On the importance of prereactive complexes in molecule-radical reactions: Hydrogen abstraction from aldehydes by OH

In this work, the OH + formaldehyde and OH + acetaldehyde reactions have be en characterized using accurate ab initio methods with, large basis sets. T he results clearly indicate that the reaction occurs by hydrogen abstractio n, and that the OH addition channel is unfavorable. Close to zero (for form aldehyde) and negative (for acetaldehyde) activation energy values are obta ined, which are in excellent agreement with the experimentally observed val ues. The reaction rate constants, calculated using the classical transition -state theory as applied to a complex mechanism involving the formation of a prereactive complex, reproduce very well the reported experimental result s. Consideration of the prereactive complex is shown to be essential for th e determination of the height of the energy barrier and thus for the correc t; calculation of thy tunneling factor.