998年 | 21篇 |
1997年 | 28篇 |
1996年 | 28篇 |
1995年 | 24篇 |
1994年 | 27篇 |
1993年 | 15篇 |
1992年 | 20篇 |
1991年 | 22篇 |
1990年 | 24篇 |
1989年 | 19篇 |
1988年 | 16篇 |
1985年 | 14篇 |
1984年 | 19篇 |
1983年 | 20篇 |
1982年 | 30篇 |
1981年 | 30篇 |
1980年 | 26篇 |
1979年 | 23篇 |
1978年 | 18篇 |
1977年 | 19篇 |
1976年 | 17篇 |
1975年 | 17篇 |
1974年 | 14篇 |
1973年 | 19篇 |
1972年 | 17篇 |
1971年 | 13篇 |
排序方式: 共有3202条查询结果,搜索用时 15 毫秒
51.
Braulio Insuasty Jairo Quiroga Juan Carlos Argoti Samuel Gmez Roberto Martínez Enrique Angeles Rubn Gabio Manuel Nogueras Adolfo Snchez 《Journal of heterocyclic chemistry》1998,35(6):1397-1399
The reaction of 1-aryl-3-(dimethylamino)-1-propanones 1 with one equivalent of 4,5-diamino-1H-pyrimidin-6-ones 2 , in acidic medium, leads to the formation of 4-aryl-2,3,6,7-tetrahydro-1H-pyrimido[4,5-b]-[1,4]diazepin-6-ones 3 . The structure elucidation of the products is based on detail nmr analysis of experiments such as 13C, 1H and DEPT including selective 13C{1H} decoupling experiments. 相似文献
52.
Samuel González-Mancebo Javier Lacadena Yolanda García-Alonso Jesús Hernández-Benito Emilio Calle Julio Casado 《Monatshefte für Chemie / Chemical Monthly》2002,133(2):157-166
Summary. Nitrosation reactions of phenol, o-cresol, 2,6-dimethylphenol, o-tert-butylphenol, 2-hydroxyacetophenone, and 2-allylphenol in water and water/acetonitrile were studied. Kinetic monitoring of
the reactions was accomplished by spectrophotometric analysis of the nitrosated products at 345 nm. The dominant reaction
was C-nitrosation via a mechanism consisting of an attack on the nitrosatable substrate by NO+/NO2H2
+ followed by a slow proton transfer. The values of the rate constants of phenolic C-nitrosation were increased by electron
donating substituents, and a good Hammett correlation was observed with ρ = −6.1. The results also revealed the strong effect of pH and the permitivity of the reaction medium on the rate constant, whose maximum values were observed for pH ≈ 3, decreasing strongly for higher pH values. The study in water/acetonitrile with up to 25% acetonitrile showed that it is possible to inhibit the reaction strongly
by increasing the percentage of the organic component. The conclusions drawn show that (i) it is possible to predict the rate of nitrosation of phenolics as a function of the meta-substituents on the phenol ring and (ii) the nitrosation of phenolics can be strongly inhibited by increasing the pH of the reaction medium as well as by lowering its dielectric constant.
Received July 13, 2001. Accepted (revised) September 18, 2001 相似文献
53.
Samuel N. Sanders Elango Kumarasamy Kealan J. Fallon Matthew Y. Sfeir Luis M. Campos 《Chemical science》2020,11(4):1079
Singlet fission (SF) is an exciton multiplication process with the potential to raise the efficiency limit of single junction solar cells from 33% to up to 45%. Most chromophores generally undergo SF as solid-state crystals. However, when such molecules are covalently coupled, the dimers can be used as model systems to study fundamental photophysical dynamics where a singlet exciton splits into two triplet excitons within individual molecules. Here we report the synthesis and photophysical characterization of singlet fission of a hexacene dimer. Comparing the hexacene dimer to analogous tetracene and pentacene dimers reveals that excess exoergicity slows down singlet fission, similar to what is observed in molecular crystals. Conversely, the lower triplet energy of hexacene results in an increase in the rate of triplet pair recombination, following the energy gap law for radiationless transitions. These results point to design rules for singlet fission chromophores: the energy gap between singlet and triplet pair should be minimal, and the gap between triplet pair and ground state should be large.We report the synthesis and photophysical characterization of highly exoergic singlet fission in a hexacene dimer revealing exciton dynamics that follow the energy gap law. 相似文献
54.
Njardarson JT Biswas K Danishefsky SJ 《Chemical communications (Cambridge, England)》2002,(23):2759-2761
A total synthesis of Epothilone 490 and a synthesis of 11-hydroxy dEpoB utilizing a vinyl-boronate cross-metathesis followed by a Suzuki macrocyclization. A mild route to reach aldehydes from terminal olefins, anticipating Nozaki-Kishi macrocyclization is described. 相似文献
55.
