O-17 NMR spectroscopic characterization and the mechanism of formation of alkyl hydrotrioxides (ROOOH) and hydrogen trioxide (HOOOH) in the low-temperature ozonation of isopropyl alcohol and isopropyl methyl ether: Water-assisted decomposition

Low-temperature ozonation of isopropyl alcohol (1a) and isopropyl methyl et her (1b) in [D-6]acetone, methyl acetate, and tert-butyl methyl ether at -7 8 degrees C produced the corresponding hydrotrioxides, Me2C(OH)(OOOH) (2a) and Me2C(OMe)(OOOH) (2b), along with hydrogen trioxide (HOOOH). All the pol yoxides investigated were characterized for the first time by O-17 NMR spec troscopy of highly O-17-enriched species. The assignment was confirmed by G IAO/MP2/6-31++G* calculations of O-17 NMR chemical shifts, which were in ex cellent agreement with the experimental values. Ab initio density functiona l (DFT) calculations at the B3LYP/6-31G*+ZPE level have clarified the trans ition structure (TS1, Delta E-not equal = 7.4 and 10.6 kcalmol(-1), relativ e to isolated reactants and the complex la-ozone, respectively) for the ozo nation of la; this, together with the formation of HOOOH and some other pro ducts, indicates the involvement of radical intermediates (R-., (.)000H) in the reaction. The activation parameters for the decomposition of the hydro trioxides 2a and 2b (E-a = 23.5 +/- 1.5 kcalmol(-1), logA = 16 +/- 1.8) wer e typical for a homolytic process in which cleavage of the ROOOH molecule o ccurs to yield a radical pair [(ROOOH)-O-..] and represents the lowest avai lable energy pathway. Significantly the lower activation parameters for the decomposition of HOOOH (E-a = 16.5 +/- 2.2 kcalmol(-1), logA = 9.5 +/- 2.0 ) relative to those expected for the homolytic scission of the HO-OOH bond [bond dissociation energy (BDE)= 29.8 kcalmol(-1), CCSD(T)/6-311++G**] are in accord with the proposal that water behaves as a bifunctional catalyst a nd therefore participates in a "polar" (non-radical) decomposition process of this polyoxide. A relatively large acceleration of the decomposition of the hydrotrioxide 2a in [D-6]acetone, accompanied by a significant lowering of the activation energies, was observed in the presence of a large excess of water. Thus intramolecular 1,3-proton transfer probably also involves t he participation of water and is similar to the mechanism proposed for the decomposition of HOOOH. This hypothesis was further substantiated by the B3 LYP/6-31++G*+ZPE calculations for the participation of water in the decompo sition of CH3OOOH, which revealed two stationary points on the potential en ergy surface corresponding to a CH3OOOH-HOH complex and a six-membered cycl ic transition state TS2. The energy barriers were comparable with those cal culated for HOOOH, that is, Delta E-not equal = 15.0 and 21.5 kcalmol(-1) r elative to isolated reactants and the CH3OOOH-HOH complex, respectively.