全文获取类型
收费全文 | 33184篇 |
免费 | 3775篇 |
国内免费 | 6088篇 |
专业分类
化学 | 29314篇 |
晶体学 | 1331篇 |
力学 | 808篇 |
综合类 | 236篇 |
数学 | 2082篇 |
物理学 | 9276篇 |
出版年
2024年 | 71篇 |
2023年 | 382篇 |
2022年 | 953篇 |
2021年 | 902篇 |
2020年 | 1123篇 |
2019年 | 1107篇 |
2018年 | 917篇 |
2017年 | 1265篇 |
2016年 | 1293篇 |
2015年 | 1171篇 |
2014年 | 1572篇 |
2013年 | 2830篇 |
2012年 | 1934篇 |
2011年 | 2383篇 |
2010年 | 1997篇 |
2009年 | 2275篇 |
2008年 | 2315篇 |
2007年 | 2300篇 |
2006年 | 2147篇 |
2005年 | 2008篇 |
2004年 | 1775篇 |
2003年 | 1467篇 |
2002年 | 1252篇 |
2001年 | 1056篇 |
2000年 | 1007篇 |
1999年 | 810篇 |
1998年 | 688篇 |
1997年 | 608篇 |
1996年 | 520篇 |
1995年 | 526篇 |
1994年 | 476篇 |
1993年 | 399篇 |
1992年 | 310篇 |
1991年 | 218篇 |
1990年 | 139篇 |
1989年 | 137篇 |
1988年 | 118篇 |
1987年 | 72篇 |
1986年 | 76篇 |
1985年 | 67篇 |
1984年 | 54篇 |
1983年 | 33篇 |
1982年 | 43篇 |
1981年 | 47篇 |
1980年 | 49篇 |
1979年 | 43篇 |
1978年 | 32篇 |
1977年 | 27篇 |
1976年 | 14篇 |
1973年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Summary The first-order polarized basis sets for the use in high-level-correlated investigations of molecular electric properties have been generated for Pb, Bi, Po, and At. The performance of the standard [10.17.14.5/13.11.8.2] and extended [20.17.14.9/13.11.8.4] basis sets has been examined in nonrelativistic and quasirelativistic calculations for atoms and simple closed-shell hydrides. The relativistic contributions to electric dipole properties of those systems have been evaluated by using the recently developed quasirelativistic scheme. The predicted dipole polarizability of At is in good agreement with the results of other relativistic calculations. The calculated quasirelativistic dipole moments of BiH3 (–0.499 a.u.), PoH2 (–0.207 a.u.), and AtH (+0.036 a.u.) involve a significant relativistic contribution which amounts to —0.230 a.u., –0.177 a.u., and –0.097 a.u., respectively. The basis set details append this paper. They are also available as a part of the basis set library of the MOLCAS system. 相似文献
992.
本文研究了十七种2-芳基吲哚(1a-1q)在甲醇-乙酸介质中的亚甲基蓝(MB)敏化光氧化反应, 发现有十五种吲哚(1a-1o)以85%-95%的产率给出2,2'-二芳基-[2,3'-联-1H-吲哚]-3(2H)-酮(2a-2o), 而2-(4-硝基苯基]吲哚(1p)和2-联苯基吲哚(1q)则分别生成2-甲氧基-2-(4-硝基苯基)-1,2-二氢-3H-吲哚-3-酮(7p)和2-联苯基-4H-3,1-苯并恶嗪-4-酮(11q), 其中7p在分离过程中失去甲醇分子给出2-(4-硝基苯基)-3H-吲哚-3-酮(10p)。 相似文献
993.
R. Miletich 《Monatshefte für Chemie / Chemical Monthly》1995,126(4):417-430
Summary The crystal structure investigations of hydrothermally synthesized Na2[Zn2(TeO3)3]·3H2O (NZT) and Na2[Co2(TeO3)3]·3H2O (NCT) using single-crystal X-ray diffraction methods revealed a microporous framework structure of hexagonal symmetry with tubular channels running parallel to the c-axis (zemannite type). The statistically occupied sites of the Na+ ions and H2O molecules within the channels as well as the channel dimensions indicate a possible zeolitic behaviour of the compounds. Thermoanalytical investigations ofNZT show an irreversible breakdown of the framework already atT=533 K caused by complete dehydration. A reversible rehydration was solely observed for partially dehydratedNZT (T493 K); a remarkable ion exchange rate from alkali-chloride solutions was observed only for Li+. Both the rehydration and the ion exchange properties are characterized by relatively long reaction times. The relatively poor zeolitic properties compared to typical zeolitic compounds could be related to the stereochemical activity of the lone-pair electrons of the Te4+ atoms which claim a certain volume within the channels and constrict the effective channel dimension to a free diameter of approximately 2.5 Å.
