Information on the Proton Affinity and Protolysis of Propane From Measurement of the Ion Cluster Equilibrium: C2H5+ + CH4 = C3H9+

The clustering equilibrium C2H5+ + CH4 = C3H9+ [1] was measured between −60 and −110 °C in 0.5–8 Torr methane. A van't Hoff plot of K1 leads to ΔH10 = −6.6 kcal/mol, ΔS10 = −23.4 e.u.(std. st. 1 atm), and ΔG10 (300 K) ≈ +0.4 kcal/mol. The C3H9+formed by [1] is probably the most stable structure of protonated propane which corresponds to a three center bond of the proton with the electrons of a C—C bond. This structure is also similar to an ethyl ion forming a three center bond with C—H in CH4i.e. the product of reaction 1. With this assumption ΔHf(C3H9+) = 194.5 kcal/mol and p.a.(C3H8) = 147 kcal/mol are obtained. The pathways for the decomposition of C3H9+ (protolysis of propane) are examined. The reaction to C3H7+ + H2 is shown to involve a 9 kcal/mol energy barrier compared to the decomposition to CH4 and C2H5+. The latter is favored at low temperature and low excitation of C3H9+.