全文获取类型
收费全文 | 3669篇 |
免费 | 139篇 |
国内免费 | 26篇 |
专业分类
化学 | 2548篇 |
晶体学 | 20篇 |
力学 | 58篇 |
数学 | 625篇 |
物理学 | 583篇 |
出版年
2023年 | 27篇 |
2022年 | 65篇 |
2021年 | 99篇 |
2020年 | 89篇 |
2019年 | 108篇 |
2018年 | 121篇 |
2017年 | 86篇 |
2016年 | 144篇 |
2015年 | 129篇 |
2014年 | 153篇 |
2013年 | 209篇 |
2012年 | 249篇 |
2011年 | 268篇 |
2010年 | 181篇 |
2009年 | 122篇 |
2008年 | 246篇 |
2007年 | 245篇 |
2006年 | 216篇 |
2005年 | 191篇 |
2004年 | 153篇 |
2003年 | 121篇 |
2002年 | 100篇 |
2001年 | 46篇 |
2000年 | 32篇 |
1999年 | 37篇 |
1998年 | 34篇 |
1997年 | 22篇 |
1996年 | 38篇 |
1995年 | 26篇 |
1994年 | 31篇 |
1993年 | 17篇 |
1992年 | 14篇 |
1991年 | 11篇 |
1989年 | 10篇 |
1988年 | 11篇 |
1987年 | 6篇 |
1986年 | 10篇 |
1985年 | 9篇 |
1984年 | 11篇 |
1983年 | 11篇 |
1982年 | 13篇 |
1981年 | 8篇 |
1980年 | 10篇 |
1979年 | 9篇 |
1978年 | 7篇 |
1977年 | 7篇 |
1976年 | 13篇 |
1975年 | 13篇 |
1973年 | 7篇 |
1954年 | 5篇 |
排序方式: 共有3834条查询结果,搜索用时 15 毫秒
61.
Melanin offers protection against induction of cyclobutane pyrimidine dimers and 6-4 photoproducts by UVB in cultured human melanocytes 总被引:3,自引:0,他引:3
Smit NP Vink AA Kolb RM Steenwinkel MJ van den Berg PT van Nieuwpoort F Roza L Pavel S 《Photochemistry and photobiology》2001,74(3):424-430
The goal of this investigation was to correlate the melanin content in human pigmentary cells with the generation of UVB-induced photoproducts and to examine the relationship between the melanin content and the removal of the photoproducts. Cultured melanocytes from light-skinned individuals synthesized less melanin and produced more cyclobutane pyrimidine dimers and 6-4 photoproducts upon UVB exposure than did melanocytes from black skin. Tyrosine-stimulated melanogenesis provided protection against DNA damage in both cell types. In another set of pigmented cell lines a ratio between eumelanin and pheomelanin was determined. The assessment of association between DNA damage induction and the quantity and quality of melanin revealed that eumelanin concentration correlated better with DNA protection than pheomelanin. Skin type-I and skin type-VI melanocytes, congenital nevus (CN)-derived cells and skin type-II melanocytes from a multiple-melanoma patient were grown in media with low or high L-tyrosine concentration. The cells were irradiated with 200 J/m2 UVB, and the levels of the photoproducts were determined immediately and after 6 and 24 h. Once again the induction of the photoproducts was mitigated by increased melanogenesis, and it was inversely correlated with the skin type. No significant differences were found for the removal of photoproducts in the cultures of skin types I and VI and CN cells. No indications of a delay in the removal of photoproducts in the melanocytes from the multiple-melanoma patient were found either. 相似文献
62.
