Theoretical evidence for a bound doubly-excited B21(C 1s,n→π*2) state in H2CO below the C 1s ionization threshold

Abstract

The group of three lowest singlet C 1s-excited states of formaldehyde H2CO is studied theoretically. The equilibrium geometries are determined at the restricted open-shell Hartree–Fock (ROHF) level and refined total energies are obtained using the multireference configuration interaction (MRCI) approach. In agreement with an earlier prediction [Chem. Phys. 122, 9 (1988)] the second lowest singlet state, B21, is characterized by a doubly excited, “two particle–two hole” (2p–2h), configuration C 1s,n→π*2. Our calculations predict that H2CO in the B21(2p–2h) state has a stable pyramidal equilibrium structure with a barrier to inversion of 0.28 eV, the valence angle being close to 107°. The calculated length of the CO bond is 1.390 Å. The B21(2p–2h) state is shown to be also bound with respect to all possible dissociation and rearrangement processes. The lowest predicted dissociation energy for the B21 state (H2CO*→H2+CO* reaction) is 0.29 eV (6.69 kcal/mol). The rationalization of the great stability of the B21(2p–2h) state is the similarity of its electronic structure to that of the first singly-excited state A″2(n) of nitroxyl radical H2NO⋅. The neighboring states B11(C 1s→π*) and A11(C 1s→3s) are characterized within the same framework. Spectroscopic implications and possibilities for the experimental identification of the B12(2p–2h) state are discussed.