全文获取类型
收费全文 | 1509篇 |
免费 | 49篇 |
国内免费 | 8篇 |
专业分类
化学 | 911篇 |
晶体学 | 9篇 |
力学 | 98篇 |
数学 | 218篇 |
物理学 | 330篇 |
出版年
2023年 | 9篇 |
2022年 | 32篇 |
2021年 | 51篇 |
2020年 | 39篇 |
2019年 | 33篇 |
2018年 | 29篇 |
2017年 | 29篇 |
2016年 | 57篇 |
2015年 | 48篇 |
2014年 | 55篇 |
2013年 | 110篇 |
2012年 | 118篇 |
2011年 | 127篇 |
2010年 | 82篇 |
2009年 | 97篇 |
2008年 | 121篇 |
2007年 | 90篇 |
2006年 | 86篇 |
2005年 | 73篇 |
2004年 | 47篇 |
2003年 | 47篇 |
2002年 | 42篇 |
2001年 | 15篇 |
2000年 | 15篇 |
1999年 | 13篇 |
1998年 | 10篇 |
1997年 | 8篇 |
1996年 | 16篇 |
1995年 | 7篇 |
1994年 | 5篇 |
1993年 | 7篇 |
1992年 | 4篇 |
1991年 | 4篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 1篇 |
1972年 | 1篇 |
1961年 | 1篇 |
排序方式: 共有1566条查询结果,搜索用时 156 毫秒
991.
992.
Paulo L Freitas AA da Silva PF Shimizu K Quina FH Maçanita AL 《The journal of physical chemistry. A》2006,110(6):2089-2096
Synthetic and natural hydroxyflavylium salts are super-photoacids, exhibiting values of the rate constant for proton transfer to water in the excited state as high as 1.5 x 10(11) s(-1). The synthetic flavylium salt 4-carboxy-7-hydroxy-4'-methoxyflavylium chloride (CHMF) has an additional carboxyl group at the 4-position of the flavylium cation that deprotonates in the ground state at a lower pH (pK(a1) = 0.73; AH2+ --> Z) than the 7-hydroxy group (pK(a2) = 4.84; Z --> A-). Ground-state deprotonation of the carboxyl group of the acid (AH2+) to form the zwitterion (Z) is too fast to be detected by nanosecond laser flash perturbation of the ground-state equilibrium, while deprotonation of the hydroxyl group of Z to form the anionic base (A-) occurs in the microsecond time range (k(d2) = 0.6 x 10(6) s(-1) and k(p2) = 4.2 x 10(10) M(-1) x s(-1)). In the excited state, the cationic form (AH2+) deprotonates in approximately 9 ps, resulting in the excited neutral base form (AH), which is unstable in the ground state. Deprotonation of Z occurs in 30 ps (k(d2) = 2.9 x 10(10) s(-1)), to form excited A-, which either reprotonates (k(p3)* = 3.7 x 10(10) M(-1) x s(-1)) or decays in 149 ps, and shows an important contribution from geminate recombination to give the excited neutral base (AH). Predominant reprotonation of A- at the carboxylate group reflects both the presence of the negative charge on the carboxylate and the increase in the excited-state pK(a) of the carboxyl group. Thus, while the hydroxyl pK(a) decreases by approximately 5 units upon going from the ground state (pK(a) = 4.84) to the excited state (pK(a) = -0.2), that of the carboxyl group increases by at least this much. Consequently, the excited state of the Z form of CHMF acts as a molecular proton transporter in the picosecond time range. 相似文献
993.
de Oliveira KT Júnior VL Constantino MG Donate PM da Silva GV Brocksom TJ Frederico D 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2006,63(3):709-713
A detailed analysis with total assignment of (1)H and (13)C NMR spectral data for a cycloheptenone derivative, a key intermediate for the synthesis of perhydroazulene terpenoids, is related. These assignments are based on 1D (1)H and (13)C NMR and on 2D NMR techniques including gCOSY, gHSQC, gHMBC, J-resolved and NOEDIF experiments. The unequivocal assignments were supported by theoretical chemical shifts and scalar coupling constant calculations at GIAO B3LYP/cc-pVDZ level from optimized structures at the same level of theory. 相似文献
994.
Vale JA Faustino WM Menezes PH de Sá GF 《Chemical communications (Cambridge, England)》2006,(31):3340-3342
The use of a new chiral lanthanide complex derived from a europium dithiocarbamate complex and a N-tosylated amino acid for the asymmetric synthesis of cyanohydrins is described. In some cases, high enantioselectivities were observed. 相似文献
995.
Indole 25, an advanced intermediate in a projected enantioselective total synthesis of lyngbyatoxin A 1, was prepared from allylic alcohol 11 in 9 steps and >95% ee, key transformations being the enantiospecific rearrangement of vinyl epoxide 14 and the Hemetsberger-Knittel reaction of azide 24. 相似文献
996.
Using molecular dynamics simulations we investigate the structure of a system of particles interacting through a continuous core-softened interparticle potential. We found for the translational order parameter t a local maximum at a density rho(t-max) and a local minimum at rho(t-min)>rho(t-max). Between rho(t-max) and rho(t-min), the t parameter anomalously decreases upon increasing pressure. For the orientational order parameter Q(6) a maximum was observed at a density rho(t-max)相似文献
997.
998.
999.
Marco César Goldbarg Paulo Henrique Asconavieta Elizabeth Ferreira Gouvêa Goldbarg 《Memetic Computing》2012,4(2):89-108
The Traveling Car Renter Problem (CaRS) is a generalization of the Traveling Salesman Problem where the tour can be decomposed into contiguous paths that are travelled by different rented cars. When a car is rented in a city and delivered in another, the renter must pay a fee to return the car to its home city. Given a graph G and cost matrices associated to cars available for rent, the problem consists in determining the minimum cost Hamiltonian cycle in G, considering also the cost paid to deliver a car in a city different from the one it was rented. The latter cost is added to the cost of the edges in the cycle. This paper describes the general problem and some related variants. Two metaheuristic approaches are proposed to deal with CaRS: GRASP hybridized with Variable Neighborhood Descent and Memetic Algorithm. A set of benchmark instances is proposed for the new problem which is utilized on the computational experiments. The algorithms are tested on a set of 40 Euclidean and non-Euclidean instances. 相似文献
1000.