全文获取类型
收费全文 | 12293篇 |
免费 | 622篇 |
国内免费 | 4篇 |
专业分类
化学 | 9087篇 |
晶体学 | 70篇 |
力学 | 211篇 |
数学 | 1729篇 |
物理学 | 1822篇 |
出版年
2023年 | 117篇 |
2022年 | 61篇 |
2021年 | 114篇 |
2020年 | 194篇 |
2019年 | 207篇 |
2018年 | 133篇 |
2017年 | 125篇 |
2016年 | 427篇 |
2015年 | 446篇 |
2014年 | 469篇 |
2013年 | 892篇 |
2012年 | 1006篇 |
2011年 | 931篇 |
2010年 | 542篇 |
2009年 | 407篇 |
2008年 | 858篇 |
2007年 | 812篇 |
2006年 | 813篇 |
2005年 | 738篇 |
2004年 | 588篇 |
2003年 | 504篇 |
2002年 | 441篇 |
2001年 | 176篇 |
2000年 | 175篇 |
1999年 | 99篇 |
1998年 | 76篇 |
1997年 | 80篇 |
1996年 | 114篇 |
1995年 | 73篇 |
1994年 | 61篇 |
1993年 | 83篇 |
1992年 | 67篇 |
1991年 | 50篇 |
1990年 | 49篇 |
1989年 | 40篇 |
1988年 | 39篇 |
1987年 | 38篇 |
1986年 | 59篇 |
1985年 | 85篇 |
1984年 | 67篇 |
1983年 | 38篇 |
1982年 | 56篇 |
1981年 | 58篇 |
1980年 | 62篇 |
1979年 | 48篇 |
1978年 | 53篇 |
1977年 | 39篇 |
1976年 | 48篇 |
1975年 | 28篇 |
1974年 | 28篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
291.
Rastelli A Gandolfi R Sarzi-Amadé M Carboni B 《The Journal of organic chemistry》2001,66(7):2449-2458
Structures and energetics of reactants, reactant complexes, concerted transition structures, and products of the cycloaddition of the prototypical nitrone with vinylborane have been produced and discussed. Structure optimizations have been performed at the B3LYP/6-31G(d) and B3LYP/AUG-cc-pVDZ levels of approximation, and single-point calculations on the B3LYP geometries have been carried out at the MP4(SDTQ) level with the same basis sets. Kinetic contributions to standard enthalpies, entropies, and free enthalpies have been computed at the same levels of geometry optimizations. The effects of methyl and chloro substitution on the BH2 group and of methyl substitution on the vinyl moiety has been also explicitly considered. The most striking theoretical features of this cycloaddition are (i) the formation of reactant complexes where the nitrone oxygen is strictly bound up to the boron atom (B...O interactions), (ii) their persistence in the endo/exo transition structures, and (iii) energy profiles suggesting very high reaction rates, regiospecificity (5-borylisoxazolidines) and complete endo-stereoselectivity. The BH2 (BX2) substituent appears to induce a sort of intramolecular catalysis which is also largely selective in favor of the endo reaction path. Possible competitive reaction paths such as cyclization, organoboration, and oxidation have equally been investigated, on the same grounds, both with prototypical reagents and with dimethylvinylborane, dichlorovinylborane, 2-methyl-1-propenylborane, and 2-methyl-1-propenyldichloroborane. The transition structures for these reaction paths are significantly higher in energy than those of the corresponding 1,3-dipolar cycloadditions in the sequence oxidation > cyclization > boration > cycloaddition, whereas the resulting reaction products show the reversed sequence. Polar solvents appear to increase the competition of boration although maintaining its character of secondary reaction. As expected, the reaction rate of 1,3-dipolar cycloaddition is lowered by dimethyl substitution on the vinyl CH2 reacting center (i.e., for the reaction of 2-methyl-1-propenylborane and 2-methyl-1-propenyldichloroborane) whereas the reaction rate of boration is increased, the boration results being significantly competitive even in the gas phase. Experiments for the control of the above predictions are not yet available. 相似文献
292.
