全文获取类型
收费全文 | 2380篇 |
免费 | 130篇 |
国内免费 | 4篇 |
专业分类
化学 | 2099篇 |
晶体学 | 5篇 |
力学 | 27篇 |
数学 | 212篇 |
物理学 | 171篇 |
出版年
2024年 | 3篇 |
2023年 | 26篇 |
2022年 | 116篇 |
2021年 | 128篇 |
2020年 | 61篇 |
2019年 | 66篇 |
2018年 | 54篇 |
2017年 | 40篇 |
2016年 | 113篇 |
2015年 | 82篇 |
2014年 | 109篇 |
2013年 | 140篇 |
2012年 | 153篇 |
2011年 | 212篇 |
2010年 | 111篇 |
2009年 | 115篇 |
2008年 | 174篇 |
2007年 | 156篇 |
2006年 | 134篇 |
2005年 | 125篇 |
2004年 | 94篇 |
2003年 | 79篇 |
2002年 | 59篇 |
2001年 | 20篇 |
2000年 | 10篇 |
1999年 | 16篇 |
1998年 | 15篇 |
1997年 | 11篇 |
1996年 | 14篇 |
1995年 | 13篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1991年 | 5篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1983年 | 10篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1972年 | 2篇 |
1938年 | 1篇 |
1933年 | 2篇 |
1932年 | 1篇 |
排序方式: 共有2514条查询结果,搜索用时 31 毫秒
51.
A previously described pyrolysis device with a movable reaction zone was used in the present work for thermal degradation of cured novolac and resol resins. After the pyrolysis the usual major components were identified, as well as relatively low-volatile aromatic compounds which were revealed after their separation in a capillary chromatographic column. Among the latter components, for both types of cured resins, xanthene and its methyl derivatives were found. The isomers of bis(hydroxyphenyl)methane were found in considerable quantities only in the case of partially cured novolac resins which contained any linear sequences in their structure. 相似文献
52.
53.
Regina Y. Moriya Adilson R. Gonçalves Marta C. T. Duarte 《Applied biochemistry and biotechnology》2005,121(1-3):171-181
Organosolv (ethanol/water and acetosolv) pulps were treated with Bacillus pumilus xylanase for 4, 8, and 12 h and compared with commercial Cartazyme HS xylanase-treated pulps. Treatment of ethanol/water pulps with B. pumilus xylanase increased viscosity by 40% in 8 h of treatment compared with pulps treated without enzyme. However, acetosolv pulps treated with B. pumilus xylanase lost viscosity. Ethanol/water pulps treated with Cartazyme had a viscosity of 18.5 cP in 4 h of treatment. In the acetosolv pulps treated with commercial enzyme, the loss of viscosity was 20% compared with pulps treated without enzyme. Ethanol/water pulps treated with B. pumilus and Cartazyme had similar effects: a 44% reduction in kappa number for pulps treated with enzyme followed by alkaline extraction compared with pulps treated with alkaline extraction. In acetosolv pulps treated with B. pumilus, the kappa number was from 12 to 18, compared with pulps treated without enzyme, which had a 40% reduction in 4 and 12 h and a 60% reduction in 8 h. Cartazyme-treated acetosolv pulps had a kappa number of 14 in 4 and 8 h of treatment. For 12 h of treatment, the kappa number was 8. Fourier transform infrared spectra of the pulps showed that enzyme-treated pulps had changes in the 1000 cm?1 absorption owing to a C-O bond present in esters. Using principal component analysis, it is possible to differentiate the unbleached pulps and enzyme-treated pulps. 相似文献
54.
