Theoretical Prediction on the Thermal Stability of Cyclic Ozone and Strong Oxygen Tunneling

Dual-level dynamics calculation with variational transition state theory including multidimensional tunneling has been performed on the isomerization reaction of cyclic ozone -> normal (open) ozone, which was believed to be the stability-determining reaction of the elusive cyclic ozone molecule under thermal condition. The high-level potential energy surface data were obtained from the calculation using the MRCISD+Q theory with the aug-cc-pVQZ basis set, while the low-level reaction path information was obtained using the hybrid density functional theory B3LYP with the cc-pVTZ basis set. the calculated results showed very significant tunneling effects below 300 K (a factor of similar to 200 at 300 K and over 10(7) at 200 K). Because of the strong tunneling effects and the potential energy surface crossing of a 1A(1) and 1A(2) states, the isomerization reactions were found to be significantly faster than previously believed. The half-life of the cyclic ozone was estimated only similar to 10 s at 200 K and similar to 70 s below 100 K, which might partly explain the unsuccessful attempts for its experimental indentification. The kinetic isotope effects (KIEs) for various O-18 substitution reactions were also calculated as function of temperature and were as high as 10 at very low temperature. Because of the large KIEs, the experimental identification of the cyclic O-18(3) seems more promising.