共查询到20条相似文献,搜索用时 31 毫秒
1.
Energy transfer from Bi3+ to Nd3+ is reported in germanate glass. It was found that the excitation range and intensities of the 4F → 4I, 4I emissions are increased several fold when excited through 1S0 → 3P1 absorption of Bi3+. It is shown that the energy transfer is nonradiative. The energy transfer probability and efficiency were calculated from the Bi3+ fluorescence decay rates and intensities. The Bi3+ → Nd3+ energy transfer may be utilized in Nd3+ glass laser. 相似文献
2.
3.
An amide group is shown to be capable of intramolecular participation in the formation of an 1,3-dioxolan 2-ylium cation giving a tricyclic organic cation of a new type ,. This cation may be considered as a model of an electrophilic intermediate which might be formed from a peptidyl t-RNA during protein biosynthesis.The tricyclic salt , reacts with water and methanol like a classical dioxolenium salt. An amide acetal can be obtained from dimethylamine and 5a; hydrolysis of does not lead to the corresponding amide but to the cleavage of the CN. bond. 相似文献
4.
The conformations and the rotational barriers of the 2-substituted 1,3-diphenylallylanions – (Tab. 2) have been determined. Increasing size of the substituents leads to more ,- and ,-conformers at the cost of the ,-species. This trend is connected with decreasing ΔG3-value sof the rotational barriers; the barriers are essentially not affected by ion pair effects, which is in contrast to the parent “allyl anion”. 相似文献
5.
Two cleavage products, -dimethylmuconate () and -dimethyl-muconate (), are inefficiently produced in the 9,10-dicyanoanthracene (DCA) sensitized photooxidation of -dimethoxybenzene (-DMB) in polar solvents. An electron transfer mechanism is proposed in which superoxide ion (O?20 combines with ortho-dimethoxybenzene to form a dioxetane (3), which cleaves to form (). The initial products is , which is rapidly converted to and other products under the conditions; cis-trans isomer is formed. 相似文献
6.
Perfluoroalkyl iodides react readily with simple olefins in the presence of catalytic amounts of copper in diglyme or acetic anhydride to give adducts in high yields. The reaction can be moderately accelerated by irradiation, partly suppressed with -DNB and inhibited completely by hydroquinone. Fluorine-containing non-volatile amorphous solids were obtained when easily polymerized olefins, such as styrene and acrylates, were used as substrates. The reaction of perfluoroalkyl iodides and diallylic ether gave five-membered ring products under the same conditions. All the results seem to indicate that the reaction is a radical chain process induced by electron transfer. In DMSO, the main reaction may involve perfluoroalkylcopper intermediates which can be trapped by iodobenzene and hydroquinone and -DNB show little inhibition effects. 相似文献
7.
In electrothermal atomic absorption spectrometry, the signals corresponding to total absorption (TA), background(BG) adn the required atomic absorption signal (TA - BG) show good temporal reproducibility. For improvement of the detection power, the time-resolved signals can be summed with the aid of a microcomputer. The summed signal height is exponentially related to the number (N) of measurements up to N = 40. The detection power is improved according to the N law with a practical limit at summation of ca. 20 signals. Concentrations which are around the detection limit in single measurements (lead and cadmium in drinking water or lead in urine) can readily be determined by summation of signals from, for example, sixteen 20-μl injections of urine. Resulting high summed backgrounds, giving absorbances > 5, can be compensated. 相似文献
8.
O. Glemser 《Journal of fluorine chemistry》1984,24(3):319-325
The structures of the solid fluorides MF2, MF3, MF4 and MF5, in which M has the coordination number 6 and belongs to the 3, 4- and 5-periods and the Vb to VIII groups, can be divided into 3 types: (a) cubic close packing () of F with an MFM bridging angle of 180°; (b) hexagonal close packing () with an MFM-bonding angle of 132°; (c) intermediate packing between (a) and (b). The linear bridging is assumed to be a consequence of π-back bonding (or charge transfer) between F-orbitals and -orbitals of the metal. If such bonding is not possible then with the bridging angle of 132° will result. Weaker π-interactions give the intermediates (c). 相似文献
9.
Unsymmetrical disulfides formed from -cysteine esters and -Cbz--alaninyloxy-benzenethiol , 2-Cbz--alaninyloxy-5-chlorophenylmethylenethiol ,4-acetoxyxanthenylmethylene thiol , and 1,5-diacetoxy-2-methyl-3-methoxy-4-thioxanthone are observed to undergo intramolecular O,N-acyl transfer in yields up to 60%, with accompanying disulfide interchange. The significance of these results for a general amide forming strategy of prior cysteine capture are discussed. 相似文献
10.
The first enantioselective syntheses of (+) dictyopterene A and (+)dictyopterene C′ were reported. The key reaction was based on palladium promoted cyclisation of chiral allylic benzoate with transfer of chirality (anti attack of the palladium with respect to the leaving group) to give optically active vinylcyclopropane with (R) configuration which contains proper functionality for further elaboration into and 相似文献
11.
