全文获取类型
收费全文 | 1955篇 |
免费 | 50篇 |
国内免费 | 12篇 |
专业分类
化学 | 1390篇 |
晶体学 | 19篇 |
力学 | 49篇 |
数学 | 151篇 |
物理学 | 408篇 |
出版年
2022年 | 9篇 |
2021年 | 24篇 |
2020年 | 24篇 |
2019年 | 29篇 |
2018年 | 22篇 |
2017年 | 13篇 |
2016年 | 37篇 |
2015年 | 37篇 |
2014年 | 39篇 |
2013年 | 84篇 |
2012年 | 95篇 |
2011年 | 114篇 |
2010年 | 58篇 |
2009年 | 56篇 |
2008年 | 96篇 |
2007年 | 106篇 |
2006年 | 101篇 |
2005年 | 122篇 |
2004年 | 104篇 |
2003年 | 107篇 |
2002年 | 101篇 |
2001年 | 55篇 |
2000年 | 38篇 |
1999年 | 24篇 |
1998年 | 16篇 |
1997年 | 16篇 |
1996年 | 26篇 |
1995年 | 14篇 |
1994年 | 19篇 |
1993年 | 23篇 |
1992年 | 19篇 |
1991年 | 23篇 |
1990年 | 18篇 |
1989年 | 19篇 |
1988年 | 25篇 |
1987年 | 19篇 |
1986年 | 12篇 |
1985年 | 24篇 |
1984年 | 27篇 |
1983年 | 20篇 |
1982年 | 20篇 |
1981年 | 19篇 |
1980年 | 11篇 |
1979年 | 18篇 |
1978年 | 17篇 |
1977年 | 22篇 |
1976年 | 15篇 |
1975年 | 12篇 |
1974年 | 18篇 |
1973年 | 11篇 |
排序方式: 共有2017条查询结果,搜索用时 773 毫秒
921.
Direct organocatalytic Mannich-type reactions of thioesters provide for the expedient and diastereoselective synthesis of protected beta-amino acids. A variety of thioesters were found to be reactive with different imines under mild conditions to provide beta-amino acids in good yields. This chemistry was extended to a diastereo- and enantioselective variant. 相似文献
922.
The insertion reaction of isocyanide into a C-S bond of dithioacetals is catalyzed by GaCl3 or TiCl4 to afford thioimidates containing an alpha-alkylthio group. Balanced thiophilicity of these Lewis acids is critical for efficient catalysis. 相似文献
923.
The reaction of 2-(chloromethyl)phenylboronic acid (1) with alkynes in the presence of a Rh(I) complex gave indene derivatives in high yields. The regioselectivity depends on the steric nature of the substituent on the alkynes. A bulky group favors the alpha-position of indenes. 相似文献
924.
The electronic structures and structural morphologies of naphthalene cluster anions, (naphthalene)(n)(-) (n=3-150), and its related aromatic cluster anions, (acenaphthene)(n)(-) (n=4-100) and (azulene)(n)(-) (n=1-100), are studied using anion photoelectron spectroscopy. For (naphthalene)(n) (-) clusters, two isomers coexist over a wide size range: isomers I and II-1 (28 < or = n < or =60) or isomers I and II-2 (n > or = ~60). Their contributions to the photoelectron spectra can be separated using an anion beam hole-burning technique. In contrast, such an isomer coexistence is not observed for (acenaphthene)(n) (-) and (azulene)(n) (-) clusters, where isomer I is exclusively formed throughout the whole size range. The vertical detachment energies (VDEs) of isomer I (7 < or = n < or = 100) in all the anionic clusters depend linearly on n(-13) and their size-dependent energetics are quite similar to one another. On the other hand, the VDEs of isomers II-1 and II-2 produced in (naphthalene)(n)(-) clusters with n > or = approximately 30 remain constant at 0.84 and 0.99 eV, respectively, 0.4-0.6 eV lower than those of isomer I. Based upon the ion source condition dependence and the hole-burning photoelectron spectra experiments for each isomer, the energetics and characteristics of isomers I, II-1, and II-2 are discussed: isomer I is an internalized anion state accompanied by a large change in its cluster geometry after electron attachment, while isomers II-1 and II-2 are crystal-like states with little structural relaxation. The nonappearance of isomers II-1 and II-2 for (acenaphthene)(n)(-) and (azulene)(n)(-) and a comparison with other aromatic cluster anions indicate that a highly anisotropic and symmetric pi-conjugated molecular framework, such as found in the linear oligoacenes, is an essential factor for the formation of the crystal-like ordered forms (isomers II-1 and II-2). On the other hand, lowering the molecular symmetry makes their production unfavorable. 相似文献
925.
