全文获取类型
收费全文 | 350531篇 |
免费 | 3595篇 |
国内免费 | 1001篇 |
专业分类
化学 | 186158篇 |
晶体学 | 5436篇 |
力学 | 15879篇 |
综合类 | 18篇 |
数学 | 39551篇 |
物理学 | 108085篇 |
出版年
2020年 | 3033篇 |
2019年 | 3456篇 |
2018年 | 4704篇 |
2017年 | 4638篇 |
2016年 | 6939篇 |
2015年 | 3965篇 |
2014年 | 6344篇 |
2013年 | 14745篇 |
2012年 | 11454篇 |
2011年 | 13654篇 |
2010年 | 10076篇 |
2009年 | 9839篇 |
2008年 | 12661篇 |
2007年 | 12551篇 |
2006年 | 11617篇 |
2005年 | 10605篇 |
2004年 | 9737篇 |
2003年 | 8823篇 |
2002年 | 8613篇 |
2001年 | 9401篇 |
2000年 | 7135篇 |
1999年 | 5481篇 |
1998年 | 4756篇 |
1997年 | 4786篇 |
1996年 | 4570篇 |
1995年 | 4237篇 |
1994年 | 4145篇 |
1993年 | 4050篇 |
1992年 | 4612篇 |
1991年 | 4562篇 |
1990年 | 4396篇 |
1989年 | 4475篇 |
1988年 | 4345篇 |
1987年 | 4387篇 |
1986年 | 4164篇 |
1985年 | 5442篇 |
1984年 | 5597篇 |
1983年 | 4685篇 |
1982年 | 4964篇 |
1981年 | 4735篇 |
1980年 | 4609篇 |
1979年 | 4934篇 |
1978年 | 5036篇 |
1977年 | 5156篇 |
1976年 | 5135篇 |
1975年 | 4851篇 |
1974年 | 4642篇 |
1973年 | 4878篇 |
1972年 | 3439篇 |
1971年 | 3024篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The INDO and CNDO/S methods were used to calculate changes in the energy of the ground state and the energy of the first singlet-singlet transition of phenyl benzoate permitted with respect to its multiplet nature with consistent rotation of phenyl groups of the molecule in the cis conformation. It was found that in the lowest excited singlet state of the phenyl benzoate molecule an adiabatic photoreaction can occur, as a result of which the energy of the S0-S1 electron transition decreases to values of 2 eV. With motion of the molecular system along the coordinate of the photoreaction, an internal-conversion process is initiated, which leads to the experimentally observed absence of luminescence of phenyl benzoate solutions.Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 21, No. 3, pp. 365–367, May–June, 1985. 相似文献
992.
Abstract— The fluorescence decay kinetics of the reduced nicotinamides NMNH, NADH and NADPH in aqueous solution were investigated using an Ar ion laser, mode locked in the UV, as source of excitation and single photon counting electronics in the detection system allowing for a time resolution in the picosecond range. Analysis of the experimental fluorescence decay showed that the dinucleotides did not follow a single exponential decay law. Good fitting was accomplished with a sum of two exponentials. The mononucleotide fluorescence decay was a single exponential for at least 95% of its amplitude.
The heterogeneity in lifetimes of the fluorescence of the dinucleotides was interpreted in terms of an exciplex mechanism. 相似文献
The heterogeneity in lifetimes of the fluorescence of the dinucleotides was interpreted in terms of an exciplex mechanism. 相似文献
993.
P. J. Hauser A. F. Schreiner J. D. Gunter W. J. Mitchell M. K. Dearmond 《Theoretical chemistry accounts》1972,24(1):78-88
MCD, electronic absorption, external heavy atom, and crystal field data are presented for the low energy region (2
E
g, 2
T
1g, 4
T
2g) and high energy region (2
T
2g, 4
T
1g) of Cr(dtp)3, Cr(dtc)3, and Cr(exan)3. At low energy, MCD intensities of 2
E(2
E
g) and 2
E(2
T
g) are as large or larger than 4
T
2g, and the MCD technique is advantageous over electronic absorption in this respect. The MCD positions of 2
E
g and 2
T
1g are nearly the same for these molecules ( 13 kK and 13.6 kK) · 4
T
2g of this region appears trigonally split ( 500 cm–1) in the MCD of dtp but to a smaller extent than in the electronic crystal spectrum of Lebedda and Palmer ( 600 cm–1). MCD did not resolve such components for exan and dtc. The higher energy region includes 2
T
2g and 4
T
1g, and the combined MCD and electronic absorption data of the three compounds taken together lead us to conclude the ordering 2
A
1(2
T
2g)<2
E(2
T
2g)<4
E(4
T
1g). The potentially useful external heavy atom affect on the solution-observed electronic 2
E and 4
E bands of Cr(dtp)3 did not shed additional light on this order of E states. Finally, it is concluded that the order of 4
T
1g and 2
T
2g cannot be decided from O
h crystal field calculations because of experimental uncertainties about choosing centers of gravity. In addition, 4
T
1g and 2
T
2g are close together so that ordering 2
E<4
E does not guarantee 2
T
2g<4
T
1g. However, it can be concluded that the ratio C/B4 is not correct, whereas the larger 7<(C/B)<8 is consistent with the data of all three molecules because of small B parameters ( 0.4). Locating OO transitions may somewhat decrease C/B and Dq.