Waldemar Adam Klaus Zinner Adolf Krebs Hermann Schmalstieg 《Tetrahedron letters》1981,22(46):4567-4570
The rather stable 1,2-dioxetanes () and (), derived from the sterically stabilized cyclobutadiene (), exhibit distinct enhanced chemiluminescence behavior, namely energy transfer chemiluminescence (ETC) for () and electron exchange chemiluminescence (EEC) for (). 相似文献
12.
Thioanisole derivatives () were found to be oxygenated by a reconstituted system of purified cytochrome P-450 to give sulfoxides (). Logarithms of Vmax were found to be correlated linearly with one electron oxidation potentials of the sulfides (Ep) suggesting the oxygenation to proceed via one electron transfer from sulfides to the active species of the enzyme. 相似文献
13.
Ernst-Ulrich Würthwein Jayaraman Chandrasekhar Eluvathingal D Jenmis Paul von Ragué Schleyer 《Tetrahedron letters》1981,22(9):843-846
Semi-empirical (MINDO/3 and UNDO) MO calculations on highly strained planar tetracoordinate carbon candidates indicate the central carbons in - [4.4.4.4] fenestrane () to have pyramidal () and in trans-[4.4.4.]fenestrane () to have distorted tetrahedral () geometries. In [2.2.2.2]paddlane (), the two central carbons are pentacoordinate. Each is nearly coplanar with four carbon neighbours; additionally, the two bridgehead carbons are connected by a single bond (). 相似文献
14.
Diastereomeric -2-norbornylmagnesium bromide and mixtures of - and -2-norbornylmagnesium bromide react stereospecifically with retention of configuration at the carbon-magnesium bond with formaldehyde to form the expected primary alcohol. This stereochemical result, which is comparable to that observed previously in electrophilic substitutions of stereoisomeric organomagnesium reagents, is observed even when 5 mole % ferric chloride is added. The stereospecificity observed both with and without added metal salts suggests that single electron transfer is not important in 1,2 addition of -alkylmagnesium reagents to simple aliphatic aldehydes. 相似文献
15.
Simon S. Jones Colin B. Reese Samson Sibanda Aiko Ubasawa 《Tetrahedron letters》1981,22(47):4755-4758
The protection of uracil and 2--acyl guanine residues with 4--phenyl [or 4--(2,4-dimethylphenyl)] and 6--(2-nitrophenyl) groups as in [or ] and , respectively, is described. These -aryl protecting groups, which appear to withstand the usual conditions of oligonucleotide synthesis, may readily be removed by treatment with 2-nitrobenzaldoximate ions. 相似文献
16.
Anions of β,β-disubstituted acetyl acetates (, CH2COCR1R2COOR3; R1= R2 = Me2 R1 = Et, R2 = COOMe, R1 = Et, R2 = SPh) add to 1-tosyl-3,4,4-trimethyl-Δ -imidazolinium iodide () to give the corresponding imidazolines (), which are regarded as models of methylenetetrahydrofolate These models transfer C(2) of the imidazalodine -with its appended carbon fragment- to tryptamine and 2-(3,4-dimethoxybenzene)ethyl amine to yield enaminoketo ester intermediates which can be converted into benzo[a]quinolizine and indolo[2,3-a]quinolizine derivatives, respectively, in two cyclization steps. The intermediate derived from transfer of carbon fragment CH-CH2COC(SPh) (Et) COOMe to tryptamine exhibits reactions which involve nucleophilic displacement at a sulfur atom of a thio ether bond. 相似文献
17.
The combination of orthogonal collocation and the heterogeneous equivalent technique is extended to simulate cyclic voltammograms of fast second-order follow-up reactions coupled to an electron transfer at an electrode surface. The (reversible electron transfer with irreversible follow-up dimerization) and 2 (reversible electron transfer with irreversible second-order follow-up reaction) models are considered. The non-linear boundary equations are solved numerically. No linear approximation of the concentration profiles is required. The use of non-linear space coordinate transformations is described. Peak potential and peak current function results are compared with literature values and agreement is found. The transition between the second-order 2 and the corresponding first-order mechanisms is discussed. 相似文献
18.
The structures and syntheses of prenylhydroquinone (), 6-hydroxy-2,2-dimethylchromeme (), and prenylquinone (), which are natural products isolated from the marine colonial tunicate , are described. Prenylhydroquinone shows activity against P388 lymphocytic leukemia (T/C=138). Both and significantly inhibit the mutagenic effects of benzo(a)pyrene, aflatoxin B1 and ultraviolet light on , and may be cancer protective agents. 相似文献
19.
Virtually complete chirality transfer is observed in the [2,3] sigmatropic rearrangement of the anion derived from trialkylstanylmethyl ()- or ()-allylic ethers. 相似文献
20.
The title compound // may be transformed into the 3-/methylaminomethylene/--2,3,6,7,8,9-hexahydro-4H-pyrido[1,2-a]pyrimidine-2,4-dione /E, Z/ which is in equilibrium with the 1,2,3,6,7,8-hexahydro tautomer /E, Z/ in DMSO-d6. 相似文献