Communication: Effective spectroscopic Hamiltonian for multiple minima with above barrier motion: Isomerization in HO2

We present a two-dimensional potential surface for the isomerization in the hydroperoxyl radical HO2 and calculate the vibrational spectrum. We then show that a simple effective spectroscopic fitting Hamiltonian is capable of reproducing large scale vibrational spectral structure above the isomerization barrier. Polyad breaking with multiple resonances is necessary to adequately describe the spectral features of the system. Insight into the dynamical nature of isomerization related to the effective Hamiltonian is gained through classical trajectories on the model potential. Contrary to physical intuition, the bend mode is not a reaction mode, but rather isomerization requires excitation in both stretch and bend. The dynamics reveals a Farey tree formed from the 2:1 and 3:1 resonances, corresponding to the resonance coupling terms in the effective Hamiltonian, with the prominent 5:2 (2:1+3 :1) feature dividing the tree into parts that we call the 3:1 and 2:1 portions. (c) 2010 American Institute of Physics. [doi:10.1063/1.3480017]