The low-lying singlet and triplet states of H2CBe and HCBeH are examined using ab inito molecular orbital theory. In agreement with earlier results, the lowest-lying structure of H2CBe has C2v symmetry and is a triplet with one π electron (3 B1). The results presented here suggest that the lowest-energy singlet structure is the (1B1) open-shell singlet, also with C2v symmetry, at least 2.5 kcal/mol higher in energy. The singlet C2v structure with two π electrons (1A1) is 15.9 kcal/mol higher than 3B1. All of these structures are bound with respect to the ground state of methylene and the beryllium atom. In HCBeH, linear equilibrium geometries are found for the triplet (3Σ) and singlet (1Δ) states. The triplet is more stable than the singlet (1Δ) by 35.4 kcal/mol, and is only 2.9 kcal/mol higher in energy than triplet H2 CBe. Since the transition structure connecting these two triplet molecules is found to be 50.2 kcal/mol higher in energy than H2 CBe, both triplet equilibrium species might exist independently. The harmonic vibrational frequencies of all structures are also reported. 相似文献
293.
Analysis of cyanobacterial toxins (anatoxin-a, cylindrospermopsin, microcystin-LR) by capillary electrophoresis 总被引:4,自引:0,他引:4
Capillary zone electrophoresis (CZE) and micellar electrokinetic chromatography (MEKC) were applied to the simultaneous separation of cyanobacterial toxins (anatoxin-a, microcystin-LR, cylindrospermopsin). The analytical performance data of both methods, optimized for the three toxins, were similar with a precision of migration times smaller than 0.8 RSD% and a detection limit in the range of 1-4 microg/mL, using spectrophotometric detection at 230 nm. Both methods were applied to an analysis of cyanotoxins in water bloom samples and crude cyanobacterial extracts. The results obtained indicate that, for complex matrices, the sequential application of CZE and MEKC is necessary. It is recommended to use both CE techniques for the analysis of the same sample in order to confirm the results by an orthogonal approach. 相似文献
294.
[reaction: see text] Enantiomerically pure gamma-fluoroalkyl beta-amino sulfones are readily synthesized in three steps starting from fluorinated imidoyl chlorides and arylmethyl sulfones. A complementary two-step sequence starting from chiral fluorinated beta-amino sulfoxides has also been developed. To illustrate the application of this procedure, a new method for the synthesis of alpha-fluoroalkyl allylic amines in optically pure form involving a Julia methylenation-desulfonylation reaction is presented. 相似文献
295.
Kinetic studies of cyanide exchange on [M(CN)(4)](2-) square-planar complexes (M = Pt, Pd, and Ni) were performed as a function of pH by (13)C NMR. The [Pt(CN)(4)](2-) complex has a purely second-order rate law, with CN(-) as acting as the nucleophile, with the following kinetic parameters: (k(2)(Pt,CN))(298) = 11 +/- 1 s(-1) mol(-1) kg, DeltaH(2) (Pt,CN) = 25.1 +/- 1 kJ mol(-1), DeltaS(2) (Pt,CN) = -142 +/- 4 J mol(-1) K(-1), and DeltaV(2) (Pt,CN) = -27 +/- 2 cm(3) mol(-1). The Pd(II) metal center has the same behavior down to pH 6. The kinetic parameters are as follows: (k(2)(Pd,CN))(298) = 82 +/- 2 s(-1) mol(-1) kg, DeltaH(2) (Pd,CN) = 23.5 +/- 1 kJ mol(-1), DeltaS(2) (Pd,CN) = -129 +/- 5 J mol(-1) K(-1), and DeltaV(2) (Pd,CN) = -22 +/- 2 cm(3) mol(-1). At low pH, the tetracyanopalladate is protonated (pK(a)(Pd(4,H)) = 3.0 +/- 0.3) to form [Pd(CN)(3)HCN](-). The rate law of the cyanide exchange on the protonated complex is