全文获取类型
收费全文 | 1902篇 |
免费 | 31篇 |
国内免费 | 6篇 |
专业分类
化学 | 1373篇 |
晶体学 | 29篇 |
力学 | 11篇 |
数学 | 126篇 |
物理学 | 400篇 |
出版年
2022年 | 11篇 |
2021年 | 9篇 |
2020年 | 8篇 |
2019年 | 22篇 |
2018年 | 14篇 |
2017年 | 11篇 |
2016年 | 24篇 |
2015年 | 27篇 |
2014年 | 35篇 |
2013年 | 99篇 |
2012年 | 75篇 |
2011年 | 106篇 |
2010年 | 54篇 |
2009年 | 84篇 |
2008年 | 124篇 |
2007年 | 114篇 |
2006年 | 100篇 |
2005年 | 119篇 |
2004年 | 108篇 |
2003年 | 88篇 |
2002年 | 72篇 |
2001年 | 37篇 |
2000年 | 37篇 |
1999年 | 21篇 |
1998年 | 14篇 |
1997年 | 9篇 |
1996年 | 37篇 |
1995年 | 24篇 |
1994年 | 19篇 |
1993年 | 21篇 |
1992年 | 20篇 |
1991年 | 16篇 |
1990年 | 11篇 |
1989年 | 14篇 |
1988年 | 12篇 |
1987年 | 15篇 |
1986年 | 21篇 |
1985年 | 37篇 |
1984年 | 32篇 |
1983年 | 12篇 |
1982年 | 21篇 |
1981年 | 24篇 |
1980年 | 24篇 |
1979年 | 35篇 |
1978年 | 17篇 |
1977年 | 28篇 |
1976年 | 21篇 |
1975年 | 16篇 |
1974年 | 8篇 |
1973年 | 10篇 |
排序方式: 共有1939条查询结果,搜索用时 937 毫秒
81.
Ando M Takizawa M Suwabe S Yamato S Shimada K 《European journal of mass spectrometry (Chichester, England)》2003,9(6):619-622
Analysis of selenium in biological samples is very important and numerous analytical methods for the element have been developed. One of the most convenient and widely used methods for routine determination of serum selenium is a fluorometric method using 2,3-diaminonaphthalene (DAN); however, this method lacks specificity. We observed that 4,5-benzopiazselenol (BPS), a selenium derivative of DAN, is ionized with electron capture in an atmospheric pressure chemical ionization (APCI) interface, and subsequently established a method for determining total human serum selenium by means of liquid chromatography/atmospheric pressure chemical ionization mass spectrometry. All pretreatment procedures were carried out in a single test tube to minimize selenium loss. The recovery of organic or inorganic selenium spiked to human serum was 97-103%.The detection limit of BPS was equivalent to 0.2 ng of selenium and the lower quantitative limit of serum selenium was 10 ng mL(-1). The coefficient of variation of standard concentrations in control serum samples was 4.5%. The purity of the observed peak obtained from serum samples was confirmed using the ion cluster technique. 相似文献
82.
The mechanism for the activation of the sigma bonds, the O-H of H2O, C-H of CH4, and the H-H of H2, and the pi bonds, the C[triple bond]C of C2H2, C=C of C2H4, and the C=O of HCHO, at the Pd=X (X = Sn, Si, C) bonds of the model complexes (H2PC2H4PH2)Pd=XH2 5 has been theoretically investigated using a density functional method (B3LYP). The reaction is significantly affected by the electronic nature of the Pd=X bond, and the mechanism is changed depending on the atom X. The activation of the O-H bond with the lone pair electron is heterolytic at the Pd=X (X = Sn, Si) bonds, while it is homolytic at the Pd=C bond. The C-H and H-H bonds without the lone pair electron are also heterolytically activated at the Pd=X bonds independent of the atom X, where the hydrogen is extracted as a proton by the Pd atom in the case of X = Sn, Si and by the C atom in the case of X=C because the nucleophile is switched between the Pd and X atoms depending on the atom X. In contrast, the pi bond activation of C[triple bond]C and C=C at the Pd=Sn bond proceeds homolytically, and is accompanied by the rotation of the (H2PC2H4PH2)Pd group around the Pd-Sn axis to successfully complete the reaction by both the electron donation from the pi orbital to Sn p orbital and the back-donation from the Pd dpi orbital to the pi orbital. On the other hand, the activation of the C=O pi bond with the lone pair electron at the Pd=Sn bond has two reaction pathways: one is homolytic with the rotation of the (H2PC2H4PH2)Pd group and the other is heterolytic without the rotation. The role of the ligands controlling the activation mechanism, which is heterolytic or homolytic, is discussed. 相似文献
83.
