THEORETICAL INVESTIGATION OF THE LOW-LYING ELECTRONIC STATES OF DIOXIRANE - RING-OPENING TO DIOXYMETHANE AND DISSOCIATION INTO CO2 AND H-2

The low-lying electronic states of dioxirane (1), their ring opening t o dioxymethane (2), and the dissociation of 2 into CO2 and H-2 have be en investigated by means of CASSCF and MRD-CI+Q quantum chemistry calc ulations. The ground state of 1 is a singlet with 4 pi electrons, 1(1) A(1)(4 pi), while the ground state of 2 is a 2 pi-electron singlet, 1( 1)A(1)(2 pi), lying 5.8 kcal/mol higher than 1 in energy. A 0 K activa tion energy of 21.4 kcal/mol is predicted for the thermal ring opening of 1 into 2, which takes place via a transition structure approximate ly corresponding to the crossing between the lower (1)A(1)(4 pi) and ( 1)A(1)(2 pi) states of both molecules. Twelve excited states have been calculated for 1 with vertical excitation energies ranging from 3.07 to 13.11 eV. The energy ordering of these states changes dramatically upon relaxation of the molecular geometries. The optimum geometries of these excited states show an angle OGO in the 106.3-120.1 degrees ran ge, so they should be considered as excited states of 2. Minimum energ y points of the intersection seam between the 1(1)A(2)(3 pi)/B-1(1)(3 pi), 1(1)B(1)(3 pi)/(1)A(1)(2 pi), 1(1)A(2)(3 pi)/(1)A(1)(4 pi), and 1 (1)B(1)(3 pi)/(1)A(1)(4 pi) potential energy surfaces have been locate d in an angle OGO range of 91.0-104.6 degrees. The photochemical ring opening of 1 into 2 may occur through vertical excitation to either th e 1(1)B(1)(3 pi) or 1(1)A(2)(3 pi) states of 1 and subsequent radiatio nless decay to groundstate 2 via minimum energy intersection points on the potential energy surfaces of the appropriate states. The dissocia tion of ground-state 2 into CO2 and H-2 is predicted to be exothermic by 105.2 kcal/mol with a 0 K activation energy of 3.2 kcal/mol, while the dissociations of the first four excited states of 2 are all predic ted to be endothermic.