| Abstract
| - The equilibrium constant for the reaction, Br2(g) + Cl2(g) ⇄ BrCl(g), is estimated using the classicspectrophotometric method with precise data and a multispectrum fitting approach. Analysis of spectra for18 Cl2:Br2 mixing ratios yields K° = 9.1 at 22 °C, with a nominal statistical error (1 σ) of 0.04. This errortranslates into a remarkable precision of 0.4 cm-1 in the dissociation energy of BrCl, easily beating that forthe current best spectroscopic estimate (±5 cm-1). However, a sensitivity analysis shows that K° is susceptibleto small systematic errors and minor changes in the least-squares fit model, leading to a more conservativeestimate of 2% error. The derived K° is consistent with statistical mechanical estimates that employ the currentvalue for the BrCl dissociation energy, but it differs considerably from some recently measured and usedvalues, which range from 6.4 to 10.4. The revised estimate of the BrCl dissociation energy is 𝒟e = 18 248± 2 cm-1.
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