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 lớn 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.
George L, Kalume A, El-Khoury PZ, Tarnovsky A, Reid SA. George L, et al. J Chem Phys. 2010 Feb 28;132(8):084503. doi: 10.1063/1.3319567. J Chem Phys. 2010. PMID: 20192303
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248 nm photolysis of CH2Br2 by using cavity ring-down absorption spectroscopy: Br2 molecular elimination at room temperature.
Wei PY, Chang YP, Lee WB, Hu Z, Huang HY, Lin KC, Chen KT, Chang AH. Wei PY, et al. J Chem Phys. 2006 Oct 7;125(13):133319. doi: 10.1063/1.2218514. J Chem Phys. 2006. PMID: 17029472
Mechanistic aspects of the reaction between Br2 and chalcogenone donors (LE; E=S, Se): competitive formation of 10-E-3, T-shaped 1:1 molecular adducts, charge-transfer adducts, and [ (LE)2]2+ dications.
Aragoni MC, Arca M, Demartin F, Devillanova FA, Garau A, Isaia F, Lelj F, Lippolis V, Verani G. Aragoni MC, et al. Chemistry. 2001 Jul 16;7(14):3122-33. doi: 10.1002/1521-3765(20010716)7:143122::aid-chem3122>3.0.co;2-h. Chemistry. 2001. PMID: 11495439
Br2 elimination in 248-nm photolysis of CF2Br2 probed by using cavity ring-down absorption spectroscopy.
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Hsu CY, Huang HY, Lin KC. Hsu CY, et al. J Chem Phys. 2005 Oct 1;123(13):134312. doi: 10.1063/1.2047570. J Chem Phys. 2005. PMID: 16223293
Halogen exchange and scrambling between C-X and M-X' bonds in copper, nickel, and cobalt complexes of 6,6'-bis(bromo/chloromethyl)-2,2'-bipyridine. structural, electrochemical, and photochemical studies.
Ghosh M, Biswas Phường, Flörke U, Nag K. Ghosh M, et al. Inorg Chem. 2008 Jan 7;47(1):281-96. doi: 10.1021/ic7014786. Epub 2007 Dec 5. Inorg Chem. 2008. PMID: 18052246