PROTON TUNNELING IN THE LOSS OF HYDROGEN BROMIDE FROM ENERGY-SELECTEDGAS-PHASE 2-BROMOBUTANE CATIONS

The dissociation dynamics of energy-selected 2-bromobutane ions has be en investigated by photoelectron photoion coincidence (PEPICO). The da ta were collected with a conventional gas discharge light source at th e University of North Carolina and at the SuperACO synchrotron storage ring of LURE in Orsay, France. The 2-bromobutane ion dissociates via Br and HBr loss within 0.3 eV of the ionization limit. The Br loss cha nnel produces the sec-butyl ion C4H9+ via a simple bond cleavage and i s found to proceed at a fast rate. At lower ion energies, the HBr loss channel produces the 2-butene ion via a concerted mechanism with rate constants in the microsecond range; A model that involves tunneling a ssociated with the proton transfer is proposed. Asymmetric time-of-fli ght distributions and breakdown diagrams are fitted to the statistical RRKM theory with the aid of GAUSSIAN ab initio molecular orbital calc ulations al the MP2/6-31G level. The fitting of the data was optimize d for a barrier height of 0.438 eV and an imaginary frequency associat ed with the H atom transfer step of 1245 cm(-1). This compares with th e ab initio results of 0.796 eV and 1265 cm(-1) Heats of formation at 0 K for 2-bromobutane cation (880.1 +/- 2.7 kJ mol(-1)) and sec-butyl cation (794 +/- 3 kJ mol(-1)) were derived from this work.