全文获取类型
收费全文 | 4605篇 |
免费 | 139篇 |
国内免费 | 16篇 |
专业分类
化学 | 2925篇 |
晶体学 | 13篇 |
力学 | 242篇 |
数学 | 804篇 |
物理学 | 776篇 |
出版年
2023年 | 50篇 |
2022年 | 138篇 |
2021年 | 133篇 |
2020年 | 109篇 |
2019年 | 97篇 |
2018年 | 82篇 |
2017年 | 91篇 |
2016年 | 185篇 |
2015年 | 144篇 |
2014年 | 142篇 |
2013年 | 246篇 |
2012年 | 260篇 |
2011年 | 288篇 |
2010年 | 173篇 |
2009年 | 150篇 |
2008年 | 279篇 |
2007年 | 270篇 |
2006年 | 221篇 |
2005年 | 236篇 |
2004年 | 165篇 |
2003年 | 157篇 |
2002年 | 112篇 |
2001年 | 50篇 |
2000年 | 50篇 |
1999年 | 45篇 |
1998年 | 32篇 |
1997年 | 32篇 |
1996年 | 52篇 |
1995年 | 45篇 |
1994年 | 37篇 |
1993年 | 36篇 |
1992年 | 28篇 |
1991年 | 27篇 |
1990年 | 30篇 |
1989年 | 32篇 |
1988年 | 31篇 |
1987年 | 47篇 |
1986年 | 24篇 |
1985年 | 40篇 |
1984年 | 38篇 |
1983年 | 19篇 |
1982年 | 25篇 |
1981年 | 27篇 |
1980年 | 27篇 |
1979年 | 19篇 |
1978年 | 17篇 |
1977年 | 31篇 |
1976年 | 13篇 |
1975年 | 14篇 |
1974年 | 14篇 |
排序方式: 共有4760条查询结果,搜索用时 15 毫秒
91.
Maria L. Foresti Francesco Pergola Rolando Guidelli 《Journal of Electroanalytical Chemistry》1980,107(2):307-321
Both the oxidation of Cu0 at dropping amalgam electrodes immersed in solutions of benzotriazole (BTA) and the reduction of CuII at a dropping mercury electrode from BTA solutions have been investigated by the single potential-step chronocoulometric technique. The dependence of the charge Q(t) flowing as a consequence of a given potential jump Ei→Ef upon the initial and final potentials Ei and Ef, as well as upon the time t elapsed from the instant of the potential jump provides direct evidence for the presence of a single adsorbed monolayer of a CuI compound on a mercury electrode immersed in a CuII solution containing BTA, at applied potentials positive to ≈?0.4 V/SCE. Analogous measurements carried out at dropping amalgam electrodes reveal the presence of a single adsorbed monolayer of a CuI compound, or else of an adsorbed multilayer, depending on the potential range investigated. The results of the chronocoulometric measurements are in agreement with those of the polarographic measurements of Part I. 相似文献
92.
Renato Noto Giuseppe Werber Francesco Buccheri Caterina Arnone 《Journal of heterocyclic chemistry》1987,24(5):1457-1459
The dissociation constants (K1) of both acids 4a-c and esters 5a-c and the rate constants of the decarboxylation reaction of acids 4a-c have been measured at various high concentrations of hydrochloric acid (0.5-8.0 M range). The results obtained have enabled us to suggest the probable structure of the zwitterion which undergoes decarboxylation. 相似文献
93.
Van den Eynde I Van Rompaey K Lazzaro F Tourwé D 《Journal of combinatorial chemistry》2004,6(4):468-473
Starting from Boc-o-aminomethylphenylalanine, a solution-phase parallel synthesis of 2,4-substituted 4-amino-1,2,4,5-tetrahydro-2-benzazepine-3-ones is described. This heterocycle has two nitrogen functions, which are differentiated and can be selectively substituted. The sources of diversity are aldehydes for the R(1) position and carboxylic acids, sulfonyl chlorides, or isocyanates for the R(2) position. High-throughput synthesis and purification of this multistep synthetic sequence was accomplished using polymer-bound reagents and scavengers and liquid-liquid extraction protocols, and a small library of compounds was prepared. Polymer-bound cyanoborohydride was found to work well for the reductive amination. Scavenging of excess of amine was performed by polymer-bound benzaldehyde, and cyclization was performed in the presence of polymer-bound coupling reagent 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC). After Boc-deprotection, the second nitrogen can be acylated using carboxylic acids, sulfonylated or converted to a urea. The acylation is again performed by polymer-bound EDC. Excellent yields and purities were obtained. 相似文献
94.