A zwitterionic trichloroplatinum(II) complex PtCl3(4,4′-bpy-N-nBu) (1) was prepared by an aqueous reaction of [4,4′-bpy-N-nBu]Cl with K2PtCl4, and was characterized by 1H-n.m.r. and IR spectroscopy as well as elemental analysis. Dissolving (1) into dmso-d
6 at 25 °C yields a mixture of the complexes, cis-PtCl2(4,4′-bpy-N-nBu)(dmso-d
6) (2-cis), trans-PtCl2(4,4′-bpy-N-nBu)(dmso-d
6) (2-trans), [4,4′-bpy-N-nBu]Cl and PtCl2(dmso-d
6)2. Ratio of the products in the dmso-d
6 solution changed depending on the temperature and the total concentration of the complexes. These compounds are in equilibrium
via isomerization reaction between (2-cis) and (2-trans) and via displacement reaction of the alkylbipyridinium ligand of
(2-cis) and (2-trans) with dmso-d
6 to form [4,4′-bpy-N-nBu]Cl and PtCl2(dmso-d
6)2. 相似文献
84.
Shinya Nakashima Dr. Hiromune Ando Risa Saito Hideki Tamai Dr. Hideharu Ishida Dr. Makoto Kiso 《化学:亚洲杂志》2012,7(5):1041-1051
The first total synthesis of the hybrid ganglioside X2, which consisted of a highly branched octasaccharide and ceramide moieties, was accomplished by using a glucosyl ceramide cassette approach. With a disaccharyl donor, the heptasaccharide could not be constructed by glycosylation of the C4 hydroxy group of galactose at the reducing end of the pentasaccharide. In contrast, through an alternative approach with two branched glycan units, a GM2-core trisaccharide, and a lacto-ganglio tetrasaccharide, the heptasaccharyl donor could be prepared and subsequently joined with a glucosyl ceramide cassette to afford the protected ganglioside, X2. Finally, global deprotection completed the synthesis, thus affording the pure ganglioside X2. 相似文献
85.
A practical synthetic route to 7-formamidino-7-deazaguanosine (archaeosine), a hypermodified nucleoside observed in archaeal tRNA, has been developed, which involves the addition of hydroxylamine to the cyano group of 7-cyano-7-deazaguanosine (preQ0-nucleoside) and a subsequent Pd-catalyzed hydrogenation. PreQ0-nucleoside was obtained from an optimized β-selective glycosylation developed by Hocek et al. The corresponding archaeosine base was subsequently synthesized in high yield from its precursor 7-cyano-7-deazaguanine (preQ0). 相似文献
86.
87.
We propose a unique optical system for measuring the retardation of birefringent films using a pair of liquid crystal (LC) gratings; that is, the examined birefringent films are inserted between two LC gratings. Because the LC grating functions as a polarization beam splitter for circularly polarized light, the proposed system is optically equivalent to the measurement system using a pair of two circular polarizers. First, the polarization splitting performance of the LC grating is discussed. It is found that a sufficiently high voltage (such that the retardation is less than a half wavelength) has to be applied for the almost pure circularly polarized diffracted light. Next, the measurement of the retardation of a homogeneous LC cell as an examined birefringent film was demonstrated using the proposed method. The proposed method is revealed to have the same measurement performance as that of the conventional method using a pair of linear polarizers and has an advantage that there is no need for the optic axis of the test birefringent specimen to be set at a specific angle. 相似文献
88.
