Pseudo Jahn-Teller origin of formaldehyde molecular geometry in excited states

The pseudo Jahn-Teller effect is employed to explain the origin of the out- of-plane displacement of the oxygen atom (bending of the C-O group) in the excited singlet (1)A(2) and triplet (3)A(2) electronic states of formaldehy de which is planar in the ground state. The ground state, the excited (1)A( 2) and (3)A(2) states, and several higher excited B-1(2) and B-3(2) states formed by one-electron MO excitations are calculated in a single-transition approximation and by including configuration interaction with single and d ouble excitations using the GAMESS program. With these data the correspondi ng pseudo Jahn-Teller coupling constants and the contribution of the B-2 st ates to the instability of the A(2) states with respect to the oxygen out-o f-plane displacements is estimated, and an explanation of both the larger i nstability of the triplet state as compared with the singlet state and the elongation of the C-O bond in the bent states is obtained. (C) 2001 Elsevie r Science B.V. All rights reserved.