Application of the HPLC hyphenated techniques of LC-two-dimensional (2D) NMR using pre-concentration column trapping and LC-MS was demonstrated by the identification of two major degradation products, DP-1 and DP-2, in stressed commercial tablets of amlodipine maleate. The molecular formulas were estimated by LC-MS. Sample pre-concentration by column trapping was conducted to obtain adequate 2D-NMR signals by reducing the peak widths of the degradation products and making sure that the maximum amount of each component was inside the flow cell for NMR detection. Double-quantum filtered correlation spectroscopy (DQF-COSY) was applied to identify DP-1 as beta-N-lactosylamlodipine by suppressing the residual water signal without affecting the sample signal and by measuring the coupling constant of the lactose anomeric proton. Heteronuclear multiple bond coherence spectroscopy (HMBC) was applied to characterize DP-2 as an aspartic acid derivative of amlodipine by detecting long-range CH correlations. The chemical structures of the degradation products could be successfully elucidated unambiguously without an isolation process. 相似文献
926.
Triphosphazene, 1,3,5-triazo-2,4,6-triphosphorine-2,2,4,4,6,6-chloride (TAPC), was found to be an efficient catalyst for the Beckmann rearrangement of cyclohexanone oxime and cyclododecanone oxime to epsilon-caprolactam and laurolactam, which are raw materials of nylon-6 and nylon-12, respectively. 相似文献
927.
Ishii N Okamura Y Chiba S Nogami T Ishida T 《Journal of the American Chemical Society》2008,130(1):24-25
A metal-radical polymer [Co(hfac)2.BPNN] showed a very large coercive field of 52 kOe (4.1 MA m-1) at 6 K, indicating that it is the hardest magnet ever reported. Above 10 K, a soft character appeared, owing to the fast dynamics of magnetization reorientation. 相似文献
928.
Mass spectrometry and photoelectron spectroscopy of o-, m-, and p-terphenyl cluster anions, (o-TP)n(-) (n = 2-100), (m-TP)n(-) (n = 2-100), and (p-TP)n(-) (n = 1-100), respectively, are conducted to investigate the effect of molecular shape on the molecular aggregation form and the resultant ion core character of the clusters. For (o-TP)n(-) and (m-TP)n(-), neither magic numbers nor discernible isomers are observed throughout the size range. Furthermore, their vertical detachment energies (VDEs) increase up to large n and depend linearly on n(-1/3), implying that they possess a three-dimensional (3D), highly reorganized structure encompassing a monomeric anion core. For (p-TP)n(-), in contrast, prominent magic numbers of n = 5, 7, 10, 12, and 14 are observed, and the VDEs show pronounced irregular shifts below n = 10, while they remain constant above n = 14 (isomer A). These results can be rationalized with two-dimensional (2D) orderings of p-TP molecules and different types of 2D shell closure at n = 7 and 14, the monomeric and multimeric anion core, respectively. Above n = 16, the new feature (isomer B) starts to appear at the higher binding side of isomer A, and it becomes dominant with n, while isomer A gradually disappears for larger sizes. In contrast to isomer A, the VDEs of isomer B continuously increase with the cluster size. This characteristic size evolution suggests that the transition to modified 2D aggregation forms from 2D ones occurs at around n = 20. 相似文献
929.
Kakiuchi F Matsuura Y Kan S Chatani N 《Journal of the American Chemical Society》2005,127(16):5936-5945
When the reaction of aromatic ketones with arylboronates (arylboronic acid esters) using RuH(2)(CO)(PPh(3))(3) (3) as a catalyst was conducted in toluene, the corresponding arylation product was obtained in moderate yields. In this case, a nearly equivalent amount of a benzyl alcohol derived from a reduction of an aromatic ketone was also formed. The use of aliphatic ketones, such as pinacolone and acetone, as an additive or a solvent dramatically suppressed the reduction of the aromatic ketones and, as a result, ortho-arylation products were obtained in high yield based on the aromatic ketones. In these reactions, the aliphatic ketone functioned as a scavenger of ortho-hydrogens of the aromatic ketones and the B(OR)(2) moiety of the arylboron compound (HB species). A variety of aromatic ketones, such as acetophenones, acetonaphthones, tetralones, and benzosuberone, could also be used in this coupling reaction. Several arylboronates containing electron-donating (NMe(2), OMe, and Me) and -withdrawing (CF(3) and F) groups were also applicable to this coupling reaction. Intermolecular competitive reaction using pivalophenone-d(0)() and -d(5) and intramolecular competitive reaction using pivalophenone-d(1) were carried out using 3 as a catalyst. The k(H)/k(D) value for the intermolecular competitive reaction was substantially different, compared with intramolecular competitive reaction. This strongly suggests the production of an intermediate where the ketone carbonyl is coordinated to the ruthenium involved in this catalytic reaction. (1)H and (11)B NMR studies using 2'-methylacetophenone, phenylboronate (2), and pinacolone (6) indicate that 6 functions effectively as a scavenger of the HB species. 相似文献
930.
A phase demodulation scheme using a discrete Hilbert transform that can change the interferometric phase by /2 has been investigated. In-quadrature components of a fringe pattern are obtained from one captured interferogram using a one-dimensional (1-D) discrete Hilbert transform and a 1-D discrete high-pass filtering that are based on a digital signal processing technique. The phase distribution in the range of 15 (rad) can be demodulated with the proposed method. The 1-D discrete Hilbert transform can be extended to two-dimensional calculation with a raster scanning procedure. © 2005 The Optical Society of Japan 相似文献