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1.
When α,β-unsaturated γ-dimethoxymethyl cyclohexenones are excited to the S2(π,π*) state, certain unimolecular reactions can be observed to compete with S2 → S1 internal conversion. These reactions do not occur from the S1(n,π*) or the lowest T(π,π* and n,π*) states. They comprise the radical elimination of the formylacetal substituent (cf. 8 , 9 → 32 + 33 ), γ → α formylacetal migration (cf. 6 → 27 , 8 → 30 , 9 → 34 , 12 → 37 ), and a cyclization process involving the transfer of a methoxyl hydrogen to the α carbon and ring closure at the β position (cf. 6 → 28 , 8 → 31 , 12 → 38 , 20 → 40 + 41 ). The quantum yield of the ring closure 20a → 40a + 41a is 0.016 at ≤ 0.05M concentration. It is independent of the excitation wavelength within the π→π* absorption band (238–254 nm), but Φ ( 40a + 41a ) decreases at higher concentrations. According to the experimental data the reactive species of these specifically π→π*-induced transformations is placed energetically higher than the S1(n,π*) state, and it is either identical with the thermally equilibrated S2(n,π*) state, or reached via this latter state. The linear dienone 14 undergoes a similar π→π*-induced cyclization (→ 42 ) whereas the benzohomologue 26 proved unreactive, and the dienone 22 at both n → π and π→π* excitation only gives rise to rearrangements generally characteristic of cross-conjugated cyclohexadienones. 相似文献
2.
The pulsed CO2-laser-induced multiphoton dissociation of n-butyl vinyl ether is studied. Comprehensive measurements have been performed for 9.6.μm and 10.6 μm irradiation. The corresponding dissociation product distributions show distinct differences for the two irradiation wavelengths, the most important finding being that acetylene is only produced at 9.6 μm. 相似文献
3.
W.E. Cleland A. Delfosse P.-A. Dorsaz J.-L. Gloor M.N. Kienzle-Focacci G. Mancarella A.D. Martin M. Martin P. Muhlemann C. Nef T. Pal J. Rutschmann H. Zeidler 《Nuclear Physics B》1982,208(2):228-261
The mass and momentum transfer spectra of the charged system produced in the reaction π±p→Ks0K±p are analyzed. The data have been collected at the CERN SPS with the Geneva-Lausanne two-arm, non-magnetic spectrometer at 30 and 50 GeV/c incident momenta. The general features of the reactions at these energies and the results of partial-wave analyses of the two kaon system are presented.The channel is dominated by the diffractive production of even spin resonances. The spin 4 recurrence of the A2(1320) is clearly observed at 2040 MeV (Γ=380 MeV. A new resonance is observed with a mass M=2450MeV and a width Γ=400 MeV; the quantum numbers of this state are found to be IG(JPC)=1?(6++). The analysis also shows the decay of the decay of the meson ?′(1600) through the channel at both energies.The production amplitudes are determined both as a function of the effective mass and of the momentum transfer. Isoscalar natural parity exchange is dominant. The energy dependence between 10 and 50 GeV/c is shown to be well described by a Regge pole model based on the f-dominated pomeron hypothesis. We compare the production mechanisms of the 2+ resonances A2(1320) and K1(1430). Finally, we estimate the branching ratios of the spin 4 A2(2040) and spin 6 A2(2450) resonances. 相似文献
4.
The kinetics of the second step in the formation of a dis-azocompound from 4,4′-bis-diazobiphenyl (bisdiazotized benzidine) with 2-naphthol-3,6-disulfonic acid have been investigated at 25°C, ionic strength I = 0.01 in aqueous buffers (pH 6.78 to 7.71). As expected the reaction is first order with respect to the naphtholdisulfonate (triple) anion, but astonishingly it has only an apparent order n = 0.65 with respect to the electrophilic reagent. This order is due to an aggregation equilibrium in which dimeric and oligomeric aggregates are formed whose electrophilicity is at least 50 times lower than that of the monomeric diazonium ion. 相似文献
5.
(1) Phylloquinone (K), menaquinone-4 (MK-4) and α-tocopherolquinone (α-TQ) were compared with d,l-α-tocopherol as regards absorption, retention and distribution in various organs. 相似文献
6.
