Determination of the proton affinity and absolute heat of formation of cyclopropenylidene

Abstract

The proton affinity and absolute heat of formation of cyclopropenylidene (c-C 3H 2) have been derived from the translational energy threshold for endothermic proton transfer from c-C 3H 3 + to ammonia in a flowing afterglow triple quadrupole instrument: c-C 3H 3 + + NH 3 → c-C 3H 2 + NH 4 +. The cyclopropenium cation C 3H 3 + was prepared in a helium flow reactor at room temperature by the reaction of ionized ethylene with acetylene, and from dissociative electron ionization of bromocyclopropane. The kinetic energy dependence of the cross-sections for proton transfer from this ion to ammonia and other neutral amines was characterized in a triple quadrupole mass analyzer. The endothermicity for the reaction with ammonia was determined to be 23.3 ± 1.8 kcal mol −1. Combining this with the known proton affinity (PA) of ammonia (204.0 ± 1.0 kcal mol −1) gives a value for PA(c-C 3H 2) of 227.3 ± 2.1 kcal mol −1. From the measured proton affinity and the known heats of formation of c-C 3H 3 + and the proton, the 298 K heat of formation of cyclopropenylidene is determined to be 119.5 ± 2.2 kcal mol −1. This value is slightly higher than a previous experimental estimate, but is in good agreement with the 298 K heat of formation predicted by high level molecular orbital calculations.