also purely second order, with (k(2)(PdH,CN))(298) = (4.5 +/- 1.3) x 10(3) s(-1) mol(-1) kg. [Ni(CN)(4)](2-) is involved in various equilibrium reactions, such as the formation of [Ni(CN)(5)](3-), [Ni(CN)(3)HCN](-), and [Ni(CN)(2)(HCN)(2)] complexes. Our (13)C NMR measurements have allowed us to determine that the rate constant leading to the formation of [Ni(CN)(5)](3-) is k(2)(Ni(4),CN) = (2.3 +/- 0.1) x 10(6) s(-1) mol(-1) kg when the following activation parameters are used: DeltaH(2)() (Ni,CN) = 21.6 +/- 1 kJ mol(-1), DeltaS(2) (Ni,CN) = -51 +/- 7 J mol(-1) K(-1), and DeltaV(2) (Ni,CN) = -19 +/- 2 cm(3) mol(-1). The rate constant of the back reaction is k(-2)(Ni(4),CN) = 14 x 10(6) s(-1). The rate law pertaining to [Ni(CN)(2)(HCN)(2)] was found to be second order at pH 3.8, and the value of the rate constant is (k(2)(Ni(4,2H),CN))(298) = (63 +/- 15) x10(6) s(-1) mol(-1) kg when DeltaH(2) (Ni(4,2H),CN) = 47.3 +/- 1 kJ mol(-1), DeltaS(2) (Ni(4,2H),CN) = 63 +/- 3 J mol(-1) K(-1), and DeltaV(2) (Ni(4,2H),CN) = - 6 +/- 1 cm(3) mol(-1). The cyanide-exchange rate constant on [M(CN)(4)](2-) for Pt, Pd, and Ni increases in a 1:7:200 000 ratio. This trend is modified at low pH, and the palladium becomes 400 times more reactive than the platinum because of the formation of [Pd(CN)(3)HCN](-). For all cyanide exchanges on tetracyano complexes (A mechanism) and on their protonated forms (I/I(a) mechanisms), we have always observed a pure second-order rate law: first order for the complex and first order for CN(-). The nucleophilic attack by HCN or solvation by H(2)O is at least nine or six orders of magnitude slower, respectively than is nucleophilic attack by CN(-) for Pt(II), Pd(II), and Ni(II), respectively. 相似文献
296.
Chadim M Díaz P García-España E Hodacová J Latorre J Liu-González M Luis SV Llinares JM Závada J 《Inorganic chemistry》2005,44(21):7503-7510
Three crystal structures of a ditopic cyclophane ligand (L) in which two 1,5,8,12-tetraamine molecules have been attached through methylene spacers to the ortho positions of a benzene ring are reported. The first one (1) corresponds to the tetraprotonated free macrocycle (H4L4+) having two tetrachlorozincate(II) counteranions (C24H54O2N8Cl8Zn2, a = 9.1890(2) A, b = 14.0120(3) A, c = 15.3180(3) A, alpha = 89.2320(7) degrees , beta = 82.0740(6) degrees , gamma = 83.017(1) degrees , Z = 2.00, triclinic, P); the second one (2) is of a binuclear Cu2+ complex having coordinated chloride anions and perchlorate counteranions (C24H58O14N8Cl4Cu2 a = 9.9380(2) A, b = 30.2470(6) A, c = 53.143(1) A, orthorhombic, F2dd, Z = 18), and the third one (3) corresponds to an analogous Zn2+ complex that has been crystallized using triflate as counteranion (C26H(51.2)O(6.6)N8Cl2F6S2Zn2 a = 8.472(5) A, b = 9.310(5), c = 13.745(5) A, alpha = 84.262(5) degrees , beta = 77.490(5) degrees , gamma = 73.557(5) degrees , triclinic, P, Z = 2). The analysis of the crystallographic data clearly shows that the conformation of the macrocycle and, in consequence, the overall architecture of the crystals are controlled by the anions present in the moiety, pi-pi-stacking associations, and hydrogen bonding interactions. The protonation and stability constants for the formation of the Cu2+ and Zn2+ complexes in aqueous solution have been determined potentiometrically in 0.15 mol dm(-3) NaClO4 at 298.1 K. Intramolecular hydrogen bonding defines the protonation behavior of the compound. Positive cooperativity is observed in the formation of the Cu2+ complexes. 相似文献
297.
