Knowledge Bank

The halogens exhibit concentration-dependent absorption bands in the UV that were long ago attributed to the formation of dimers. Although the spectral profiles of these bands have been moderately well characterized, the equilibrium constants K for their formation have mostly remained ill-defined. In particular, spectrophotometric data have been dismissed as inherently incapable of providing estimates of K. Here this presumption is shown to be incorrect in the cases of Br$2$(g) dimer absorption near 205 nm and I$2$(soln) absorption near 300 nm. The analysis is enabled by precise spectrophotometric data for 20 concentrations, analyzed via a multiwavelength global approach. The Br$2$ dimerization constant is found to be K = 2.5(3) L/mol at 22C, giving less than 2 percent dimerization at the maximum studied pressure (119 Torr). The K value is more than a factor of 2 larger than a previous estimate from analysis of PVT data -- an approach that can be shown to yield at best a lower limit.