全文获取类型
收费全文 | 741篇 |
免费 | 29篇 |
国内免费 | 25篇 |
专业分类
化学 | 587篇 |
晶体学 | 7篇 |
力学 | 6篇 |
数学 | 51篇 |
物理学 | 144篇 |
出版年
2023年 | 4篇 |
2022年 | 7篇 |
2021年 | 5篇 |
2020年 | 8篇 |
2019年 | 5篇 |
2018年 | 8篇 |
2017年 | 8篇 |
2016年 | 12篇 |
2015年 | 18篇 |
2014年 | 13篇 |
2013年 | 53篇 |
2012年 | 33篇 |
2011年 | 41篇 |
2010年 | 20篇 |
2009年 | 16篇 |
2008年 | 54篇 |
2007年 | 48篇 |
2006年 | 53篇 |
2005年 | 41篇 |
2004年 | 40篇 |
2003年 | 29篇 |
2002年 | 26篇 |
2001年 | 10篇 |
2000年 | 13篇 |
1999年 | 16篇 |
1998年 | 8篇 |
1997年 | 10篇 |
1996年 | 8篇 |
1995年 | 10篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1992年 | 15篇 |
1991年 | 6篇 |
1990年 | 9篇 |
1989年 | 6篇 |
1988年 | 7篇 |
1987年 | 7篇 |
1986年 | 6篇 |
1985年 | 14篇 |
1984年 | 15篇 |
1983年 | 9篇 |
1982年 | 6篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1979年 | 7篇 |
1978年 | 7篇 |
1977年 | 4篇 |
1976年 | 9篇 |
1973年 | 3篇 |
1971年 | 7篇 |
排序方式: 共有795条查询结果,搜索用时 15 毫秒
711.
Native DL-pantothenic acid, having a 1,3-diol structure, was chirally resolved by ligand exchange capillary electrophoresis using (S)-3-amino-1,2-propanediol as a chiral selector and the borate anion as a central ion. The optimum conditions for both high resolution and short migration time of DL-pantothenic acid were found to be 200 mM (S)-3-amino-1,2-propanediol and 200 mM borate buffer (pH 9.2) containing 15% methanol with an applied voltage of +25 kV at 20 degrees C, using direct detection at 200 nm. With this system, the resolution (Rs) of racemic pantothenic acid was approximately 1.7. When (S)-1,2-propanediol, (S)-1,2,3-propanetriol, (S)-1,3-butanediol or (S)-1-amino-2-propanol were used as chiral ligand instead of (S)-3-amino-1,2-propanediol, DL-pantothenic acid was not enantioseparated. When borate was replaced with Tris or butylborate, no chiral separation was achieved. Therefore, the ionic interaction between the amino and carboxyl groups of the ternary complex may play an important role in the enantioseparation of DL-pantothenic acid by the proposed CE system. 相似文献
712.
The dipole moments (μ), polarizabilities (α), hyperpolarizabilities (β), and second-order hyperpolarizabilities (γ) of polyenes, polyynes, and polyphenyls have been calculated by a finitefield method with the PM -3 parameterization of the semiempirical MNDO Hamiltonian at the optimum geometries. These results were compared to experimental values obtained from EFISH and THG measurements. The calculations reproduce the magnitudes of β and γ, as well as the effect of the substituents and the effect of bond alternation on β and γ. The coefficients of the power law, which describes the dependence of β and γ on the number of π centers, were calculated. For β, exponents of 1.5–2.2 and 0.03–0.04 were obtained for polyenes and polyynes, respectively, and for γ, exponents of 3.9–4.9, 2.9–3.3, and 2.5–2.7 were obtained for polyenes, polyynes, and polyphenyls, respectively. These results confirm the efficiency of enhancing γ by insertion of C = C double bonds into a chain. © 1992 John Wiley & Sons, Inc. 相似文献
713.
Hiromichi Tanaka Hiroyuki Hayakawa Shuji Iijima Kazuhiro Haraguchi Tadashi Miyasaka 《Tetrahedron》1985,41(5):861-866
Synthesis of 6-substituted 2'-deoxyuridines can be effected by lithiation of 3',5'--(tetraisopropyldisiloxane-1,3-diyl)-2'-deoxyuridine with LDA followed by the reaction of its lithio derivative with electrophiles. This method provides a general, regiospecific and simple route to various types of 6-substituted 2'-deoxyuridines which have, so far, been known to be difficult to prepare. 相似文献
714.
Masumoto S Usuda H Suzuki M Kanai M Shibasaki M 《Journal of the American Chemical Society》2003,125(19):5634-5635
A new method for the catalytic enantioselective Strecker reaction (cyanation) of N-diphenylphosphinoyl ketoimines is described. The asymmetric catalyst is a chiral gadolinium complex prepared from Gd(OiPr)3 and the d-glucose-derived ligand 3 in a 1:2 ratio. The reaction has a broad substrate generality, giving high enantioselectivity from aromatic, ethyl, primary alkyl, and alpha,beta-unsaturated ketoimines. The products could be easily converted to disubstituted alpha-amino acids and their derivatives. 相似文献
715.
