The structures, electron affinities, and energetic stabilities of TiOn and TiOn- (n = 1-3)

By use of density functional and coupled cluster methods, we report energetic and structural information concerning the ground states of TiOn and TiOn- (n = 1-3), much of which has not previously been observed experimentally or predicted theoretically. This study establishes the following geometrical symmetries and electronic ground states: (X) over tilde (1)A' TiO3 (C-s), (X) over tilde (2)A'(2) or B-2(2) TiO3- (D-3h or C-2 nu), and (X) over tilde (2)A(1) TiO2- (C-2 nu). In addition, the electronic ground state of TiO- is established as 2 Delta, arising from the 9 sigma(2) 1 delta configuration. This finding is contrary to the suggestion of Wu and Wang, contained in their report of recent photoelectron experiments, that ground state TiO- has a 9 sigma 1 delta(2) electronic configuration and (4)Sigma(-) symmetry. The ground state minimum-energy structure for TiO3- contains no oxygen-oxygen bonds and has D-3h or C-2 nu symmetry. The first theoretical adiabatic electron affinities, as predicted by the CCSD(T)//B3LYP level of theory, for TiO, TiO2, and TiO3 are 1.25 eV, 1.60 eV, and 3.34 eV, while Wu and Wang's photoelectron measurements for these were 1.30 eV, 1.59 eV, and 4.2 eV, respectively. The results for the mon- and dioxide cases are in excellent accord with experiment; however, the experimental result for TiO3 is approached with similar accuracy only when compared with our CCSD(T)//B3LYP value for the vertical detachment energy for TiO3- of 4.02 eV. The latter prediction reflects the fact that the theoretical structures for TiO3 and TiO3- are qualitatively different. Finally, several of the lowest-lying minima on the potential energy surfaces of TiO3 and TiO3- are elucidated. (C) 1999 American Institute of Physics. [S0021-9606(99)30411-6].