title: Computations on the primary photoreaction of Br2 with CO2: stepwise vs concerted addition of Br atoms.
abstract: 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 (C2v) 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.
authors.affiliation: Chemistry Department, Syracuse University, Syracuse, New York 13244, United States.
authors.forename: Kewei
authors.initials: K
authors.lastname: Xu
authors.affiliation: Chemistry Department, Syracuse University, Syracuse, New York 13244, United States.
authors.forename: Timothy M
authors.initials: TM
authors.lastname: Korter
authors.affiliation: Chemistry Department, Syracuse University, Syracuse, New York 13244, United States.
authors.forename: Mark S
authors.initials: MS
authors.lastname: Braiman