Pavel V. Pasternak 《Journal of fluorine chemistry》2004,125(12):1853-1868
A new CF2X-analogue of 1,1-bis(trifluoromethyl)-2,2-dicyanoethylene (X = P(O)(OEt)2), diethyl (1,1-difluoro-3,3-dicyano-2-trifluoromethylallyl)phosphonate, has been synthesized from diethoxyphosphoryl pentafluoroacetone 1. A similar phosphoryl analogue of ethyl 3,3-dicyano-2-trifluoromethylacrylate, ethyl 3,3-dicyano-2-[(diethoxyphosphoryl)difluoromethyl]acrylate, has been obtained from ethyl 3-(diethoxyphosphoryl)-3,3-difluoro-2-oxopropionate 2. By heterocyclization of these new ethylenes with 3-methyl-2-pyrazoline-5-ones, 3(5)-aminopyrazoles, dimedone, 2-aminopyridines, 1-aryl-3-methyl-5-aminopyrazoles, 1,3,3-trimethylisoquinolines, as well as by condensation with anilines and ketones, the difluoromethylphosphonate-substituted derivatives of 1,4-dihydropyrano[2,3-c]pyrazole, 4,5-dihydropyrazolo[1,5-a]pyrimidine, 5,6,7,8-tetrahydro-4H-chromene, 2H-pyrido[1,2-a]pyrimidine, 4,7-dihydro-1H-pyrazolo[3,4-b]pyridine, 1,4-dihydropyridine, 4,5,6,7-tetrahydro-1H-[1]pyrindine, 1,4,5,6,7,8-hexahydroquinoline, and 6,7-dihydro-2H-pyrido[2,1-a]isoquinoline have been obtained in one stage. 相似文献
63.
Konstantin N. Gavrilov Pavel V. Petrovskii AntonS. Safronov Vadim A. Davankov 《Tetrahedron》2005,61(44):10514-10520
Novel P-monodentate aryl phosphite ligands have been synthesised in one step from (R)-BINOL, (R)-H8-BINOL and (R)-H8-3,3′-dibromo-BINOL. With the new aryl phosphites, up to 86% ee was observed in the asymmetric Pd-catalysed amination of 1,3-diphenyl-2-propenyl acetate with sodium diformylamide. In the enantioselective alkylation of 1,3-diphenyl-2-propenyl acetate with dimethyl malonate, up to 97% enantioselectivity was achieved. 相似文献
64.
Jan Hlav
Miroslav Soural Pavel Hradil Iveta Fryov Jan Slouka 《Journal of heterocyclic chemistry》2004,41(4):633-636
The reactions of 5‐nitroisatine were studied with nucleophiles like heterocyclic amines and alkaline hydroxide. With the use of alkaline hydroxide it was converted into 2‐amino‐5‐nitrophenylglyoxylic acid 2 , with piperidine, morpholine and carbethoxypiperazine to its amides 4a‐4c or by oxidation to 5‐nitroanthranilic acid 7. This acid was used for synthesis of 3‐hydroxy‐6‐nitro‐2‐phenyl‐1H‐quinolin‐4‐one 10. Semicarbazone of 5‐nitroisatine 11 was converted to 5‐(2‐amino‐5‐nitrophenyl)‐2,3,4,5‐tetrahydro‐1,2,4‐triazine‐3,5‐dione 12. Cyclocondensation of this compound to afford 8‐nitro‐2,3‐dihydro‐5H‐[1,2,4]triazino‐[5,6‐b]indol‐3‐one 13 was unsuccessful. 相似文献
65.
Paul Von Ragu Schleyer Andrzej Sawaryn Alan E. Reed Pavel Hobza 《Journal of computational chemistry》1986,7(5):666-672
Electron correlation corrections have a considerable influence on the relative stabilities of lithium isocyanide ( 1 ), lithium cyanide ( 2 ), and the bridged form, 3 . While Hartree-Fock theory finds 1 to be most stable and 3 not to be a minimum, MP2/6-31G* optimization indicates 3 to be the global minimum. At higher levels employing full fourth-order Møller-Plesset theory and a quadruply split valence and polarized basis set (MP4STDQ/6-311+G*), 2 is only about 2 kcal/mol less stable than 1 and 3 , which are indicated to have nearly the same energy. LiNC thus is similar to C(Na)N and C(K)N, both of which are known to prefer T-shaped (bridged) structures in the gas phase. However, to an even greater extent than formerly realized, rotation of the lithium cation around the cyanide anion nucleus should be practically free. ΔH (LiCN) = 32.8 kcal/mol is estimated from the calculated lithium cation affinity of 151.2 kcal/mol. In addition, we find at the MP4SDTQ/6-31+G*//MP2/6-31G* level that the bridged form of NaCN is favored by 2–3 kcal/mol over the corresponding linear forms, which have nearly the same energy. 相似文献
66.
