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Environmental fate & pathways

Phototransformation in air

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Description of key information

HBr undergoes photolysis via a series of mechanisms to form hydrogen and bromine in air.

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Additional information

Falconer and Sunder (1968) showed that the photolysis reaction of HBr could progress via one or more of the following reaction schemes:

[1]       HBr +hν → HBr*

[2]      HBr* + M → HBr + M

[3]       HBr* → H(2S) + (Br(2P3/2)

[4]       [HBr + HBr]c+hν → [HBr…H…Br]c

[5]       [HBr…H…Br]c→ HBr + HBr

[6]       [HBr…H…Br]c→ H2+ 2[Br]c

[7]       [HBr…H…Br]c→ HBr + H+ [Br]c

[8]       H + H + M → H2+ M

[9]       H + HBr → H2+ [Br]c

[13]     [Br2+ HBr]c+hν → [HBr…H…Br]c

[14]     [HBr…H…Br]c→ Br2+ HBr

[15]     [HBr…H…Br]c→ H + Br2+ [Br]c

[16]     H + Br2→ HBr + Br

Dotted lines indicate complexes and the subscript c indicates caged or matrix restrained species. 

Baumfalk R, et al (1997) measured kinetic energies the dissociation reaction and obtained values of 1.36 ± n0.06 eV (Br) and 0.90 ± 0.05 eV (Br*) which were in good agreement with the calculated values of 1.35 eV and 0.89 eV obtained using the following equation:

               hν(243.1nm) –D0(HBr) =Ekin(H) +Ekin(Br) +Eint(Br)

WhereD0is the dissociation energy of the HBr molecule andEkinandEintthe kinetic and internal energy of the fragment atoms.

The H atom photofragmented with zero kinetic energy after the photo-dissociation