DECOMPOSITION MODES OF DIOXIRANE, METHYLDIOXIRANE AND DIMETHYLDIOXIRANE - A CCSD(T), MR-AQCC AND DFT INVESTIGATION

Formation and decomposition of dioxirane (2a), methyldioxirane (2b) an d dimethyldioxirane (2c) in the gas phase were investigated by carryin g out CCSD(T), MR-AQCC and B3LYP calculations with the 6-31G(d, p), 6- 311 + G(3df, 3pd) and cc-VTZ2P + f,d basis sets. The inclusion of f fu nctions in the basis set was essential to determine the heat of format ion a H-f(o)(298) of carbonyl oxide (1a) and 2a to be 27.0 and - 0.3 k cal/mol, respectively. With the latter value, we calculate the same ri ng strain energy for cyclopropane, oxirane and 2a. Molecule 2a decompo ses at 298 K with an activation enthalpy of 18 kcal/mol to methylenebi s(oxy) (3a), which is calculated to be 1.2 kcal/mol less stable than 2 a, in contrast to previous investigations. Two methyl substituents inc rease the ring opening barrier to 23 kcal/mol and, thereby guarantee t he kinetic stability of 2c. The biradicals 3 decompose with barriers s maller than 4 kcal/mol to esters and therefore will be difficult to in tercept in dioxirane decomposition reactions. (C) 1998 Elsevier Scienc e B.V. All rights reserved.