Motohiro Akazome Toshiaki Takahashi Ryo-Ichi Sonobe Katsuyuki Ogura 《Supramolecular chemistry》2013,25(1):109-136
Abstract Crystallization of (R)-(1-naphthyl)glycyl-(R)-phenyl-glycine [(R,R)-1] in the presence of oligo(ethylene glycol) dimethyl ethers 2(n) or poly(ethylene glycol)s (PEGs, 3(Mn )) afforded inclusion compounds. The ratio of (R,R)-1/the guest polymer (2 or 3) was proportional to the length of the polymer chain. The crystal structure of a hepta(ethylene glycol) dimethyl ether-included compound was disclosed by X-ray crystallography which showed that (R,R)-1 molecules form a sheet and the guest molecule penetrates the crystal lattice of (R,R)-1 through a one-dimensional channel on the sheet. Powder X-ray analysis revealed that, regardless of the length of the guest polymer, the distance between the neighboring sheets remains unchanged (12.0–12.3 Å) in these inclusion crystals. By thermal analysis, it was shown that the decomposition points of these inclusion compounds became higher with the longer PEG included. The inclusion phenomenon enabled the fractionation of PEGs with various molecular weights, among which longer PEG was preferably included. 相似文献
89.
Masanobu Yamamoto Ryoko Maruyama Yoko Murakami Yuki Sakamoto Rei Yamakawa Tetsu Ando 《Analytical and bioanalytical chemistry》2013,405(23):7405-7414
trans-11,12-Epoxy-(6Z,9Z)-6,9-henicosadiene (posticlure) has been identified from a pheromone gland of the lymantriid species, Orgyia postica. Since the diversity of Lepidoptera suggests that some species utilize the structure-related epoxy compound as a sex pheromone component, epoxydienes and epoxytrienes derived from (6Z,9Z,11E)-6,9,11-trienes and (3Z,6Z,9Z,11E)-3,6,9,11-tetraenes with a C19–C21 chain were systematically synthesized and the chemical data were accumulated in order to contribute to a new pheromone research. Peracid oxidation of each triene and each tetraene produced, respectively, a mixture of three epoxydienes (cis-6,7-epoxy-9,11-diene; cis-9,10-epoxy-6,11-diene; and trans-11,12-epoxy-6,9-diene) and four epoxytrienes (cis-3,4-epoxy-6,9,11-triene; cis-6,7-epoxy-3,9,11-triene; cis-9,10-epoxy-3,6,11-triene; and trans-11,12-epoxy-3,6,9-triene). While the 9,10-epoxy compounds were unstable and, interestingly, converted into 9-ketone derivatives after chromatography over SiO2, each positional isomer was isolated by HPLC equipped with an ODS column, and the chemical structure was determined by NMR analysis. On the GC-MS analysis with a DB-23 column, the positional isomers were also eluted separately and characteristic mass spectra were proposed. By comparing the spectral data of the epoxy compounds with a different carbon chain, diagnostic fragment ions reflecting the chemical structure were determined as follows: m/z 79, 109, 113, and M-114 for the 6,7-epoxydienes; m/z 69, 97, 111, 139, and M-111 for the 9,10-epoxydienes; m/z 57, 79, 109, 136, M-151, and M-111 for the 11,12-epoxydienes; m/z 79, 91, 105, and 119 for the 3,4-epoxytrienes; m/z 79, 124, M-124, M-96, and M-69 for the 6,7-epoxytrienes; m/z 79, 95, 109, 137, and M-108 for the 9,10-epoxytrienes; and m/z 79, 134, M-149, M-109, and M-95 for the 11,12-epoxytrienes. 相似文献
90.
Alkylesters of orthophosphoric acids have been used for alkylation of phenols1, anilines2, and nitrogen heterocyclic compounds.3 However, no example is reported on their use as an alkoxylating agent for aromatic amines. We have found that alkyl phosphates can act as alkoxylating agents for aromatic diamines in the presence of H2O or aqueous H3PO4. We now report the one-step dialkoxylation of aromatic diamines such as diaminoanthracene and -naphthalene by trialkyl phosphates. 相似文献