Die synthetischen zeolithartigen Tellurite Na2[Me 2(TeO3)3]·3H2O (Me=Zn, Co): Kristallstruktur, De- und Rehydratisierung und Ionenaustauschvermögen
Zusammenfassung Die Kristallstrukturuntersuchungen mittels Röntgen-Einkristalldiffraktionsmethoden von hydrothermal dargestelltem Na2[Zn2(TeO3)3]·3H2O (NZT) und Na2[Co2(TeO3)3]·3H2O (NCT) ergaben eine zeolithartige Gerüststruktur hexagonaler Symmetrie mit parallel zur c-Achse angeordneten röhrenartigen Kanälen (Zemannittyp). Die statistisch besetzten Punktlagen der Na+-Atome und H2O Moleküle innerhalb der Kanäle wie auch die Kanaldimensionen selbst weisen auf mögliche zeolithische Eigenschaften der Verbindungen hin. Thermoanalytische Untersuchungen anNZT ergaben einen irreversiblen Zusammenbruch der Gerüststruktur bereits beiT=533 K durch vollständige Entwässerung. Eine reversible Rehydratisierung wurde nur für partiell entwässertesNZT (T493 K) beobachtet; ein nennenswerter Ionenaustausch aus Alkalichlorid-Lösungen konnte nur für Li+ bestimmt werden. Sowohl die Rehydratisierung als auch das Ionenaustauschvermögen sind durch relativ lange Reaktionszeiten charakterisiert. Die im Vergleich zu typisch zeolithischen Verbindungen relativ schlechten zeolithischen Eigenschaften konnten auf die stereochemische Aktivität der einsamen Elektronenpaare der Te4+ Atome zurüchgefürt werden, die einen gewissen Platz innerhalb der Kanäle beanspruchen und den effektiven freien Durchmesser der Kanäle auf etwa 2.5 Å einengen.相似文献
994.
FAN Jian-Fen② LU Yun-Xiang WANG Qiu-Xia WU Li-Fen SUN Yun-Peng 《结构化学》2005,24(4):413-418
Density functional theory (DFT) has been applied to study the enantioselective reduction of 3-morpholin-4-yl-l-phenyl-l-propanone with borane catalyzed by (S)-4-benzyl-5,5-diphenyl-l,3,2-oxazaborolidine at the B3LYP/6-31G* level. All molecular species involved in the four reaction steps have been fully optimized and the structural parameters are provided, and the micro process of reaction was also investigated. The catalyst-alkoxyborane adduct formed in step Ⅲ exhibits a B-O-B-N tetra-atomic ring. Reaction coordination calculations show that BH3 can react with 3-morpholin-4-yl-l-phenyl-l-propanone spontaneously, resulting in the need of 2 tool BH3 in the reaction. 相似文献
995.
Dongsen Mao Guanzhong Lu Qingling Chen 《Journal of molecular catalysis. A, Chemical》2005,240(1-2):164-171
The deactivation and regeneration of B2O3/TiO2-ZrO2 catalyst for the vapor phase Beckmann rearrangement of cyclohexanone oxime to -caprolactam were studied. The fresh, deactivated and regenerated catalysts were characterized by using adsorption of nitrogen, X-ray diffraction (XRD), thermogravimetry (TG) and NH3-temperature-programmed desorption (NH3-TPD) techniques. The crystal structure and pore size distribution of the catalyst were retained after reaction, but the number of acid sites decreased significantly. There was a relationship between the amount of coke deposited on the catalyst and the decline in catalytic activity. These results suggest that the coke deposition on the surface of catalyst is mainly responsible for the catalyst deactivation. The catalytic activity can be recovered completely after calcining the deactivated catalyst in air flow at 600 °C for 8 h. 相似文献
996.
Valentine G. Nenajdenko Sergey V. Druzhinin Elizabeth S. Balenkova 《Tetrahedron letters》2005,46(51):8853-8855
Reactions of α,β-unsaturated CF3-ketones with nitromethane and ethyl nitroacetate have been investigated. We found that α,β-unsaturated trifluoromethylketones react with ethyl nitroacetate in the presence of calcinated potassium fluoride to form two classes of 1,4-conjugated addition products: CF3-γ-nitroketones and 6,6,6-trifluoro-2-nitro-5-oxohexanoates in nearly quantitative yields. The products obtained are precursors for CF3-pyrrolidine synthesis. 相似文献
997.
A robust tandem HPLC method coupling size-exclusion (Shodex Asahipak GS-320HQ) and reversed phase (Vydac 218TP54) columns with ultraviolet detection was developed for quantitative determination of interferon-gamma (IFN-gamma) in Chinese hamster ovary cell culture supernatant. The 2D-HPLC system was linked up by a 6-port 2-position low hold-up volume switch valve. Compared to a commercial ELISA kit for IFN-gamma, the coupled column LC approach was able to detect and quantify soluble IFN-gamma, regardless of the glycoprotein's molecular/conformational variability and sample background. Each LC-LC analysis took 90 minutes inclusive of column regeneration. The relative standard deviation of measurements (n = 5) was less than 3%. The limit of detection (LOD) was determined to be 0.35 microg IFN-gamma. 相似文献
998.