Phylloquinone, menaquinone-4, ubiquinone-9 and hexahydroplastoquinone-4, i.e. compounds with isoprenoid side-chains which differ by their length and degrees of unsaturation, are degraded in the rat in the same way as tocopherols. After oxidative shortening of the side-chain to seven C-atoms, the metabolites are excreted in the urine as conjugates, presumably glucuronides, which, after hydrolysis, can be isolated as γ-lactones. 相似文献
7.
T.A. Gloor M.E. Zhitomirsky T.M. Rice 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(4):491-497
We investigate multiexciton bound states in a semiconducting phase of divalent hexaborides. Due to three degenerate valleys
in both the conduction and valence bands the binding energy of a 6-exciton molecule is greatly enhanced by the shell effect.
The ground state energies of multiexciton molecules are calculated using the density functional formalism. We also show that
charged impurities stabilize multiexciton complexes leading to condensation of localized excitons. These complexes can act
as nucleation centers of local moments.
Received 24 October 2000 相似文献
8.
D. Weiss W. Shotyk H. Sch?fer U. Loyall E. Grollimund M. Gloor 《Fresenius' Journal of Analytical Chemistry》1999,363(3):300-305
Different acid compositions (HNO3, H2O2, HF and HClO4) and low pressure microwave digestion were applied to the microwave assisted dissolution of ancient peats. The digests were
evaluated with respect to the decomposition of the inorganic and organic fraction of peats and to an optimised determination
of Pb by differential pulse anodic stripping voltammetry (DPASV). Addition of hydrofluoric acid was necessary to achieve a
complete dissolution of the resistant inorganic phase of the peat and to obtain the total Pb concentration. Addition of perchloric
acid decreased the concentration of dissolved organic carbon significantly, enabling reliable voltammetric signals for Pb
determinations with satisfying peak shape, baseline, sensitivity, and precision. The limit of detection for Pb in peats was
1 ng/mL in the acid digest, equivalent to 0.2 μg/g in solid peat. This allowed the determination of low, pre-industrial concentrations
of Pb in peat bogs from various locations (Europe and SE Asia). A chelation column used to isolate the metals from the complex
matrix to further improve the detection limits failed to provide good recoveries for the peats and plants.
Received: 12 June 1998 / Revised: 24 July 1998 / Accepted: 28 July 1998 相似文献
9.
Laser polishing of diamond plates 总被引:5,自引:0,他引:5
S.M. Pimenov V.V. Kononenko V.G. Ralchenko V.I. Konov S. Gloor W. Lüthy H.P. Weber A.V. Khomich 《Applied Physics A: Materials Science & Processing》1999,69(1):81-88
Results are reported on laser polishing of 150–400-μm-thick free-standing diamond films with either a copper vapor laser (510 nm
wavelength) or an ArF excimer laser (193 nm wavelength). Studies were focused on three particular goals. First, we aimed at
a choice of optimum conditions for laser polishing of thick diamond films. It was shown that the laser polishing conditions
and the resulting surface roughness were controlled by varying the angle of incidence of a scanning laser beam and by polishing
time. Second, the laser ablation technique was applied to remove a defective layer from the “substrate” side of the diamond
plates in order to reduce optical losses due to absorption in this layer. Third, the structure of the laser-graphitized diamond
surface was studied using UV, visible, and IR optical spectroscopy techniques in the course of the “step-by-step” oxidative
removal of the graphitic layer with increasing temperature of the oxidation in ambient air. Once the graphitic layer was removed,
the optical transmission in the UV-visible-IR spectral range of the diamond films polished under optimum conditions was measured
and compared with the optical transmission of the mechanically polished diamond films. It was shown that the optical quality
(in the long-wave infrared region) of the laser-polished diamond plates was sufficient to reach the transmittance value very
close to the theoretical limit.
Received: 20 October 1998 / Accepted: 8 March 1999 / Published online: 5 May 1999 相似文献
10.
Energy spectra for decaying 2D turbulence in a bounded domain 总被引:1,自引:0,他引:1
We use results derived in the framework of the replica approach to study the liquid-glass thermodynamic transition. The main results are derived without using replicas and applied to the study of the Lennard-Jones binary mixture introduced by Kob and Andersen. We find that there is a phase transition due to the entropy crisis. We compute both analytically and numerically the value of the phase transition point T(K) and the specific heat in the low temperature phase. 相似文献