Computations on the primary photoreaction of Br₂ with CO₂: Stepwise vs concerted addition of Br atoms

It was proposed previously that Br₂-sensitized photolysis of liquid CO₂ proceeds through a metastable primary photoproduct, CO₂Br₂. Possible mechanisms for such a photoreaction are explored here computationally. First, it is shown that the CO₂Br radical is not stable in any geometry. This rules out a free-radical mechanism, for example, photochemical splitting of Br₂ followed by stepwise addition of Br atoms to CO₂-which in turn accounts for the lack of previously observed Br₂+CO₂ photochemistry in gas phases. A possible alternative mechanism in liquid phase is formation of a weakly bound CO₂:Br₂ complex, followed by concerted photoaddition of Br₂. This hypothesis is suggested by the previously published spectroscopic detection of a binary CO₂:Br₂ complex in the supersonically cooled gas phase. We compute a global binding-energy minimum of -6.2 kJ mol^-1 for such complexes, in a linear geometry. Two additional local minima were computed for perpendicular (C_2v) and nearly parallel asymmetric planar geometries, both with binding energies near -5.4 kJ mol^-1. In these two latter geometries, C-Br and O-Br bond distances are simultaneously in the range of 3.5-3.8 Å, that is, perhaps suitable for a concerted photoaddition under the temperature and pressure conditions where Br₂ + CO₂ photochemistry has been observed.