Time-resolved resonance Raman and density functional theory study of the photochemistry of 4-benzoylpyridine in acetonitrile and 2-propanol

Abstract

A nanosecond time-resolved resonance Raman (ns-TR 3) spectroscopic investigation of the intermolecular hydrogen-abstraction reaction of the triplet state of 4-benzoylpyridine (4-BPy) in 2-propanol solvent is reported. The TR 3 results reveal a rapid hydrogen abstraction (<10 ns) by the 4-BPy triplet state (nπ*) with the 2-propanol solvent, leading to formation of a 4-BPy ketyl radical and an associated dimethyl ketyl radical partner from the solvent. The recombination of these two radical species occurs with a time constant about 200 ns to produce a para-N-LAT (light absorbing transient). The structure, major spectral features, and identification of the ketyl radical and the para-N-LAT coupling complex have been determined and confirmed by comparison of the TR 3 results with results from density functional theory (DFT) calculations. A reaction pathway for the photolysis of 4-BPy in 2-propanol deduced from the TR 3 results is also presented. The electron-withdrawing effect of the heterocyclic nitrogen for 4-BPy on the triplet state makes it have a significantly higher chemical reactivity for the hydrogen abstraction with 2-propanol compared to the previously reported corresponding benzophenone triplet reaction under similar reaction conditions. In addition, the 4-BPy ketyl radical reacts with the dimethyl ketyl radical to attach at the para-N atom position of the pyridine ring to form a cross-coupling product such as 2-[4-(hydroxy-phenyl-methylene)- 4h-pyridin-1-yl]-propan-2-ol instead of attacking at the para-C atom position as was observed for the corresponding benzophenone reaction reported in an earlier study. Copyright © 2008 John Wiley & Sons, Ltd.