全文获取类型
收费全文 | 3515篇 |
免费 | 101篇 |
国内免费 | 15篇 |
专业分类
化学 | 2176篇 |
晶体学 | 15篇 |
力学 | 195篇 |
数学 | 744篇 |
物理学 | 501篇 |
出版年
2023年 | 16篇 |
2022年 | 49篇 |
2021年 | 98篇 |
2020年 | 73篇 |
2019年 | 51篇 |
2018年 | 65篇 |
2017年 | 57篇 |
2016年 | 111篇 |
2015年 | 104篇 |
2014年 | 80篇 |
2013年 | 186篇 |
2012年 | 206篇 |
2011年 | 247篇 |
2010年 | 145篇 |
2009年 | 122篇 |
2008年 | 201篇 |
2007年 | 220篇 |
2006年 | 162篇 |
2005年 | 154篇 |
2004年 | 145篇 |
2003年 | 117篇 |
2002年 | 98篇 |
2001年 | 43篇 |
2000年 | 25篇 |
1999年 | 37篇 |
1998年 | 35篇 |
1997年 | 34篇 |
1996年 | 36篇 |
1995年 | 26篇 |
1994年 | 30篇 |
1993年 | 47篇 |
1992年 | 18篇 |
1991年 | 27篇 |
1990年 | 29篇 |
1989年 | 17篇 |
1988年 | 26篇 |
1987年 | 22篇 |
1986年 | 23篇 |
1985年 | 47篇 |
1984年 | 41篇 |
1983年 | 24篇 |
1982年 | 25篇 |
1981年 | 29篇 |
1980年 | 26篇 |
1979年 | 22篇 |
1978年 | 26篇 |
1977年 | 23篇 |
1976年 | 22篇 |
1975年 | 13篇 |
1974年 | 21篇 |
排序方式: 共有3631条查询结果,搜索用时 31 毫秒
941.
Martínez-Lillo J Armentano D De Munno G Wernsdorfer W Julve M Lloret F Faus J 《Journal of the American Chemical Society》2006,128(44):14218-14219
The reaction of [ReIVCl4(ox)]2- and fully solvated Ni2+ ions in a MeCN/i-PrOH mixture affords the heterotetranuclear complex (NBu4)4[Ni{ReCl4(ox)}3] where the rhenium precursor acts as a bidentate ligand toward the nicke(II) ion through the oxalate group. The mixed 3d-5d species exhibits intramolecular ferromagnetic coupling and it behaves like a single-molecule magnet. 相似文献
942.
Meloni G Zou P Klippenstein SJ Ahmed M Leone SR Taatjes CA Osborn DL 《Journal of the American Chemical Society》2006,128(41):13559-13567
The photoionization of alkylperoxy radicals has been investigated using a newly developed experimental apparatus that combines the tunability of the vacuum ultraviolet radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory with time-resolved mass spectrometry. Methylperoxy (CH(3)OO) and ethylperoxy (C(2)H(5)OO) radicals are produced by the reaction of pulsed, photolytically produced alkyl radicals with molecular oxygen, and the mass spectrum of the reacting mixture is monitored in time by using synchrotron-photoionization with a double-focusing mass spectrometer. The kinetics of product formation is used to confirm the origins and assignments of ionized species. The photoionization efficiency curve for CH(3)OO has been measured, and an adiabatic ionization energy of (10.33 +/- 0.05) eV was determined with the aid of Franck-Condon spectral simulations, including ionization to the lowest triplet and singlet cation states. Using the appearance energy of CH(3)(+) from CH(3)OO, an enthalpy of formation for CH(3)OO of Delta(f) (CH(3)OO) = (22.4 +/- 5) kJ mol(-1) is derived. The enthalpy of formation of CH(3)OO(+) is derived as Delta(f) = (1019 +/- 7) kJ mol(-1) and the CH(3)(+)-OO bond energy as (CH(3)(+) - O(2)) = (80 +/- 7) kJ mol(-1). The C(2)H(5)OO(+) signal is not detectable; however, the time profile of the ethyl cation signal suggests its formation from dissociative ionization of C(2)H(5)OO. Electronic structure calculations suggest that hyperconjugation reduces the stability of the ethylperoxy cation, making the C(2)H(5)OO(+) ground state only slightly bound with respect to the ground-state products, C(2)H(5)(+) and O(2). The value of the measured appearance energy of C(2)H(5)(+) is consistent with dissociative ionization of C(2)H(5)OO via the Franck-Condon favored ionization to the ? (1)A' state of C(2)H(5)OO(+). 相似文献
943.