In this paper we report the first synthesis of isoxazolidine and isoxazoline-POSS macromers by 1,3-dipolar cycloaddition reactions of vinyl- and styryl-POSS with N-methyl-C-ethoxycarbonylnitrone and ethoxycarbonyl nitrile oxide, promoted by microwave irradiation. The nature of the resulting cycloadducts has been determined by NOE experiments and supported by computational studies at PM3 and DFT B3LYP/6-31G* levels. 相似文献
95.
Bigoli F Deplano P Mercuri ML Pellinghelli MA Pilia L Pintus G Serpe A Trogu EF 《Inorganic chemistry》2002,41(20):5241-5248
New [M(R(2)pipdt)(2)](BF(4))(2) salts [R(2)pipdt = N,N'-dialkyl-piperazine-2,3-dithione; M = Pd(II), R = Me and M = Pt(II), R = Me, Et, Pr(i)] bearing redox-active cationic dithiolene complexes have been prepared and characterized. These cations react with the redox-active [M(mnt)(2)](2-) [M = Pd(II), Pt(II); mnt = maleonitrile-2,3-dithiolate] anionic dithiolenes to form salts describable as ion pair charge-transfer complexes. X-ray crystallographic studies have shown that [M(Me(2)pipdt)(2)][M(mnt)(2)] complexes, with M = Pd(II) and Pt(II), are isomorphous. Crystal data of the Pt salt (3a): triclinic, Ponemacr; (No. 2); Z = 1; T = 293(2) K; a = 6.784(7) A, b = 8.460(6) A, c = 13.510(5) A, alpha = 100.63(2) degrees, beta = 104.04(2) degrees, gamma = 96.90(2) degrees; R1 = 0.0691 [wR2 = 0.2187 (all data)]. Structural data show that approximately square-planar [Pt(Me(2)pipdt)(2)] dications and regular square-planar [Pt(mnt)(2)] dianions form an infinite anion-cation one-dimensional stack along axis a with a Pt...Pt a/2 distance of 3.392 A and a Pt...Pt...Pt angle of 180 degrees. Anions and cations arrange themselves face-to-face so as to take on a staggered arrangement. These salts exhibit strong absorptions in the visible-near-infrared region assigned to ion pair charge-transfer transitions. A relation between the optical and thermal electron transfer in the solid state is obtained using a "Marcus-Hush model", and a solid-state electrical conductivity in agreement with expectations is observed. Vibrational spectroscopy is in agreement with the existence of charge-transfer interactions between the cationic and anionic components of the salts. 相似文献
96.
Marchetti N Dondi F Felinger A Guerrini R Salvadori S Cavazzini A 《Journal of chromatography. A》2005,1079(1-2):162-172
The Reversed-phase (RP) gradient elution chromatography of nociceptin/orphanin FQ (N/OFQ), a neuropeptide with many biological effects, has been modeled under linear and non-linear conditions. In order to do this, the chromatographic behavior has been studied under both linear and nonliner conditions under isocratic mode at different mobile phase compositions--ranging from 16 to 19% (v/v) acetonitrile (ACN) in aqueous trifluoracetic acid (TFA) 0.1% (v/v)-on a C-8 column. Although the range of mobile phase compositions investigated was quite narrow, the retention factor of this relatively small polypeptide (N/OFQ is a heptadecapeptide) has been found to change by more than 400%. In these conditions, gradient operation resulted thus to be the optimum approach for non-linear elution. As the available amount of N/OFQ was extremely reduced (only a few milligrams), the adsorption isotherms of the peptide, at the different mobile phase compositions examined, have been measured through the so-called inverse method (IM) on a 5 cm long column. The adsorption data at different mobile phase compositions have been fitted to several models of adsorption. The dependence of the isotherm parameters on the mobile phase composition was modeled by using the linear solvent strength (LSS) model and a generalized Langmuir isotherm that includes the mobile phase composition dependence. The overloaded gradient separation of N/OFQ has been modeled by numerically solving the equilibrium-dispersive (ED) model of chromatography under a selected gradient elution mode, on the basis of the previously determined generalized Langmuir isotherm. The agreement between theoretical calculations and experimental overloaded band profiles appeared reasonably accurate. 相似文献
97.
