全文获取类型
收费全文 | 680篇 |
免费 | 14篇 |
国内免费 | 1篇 |
专业分类
化学 | 499篇 |
晶体学 | 4篇 |
力学 | 4篇 |
数学 | 54篇 |
物理学 | 134篇 |
出版年
2022年 | 17篇 |
2021年 | 11篇 |
2020年 | 12篇 |
2019年 | 11篇 |
2018年 | 6篇 |
2017年 | 4篇 |
2016年 | 10篇 |
2015年 | 7篇 |
2014年 | 17篇 |
2013年 | 38篇 |
2012年 | 29篇 |
2011年 | 23篇 |
2010年 | 10篇 |
2009年 | 15篇 |
2008年 | 24篇 |
2007年 | 34篇 |
2006年 | 18篇 |
2005年 | 35篇 |
2004年 | 17篇 |
2003年 | 32篇 |
2002年 | 29篇 |
2001年 | 10篇 |
2000年 | 12篇 |
1999年 | 14篇 |
1998年 | 5篇 |
1997年 | 10篇 |
1996年 | 11篇 |
1995年 | 6篇 |
1994年 | 12篇 |
1993年 | 8篇 |
1992年 | 14篇 |
1991年 | 6篇 |
1990年 | 12篇 |
1989年 | 6篇 |
1988年 | 12篇 |
1987年 | 9篇 |
1986年 | 6篇 |
1985年 | 19篇 |
1984年 | 25篇 |
1983年 | 6篇 |
1982年 | 7篇 |
1981年 | 11篇 |
1980年 | 8篇 |
1979年 | 13篇 |
1978年 | 14篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1975年 | 5篇 |
1970年 | 3篇 |
1968年 | 3篇 |
排序方式: 共有695条查询结果,搜索用时 0 毫秒
71.
72.
Six new ent-kaurane-type diterpenoids were isolated from the leaves of the endemic Vietnamese medicinal plant Croton tonkinensis GAGNEP. (Euphorbiaceae) together with three known ent-11alpha-acetoxy-7beta,14alpha-dihydroxykaur-16-en-15-one (1), ent-kaur-16-en-15-one 18-oic acid (5) and ent-18-hydroxykaur-16-ene (7). Their structures were determined by spectroscopic analyses to be ent-7beta-acetoxy-11alpha-hydroxykaur-16-en-15-one (2), ent-18-acetoxy-11alpha-hydroxykaur-16-en-15-one (3), ent-11alpha-acetoxykaur-16-en-18-oic acid (4), ent-15alpha,18-dihydroxykaur-16-ene (6), ent-11alpha,18-diacetoxy-7beta-hydroxykaur-16-en-15-one (8), and ent-(16S)-1alpha,14alpha-diacetoxy-7beta-hydroxy-17-methoxykauran-15-one (14). ent-Kaurane-type diterpenoids from Croton tonkinensis 2-4, 6, and 9-13, were tested for toxicity in the brine shrimp lethality assay. Compounds 9, 10, and 12 demonstrated significant activity, compounds 2, 3, 6, and 11 showed weak activity, and compounds 4 and 13 were inactive. 相似文献
73.