Presented in part at the 161st American Chemical Society National Meeting, Los Angeles, California, March–April, 1971.
NDEA Pre-Doctoral Fellow. 相似文献
Zusammenfassung In der vorliegenden Arbeit werden folgende Meßergebnisse mitgeteilt; MCD, elektronische Absorption, Einfluß eines äußeren schweren Atoms sowie Kristallfelddaten für den Bereich niedriger Energie (2 E g, 2 T 1g, 4 T 2g) und den Bereich hoher Energie (2 T 2g, 4 T 1g) von Cr(dtp)3, Cr(dtc)3 und Cr(exan). Bei niedriger Energie sind die MCD-Intensitäten von 2 E(2 E g) und 2 E(2 T 1g) genau so groß, oder größer als 4 T 2g, und die MCD-Technik bietet Vorteile gegenüber der elektronischen Absorptionsmethode. Die MCD-Werte von 2 E g und 2 T 1g sind für die genannten Moleküle etwa gleich ( 13 kK und 13,6 kK). 4 T 2g dieses Gebietes erscheint trigonal aufgespalten ( 500 cm–1) bei MCD von dtp, aber in einem geringeren Maß als im elektronischen Kristallspektrum von Lebedda und Palmer ( 600 cm–1) MCD löste solche Komponenten bei exan und dtc nicht auf. Der Bereich höherer Energie enthält 2 T 2g und 4 T 1g, und aus der Kombination von Daten der MCD-Methode sowie der elektronischen Absorption schlossen wir auf die Anordnung 2 A 1g(2 T 2g)<2 E(2 T 2g)<4 E(4 T 1g). Der möglicherweise nützliche Effekt eines äußeren schweren Atoms auf die in Lösung beobachteten elektronischen 2 E- und 4E-Banden von Cr(dtp)3 brachte bezüglich dieser Anordnung der E-Zustände nichts Neues. Weiterhin wird gefolgert, daß die Ordnung von 4 T 1g und 2 T 2g nicht aus O h-Kristallfeldberechnungen entschieden werden kann, da experimentelle Unsicherheiten bezüglich der Wahl von Schwerpunkten bestehen. Außerdem liegen 4 T 1g und 2 T 2g nahe zusammen, sodaß aus der Anordnung 2 E<4 E nicht notwendig 2 T 2g<4 T 1g folgt. Es kann jedoch gefolgert werden, daß das Verhältnis C/B4 nicht korrekt ist, während 7<(C/B)<8 konsistent mit den Daten aller drei Moleküle ist, da die B-parameter klein sind (0,4). Die Vokalisierung der OO-Übergänge könnten C/B und Dq etwas erniedrigen.
Presented in part at the 161st American Chemical Society National Meeting, Los Angeles, California, March–April, 1971.
NDEA Pre-Doctoral Fellow. 相似文献
994.
H. Müller 《Theoretical chemistry accounts》1971,21(1):110-114
The linear ions Br
4
2–
which has been discovered for the first time by Siepmann and Schnering [13] in W6Br16 have been studied theoretically using the free electron model for the valence electrons. Electronic structure, binding energy and charge distribution show that Br
4
2–
is a typical electron deficient compound stabilized by a 4c-6e bond.
Herrn Prof. Dr. H. Dunken bin ich für sein stetes Interesse und die rege Anteilnahme an diesen Modellrechnungen zu großem Dank verpflichtet. 相似文献
Herrn Prof. Dr. H. Dunken bin ich für sein stetes Interesse und die rege Anteilnahme an diesen Modellrechnungen zu großem Dank verpflichtet. 相似文献
995.
