Computational study of the vibrational and electronic spectroscopy of a HO 2 –H 2 O 2 complex Jaron C. Hansen a, * , Bradley A. Flowers b , John F. Stanton b a Department of Chemistry and Biochemistry, Brigham Young University, C100 Benson, Provo, UT 84602, USA b Department of Chemistry and Biochemistry, Institute for Theoretical Chemistry, The University of Texas at Austin, Austin, TX 78712, USA Received 2 May 2006; accepted 3 May 2006 Abstract Perturbations of the vibrational and electronic energy levels of isolated HO 2 by formation of a doubly hydrogen bound association complex with H 2 O 2 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 K1 . We find that complexation with H 2 O 2 blue shifts HO 2 electronic excited states by w0.1 eV and redshifts H 2 O 2 states by w0.1 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 DH 0 (270 K)Z-35.8 kJ mol -1 and DS 0 (270 K)Z-138.3 J mol -1 K -1 . The existence of a HO 2 –H 2 O 2 complex is discussed in terms of its potential role in atmospheric photochemistry and laboratory kinetic studies. q 2006 Elsevier B.V. All rights reserved. Keywords: Radical molecule complex 1. Introduction The existence of radical–molecule complexes and their impact on the kinetics and spectroscopy associated with atmospherically important processes has been the subject of a number of recent reviews and papers. In particular, the hydroperoxyl radical has been found to form complexes due to its charge distribution ( dC H–O–O %dK ) and high gas-phase acidity. The hydroperoxyl radical can attract and donate electron density for hydrogen bonding at its respective sites. It has been postulated that as much as 30% of free HO 2 radicals in the troposphere may actually be complexed with H 2 O under ideal conditions [1]. Recent work on the self-reaction of HO 2 , HO 2 C HO 2 C M/H 2 O 2 C O 2 C M (1) the principal production mechanism in the upper troposphere/ lower stratosphere (UT/LS) for H 2 O 2 , has shown the dramatic effect that radical–molecule complexes can have on chemical kinetics [2]. Christensen et al. have demonstrated the enhance- ment in the HO 2 self reaction rate coefficient due to the formation of an HO 2 –CH 3 OH complex occurring during the reaction mechanism [2,3]. The same enhancement in the HO 2 self reaction rate has also been reported by Rowley et al. [4]. Laboratory experiments have demonstrated the increased reactivity of the HO 2 radical when complexed with NH 3 and H 2 O [5–8]. Francisco et al. [9] have shown that, in addition to the enhanced reactivity observed for a number of radicals as a result of complex formation, their photochemistry can also be perturbed. One example is the single hydrogen bonded complex formed between ClO and H 2 O. The formation of the complex splits the symmetry of the ClO p system. Their calculations predict the formation of a new electronic excited state centered at 1321 nm (0.94 eV) that is unique to the complex, where this state is separated from any other excited state of either monomer ClO or H 2 O. The photolysis of H 2 O 2 under atmospheric conditions produces two OH radicals with a quantum yield of 2 below 248 nm [10,11]. A shift or new excited state for H 2 O 2 could represent an enhancement in H 2 O 2 photolysis due to lowering of the threshold wavelength for H 2 O 2 absorption and sub- sequent HO–OH bond dissociation. Due to the propensity of radical–molecule complexes to perturb both the reactivity and Journal of Molecular Structure: THEOCHEM 768 (2006) 111–118 www.elsevier.com/locate/theochem 0166-1280/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2006.05.045 * Corresponding author. Tel.: C1 801 422 4066. E-mail address: jhansen@chem.byu.edu (J.C. Hansen).