Synthesis and Biological Evaluation of Some Heterocyclic Compounds from Salicylic Acid Hydrazide 下载免费PDF全文
E. M. Sarshira N. M. Hamada Y. M. Moghazi M. M. Abdelrahman 《Journal of heterocyclic chemistry》2016,53(6):1970-1982
Different heterocyclic compounds were prepared starting from 2‐hydroxy benzohydrazide; for example, cyclization of hydrazide hydrazone 3 derived from 2‐hydroxybenzohydrazide 2 with acetic anhydride or concentrated sulfuric acid gave 1,3,4‐oxadiazole derivatives 4 – 5 . On the other hand, direct cyclization of 2‐hydroxy benzohydrazide 2 with one carbon cyclizing agent gave a new derivative of 1,3,4‐oxadiazole 7 , 8 , 9 , 10 , 11 . Heating of hydrazide hydrazone 3 with thioglycolic acid in pyridine gave thiazolidinone 12 . When 2‐hydroxy benzohydrazide 2 reacted with aliphatic carboxylic acids such as formic acid or acetic acid, it gave the corresponding N‐formyl or N‐acetyl derivatives 6 . Subsequent cyclization of 6 using phosphorous pentasulphide in pyridine gave 1,3,4‐thiadiazoles 13 . Cyclization of 2‐hydroxy benzohydrazide with ethyl acetoacetate gives pyrazolone derivative 14 . Finally, when an ethanolic solution of acid hydrazide 2 was treated with ammonium thiocyanate in 35% HCl, it gave the thiosemicarbazide 15 . Subsequent treatment of 15 with concentrated sulfuric acid or 10% sodium hydroxide gave 5‐amino‐1,3,4‐thiadiazole 16 and 1,2,4‐triazole 17 , respectively. The structures of all newly isolated compounds were confirmed using 1H NMR, IR spectra, and elemental analyses. The antimicrobial activities for all isolated compounds were examined against different microorganisms. 相似文献
74.
Laser induced breakdown spectrometry (LIBS) combined with laser induced fluorescence spectrometry (LIFS) has been applied for detection of trace-level phosphorus in steel. The plasma induced by irradiation of Nd:YAG laser pulse for ablation was illuminated by the 3rd harmonic of Ti:Sapphire laser tuned to one of the resonant lines for phosphorus in the wavelength region of 253–256 nm. An excitation line for phosphorus was selected to give the highest signal-to-noise ratio. Fluorescence signals, P213.62 and P214.91 nm, were observed with high selectivity at the contents as low as several tens µg g− 1. Fluorescence intensities were in a good linear correlation with the contents. Fluorescence intensity ratio of a collisionally assisted line (213.62 nm) to a direct transition line (214.91 nm) was discussed in terms of the analytical conditions and experimental results were compared with a calculation based on rate equations. Since the fluorescence signal light in the wavelength range longer than 200 nm can be transmitted relatively easily, even through fiber optics of moderate length, LIBS/LIFS would be a versatile technique in on-site applications for the monitoring of phosphorus contents in steel. 相似文献
75.
Turky Abdallah Bayoumi Ashraf H. Sherbiny Farag F. El-Adl Khaled Abulkhair Hamada S. 《Molecular diversity》2021,25(1):403-420
Molecular Diversity - The discovery of potent STAT3 inhibitors has gained noteworthy impetus in the last decade. In line with this trend, considering the proven biological importance of... 相似文献
76.
A new route for (2S,3S,4S)-form, the physiologically active stereoisomer of dehydroxymethylepoxyquinomicin (DHMEQ), a potent NF-κB inhibitor, was established by chemoenzymatic approach. Elaboration on the asymmetric epoxidation of a p-benzoquinone monoketal with benzylcinchonidinium tert-butylhydroperoxide yielded an epoxyenone, in 79.8% ee and 57% yield in reproducible manner. By way of the transformation of this key intermediate to enantiomerically pure (2S,3S,4S)-DHMEQ, the contaminating undesired enantiomer could be effectively removed by applying Burkholderia cepacia lipase-catalyzed hydrolysis of diacylated precursor. The above integrated combination of chemical asymmetric synthesis and enzyme-catalyzed kinetic resolution enabled us to prepare active DHMEQ in a large-scale. 相似文献
77.
