Computational study of the vibrational and electronic spectroscopy of a HO2-H2O2 complex

Perturbations of the vibrational and electronic energy levels of isolated HO2 by formation of a doubly hydrogen bound association complex with H2O2 have been investigated using ab initio calculations. Minimum potential energy surfaces for a six-membered and five-membered ring structures are found. The binding energy, vibrational frequencies and low lying electronic excited states are predicted for the lowest energy structure of the complex and are calibrated by calculations of the separated monomers. The lowest energy conformation of the complex is found to be bound by 7.9 kcal mol−1. We find that complexation with H2O2 blue shifts HO2 electronic excited states by ∼0.1 eV and redshifts H2O2 states by ∼0.1 eV. Two additional, though exceedingly weak, electronic transitions are found to borrow oscillator strength from the ground state of the hydrogen bonded complex. The enthalpy and entropy of formation of the complex is estimated to be ΔH0 (270 K)=-35.8 kJ mol-1 and ΔS0 (270 K)=-138.3 J mol-1 K-1. The existence of a HO2-H2O2 complex is discussed in terms of its potential role in atmospheric photochemistry and laboratory kinetic studies.