The open chain or chemically bonded structure of H2O4: The hydroperoxyl radical dimer

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The straight chain isomer H–O–O–O–O–H of H2O4 is of considerable current interest in combustion and atmospheric chemistry. Ab initio quantum mechanical methods have been used to study the geometrical structure, energetics, and vibrational frequencies of this species. Double zeta (DZ) and double zeta plus polarization (DZ+P) basis sets have been used in this theoretical study, the latter designated O(9s5p1d/4s2p1d), H(4s1p/2s1p). These basis sets have been employed in conjunction with self–consistent field (SCF) and configuration interaction (CI) methods, including variationally up to 470 935 configurations. For the straight chain isomer, stationary points of symmetry C2h , Ci , and C1 have been identified, and correspond to Hessian indices 3,1, and 0, respectively. The equilibrium geometry, having no elements of symmetry at all, is relatively unique. The highest level of theory (unlinked cluster corrected DZ+P CI) predicts the straight chain structure of H2O4 to lie slightly lower in total energy than the cyclic two-hydrogen bond isomer.