A. P. Gulea S. N. Spynu V. I. Tsapkov D. Poirier J. Roy 《Russian Journal of General Chemistry》2006,76(9):1458-1463
Isatin (L1) and N-methylisatin (L2) β-thiosemicarbazones react in ethanol with Cu(II) chloride and bromide in the presence of sulfanilamide (Streptocid, Sf1), N-acetylsulfanilamide (Sulfacyl, Sf2), Norsulfazole (Sf3), Aethazolum (Sf4), and Sulfadimesine (Sf5) to form coordination compounds Cu(Sf1–5)L1–2X2 · nH2O (X = Cl, Br; n = 2–5). All the complexes have a monomeric structure. Thiosemicarbazones L1 and L2 in these complexes are tridentate O,N,S ligands, and sulfanilamides Sf1–5 are monodentate ligands. Thermolysis of the substances involves the steps of dehydration (70–95°C) and complete decomposition (410–530°C). 相似文献
996.
V. N. Reshetnikova E. A. Filatova V. V. Kuznetsov 《Journal of Analytical Chemistry》2007,62(11):1013-1016
The concepts of fuzzy logic were applied to the analysis of gas-chromatographic data for 12 samples of Russian vodka products. It was found that, with the use of the truth degrees of fuzzy logic propositions on the composition of analytical samples based on a simulation of expert’s reasoning, the grade of raw materials used for the production of the analytical samples can be supposed and the conformity with standards can be established. 相似文献
997.
V. E. Proskurina M. N. Akopyan V. A. Myagchenkov 《Russian Journal of Applied Chemistry》2006,79(7):1146-1152
The kinetics of hindered sedimentation and thickening of the sediments of a real disperse system (bentonitic clay suspension) was studied. The flocculation parameters of acrylamide copolymers in aqueous and aqueous salt solutions were evaluated in relation to the chemical nature and concentration of flocculants and electrolytes. 相似文献
998.
Sadovskii Yu. S. Solomoichenko T. N. Kas'yan A. O. Golodaeva E. A. Anikanova S. V. Kas'yan L. I. Savelova V. A. 《Russian Journal of Organic Chemistry》2004,40(1):50-56
The nucleophilic reactivity of amines of the norbornane, norbornene, and adamantane series toward ptoluenesulfonyl chloride and diphenyl chlorophosphate in acetonitrile at 25°C is determined mainly by the steric factor. Parameters characterizing spatial accessibility of the reaction center in the amine molecule have been determined. Cage-like substituents show no appreciable effect on the amine reactivity, as compared to common alkyl groups. 相似文献
999.
A. A. Akhrem L. I. Ukhova A. P. Marochkin V. M. Gorul'ko N. I. Garbuz 《Chemistry of Heterocyclic Compounds》1976,12(7):780-784
A mixture of the corresponding 4-(4-dimethylamino-1,2-butadienyl)- and 4-(4-dimethylamino-1-butynyl)decahydro-4-quinolols, with predominance of the allene components, is formed from each vinylacetylenic alcohol as a result of the addition of diethylamine to epimeric (at the 2 and 4 positions) trans-2-methyl- and trans-1,2-dimethyl-4-vinylethynyldecahydro-4-quinolols. On the basis of the PMR spectra and data on the stabilities of allenic and acetylenic diamino alcohols under the conditions of their formation, it was concluded that the addition of dimethylamine to 4-vinylethynyldecahydro-4-quinolols proceeds simultaneously via two pathways — at the 1,4 and 3,4 positions of the vinylethynyl substituent.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 940–945, July, 1976.The authors thank A. S. Fridman and T. E. Prokof'ev for their participation in the discussion of the PMR spectra. 相似文献
1000.
V. P. Zvolinskii P. I. Zakharov V. K. Shevtsov A. V. Varlamov V. G. Pleshakov G. A. Vasil'ev N. S. Prostakov 《Chemistry of Heterocyclic Compounds》1978,14(2):200-203
The dissociative ionization of 4-azafluorene and its methyl and phenyl derivatives was investigated. The relative intensity of the [M — CH3]+ ion peak depends on the position of the CH3 group in the 4-azafluorene ring. It was established that the loss of an RCN particle (R=H, CH3, and C6H5) for unsubstituted 4-azafluorene takes place from the M+ and [M — H]+ ion, exclusively from the [M — H]+ ion for the methyl-substituted compounds, and from the [M — H]+ and [H — 2H]+ fragments for the phenyl-substituted derivatives. Randomization of the deuterium ions in the 9,9-d2-4-azafluorene molecular ion was observed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 246–250, February, 1978. 相似文献