Pseudo Jahn-Teller coupling in trioxides XO3(0,1,−1) with 22 and 23 valence electrons

Abstract

D3h and C2v geometries and energies, vertical excitation energies, as well as minimal energy paths as function of the O(1)(z)-X-O(2) angle α were obtained for XO3((0,1,-1)) (X = B, Al, Ga; C, Si, Ge; N, P, As; S, Se) molecules and ions with 22 and 23 valence electrons (VE), using density functional theory (DFT), coupled cluster with single and double substitutions with noniterative triple excitations (CCSD(T)), equation of motion (EOM)-CCSD, time-dependent DFT, and multi-reference configuration interaction methods. It is shown that pseudo Jahn-Teller (PJT) coupling increases as the central atom X becomes heavier, due to decreases in excitation energies. As is well known for CO3, the excited (1)E' states of the 22 VE systems SiO3, GeO3; NO3(+), PO3(+), AsO3(+); BO3(-), AlO3(-), GaO3(-) have strong vibronic coupling with the (1)A1' ground state via the e' vibrational modes, leading to a C2v minimum around α = 145°. For first and second row X atoms, there is an additional D3h minimum (α = 120°). Interacting excited states have minima around 135°. In the 23 VE systems CO3(-), SiO3(-); NO3, PO3; SO3(+), coupling of the excited (2)E' with the (2)A2' ground state via the e' mode does not generate a C2v state. Minima of interacting excited states are close to 120°. However, due to very strong PJT coupling, a double-well potential is predicted for GeO3(-), AsO3, and SeO3(+), with a saddle point at D3h symmetry. Interaction of the b2 highest occupied molecular orbital with the b2 lowest unoccupied molecular orbital, both oxygen lone pair molecular orbitals, is seen as the reason for the C2v stabilization of 22 VE molecules.