Hydrogen-bonded gas-phase molecular clusters of dihydrogen trioxide (HOOOH) have been investigated using DFT (B3LYP/6-311++G(3df,3pd)) and MP2/6-311++G(3df,3pd) methods. The binding energies, vibrational frequencies, and dipole moments for the various dimer, trimer, and tetramer structures, in which HOOOH acts as a proton donor as well as an acceptor, are reported. The stronger binding interaction in the HOOOH dimer, as compared to that in the analogous cyclic structure of the HOOH dimer, indicates that dihydrogen trioxide is a stronger acid than hydrogen peroxide. A new decomposition pathway for HOOOH was explored. Decomposition occurs via an eight-membered ring transition state for the intermolecular (slightly asynchronous) transfer of two protons between the HOOOH molecules, which form a cyclic dimer, to produce water and singlet oxygen (Delta (1)O 2). This autocatalytic decomposition appears to explain a relatively fast decomposition (Delta H a(298K) = 19.9 kcal/mol, B3LYP/6-311+G(d,p)) of HOOOH in nonpolar (inert) solvents, which might even compete with the water-assisted decomposition of this simplest of polyoxides (Delta H a(298K) = 18.8 kcal/mol for (H 2O) 2-assisted decomposition) in more polar solvents. The formation of relatively strongly hydrogen-bonded complexes between HOOOH and organic oxygen bases, HOOOH-B (B = acetone and dimethyl ether), strongly retards the decomposition in these bases as solvents, most likely by preventing such a proton transfer.
Multireference configuration interaction, MRD-CI, methods with the cc-pVDZ+sp and cc-pVTZ+sp basis sets were employed to determine the low-lying singlet and triplet electronic states of nitryl bromide, BrNO 2 . The calculations predict two very strong transitions, 3 1 A 1 r X 1 A 1 and 3 1 B 2 r X 1 A 1 , at 6.15 and 7.27 eV, respectively. At wavelengths that are atmospherically relevant to the BrNO 2 photolysis, 1 1 B 2 and 1 1 B 1 singlet states with vertical transition energies of 3.43 and 3.85 eV, respectively, are computed. The corresponding triplet states are lower in energy by 0.27 and 0.36 eV, respectively. Both singlet and triplet states are found to be highly repulsive along the Br-N coordinate and they dissociate to the ground-state fragments Br and NO 2 . The comparison between the first singlet excited state of BrNO 2 and ClNO 2 demonstrates that the transition is red-shifted by 80 nm from ClNO 2 upon replacement of chlorine by bromine. Furthermore, the most intense transition 3 1 A 1 r X 1 A 1 of BrNO 2 has a 0.9 eV lower energy than that of ClNO 2 . These energy differences can be explained by the large halogen character of the orbitals involved in the electron excitations.
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