Francesco S. Gentile Alexander Platonenko Khaled E. El-Kelany Michel Rérat Philippe D'Arco Roberto Dovesi 《Journal of computational chemistry》2020,41(17):1638-1644
The infrared (IR) and Raman spectra of eight substitutional carbon defects in silicon are computed at the quantum mechanical level by using a periodic supercell approach based on hybrid functionals, an all electron Gaussian type basis set and the CRYSTAL code. The single substitutional C s case and its combination with a vacancy (C sV and C sSiV) are considered first. The progressive saturation of the four bonds of a Si atom with C is then examined. The last set of defects consists of a chain of adjacent carbon atoms C, with i = 1–3. The simple substitutional case, C s, is the common first member of the three sets. All these defects show important, very characteristic features in their IR spectrum. One or two C related peaks dominate the spectra: at 596 cm−1 for C s (and C sSiV, the second neighbor vacancy is not shifting the C s peak), at 705 and 716 cm−1 for C sV, at 537 cm−1 for C and C (with additional peaks at 522, 655 and 689 for the latter only), at 607 and 624 cm−1, 601 and 643 cm−1, and 629 cm−1 for SiC, SiC, and SiC, respectively. Comparison with experiment allows to attribute many observed peaks to one of the C substitutional defects. Observed peaks above 720 cm−1 must be attributed to interstitial C or more complicated defects. 相似文献
98.
Amantini D Fringuelli F Piermatti O Pizzo F Vaccaro L 《The Journal of organic chemistry》2003,68(24):9263-9268
The [4 + 2] cycloadditions of 3-nitrocoumarin (1a), 6-chloro-3-nitrocoumarin (1b), and 6-, 7-, and 8-hydroxy-3-nitrocoumarins (1c, 5, and 6) with (E)-piperylene (7), isoprene (8), 2,3-dimethyl-1,3-butadiene (9), 2-methoxy-1,3-butadiene (10), 2,3-dimethoxy-1,3-butadiene (11), and cyclopentadiene (12) were investigated in aqueous medium, in organic solvent and under solventless conditions. The reactions performed in water occurred in heterogeneous phase but were faster than those executed in toluene or dichloroethane (DCE). 1a-c, 5, and 6 behaved as 2pi components in the Diels-Alder cycloadditions with 7-10 and 12, and exo adducts were preferentially or exclusively produced. Surprisingly 1a, behaved as a 4pi component in the cycloaddition in water with 11 and 4-substituted 3-nitrochromanones 20 and 21 were isolated. The cycloadditions of hydroxy-3-nitrocoumarins 1c, 5, and 6 with 1,3-diene 9 did not work in water or in organic solvent, but did work under solventless conditions. Nitrotetrahydrobenzo[c]chromenones 13-16, 24, and 25, originating from the normal electron-demand Diels-Alder reactions, were converted into dihydrodibenzo[b,d]furans 27-31 in water, via one-pot Nef-cyclodehydration reactions. 相似文献
99.
Alberto Bargagna Pietro Schenone Francesco Bondavalli Mario Longobardi 《Journal of heterocyclic chemistry》1980,17(1):33-37
Cycloaddition of sulfene to N,N-disubstituted 4-amino-3-methyl-3-buten-2-ones (III) occurred in fair to good yield only in the case of aliphatic N-substitution to give 4-dialkylamino-3,4-dihydro-5,6-dimethyl-1,2-oxathiin 2,2-dioxides, whereas N,N-disubstituted 1-amino-1-penten-3-ones (II) did not react at all. Cycloaddition of dichloroketene to II, III and N,N-disubstituted 4-amino-3-buten-2-ones occurred only in the case of the methylphenylamino derivative, giving in good to moderate yield 6,(5)(di)alkyl-3,3-dichloro-3,4-dihydro-4-methylphenylamino-2-Hpyran-2-ones, which were dehydrochlorinated with DBN to 6,(5)(di)alkyl-3-chloro-4-methylphenylamino-2H-pyran-2-ones. 相似文献
100.
In presence of tin(II) bromide, noble metals give coloured products which are suitable for spcctrophotometric determinations. The colours are red (platinum), yellow-orange (rhodium), yellow-brown (palladium), yellow (iridium) and violet (gold) They are extracted, except for gold, with isoamyl alcohol Platinum, rhodium and palladium can be separated from irdium, and rhodium and platinum from palladium. Rhodium and platinum can be determined simultaneously. 相似文献