Ion/molecule reactions in propyne, 1-butyne and allene by photoionization, ion cyclotron resonance and trapped-ion mass spectrometry

Ion/molecule reactions of the title compounds have been studied by several techniques. Several reaction sequences previously observed are verified and additional new ones suggested. The C₃H₄ isomers demonstrate, among others, the following sequence: C₃H₄⁺ → C₆H₇⁺ → C₇H₇⁺ → C_1OH₁₁⁺. The major primary step for 1-butyne is C₄H₆(C₄H₆, CH₃)C₇H₉⁺. A model is put forward in which proton transfer takes place via a loose collision complex, which is an intermediate step towards formation of a tight ("intimate") collision complex, which in turn is responsible for production of the condensation products. The relatively low rate coefficient for C₃H₄⁺ + C₃H₄ → products in allene, (6.4 ± 0.5) × 10^-10 cm³ molecule^-1 s^-1, as compared to propyne, (15.4 ± 0.5) × 10^-10 cm³ molecule^-1 s^-1, is due to the fact that the propyne reaction system is 12 kcal mol ^-1 more exothermic. Back- reaction can compete effectively with product formation once the internal energy of the ion is increased but only in allene.