The heat of formation of C 3H 2+

Abstract

Reaction rate coefficients have been measured in a variable temperature selected-ion flow drift tube (SIFDT) for the reaction C 3H + + H 2 → C 3H 2 + + H, which is found to be 1 kcal mol −1 endothermic. This implies a substantially lower value for Δ H f(C 3H 2 +) (329±5 kcal mol −1) than has been reported previously (∼ 357 kcal mol −1). The C 3H 2 + ion reacts with H 2 at elevated ion kinetic energy to produce C 3H 3 + with ∼4 kcal mol −1 endothermicity. This implies that the C 3H 2 + ion reacts to form the linear propargyl ion, CH 2CCH +, Δ H f = 281 kcal mol −1, and not the more stable cyclic C 3H 3 + ion, Δ H f = 257 kcal mol −1. This would seem to be strong evidence that the C 3H 2 + ion is linear and not cyclic. The reaction C 2H 2 + + H 2 → C 2H 3 + + H is found to be 1.6 kcal mol −1 endothermic which implies Δ H f(C 2H 3 +) = 267±1 kcal mol −1, in excellent agreement with more recent determinations that are slightly lower than older values.