Titanium Superoxide as a Carrier of a "Long-Lived" Superoxide Anion: An Ab Initio Investigation.

The photoinduced generation of a superoxide anion on the surface of a semiconductor photocatalyst is usually attributed to the reduction of O 2 with conduction-band electrons. In the current work, the reaction of TiO 2 with O 2 giving rise to TiO 4 in superoxide and peroxide states has been investigated with ab initio (CAS, CCSD) and DFT (B3LYP) calculations. The ground triplet state and two substates (open-shell singlet (OSS) and closed-shell singlet (CSS)) of a doubly degenerate excited singlet state ( a 1 Δ g ) are considered as reactive states of oxygen, participating in spontaneous or photoinduced processes, respectively. The triplet and OSS singlet states of TiO 4 contain O 2 - as structural units and can be defined as titanium superoxides. Both states have energy less than the level of the initial pair TiO 2 +O 2 by about 30 kcal/mol. The CSS state of TiO 4 has a diperoxide structure Ti 4+ ( O 2 2- ) 2 and also lies in energy below the initial pair TiO 2 + 3 O 2 . Titanium superoxide is considered to be the carrier of an "exceptionally stable" and "long-lived" superoxide anion, which was earlier synthesized or detected on the surface of TiO 2 . The low-energy location of the conical intersections on the way from reagents to 3 TiO 4 allows us to explain the literature data on the spontaneous generation of the "long-lived" superoxide anion on the TiO 2 surface.