Mikrochemie

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The temperature dependence of the cotton-mouton effect of ethane,ethene and ethyne

The magnetic-field-induced birefringence of the three gases ethane, ethene, and ethyne has been measured between ?80 and +120°C, at pressures up to 1 bar. The molar Cotton-Mouton constants depend linearly on the reciprocal absolute temperature which is in agreement with the well-known theory for non-interacting molecules. Magnetic hyperpolarizability anisotropy parameters, susceptibility anisotropies, and some secondary quantities of these molecules have been determined for the first time.     

Bücherbesprechungen

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Bücherbesprechungen

Ohne Zusammenfassung     

The role of odd-electron intermediates and in-cage electron transfer in ultrafast photochemical disproportionation reactions in Lewis bases

Femtosecond visible pump-IR probe studies of Cp2W2(CO)6 in P(OMe)3 and CH2Cl2 have allowed direct observation of a 19-electron intermediate and of disproportionation into CpW(CO)3- and CpW(CO)3P(OMe)3+ on the ultrafast time scale. A new disproportionation mechanism involving in-cage electron transfer between a 19-electron intermediate and a 17-electron radical has been proposed.     

A seemingly well understood light-induced peroxide decarboxylation reaction reinvestigated with femtosecond time resolution

The photoinduced (266 nm) ultrafast decarboxylation of the peroxyester tert-butyl 9-methylfluorene-9-percarboxylate (TBFC) in solution has been studied with femtosecond time resolution. While the photodissociation of TBFC occurs too fast to be resolved, the intermediate 9-methylfluorenylcarbonyloxy radical (MeFl-CO(2)) decarboxylates on a picosecond time scale. The latter process is monitored by pump-probe absorption spectroscopy at wavelengths between 400 and 883 nm. The measured transient absorbance signals reveal a dominant fast decay with a lifetime of a few picoseconds and, to a minor extent, a slow component with a lifetime of about 55 ps. Statistical modeling of MeFl-CO(2) decarboxylation employing molecular parameters calculated by density functional theory suggests that the fast component is associated with the decarboxylation of vibrationally hot radicals, whereas the 55 ps decay reflects the dissociation of thermally equilibrated MeFl-CO(2) at ambient temperature. The vast majority of MeFl-CO(2) radicals thus decarboxylate on a time scale about an order of magnitude faster than expected from the time constant of 55 ps reported by Falvey and Schuster for this reference reaction. This literature value turns out to refer to decarboxylation rate of MeFl-CO(2) at ambient temperature.