Molecular structures and energetics for the lowest triplet states of glyoxal

The equilibrium geometries of the lowest three triplet states of trans glyoxal (HCOHCO) have been predicted at the self-consistent-field (SCF) level of theory using a double zeta plus polarization (DZ+P) basis set, designated C,O(9s5p1d/4s2p1d), H(4s1p/2s1p). Two of the molecular structures thus predicted differ radically from the known 1Ag ground state equilibrium geometry. The predicted relative energies of the three triplet states are notably altered when the effects of electron correlation are treated theoretically. Using configuration interaction (CI) including single and double excitations, the predicted adiabatic excitation energies are 23 000 (ã Au3 n→π*), 31 000 (3Bg n→π*), and 31 000 cm−1 (3Bu π→π*), when corrected for unlinked cluster effects.