Ultraviolet photochemistry of hydrogen-bonded HBr center dot acetone complexes in argon matrices

We have studied the ultraviolet photoreaction of HBr . acetone complexes is olated in argon matrices using Fourier-transform infrared spectroscopy. Pho toexcitation of the acetone pi*<--n(0) transition, using either 266 or 309 nm, results in the reaction of hydrogen-bonded HBr<bullet>acetone complexes to produce HBr . propen-2-ol complexes through a novel photoenolization me chanism. Only the hydrogen-bonded HBr . acetone complexes undergo reaction following mid-UV irradiation. No significant loss of isolated reactant mole cules occurs using either 266 or 309 nm excitation. Detailed ab initio calc ulations of HBr . propen-2-ol complex are presented to support these findin gs. Photoexcitation of similar matrices at 193 nm, which excites the aceton e 3s<--n(0) transition and HBr directly, shows extensive reaction of HBr, a cetone, and their complexes by a photofragmentation mechanism. We discuss t he mechanism of HBr assisted acetone photoenolization in detail and report experimental and calculated band frequencies and structures of HBr<bullet>a cetone complexes. (C) 2001 American Institute of Physics.