Dissociation Energy of Aluminium Monobromide

Abstract

IN a recent review¹ of the dissociation energies of the gaseous monohalides of boron, aluminium, gallium and thallium, it was concluded that the best value for D ₀″ (aluminium monobromide) is 105 k.cal. mole⁻¹. Dr. P. Gross has now kindly directed my attention to the work of Semenkovich², who has determined the standard heat of formation of gaseous aluminium monobromide by a thermochemical method similar to that used by Gross, Campbell, Kent and Levi³ for aluminium monofluoride and monochloride. Semenkovich obtains Q _f(AlBr_g) = + 1.1 k.cal. mole⁻¹. With Δ_S H(Al) = 77.4, Q _f(Br_g) = 26.71, and D ₂₉₈″ – D ₀″ = 0.85, D ₀″(AlBr) = 102.2 k.cal mole⁻¹. As with AlF and AlCl, this value is a little lower than two of the spectroscopic estimates from the state A ¹Π, which, for AlBr, are 106.9 k.cal. by extrapolation, and >104.4 k.cal. from the highest observed vibrational level: it is, however, consistent with a predissociation limit which leads to D ₀″ ⩽ 105.7 k.cal.