全文获取类型
收费全文 | 1893篇 |
免费 | 61篇 |
国内免费 | 6篇 |
专业分类
化学 | 1365篇 |
晶体学 | 21篇 |
力学 | 17篇 |
数学 | 131篇 |
物理学 | 426篇 |
出版年
2023年 | 7篇 |
2022年 | 20篇 |
2021年 | 29篇 |
2020年 | 28篇 |
2019年 | 29篇 |
2018年 | 27篇 |
2017年 | 24篇 |
2016年 | 33篇 |
2015年 | 30篇 |
2014年 | 51篇 |
2013年 | 92篇 |
2012年 | 105篇 |
2011年 | 115篇 |
2010年 | 69篇 |
2009年 | 62篇 |
2008年 | 110篇 |
2007年 | 129篇 |
2006年 | 102篇 |
2005年 | 104篇 |
2004年 | 119篇 |
2003年 | 90篇 |
2002年 | 82篇 |
2001年 | 37篇 |
2000年 | 38篇 |
1999年 | 31篇 |
1998年 | 21篇 |
1997年 | 27篇 |
1996年 | 24篇 |
1995年 | 16篇 |
1994年 | 17篇 |
1993年 | 16篇 |
1992年 | 15篇 |
1991年 | 13篇 |
1990年 | 11篇 |
1989年 | 17篇 |
1988年 | 7篇 |
1987年 | 12篇 |
1986年 | 12篇 |
1985年 | 32篇 |
1984年 | 22篇 |
1983年 | 14篇 |
1982年 | 16篇 |
1981年 | 10篇 |
1980年 | 18篇 |
1979年 | 17篇 |
1978年 | 12篇 |
1977年 | 9篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1973年 | 7篇 |
排序方式: 共有1960条查询结果,搜索用时 375 毫秒
51.
Shinji Toyota Takashi Nakagawa Masashi Kotani Michinori
ki Hidehiro Uekusa Yuji Ohashi 《Tetrahedron》2002,58(52):10345-10351
All the three possible rotamers of the title compound were separated by chromatography, and unambiguously identified by NMR and X-ray analysis. One of the isomers was optically inactive Ci conformation. The other optical active forms were resolved to give a pair of enantiomers, which were characterized by optical rotation and CD spectra. Thus the optical inactivity of a compound such as meso-tartaric acid that can take Ci conformation in solution, is now ascribed to that the molecule has an optically inactive Ci conformer and equal amounts of optically active conformers, that are enantiomers, in solution. 相似文献
52.
Mineo Hashizume Isamu Saeki Masashi Otsuki Jun-Ichi Kikuchi 《Journal of Sol-Gel Science and Technology》2006,40(2-3):227-232
To extend the concept of the Cerasome, an organic-inorganic vesicular nanohybrid, this paper investigates the preparation
and characterization of a “mixed” Cerasome. The system consists of a Cerasome-forming lipid 1, a cationic synthetic lipid 2, and a zwitterionic phospholipid 3. Lipid mixtures of 1 and 2 or 1 and 3 were used to prepare the mixed Cerasomes. Their lipid distributions were examined using differential scanning calorimetry
(DSC), which showed that 1 and 2 (or 1 and 3) were phase-separated in the mixed Cerasomes. These results seem to be mainly attributable to the polymerizable nature of
1. Results of scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX) showed that 1 and 3 were both incorporated into a single Cerasome, not macroscopically separated to form separate vesicles from each lipid component.
Mixed Cerasomes of 1 and 2 showed high morphological stability against a membrane-solubilizing surfactant, incorporating up to 70% of 2. On the other hand, the mixed Cerasomes from 1 and 3 were less stable than the mixed Cerasomes from 1 and 2. This relative instability might be attributable to differences between the mixed Cerasomes from 1 and 2 and 1 and 3 in terms of their vesicular sizes, lipid domain sizes, and their relative effectiveness for siloxane network formation. These
results strongly support the formation of mixed Cerasomes that have lipid domains in-plane. Systems described in this study
are useful to prepare variously mixed Cerasomes that have different surface functionalities and in-plane lipid distribution,
but which have high morphological stability. 相似文献
53.
