Calculations of the energies of the low-lying electronic states of dioxatrimethylenemethane (H₂CCO₂) and prediction of the negative ion photoelectron (NIPE) spectrum of its radical anion †

In order to predict the energies of the low-lying electronic states of the dioxa derivative of trimethylenemethane (DOTMM), we have carried out (U)B3LYP, CASPT2, and (U)CCSD(T) calculations, using the aug-cc-pVTZ basis set. Our calculations predict that DOTMM has a triplet ground state, with one unpaired electron occupying a b₂ σ MO and the other a b₁ π MO. An open-shell singlet state, with the same orbital occupancy as the triplet, is calculated to lie very close to the ³A₂ ground state. However, this open-shell singlet (¹A₂) is predicted to be the transition structure for methylene rotation and to lead to the barrierless formation of an equivalent pair of α-lactones. We also report the results of some calculations on the fragmentation of DOTMM to CH₂ + CO₂. Our predictions about DOTMM could be tested experimentally by generating the DOTMM^•− radical anion in the gas phase and obtaining its negative ion photoelectron spectrum.