DETERMINATION OF THE PROTON AFFINITY AND ABSOLUTE HEAT OF FORMATION OF CYCLOPROPENYLIDENE

The proton affinity and absolute heat of formation of cyclopropenylidene (c-C3H2) have been derived from the translational energy threshold for endothermic proton transfer from c-C3H3+ to ammonia in a flowing afterglow triple quadrupole instrument: c-C3H3+ + NH3 --> c-C3H2 + NH4+. The cyclopropenium cation C3H3+ 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-C3H2) of 227.3 +/- 2.1 kcal mol(-1). From the measured proton affinity and the known heats of formation of c-C3H3+ 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.