Arya P Couve-Bonnaire S Durieux P Laforce D Kumar R Leek DM 《Journal of combinatorial chemistry》2004,6(5):735-745
A solid-phase synthesis of tetrahydroquinoline-derived polycyclic 4, having a medium size ring with an enamide functionality, was achieved from tetrahydroquinoline derivative 3 in five steps with overall 40-45% yield. An enantiopure, tetrahydroquinoline-derived beta-amino ester, 1, was converted into compound 2 that has a free phenolic hydroxyl group as an anchoring site for solid-phase synthesis. The solid-phase worked well for this sequence, in which the synthesis of the unsaturated eight-membered enamide lactam was obtained by a ring-closing metathesis approach. Compound 4 is a novel, natural product-like polycyclic derivative that could further be utilized in library generation for developing small molecule chemical probes. 相似文献
56.
57.
Marcelo Samuel Berman 《International Journal of Theoretical Physics》1992,31(8):1451-1454
We discuss various possible scenarios where 2q, where stands for the density parameter andq for the deceleration parameter. We further estimate the corrections necessary when a variable cosmological constant is considered in the theory. 相似文献
58.
Amber L. H. Gray Aleksandra Antevska Benjamin A. Link Bryan Bogin Susan J. Burke Samuel D. Dupuy J. Jason Collier Zachary A. Levine Michael D. Karlstad Thanh D. Do 《Chemical science》2021,12(16):5853
Despite being relatively benign and not an indicative signature of toxicity, fibril formation and fibrillar structures continue to be key factors in assessing the structure–function relationship in protein aggregation diseases. The inability to capture molecular cross-talk among key players at the tissue level before fibril formation greatly accounts for the missing link toward the development of an efficacious therapeutic intervention for Type II diabetes mellitus (T2DM). We show that human α-calcitonin gene-related peptide (α-CGRP) remodeled amylin fibrillization. Furthermore, while CGRP and/or amylin monomers reduce the secretion of both mouse Ins1 and Ins2 proteins, CGRP oligomers have a reverse effect on Ins1. Genetically reduced Ins2, the orthologous version of human insulin, has been shown to enhance insulin sensitivity and extend the life-span in old female mice. Beyond the mechanistic insights, our data suggest that CGRP regulates insulin secretion and lowers the risk of T2DM. Our result rationalizes how migraine might be protective against T2DM. We envision the new paradigm of CGRP : amylin interactions as a pivotal aspect for T2DM diagnostics and therapeutics. Maintaining a low level of amylin while increasing the level of CGRP could become a viable approach toward T2DM prevention and treatment.CGRP concentration is elevated in migraine conditions. The protective effect of migraine against type 2 diabetes is attributed to the ability of CGRP to remodel human amylin aggregation and to suppress the secretion of mouse insulin 2 (the orthologue of human insulin). 相似文献
59.
García-Santos Mdel P González-Mancebo S Hernández-Benito J Calle E Casado J 《Journal of the American Chemical Society》2002,124(10):2177-2182
Nitrosation reactions of amino acids with an -NH(2) group [namely, six alpha-amino acids (glycine, alanine, alpha-aminobutyric acid, alpha-aminoisobutyric acid, valine, and norvaline); two beta-amino acids (beta-alanine and beta-aminobutyric acid), and one gamma-amino acid (gamma-aminobutyric acid)] were studied. Nitrosation was carried out in aqueous acid media, mimicking the conditions of the stomach lumen. The rate equation was r = k(3)(exp)[amino acid][nitrite](2), with a maximum k(3)(exp) value in the 2.3-2.7 pH range. The existence of an isokinetic relationship supports the argument that all the reactions share a common mechanism. A nitrosation mechanism is proposed, and the following conclusions are drawn: (i) Nitrosation reactions of amino acids with a primary amino group in acid media occur with dinitrogen trioxide as the main nitrosating agent. The finding that the nitrosation rate is proportional to the square of the nitrite concentration suggests that the yield of nitrosation products in the stomach would increase sharply with higher nitrate/nitrite intakes. (ii) Stomach hypochlorhydria could be a potential enhancer of in vivo amino acid nitrosation. (iii) The reactivity (k(3)()(exp)) [alpha-amino acids > beta-amino acids > gamma-amino acids] is the same as that found in a previous work for the alkylating potential of lactones formed from nitrosation products of the same amino acids. This implies that the nitrosation reactions of the most common natural amino acids are the most efficient precursors of the most powerful alkylating agents. (iv) The order of magnitude (10(7)-10(8) M(-1) s(-1)) of the bimolecular rate constants of nitrosation shows that such reactions occur through an encounter process. 相似文献
60.