Elomri A Michel S Koch M Seguin E Tillequin F Pierré A Atassi G 《Chemical & pharmaceutical bulletin》1999,47(11):1604-1606
Condensation of 2-chloro-3-nitrobenzoic acid with either 5-amino-7-methoxy-2,2-dimethyl-2H-chromene or 5-amino-2,2-dimethyl-2H-chromene afforded diphenylamines 14 and 15. Trifluoroacetic anhydride mediated cyclization gave the corresponding acridones 16 and 17, which were subsequently N-methylated and reduced to 11-aminoacronycine and 11-amino-6-demethoxyacronycine. These two amino compounds, which gave stable water soluble salts, were 2- to 3-fold more potent than acronycine or 6-demethoxyacronycine in inhibiting L1210 cell proliferation. 相似文献
298.
Bongui JB Elomri A Seguin E Tillequin F Pfeiffer B Renard P Pierré A Atassi G 《Chemical & pharmaceutical bulletin》2001,49(9):1077-1080
Condensation of either 2-bromobenzoic acid (4) or 2-chloro-3-nitrobenzoic acid (5) with suitable aminoquinolines 6-8 afforded phenylquinolylamines 9-13. Acid mediated cyclization gave the corresponding 12H-benzo[b][1,7]phenanthrolin-7-ones 14 and 15, and 12H-benzo[b][1,10]phenanthrolin-7-ones 16-18. Compounds 14, 16, and 17 were subsequently N-methylated to 6-demethoxyacronycine and acronycine analogues 19-21, whereas reduction of the aromatic nitro group of 18 gave the amino derivative 22. Unsubstituted 12H-benzo[b][1,10]phenanthrolin-7-ones 16, 17, 20, and 21 were devoid of significant cytotoxic activity, whereas 18 and 22, bearing a nitrogen substituent at position 11, were significantly active. Unsubstituted 12H-benzo[b][1,7]phenanthrolin-7-ones 14 and 19, which include a pyridine nitrogen in the same 4-position as the pyran oxygen of acronycine exhibited cytotoxic activities within the same range of magnitude as acronycine itself. 相似文献
299.
Marchante-Gayón JM González JM Fernández ML Blanco E Sanz-Medel A 《Analytical and bioanalytical chemistry》1996,355(5-6):615-622
A novel High Performance Liquid Chromatography (HPLC) method for the separation of selenium species with specific detection by off-line Electrothermal Atomization Atomic Absorption Spectrometry (ETAAS) or on-line focused microwave digestion (MW) Hydride Generation Atomic Absorption Spectrometry (HG-AAS) is described. Vesicular mobile phases of the cationic surfactant didodecyldimethylammonium bromide (DDAB) have been evaluated for the liquid chromatographic separation of inorganic selenium (selenite and selenate) and different selenoaminoacids (selenocystine, selenomethionine and selenoethionine) on a C(18) reversed-phase column modified by DDAB molecules. The effects of different parameters (pH, buffer and vesicle concentrations) of the mobile phase on the retention times have been determined. The detection limit for selenium with the proposed off-line HPLC-ETAAS method has been found to be 5 microg/L of Se. The detection limit using HPLC-"on line" focused microwave digestion-HG-AAS has been found to be 1 microg/L of Se, with a precision (repeatability) better than +/- 5%. The latter proved to be an exceptional on-line real-time chromatographic detector for selenium speciation purposes. 相似文献
300.
[reaction: see text] The first asymmetric direct Michael addition of aldehydes to vinyl sulfones catalyzed by N-iPr-2,2'-bipyrrolidine is described. 1,4-Adducts are obtained in good yields and enantioselectivities. The determination of absolute configuration allowed us to postulate a Si,Si transition state model, as shown previously for nitroolefins. 相似文献