Theoretical study of the H + Br2 and Mu + Br2 reactions: A new ab initio potential energy surface and quantum dynamics calculations

A new global potential energy surface for the H + Br2 → HBr + Br reaction has been developed at the MRCI+Q/aug-cc-pVTZ ab initio level of theory. It has been found that this level calculation gives a negative barrier slightly below the H + Br2 reactant energy level. Time-dependent and time-independent quantum dynamics calculations have been performed on this potential energy surface. The calculated cumulative reaction probabilities for J = 0 around the reaction threshold energy region show a very similar behavior for both the H + Br2 and Mu + Br2 reaction, in qualitative disagreement with experimental measurements, where a positive activation energy was observed for the H + Br2 reaction while a negative activation energy for the Mu + Br2 reaction. Variational calculations of vibrational energy levels have also been performed to study dynamical vibrational bonding for the Br-Mu-Br molecule. It has been found that the system has several vibrational bonding states, whose wave functions are localized around the transition-state of the Br + MuBr′ → MuBr + Br′ exchange reaction, below the lowest Br + MuBr asymptotic energy level. This result qualitatively supports the recent experimental finding that a long-lived radical containing a Mu atom is produced in the Mu + Br2 reaction.