Dissociative attachment reactions of electrons with strong acid molecules

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Abstract
Using the flowing afterglow/Langmuir probe (FALP) technique, we have determined (at variously 300 and 570 K) the dissociative attachment coefficients β for the reactions of electrons with the common acids HNO3 (producing NO−2) and H2SO4 (HSO−4), the superacids FSO3H (FSO−3), CF3SO3H (CF3SO−3), ClSO3H (ClSO−3,Cl−), the acid anhydride (CF3SO2)2O (CF3SO−3), and the halogen halides HBr (Br−) and HI (I−). The anions formed in the reactions are those given in the parentheses. The reactions with HF and HCl were investigated, but did not occur at a measurable rate since they are very endothermic. Dissociative attachment is rapid for the common acids, the superacids, and the anhydride, the measured β being appreciable fractions of the theoretical maximum β for such reactions, βmax. The HI reaction is very fast ( β∼βmax) but the HBr reaction occurs much more slowly because it is significantly endothermic. The data indicate that the extreme acidity of the (Bronsted-type) superacids has its equivalence in the very efficient gas-phase dissociative attachment which these species undergo when reacting with free electrons. The anions of the superacids generated in these reactions, notably FSO−3 and CF3SO−3, are very stable (unreactive) implying exceptionally large electron affinities for the FSO3 and CF3SO3 radicals.