An analysis of chlorine and bromine oxygen bonding and its implications for stratospheric chemistry

Trends in chlorine/oxygen and bromine/oxygen single and double bonds are examined for several molecules of interest in stratospheric halogen chemistry. Specifically, the relationships between bond distance and quadratic force constant, and bond distance and ionic bonding character are examined, together with bond energies. Similar to a previous study of FO bonding, it is found that the relationship between bond distance and force constant for Cl—O and Br—O single bonds is unusual and distinctly nonlinear. This is attributed to the through space interaction of halogen lone-pair electrons with the remainder of the molecule. Supporting evidence for this assertion is given by the fact that for X=O (X is Cl, Br) double bonds, where there are fewer halogen lone-pair electrons due to hypervalent bonding, this relationship is approximately linear. A detailed explanation for chlorine and bromine hypervalent bonding is presented which is consistent with all available data and with the trends studied here. In this model, chlorine or bromine hypervalent bonding is a result of the p → d promotion of one or two lone-pair halogen electrons followed by the formation of two or four additional pd hybrid halogen bonds. This model explains why -XO2 species are particularly stable while the -XO and -XO3 species are not. Finally, the implications of this new understanding of chlorine and bromine chemistry for stratospheric reservoir species are discussed.