The relationship between the product patterns and the configurations of 1,2-cycloheptane- and 1,2-cyclooctanediols 9 in the cyclocondensations with oxalyl chloride in the presence of triethylamine at 0 degrees C has been shown analogous to that obtained for 1,2-disubstituted acyclic ethylene glycols 1: cis-1,2-cyclooctanediol (9f) produced the cyclic oxalate 14f as the major product, while trans-1,2-cycloheptanediol (9e) and trans-1,2-cyclooctanediol (9g) formed the cyclic carbonates 12e, g as the major products. On the other hand, the cyclic oxalates 14a-d were formed as the major products from 1,2-cyclopentane- and 1,2-cyclohexanediols regardless of the configuration. These results can be accounted for by assuming the boat-like transition states for cyclizations of the half esters of comparatively rigid five- and six-membered diols 9a--d. The cyclic oxalates 14a, c may be directly formed through the resulting tetrahedral intermediates from cis-diols (9a,c), and the cyclic carbonates 12a,c as the minor products after ring inversion of the tetrahedral intermediates. The tetrahedral intermediates from the trans-isomers 9b, d cannot undergo ring inversion, producing no traces of the cyclic carbonates 12b, d. 相似文献
54.
Tsuda M Izui N Shimbo K Sato M Fukushi E Kawabata J Katsumata K Horiguchi T Kobayashi J 《The Journal of organic chemistry》2003,68(13):5339-5345
A novel cytotoxic 16-membered macrodiolide, amphidinolide X (1), has been isolated from a marine dinoflagellate Amphidinium sp. (strain Y-42). The gross structure of 1 was elucidated on the basis of spectroscopic data including one-bond and long-range (13)C-(13)C correlations. The relative and absolute stereochemistries were determined by combined analyses of NOESY data and (1)H-(1)H and (1)H-(13)C coupling constants of 1 and NMR data of the degradation products. Amphidinolide X (1) is the first macrodiolide consisting of polyketide-derived diacid and diol units from natural sources. The biosynthetic origins of 1 were investigated by means of feeding experiments with (13)C-labeled acetates. 相似文献
55.
Ishikawa Y Matsumoto Y Nishida Y Taniguchi S Watanabe J 《Journal of the American Chemical Society》2003,125(21):6558-6562
Silicon carbide (SiC) and diamond were decomposed to CO(2)(g) by the photocatalysis with TiO(2) at room temperature, although the decomposition rate of diamond was very slow. According to the XPS spectra of Si2p on the SiC surface, SiO(2) was simultaneously formed on the surface by the TiO(2) photocatalysis. The thickness of the SiO(2) formed on the SiC surface during the photocatalytic oxidation for 1 h was estimated to be about 40 A from the depth profile of the XPS spectra using Ar etching. The SiC surface was oxidized by the TiO(2) photocatalysis even under the condition without a direct contact with the TiO(2). This indicates that the photocatalytic oxidation of the SiC occurs due to active oxygen species photogenerated on the TiO(2) surface, but not by hole produced in the valence band of the TiO(2). Moreover, a remote surface treatment system using the quartz beads coated with TiO(2) was developed for the SiC surface oxidation. Consequently, the TiO(2) photocatalysis will be very useful for the surface treatment of SiC such as photopatterning without defects and damage to the substrate because the photocatalytic reaction is carried out under mild conditions. 相似文献
56.
Haru Ogawa Yoshinari Taketugu Taiji Imoto Yōichi Taniguchi Hidefumi Kato 《Tetrahedron letters》1979,20(36):3457-3460
Rearrangement of 2,4-di--pentadienals was described as a consequence of the successive alkyl substitutions on the C-2 and C-4 positions. 1H-FT nmr spectroscopy suggested that the reaction involves ketene as an intermediate. 相似文献
57.
