Abel Bernd, Buback Michael, Kling Matthias, Schmatz Stefan, Schroeder Jörg
Institut für Physikalische Chemie, Universität Göttingen, Tammannstrasse 6, 37077 Göttingen, Germany.
J Am Chem Soc. 2003 Oct 29;125(43):13274-8. doi: 10.1021/ja036304d.
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.
利用飞秒时间分辨技术研究了溶液中过氧酯叔丁基9-甲基芴-9-过羧酸酯（TBFC）的光诱导（266 nm）超快脱羧反应。虽然TBFC的光解离太快无法分辨，但中间体9-甲基芴基羰氧基自由基（MeFl-CO₂）在皮秒时间尺度上发生脱羧反应。通过泵浦-探测吸收光谱在400至883 nm波长之间监测后一过程。测得的瞬态吸收信号显示出一个主要的快速衰减，寿命为几皮秒，在较小程度上还有一个寿命约为55 ps的慢成分。采用密度泛函理论计算的分子参数对MeFl-CO₂脱羧反应进行统计建模表明，快速成分与振动热自由基的脱羧反应有关，而55 ps的衰减反映了室温下热平衡的MeFl-CO₂的解离。因此，绝大多数MeFl-CO₂自由基脱羧的时间尺度比Falvey和Schuster报道的该参考反应55 ps的时间常数预期的快约一个数量级。结果表明，该文献值指的是室温下MeFl-CO₂的脱羧速率。
