Decomposition modes of dioxirane, methyldioxirane and dimethyldioxirane - a CCSD(T), MR-AQCC and DFT investigation

Formation and decomposition of dioxirane (2a), methyldioxirane (2b) and dimethyldioxirane (2c) in the gas phase were investigated by carrying 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 functions in the basis set was essential to determine the heat of formation a H-f(o)(298) of carbonyl oxide (1a) and 2a to be 27.0 and - 0.3 kcal/mol, respectively. With the latter value, we calculate the same ring strain energy for cyclopropane, oxirane and 2a. Molecule 2a decomposes at 298 K with an activation enthalpy of 18 kcal/mol to methylenebis(oxy) (3a), which is calculated to be 1.2 kcal/mol less stable than 2a, in contrast to previous investigations. Two methyl substituents increase the ring opening barrier to 23 kcal/mol and, thereby guarantee the kinetic stability of 2c. The biradicals 3 decompose with barriers smaller than 4 kcal/mol to esters and therefore will be difficult to intercept in dioxirane decomposition reactions. (C) 1998 Elsevier Science B.V. All rights reserved.