716.
Shuji Saito 《Journal of polymer science. Part A, Polymer chemistry》1969,7(7):1789-1802
The salt effect was investigated by measurements of viscosity, cloud point, and solubility of aqueous solutions of poly(vinyl alcohol—acetate) copolymers, as these copolymers in water are sensitive in various ways to addition of various electrolytes. The major role in the salt effect is played by the anions, and water-structure breaking anions produce salting-in of the copolymers. Tetraalkylammonium halides (bromides and iodides) cause salting-in of the copolymers more effectively with increase of size of the hydrophobic cations. The Setschenow equation does not hold for the polymers except for very dilute polymer concentration. In salts of monoalkyl-substituted anions (R? COONa and R? SO4Na) and cations (R? NH3Cl and Br), so long as the alkyl chain is short, the salt effect is also dominated by the anions. With increase of the alkyl chain length, sodium salts of the monoalkyl-substituted anions exert a stronger salting-in effect upon the polymers than chlorides of similar long-chain cations. The significance of the counteranions is suggested for the interaction of nonionic polymers and the long-chain cations. The action of the salts on the copolymers is discussed in terms of medium effects (change of the water structure) and of the binding effect of the single ions to the polymers. 相似文献
717.
Bailleux S Dréan P Zelinger Z Civis S Ozeki H Saito S 《The Journal of chemical physics》2005,122(13):134302
The rotational spectra of the two isotopic species of the bromomethyl radical, CH2 79Br and CH2 81Br, have been observed in their ground electronic state 2B1 in the 180-470 GHz frequency region, corresponding to a-type transitions from N=8-7 to N=21-20. The radical was produced by hydrogen abstraction of methylbromide (CH3Br) either by chlorine or by fluorine atoms in a free space cell. Hyperfine structure due to the bromine nucleus has been resolved in the observed spectra, and the rotational constants as well as the fine and hyperfine interaction constants were accurately determined for both isotopomers. The inertial defect was determined to be 0.028 96(20) and 0.028 95(20) amu A(2), for CH2 79Br and CH2 81Br, respectively, suggesting a planar structure. By fixing the [angle]HCH bond angle at 124.5 degrees , an effective molecular structure can be derived as r0(CBr)=1.848 A and r0(CH)=1.084 A. A comparison of the molecular structure of various halogen-substituted methyl radicals with respect to the planarity of these radicals is discussed. 相似文献
718.
Gangadharar J. SanjayanAlistair Stewart Shuji HachisuRaquel Gonzalez Mark P. WattersonGeorge W.J. Fleet 《Tetrahedron letters》2003,44(31):5847-5851
Short syntheses of enantiomeric templated scaffolds of cis- and trans-tetrahydrofuran γ-amino acids from pentono-δ-lactones derived from arabinose and ribose are reported; an unexpectedly efficient synthesis of a templated tetrahydrofuran β-amino acid by azide displacement of a triflate β to an ester function proceeds with remarkably little elimination. These materials should allow evaluation of such peptidomimetics to induce predisposition towards secondary structures. 相似文献
719.
Nahomi Sakaguchi Shuji Matsuo Keiji Yamada Taku Matsuo Hisanobu Wakita 《Research on Chemical Intermediates》2004,30(9):879-887
Photocatalytic degradation process of ATP was followed by HPLC and 1H-NMR measurements. Adenine was one of the major products when the aqueous ATP sample with TiO2 dispersion was irradiated by UV light (λ = 365 nm) for 90 min. The 1H-NMR spectra indicated the presence of another product (Z), which likely came from the ribose moiety in ATP. Photocatalytic generation of OH˙ radicals on TiO2 surface was concluded to be responsible for degrading ATP. A working hypothesis of the reaction process was proposed to account for the reaction products. 相似文献
720.
Akai S Naka T Omura S Tanimoto K Imanishi M Takebe Y Matsugi M Kita Y 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(18):4255-4264
The first lipase-catalyzed domino reaction is described in which the acyl moiety formed during the enzymatic kinetic resolution of furfuryl alcohols (+/-)-3 with a 1-ethoxyvinyl ester 2 was utilized as a part of the constituent structure for the subsequent Diels-Alder reaction. The preparation of ester 2 from carboxylic acid 1 and the subsequent domino reaction were carried out in a one-pot reaction. Therefore, this procedure provides a convenient preparation of the optically active 7-oxabicyclo[2.2.1]heptene derivatives 5, which has five chiral, non-racemic carbon centers, from achiral 1 and racemic 3. The overall efficiency of this process was dependent on the substituent at the C-3 position of 3, and the use of the 3-methylfurfuryl derivatives, (+/-)-3 b and (+/-)-3 f, exclusively produced diastereoselectivity with excellent enantioselectivity to give (2R)-syn-5 (91->/=99 % ee) and (S)-3 (96->/=99 % ee). Similar procedures starting from the 3-bromofurfuryl alcohols (+/-)-3 h-j provided the cycloadducts (2R)-syn-5 j-q (93->/=99 % ee), in which the bromo group was utilized for the installation of bulky substituents to the 7-oxabicycloheptene core. 相似文献