The following paper shows results of PM10 level and Cd, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn concentrations present in this fraction for filters collected in the urban area of Campi Salentina (Apulia, Southern Italy) in 2004 in order to inquire into air quality. PM10 and lead concentrations did not exceed the standard values sanctioned by Italian Legislature with average values equal to 36.54 +/- 14.57 microg x m(-3) and 9.19 +/- 5.24 ng x m(-3) respectively. Significant correlations have been found between the pairs Fe-Mn, Ni-V, Cr-Ni and Cd-Cr. This suggests that these metals have a similar pollution source probably due to metallurgical activity. Multivariate statistical analysis showed that it is possible to distinguish the PM10 samples collected in sampling periods with different meteorological conditions. Indeed, metals concentration increases for samples characterized by scarce rains or by winds prevailing from North or North-West and is reduced due to different meteorological conditions, although there are some exceptions. 相似文献
944.
945.
Scalmani G Frisch MJ Mennucci B Tomasi J Cammi R Barone V 《The Journal of chemical physics》2006,124(9):94107
In this paper we present the theory and implementation of analytic derivatives of time-dependent density functional theory (TDDFT) excited states energies, both in vacuo and including solvent effects by means of the polarizable continuum model. The method is applied to two case studies: p-nitroaniline and 4-(dimethyl)aminobenzonitrile. For both molecules PCM-TDDFT is shown to be successful in supporting the analysis of experimental data with useful insights for a better understanding of photophysical and photochemical pathways in solution. 相似文献
946.
Arfaoui I Bermúdez V Bottari G De Nadai C Jalkanen JP Kajzar F Leigh DA Lubomska M Mendoza SM Niziol J Rudolf P Zerbetto F 《The journal of physical chemistry. B》2006,110(15):7648-7652
Surface enhanced second harmonic generation (SE SHG) experiments on molecular structures, macrocycles, catenanes, and rotaxanes, deposited as monolayers and multilayers by vacuum sublimation on silver, are reported. The measurements show that the molecules form ordered thin films, where the highest degree of order is observed in the case of macrocycle monolayers and the lowest in the case of rotaxane multilayers. The second harmonic generation activity is interpreted in terms of electric field induced second harmonic (EFISH) generation where the electric field is created by the substrate silver atoms. The measured second order nonlinear optical susceptibility for a rotaxane thin film is compared with that obtained by considering only EFISH contribution to SHG intensity. The electric field on the surface of a silver layer is calculated by using the Delphi4 program for structures obtained with TINKER molecular mechanics/dynamics simulations. An excellent agreement is observed between the calculated and the measured SHG susceptibilities. 相似文献
947.
Moggio L De Napoli L Di Blasio B Di Fabio G D'Onofrio J Montesarchio D Messere A 《Organic letters》2006,8(10):2015-2018
[structure: see text] A new and versatile on-line automated solid-phase approach to obtain cyclic PNA (I and III) and cyclic PNA-DNA chimeras (II) in highly pure form has been developed. Starting from a Tentagel matrix functionalized with a 3-chloro-4-hydroxyphenylacetic linker, the synthesis of representative, new cyclic molecules by standard peptide and phosphoramidite-based chemistry has been achieved. 相似文献
948.
Giovanni Colombo Antonio Marigonda 《Calculus of Variations and Partial Differential Equations》2006,25(1):1-31
Closed sets K ⊂
satisfying an external sphere condition with uniform radius (called ϕ-convexity or proximal smoothness) are considered. It
is shown that for
-a.e. x ∊ ∂K the proximal normal cone to K at x has dimension one. Moreover if K is the closure of an open set satisfying a (sharp) nondegeneracy condition, then the De Giorgi reduced boundary is equivalent
to ∂ K and the unit proximal normal equals
-a.e. the (De Giorgi) external normal. Then lower semicontinuous functions f :
with ϕ-convex epigraph are shown, among other results, to be locally BV and twice
-a.e. differentiable; furthermore, the lower dimensional rectifiability of the singular set where f is not differentiable is studied. Finally we show that for
-a.e. x there exists δ (x) > 0 such that f is semiconvex on B(x,δ(x)). We remark that such functions are neither convex nor locally Lipschitz, in general. Methods of nonsmooth analysis and
of geometric measure theory are used.
Work partially supported by M.I.U.R., project “Viscosity, metric, and control theoretic methods for nonlinear partial differential
equations.” 相似文献
949.
[reaction: see text] A novel synthetic pathway to benzoquinazolines from naphthylamines is reported. Benzoquinazoline nucleus was cyclized in good yield from N-protected naphthylamines using hexamethylenetetramine in TFA and potassium ferricyanide in aqueous ethanolic KOH. This method is efficient and convenient with respect to previously reported synthetic pathways. 相似文献
950.
D'Acquarica I Gasparrini F Pierini M Villani C Zappia G 《Journal of separation science》2006,29(10):1508-1516
Dynamic HPLC on enantioselective stationary phases has become a well-established technique to investigate chiral molecules with internal motions that result in stereoinversion and occur on the time scale of the separation process. Kinetic parameters for the on-column interconversion phenomena can be extracted from experimental peak profiles by computer simulation or by direct calculation methods. The technique has been used in a wide range of temperatures and is complementary in scope to dynamic NMR spectroscopy. 相似文献