Hydrogen Dibromide Radical: Infrared Detection through the Matrix Isolation Technique

When hydrogen bromide–bromine–argon mixtures are passed through a glow discharge and the products are condensed at 20°K, prominent absorptions appear at 727.4, 892.1, and 1053.0 cm⁻¹. With deuterium, these features shift to 496.1, 666, and 833 cm⁻¹. Both deuterium and bromine isotope shifts show that these features can be assigned to the BrHBr (BrDBr) free radical in a linear, symmetric ${\mathrm{(D}}_{\mathrm{\infty h}})$ structure with ${\nu }_3\text{\hspace{0.17em}=\hspace{0.17em}727.4 (496.1)}{\mathrm{cm}}^{\text{−1}}$ and ${\nu }_1\text{\hspace{0.17em}=\hspace{0.17em}164.7 (170)}{\mathrm{cm}}^{\text{−1}}$ . The evidence that these bands are not due to HBr₂⁻ is reviewed. The vibrational potential function for ${\nu }_3$ includes a large quartic term and it suggests that the hydrogen bond in HBr₂ is comparable in strength to that in HBr₂⁻.