Morales D Pérez J Riera L Riera V Miguel D Mosquera ME García-Granda S 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(17):4132-4143
Reactions of the complex [MoCl(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)] (1) (phen=1,10-phenanthroline) with potassium arylamides were used to synthesize the amido complexes [Mo(N(R)Ar)(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)] (R=H, Ar=Ph, 2 a; R=H, Ar=p-tolyl, 2 b; R=Me, Ar=Ph; 2 c). For 2 b the Mo-N(amido) bond length (2.105(4) A) is consistent with it being a single bond, with which the metal attains an 18-electron configuration. The reaction of 2 b with HOTf affords the amino complex [Mo(eta(3)-C(3)H(4)-Me-2)(NH(2)(p-tol))(CO)(2)(phen)]OTf (3-OTf). Treatment of 3-OTf with nBuLi or KN(SiMe(3))(2) regenerates 2 b. The new amido complexes react with CS(2), arylisothiocyanates and maleic anhydride. A single product corresponding to the formal insertion of the electrophile into the Mo-N(amido) bond is obtained in each case. For maleic anhydride, ring opening accompanied the formation of the insertion product. The reaction of 2 b with maleimide affords [Mo(eta(3)-C(3)H(4)-Me-2)[NC(O)CH=CHC(O)](CO)(2)(phen)] (7), which results from simple acid-base metathesis. The reaction of 2 b with (p-tol)NCO affords [[Mo(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)](2)(eta(2)-MoO(4))] (8), which corresponds to oxidation of one third of the metal atoms to Mo(VI). Complex 8 was also obtained in the reactions of 2 b with CO(2) or the lactide 3,6-dimethyl-1,4-dioxane-2,5-dione. The structures of the compounds 2 b, 3-OTf, [Mo(eta(3)-C(3)H(4)-Me-2)[SC(S)(N(H)Ph)](CO)(2)(phen)] (4), [Mo(eta(3)-C(3)H(4)-Me-2)[SC(N(p-tol))(NH(p-tol))](CO)(2)(phen)] (5 a), and [Mo(eta(3)-C(3)H(4)-Me-2)[OC(O)CH=CHC(O)(NH(p-tol))](CO)(2)(phen)] (6), 7, and 8 (both the free complex and its N,N'-di(p-tolyl)urea adduct) were determined by X-ray diffraction. 相似文献
55.
Quintela O Cruz A Concheiro M de Castro A López-Rivadulla M 《Rapid communications in mass spectrometry : RCM》2004,18(24):2976-2982
A rapid, sensitive and selective liquid chromatography/electrospray mass spectrometry (LC/ES-MS) method was developed for the quantitative determination of the anaesthetic benzodiazepine midazolam (MID) in human saliva and plasma from patients undergoing anesthesia procedures. Biological samples spiked with diazepam-d5, the internal standard, were extracted into diethyl ether. Compounds were separated on a Xterra RP18 column using a mobile phase of acetonitrile/formic acid 0.1% at a flow rate of 0.25 mL/min under a linear gradient. Column effluents were analyzed using MS with an ES source in the positive ionization mode. Calibration curves were linear in the concentration ranges of 1-250 and 0.2-25 ng/mL in plasma and saliva, respectively. The limits of detection were 0.5 ng/mL in plasma and 0.1 ng/mL in saliva, using a 0.5-mL sample volume. The recoveries of the spiked samples were above 65%. The method was applied to ten real samples from patients undergoing midazolam treatment. 相似文献
56.
Marta Catellani Sara Deledda Françoise Hénin Jacques Muzart 《Journal of organometallic chemistry》2003,687(2):473-482
A novel reaction sequence leading to the synthesis of substituted biphenyls containing a carbonyl group in an aliphatic chain has been achieved in one-pot reaction starting from iodoarenes and allylic alcohols under the catalytic action of palladium and norbornene. The latter is temporarily incorporated into a palladacycle, which directs the reaction towards the selective formation of an arylaryl bond. Norbornene spontaneously deinserts to allow the biphenylylpalladium bond thus formed to react in its turn with the allylic alcohol. 相似文献
57.
Brown SP Pérez-Torralba M Sanz D Claramunt RM Emsley L 《Journal of the American Chemical Society》2002,124(7):1152-1153
A new method for detecting hydrogen bonds in the solid state is presented. Using two-dimensional NMR correlation experiments, it is shown that a hydrogen-bond mediated J coupling can be observed in a powder under magic-angle spinning conditions, even though the J coupling is 2 orders of magnitude smaller than the dominant anisotropic interactions encountered in solid-state NMR. Specifically, the observation of a pair of peaks in a two-dimensional 15N-15N solid-state INADEQUATE experiment due to two nitrogens that have no covalent connectivity is attributed to the presence of a J coupling across a linking hydrogen bond. 相似文献
58.
