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101.
James Ferguson 《Chemical physics letters》1975,36(3):316-321
The transmission, absorption (excitation spectra) and fluorescence spectra of thin (? 47 nm) free mounted anthracene flakes have been measured. True absorption in b polarization in the region of the lowest exciton state occurs as a result of scattering by phonons. It has a minimum near the transverse exciton frequency and a maximum near the longitudinal exciton frequency, in agreement with expected polariton behaviour. Thickness dependent polariton states have finite absorption and fluorescence transition probabilities (due to crystal inhomogeneities) and are observed below the transverse exciton frequency. These polariton states represent the energy reservoir for excitation energy in the bulk of the crystal. A surface-induced exciton state is the origin of the sharp line fluorescence from pure crystals and accounts for the high efficiency of stimulated fluorescence at low temperatures. Stimulated fluorescence can also be observed from the polariton modes when excitation occurs in these modes. Resonance interactions between polariton modes and impurity levels represent an important pathway for fluorescence quenching in crystalline anthracene. 相似文献
102.
In an extension of recently reported syntheses of the I-azaphenoxathiin nucleus as well as several 7-substituted analogs, the synthesis of several 9-substituted members of this series is now reported. In addition, the first 13C-nmr spectral evidence of an interaction between a sulfur atom and the oxygen of an ortho-nilro group which has been previously observed only in X-ray erystallographic studies is also described. The possible consequences of this interaction on the reaction pathway leading to the cyclization of the 9-substituted J-azaphenoxathiin nucleus is also presented. 相似文献
103.
James Grimshaw Charles S. Sell Reginald J. Haslett 《Journal of mass spectrometry : JMS》1974,8(1):381-386
The mass spectra of some methoxy and methyl derivatives of 2-methylbenzophenone have been examined. Loss of a substituents from 3′-and 4′-positions as well as the previously known loss from 2′-positions are important fragmentation processes. Thus, these fragmentations are of little use in locating substituents on benzophenones of unknown structure. Deuterium labelling shows the [M - 1]+ ion from 3′,4,4′,5,5′-pentamethoxy-2-methyl benzophenone to be due largely to loss of hydrogen from 2′- and 6′-positions. 相似文献
104.
James M. Malloy Peter N. Keliher Malcolm S. Cresser 《Spectrochimica Acta Part B: Atomic Spectroscopy》1980,35(11-12)
The indirect evidence for a change in nebulization efficiency upon discrete sample nebulization has been critically appraised. Direct measurements of the effect of sample size upon nebulization efficiency have been made, by exploiting the reproducible droplet size obtained on allowing free-fall of droplets from a peristaltic pump outlet into a polypropylene capillary funnel. The observed enhancements in efficiency with decreasing sample size are discussed in relation to changes in measured aerosol droplet size distributions. The latter also explain the reduction in the incidence and extent of certain types of interferences. 相似文献
105.
Thermally-induced reaction between bicyclo[5.1.0]octa-2,5-diene (I) and sulphur dioxide under dry conditions is toluene-d8 as solvent leads to the unexpected formation of the hitherto unknown diene sulphone 7-thiabicyclo[4.2.1]nona-2,4-diene 7,7-dioxide (II). 相似文献
106.
The radiative lifetime of the A2Σ+(υ′ = 0, K′ = 1, J′ = 3/2) state of OH and OD has been directly measured by following the decay of fluorescence excited by light from a frequency doubled dye laser. Stern-Volmer extrapolation of the results to zero pressure gave τ(OH) = 788 ± 13 ns and τ(OD) = 754 ± 12 ns. 相似文献
107.
A rapid and sensitive method is described for the quantitative colorimetric determination of uranium(Vl). Masking agents such as EDTA, tartaric acid or thioglycerol make possible the determination of uranium in the presence of many foreign metal ions. If thorium is present, a preliminary extraction of uranium as the diethyldithiocarbamate is required. 相似文献
108.
Michael T. Cleary James L. Owens Glenn Dryhurst 《Journal of Electroanalytical Chemistry》1981,123(2):265-290
The electrochemical oxidation of 9-methylxanthine proceeds via four voltammetric oxidation peaks at the pyrolytic graphite electrode. The first voltammetric oxidation peak (peak Ia) is a 1e reaction giving a radical which dimerizes to 8,8′-bi-9-methyl-9H-purine-2,6-(1H, 3H)-dione. Peak IIa is a further 2e electrooxidation of the peak Ia dimer to another yellow dimer 8,8′-bi-9-methyl-9H-purine-2,6-(1H)-dione-3,5-(3H)-diiminylidene. This dimer is not very stable and it hydrolyzes to 1-methyl allantoin. Peak IIIa is an adsorption pre-peak to peak IVa which corresponds, overall, to a direct 4e?4H+ electrooxidation of 9-methylxanthine to an unstable diimine of 9-methyluric acid. Hydrolysis of this diimine leads to a variety of ultimate products. 相似文献
109.
James H. RigbyMona Aasuml 《Tetrahedron letters》2003,44(27):5029-5031
Highly substituted benzoxepins can be prepared via BBr3-induced rearrangement of functionalized dihydrofurans derived from carbonyl ylide [3+2] cycloaddition. 相似文献
110.
The stereochemistry and regiochemistry of cycloadditions of α-amino acid ester imines is dependent both on imine structure and on the reactivity of the dipolarophile. Phenylglycine imines undergo competing dipole stereomutation and cycloaddition with some dipolarophiles. 相似文献