Stacking energies in low-energy geometries of pyrimidine, uracil, cytosine, and guanine homodimers were determined by the MP2 and CCSD(T) calculations utilizing a wide range of split-valence, correlation-consistent, and bond-functions basis sets. Complete basis set MP2 (CBS MP2) stacking energies extrapolated using aug-cc-pVXZ (X = D, T, and for pyrimidine dimer Q) basis sets equal to -5.3, -12.3, and -11.2 kcal/mol for the first three dimers, respectively. Higher-order correlation corrections estimated as the difference between MP2 and CCSD(T) stacking energies amount to 2.0, 0.7, and 0.9 kcal/mol and lead to final estimates of the genuine stacking energies for the three dimers of -3.4, -11.6, and -10.4 kcal/mol. The CBS MP2 stacking-energy estimate for guanine dimer (-14.8 kcal/mol) was based on the 6-31G(0.25) and aug-cc-pVDZ calculations. This simplified extrapolation can be routinely used with a meaningful accuracy around 1 kcal/mol for large aromatic stacking clusters. The final estimate of the guanine stacking energy after the CCSD(T) correction amounts to -12.9 kcal/mol. The MP2/6-31G(0.25) method previously used as the standard level to calculate aromatic stacking in hundreds of geometries of nucleobase dimers systematically underestimates the base stacking by ca. 1.0-2.5 kcal/mol per stacked dimer, covering 75-90% of the intermolecular correlation stabilization. We suggest that this correction is to be considered in calibration of force fields and other cheaper computational methods. The quality of the MP2/6-31G(0.25) predictions is nevertheless considerably better than suggested on the basis of monomer polarizability calculations. Fast and very accurate estimates of the MP2 aromatic stacking energies can be achieved using the RI-MP2 method. The CBS MP2 calculations and the CCSD(T) correction, when taken together, bring only marginal changes to the relative stability of H-bonded and stacked base pairs, with a slight shift of ca. 1 kcal/mol in favor of H-bonding. We suggest that the present values are very close to ultimate predictions of the strength of aromatic base stacking of DNA and RNA bases. 相似文献
67.
Continuous chemiluminescence determination of formaldehyde in air based on Trautz-Schorigin reaction 总被引:1,自引:0,他引:1
A new continuous method for the determination of formaldehyde in air is described. A cylindrical wet effluent diffusion denuder is used for the collection of formaldehyde from air into a thin film of absorption liquid (distilled-deionized water). Formaldehyde in the denuder concentrate is on-line detected employing a chemiluminescence flow method based on a reaction of formaldehyde and gallic acid with hydrogen peroxide in an alkaline solution. The collection efficiency of formaldehyde is quantitative at the air flow rate of 0.5 L min−1 (absorption liquid flow rate of 336 μL min−1). The limit of detection (S/N = 3) is 0.60 μg m−3 HCHO (0.49 ppb). The calibration graph is linear up to 300 μg m−3 HCHO (244 ppb). The relative standard deviations of chemiluminescence method for 1 × 10−6 and 5 × 10−6 M HCHO are 2.87% and 1.49%, respectively. Acetaldehyde interferes negligible, other compounds do not interfere. The method was employed for formaldehyde measurement in ambient air. The comparison measurement illustrates the good agreement of results obtained by proposed method with those obtained by reference fluorimetric method. 相似文献
68.
Zusammenfassung Einige Änderungen an der Apparatur zur Stickstoffschnellbestimmung nachMerz wurden vorgenommen. Dadurch wird erreicht, daß zwischen den einzelnen Verbrennungen Sauerstoff das Verbrennungsrohr durchspült und vor dem Reduktionsrohr nach außen tritt. Der Verbrennungsraum ist daher bereits zu Beginn jeder Analyse mit Sauerstoff gefüllt, wodurch günstigere Bedingungen für die schlagartige Verbrennung erzielt werden. Der Umbau der von der Firma Heraeus gelieferten Apparatur und die Herstellung eines Oxydationskatalysators (Kobaltoxid-Wolframoxid) werden beschrieben.
Contribution to the rapid nitrogen determination according to merz
Summary The apparatus for rapid nitrogen determination according toMerz has been modified. Between each two combustions, oxygen is flushed through the combustion tube and is vented before the reduction tube. Therefore, at the beginning of each analysis, the combustion chamber is filled with oxygen, providing a more favorable atmosphere for the rapid ignition. The alterations in the Hereaus apparatus and the preparation of an oxidation catalyst (cobalt oxide and tungsten oxide) are described.相似文献
69.
70.