Three-dimensional quantitative structure-activity relationships of steroid aromatase inhibitors 总被引:1,自引:0,他引:1
Summary Inhibition of aromatase, a cytochrome P450 that converts androgens to estrogens, is relevant in the therapeutic control of breast cancer. We investigate this inhibition using a three-dimensional quantitative structure-activity relationship (3D QSAR) method known as Comparative Molecular Field Analysis, CoMFA [Cramer III, R.D. et al., J. Am. Chem. Soc., 110 (1988) 5959]. We analyzed the data for 50 steroid inhibitors [Numazawa, M. et al., J. Med. Chem., 37 (1994) 2198, and references cited therein] assayed against androstenedione on human placental microsomes. An initial CoMFA resulted in a three-component model for log(1/Ki), with an explained variance r2 of 0.885, and a cross-validated q2 of 0.673. Chemometric studies were performed using GOLPE [Baroni, M. et al., Quant. Struct.-Act. Relatsh., 12 (1993) 9]. The CoMFA/GOLPE model is discussed in terms of robustness, predictivity, explanatory power and simplicity. After randomized exclusion of 25 or 10 compounds (repeated 25 times), the q2 for one component was 0.62 and 0.61, respectively, while r2 was 0.674. We demonstrate that the predictive r2 based on the mean activity (Ym) of the training set is misleading, while the test set Ym-based predictive r2 index gives a more accurate estimate of external predictivity. Using CoMFA, the observed differences in aromatase inhibition among C6-substituted steroids are rationalized at the atomic level. The CoMFA fields are consistent with known, potent inhibitors of aromatase, not included in the model. When positioned in the same alignment, these compounds have distinct features that overlap with the steric and electrostatic fields obtained in the CoMFA model. The presence of two hydrophobic binding pockets near the aromatase active site is discussed: a steric bulk tolerant one, common for C4, C6-alpha and C7-alpha substitutents, and a smaller one at the C6-beta region. 相似文献
999.
Kai-Ming Chi Hung-Chang Houa Kuo-Hsien Chen Shin-Guang Shyu Yuh-Sheng Wen 《中国化学会会志》1995,42(3):547-554
Copper(I) complexes of general formula (β-diketonate)Cu(7-AcO-NBD), where 7-AcO-NBD = 7-norbornadienyl acetate and β-diketonate = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate (1), 1,1,1-trifluoro-2,4-pentanedionate (2), 2,4-pentanedionate (3), 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionate (4), 4,4,4-trifluoro-1-phenyl-1,3-butanedionate (5), were prepared from reactions of CuCl with Na(β-diketonate) in the presence of 7-AcO-NBD. AH compounds were characterized by elemental analyses, 1H, 13C, 19F NMR and IR spectra. Single-crystal strucutre of compound 1 was determined by X-ray diffraction analysis that showed a mononuclear copper species with coordination of a chelating β-diketonate ligand through two oxygen atoms and the 7-AcO-NBD through its C? C double bond and an oxygen atom in the solid state. For compound 1, the crystal data are: triclinic, space group $ {\rm P}\bar 1 $, a= 5.4519(14) Å, b= 11.852(3) Å, c= 13.304(3) Å, α = 74.721(20)° β = 80.220(20)°, γ= 76.848(19)°, Z = 2, RF = 0.060 and Rw = 0.064. Hot-wall chemical vapor deposition experiments revealed that compound 1 is suitable as a precursor for deposition of copper films in the temperature range 170–260 °C. 相似文献
1000.
S. Nezu S. Walsh V. Meille M. Yoshida J. B. Lando 《Journal of polymer science. Part A, Polymer chemistry》1995,33(6):973-977
The dimer, trimer, and tetramer of 1,11-dodecadiyne, HC?C? (CH2)8? C?CH, were synthesized. The solid-state polymerization of the dimer was investigated by infrared (IR) spectroscopy. IR bands due to the diacetylene moiety were identified through the comparison of the IR spectra of the dimer, trimer, and tetramer. The dimer was found to have two polymorphs, melt-crystallized and solution-crystallized. Both of the polymorphs undergo solid-state polymerization by exposure to γ-ray or UV irradiation. The former has higher polymerizability for the diacetylene moiety than the latter. The solid-state polymerization of the terminal acetylene group was not observed. It is shown that the previously reported dimer structure in which both the diacetylene and terminal acetylene groups are polymerized to form an inherently electrically conducting polymer is incorrect. © 1995 John Wiley & Sons, Inc. 相似文献