Oku K Watanabe H Kubota M Fukuda S Kurimoto M Tsujisaka Y Komori M Inoue Y Sakurai M 《Journal of the American Chemical Society》2003,125(42):12739-12748
Trehalose is a disaccharide that attracts much attention as a stress protectant. In this study, we investigated the mechanism of the antioxidant function of trehalose. The spin-lattice relaxation times (T(1)) of (1)H and (13)C NMR spectra were measured to investigate the interaction between trehalose and unsaturated fatty acid (UFA). We selected several kinds of UFA that differ in the number of double bonds and in their configurations (cis or trans). Several other disaccharides (sucrose, maltose, neotrehalose, maltitol, and sorbitol) were also analyzed by NMR. The T(1) values for the (1)H and (13)C signals assigned to the olefin double bonds in UFA decrease with increasing concentration of trehalose and the changes reaches plateaus at integer ratios of trehalose to UFA. The characteristic T(1) change is observed only for the combination of trehalose and UFA with cis double bond(s). On the other hand, from the (13)C-T(1) measurements for trehalose, the T(1) values of the C-3 (C-3') and C-6' (C-6) are found to change remarkably by addition of UFA. (1)H[bond](1)H NOESY measurements provide direct evidence for complexation of trehalose with linoleic acid. These results indicate that one trehalose molecule stoichiometrically interacts with one cis-olefin double bond of UFA. Computer modeling study indicates that trehalose forms a stable complex with an olefin double bond through OH...pi and CH...O types of hydrogen bonding. Furthermore, a significant increase in the activation energy is found for hydrogen abstraction reaction from the methylene group located between the double bonds that are both interacting with the trehalose molecules. Therefore, trehalose has a significant depression effect on the oxidation of UFA through the weak interaction with the double bond(s). This is the first study to elucidate the antioxidant function of trehalose. 相似文献
58.
Novel vibrational circular dichroism (VCD) studies in the CH region of a series of methyl glycosidic carbohydrates were examined. The specific CH stretching VCD band predicts absolute stereochemistries of their anomeric positions. The C-1 chiral information was extracted to the methoxy substituent as a probe. The concept of the vibrational chirality probe from a single chiral center in the presence of numerous such centers might be useful in determining the absolute configuration, when a multiplicity of chiral centers is present in a molecule such as a carbohydrate. 相似文献
59.
Tomio Shimizu Yoshiyuki Hayashi Takehiro Taniguchi Kazuhiro Teramura 《Tetrahedron》1985,41(4):727-738
A variety of symmetrically or unsymmetrically 3,4-disubstituttd furoxans such as dicyano, dialkyl, diacyl, bis(phenylsulfonyl), N.N'-dialkyldicarbamoyl, 3(or 4)-methyl-4(or -3)-phenyl(or nitro, ethoxy, phenoxy, phenylthio, pyrrolidinyl, phenylsulfonyl), 3(or 4)-ethyl-4(or -3)phcnyl, and 3(or 4)-ethoxy-4(or -3)-phenylsulfonylruroxan reacted with dipolarophiles in toluene or xylene at the refluxing temperature to give nitrone-type 1,3-dipolar cycloadducts, 5-substituted 1-aza-2,8-dioxabicyclo-[3.3.0]octanes and/or 3-substituted 2-isoxazoline 2-oxides. On the other hand, some of the furoxans gave 2-isoxazolines via nitrile oxide 1,3-dipolar cycloaddition in a toluene (or xylene)-DMF solvent at the refluxing temperature. 相似文献
60.
S. Sawamura M. Tsuchiya T. Ishigami Y. Taniguchi K. Suzuki 《Journal of solution chemistry》1993,22(8):727-732
Solubility of naphthalene in water was measured at 25°C and pressures up to 200 MPa. The solubility decreased with increasing pressure. From the pressure coefficient of the solubility, the volume change V accompanying the dissolution was estimated as 13.8±0.4 cm
3
-mol
–1
. Further we estimated the volume change V
CH
accompanying hydrophobic hydration as –0.1±0.6 cm
3
-mol
–1
using the V value, the molar volume of crystalline naphthalene, and the partial molar volume of naphthalene in n-heptane. This V
CH
is much larger (i.e., less negative) than that for hydrophobic hydration of alkyl-chain compounds and suggests that the hydration structure of naphthalene differs from that of alkyl-chain compounds. 相似文献