Density functional theory and multireference configuration interaction studies on low-lying excited states of TiO2

Using density functional theory at the BPW91/6-311+G(3df) level, optimized geometries and energies of the lowest singlet, triplet, and quintet A(1), A(2), B-1, B-2(C-2v) states of the TiO2 molecule were obtained. TiO2 has a (1)A(1) ground state in C-2v symmetry. Adiabatic excitation energies of the low-lying singlet and triplet states range from 2.1 to 3.0 eV. The (1,3)A(2) states optimize at bond angles of about 140 degrees, lying only 0.06 eV below linear (1,3)Delta(u), whereas B-1,3(1) and B-1,3(2), with bond angles of 120 degrees and 96 degrees, respectively, lie 0.3-0.4 eV below the respective (1,3)Pi(u) or (1,3)Delta(u) states. Minima with short O-O distances of similar to 1.46 angstrom, at energies of 4.2 and 4.7 eV, were found for (1)A(1) and (3)A(1). The C-2v minima of the lowest B-1(1) and B-3(1) states are saddle points, suggesting lower-energy structures in C-s symmetry. The C-2v quintet states start at energies of 5.7 eV. Multireference configuration interaction (MRCI) methods, employing a polarized valence triple-zeta basis set, lead to similar geometries and energies. MRCI vertical excitation energies up to 4.6 eV and oscillator strengths are given. The calculated excitation energy of 2.2 eV for B-1(2) agrees well with 2.3 eV from a fluorescence spectrum. The vertical electron detachment energy of TiO2- is 1.5 eV, in good agreement with 1.6 eV from anion photoelectron spectroscopy. An observed second photoelectron band corresponds to B-1(2) and/or B-3(2), but the assignment of a third band could not be verified. Vibrational frequencies, ionization energies, electron affinities, and dissociation energies are given. (c) 2007 American Institute of Physics.