Marta Catellani Gian Paolo Chiusoli Mirco Costa 《Journal of organometallic chemistry》1995,500(1-2):69-80
Chelating or metallacycle-forming substrates are very useful for directing organometallic reactions. This review covers the more recent research that has been carried out in the authors' laboratory. Rhodium(I)- and (III)-catalysed reactions of C---C coupling of butadiene with N-allylamides or N-alkylbutenamides are described. These reactions are controlled by the size and strength of the chelate ring formed by double-bond insertion into the crotyl-rhodium bond (formed from butadiene) and their regioselectivity can change with the oxidation state of the metal. Rhodium(I)-catalysed reactions of butadiene with enamides are also chelation controlled and lead to different products, depending on the substituents at nitrogen. Cobalt(II) metallacycles have been utilized for promoting some organic reactions. It has been shown that alkenes can be catalytically incorporated into cobaltacyclopentadiene rings, that spirocycles can be obtained from diynes, carbon monoxide and acrylic esters and that a Pauson-Khand-type reaction can be combined with a Michael-type reaction to prepare catalytically new cyclopentenones. The use of palladacycles, derived from norbornene insertion into aryl-palladium bonds, followed by cyclization, has allowed the selective functionalization of either end of the metallacycle and the formation of condensed rings. Conversion of a palladium(II) into a palladium(IV) metallacycle, and catalytic processes involving these intermediates, have been achieved. The formation of alkylaromatic palladacycles has also been exploited for the selective meta functionalization of the aromatic moiety by means of alkyl groups, accompanied by expulsion of the norbornene molecule. 相似文献
59.
Dyke JM Levita G Morris A Ogden JS Dias AA Algarra M Santos JP Costa ML Rodrigues P Andrade MM Barros MT 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(5):1665-1676
The thermal decompositions of methyl azidoformate (N3COOMe), ethyl azidoformate (N3COOEt) and 2-azido-N,N-dimethylacetamide (N3CH2CONMe2) have been studied by matrix isolation infrared spectroscopy and real-time ultraviolet photoelectron spectroscopy. N2 appears as an initial pyrolysis product in all systems, and the principal interest lies in the fate of the accompanying organic fragment. For methyl azidoformate, four accompanying products were observed: HNCO, H2CO, CH2NH and CO2, and these are believed to arise as a result of two competing decomposition routes of a four-membered cyclic intermediate. Ethyl azidoformate pyrolysis yields four corresponding products: HNCO, MeCHO, MeCHNH and CO2, together with the five-membered-ring compound 2-oxazolidone. In contrast, the initial pyrolysis of 2-azido-N,N-dimethyl acetamide, yields the novel imine intermediate Me2NCOCH=NH, which subsequently decomposes into dimethyl formamide (HCONMe2), CO, Me2NH and HCN. This intermediate was detected by matrix isolation IR spectroscopy, and its identity confirmed both by a molecular orbital calculation of its IR spectrum, and by the temperature dependence and distribution of products in the PES and IR studies. Mechanisms are proposed for the formation and decomposition of all the products observed in these three systems, based on the experimental evidence and the results of supporting molecular orbital calculations. 相似文献
60.
Chaudhuri RK Freed KF Hose G Piecuch P Kowalski K Włoch M Chattopadhyay S Mukherjee D Rolik Z Szabados A Tóth G Surján PR 《The Journal of chemical physics》2005,122(13):134105
Tests have been made to benchmark and assess the relative accuracies of low-order multireference perturbation theories as compared to coupled cluster (CC) and full configuration interaction (FCI) methods. Test calculations include the ground and some excited states of the Be, H(2), BeH(2), CH(2), and SiH(2) systems. Comparisons with FCI and CC calculations show that in most cases the effective valence shell Hamiltonian (H(v)) method is more accurate than other low-order multireference perturbation theories, although none of the perturbative methods is as accurate as the CC approximations. We also briefly discuss some of the basic differences among the multireference perturbation theories considered in this work. 相似文献