Masaaki?HanedaEmail author Isao?Nakamura Tadahiro?Fujitani Hideaki?Hamada 《Catalysis Surveys from Asia》2005,9(4):207-215
Comprehensive studies combining surface science and real catalyst were performed to get further insight into catalytic active
site and reaction mechanism for NO decomposition over supported palladium and cobalt oxide-based catalysts. On palladium single-crystal
model catalysts, adsorption, dissociation and desorption behavior of NO was found to be closely related to the surface structures,
the stepped surface palladium being active for dissociation of NO. In accordance with this result, the activity of powder
Pd/Al2O3 catalysts for NO decomposition was directly related to the number of step sites exposed on the surface, suggesting that the
step sites act as the catalytic active site for NO decomposition on Pd/Al2O3. NO decomposition over cobalt oxide was found to be significantly promoted by addition of alkali metals. Surface science
study and catalyst characterization led to the same conclusion that the interface between the alkali metal and Co3O4 serves as the catalytic active site. From the results of in situ Fourier transform infrared (FT-IR) spectroscopy and isotopic transient kinetic analysis, a reaction mechanism was proposed
in which the reaction is initiated by NO adsorption onto alkali metals to form NO2− species and then NO2− species react with the adsorbed NO species to form N2 over the interface between the alkali metal and Co3O4. 相似文献
78.
Prof. Dr. Tomoaki Tanase Dr. Kanako Nakamae Yume Okawa Mami Hamada Dr. Arimasa Matsumoto Prof. Dr. Takayuki Nakajima Prof. Dr. Takuya Nakashima Prof. Dr. Tsuyoshi Kawai 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(8):e202104060
A P-stereogenic linear tetraphosphine tetraoxide, (R,R)- or (S,S)-dpmppm(=O)4, was synthesized to prepare C2 dinuclear M(hfa)3 complexes (M=Eu, Tb, Y) as the first example of lanthanide(III) complexes with P-chiral multidentate phosphine oxides. The mononuclear M(hfa)3 complexes (M=Eu, Y) with a P-chiral diphosphine dioxide, tpdpb(=O)2, were also prepared, and comparison of their photophysical properties for the EuIII complexes revealed that significant chiral induction from the P-chiral centers arises on the achiral M(hfa)3 units through intramolecular π-π stacking constraint in the dinuclear system. 相似文献
79.
It has been shown by Bogdanova and Boukliev [1] that there exist a ternary [38,5,24] code and a ternary [37,5,23] code. But it is unknown whether or not there exist a ternary [39,6,24] code and a ternary [38,6,23] code. The purpose of this paper is to prove that (1) there is no ternary [39,6,24] code and (2) there is no ternary [38,6,23] code using the nonexistence of ternary [39,6,24] codes. Since it is known (cf. Brouwer and Sloane [2] and Hamada and Watamori [14]) that (i) n3(6,23) = 38> or 39 and d3(38,6) = 22 or 23 and (ii) n3(6,24) = 39 or 40 and d3(39,6) = 23 or 24, this implies that n3(6,23) = 39, d3(38,6) = 22, n3(6,24) = 40 and d3(39,6) = 23, where n3<>(k,d) and d<>3(n,k) denote the smallest value of n and the largest value of d, respectively, for which there exists an [n,k,d] code over the Galois field GF(3). 相似文献
80.
Mohamed A. F. Elmosallamy Mohamed M. Ghoneim Hamada M. A. Killa Amr L. Saber 《Mikrochimica acta》2005,151(1-2):109-113
A potentiometric poly(vinyl chloride) membrane sensor for determination of saccharin is described. It is based on the use of Aliquat 336S-saccharinateion-pair as an electroactive material in plasticized PVC membranes with o-nitrophenyloctylether or dioctylphthalate. The sensor is conditioned for at least two days in 0.1 mol L−1 sodium saccharinate before use. It exhibits fast, stable and Nernstian response for saccharinate ions over the concentration range of 1.0 × 10−1–5.0 × 10−5 mol L−1 and pH range of 4.5–11. The sensor is used for determination of saccharin in some dosage forms. Results with an average recovery of 101% and a mean standard deviation of 0.2% are obtained which is compared favourably with data obtained using the British pharmacopoeia method. The sensor shows reasonable selectivity towards saccharin in presence of many anions and natural sweeteners. 相似文献