Positive Ion–Molecule Reactions in Perfluoropropane

Abstract

Positive ion–molecule reactions have been studied in C3F8 by tandem and high-pressure mass spectrometry. Rate constants at a nominal 0.3-eV ion energy and kinetic energy dependences of the reactions are reported. The most common reaction is F− transfer which has a typical rate constant of 5 × 10−11 cm3 molecule−1·sec−1. These rate constants are generally more than an order of magnitude lower than those for analogous H− transfer reactions in alkanes. Other reactions, including F transfer to CF2+ and F2 transfer to CF+ as well as several condensation reactions involving C–C bond formation are also observed. Endothermic collision-induced dissociation (CID) reactions are observed for CF2+, C2F3+, C2F4+, C2F5+, and C3F7+ even at a nominal 0.3-eV ion kinetic energy. The effects of ionizing electron energy and of changing the molecular source of the ionic species show that the fragment ions are formed with considerable internal excitation. For ions formed by 76-eV electrons, the ratio of CID to F− transfer rate constants is 0.06 for CF2+, 1.0 for C2F3+, and 5.5 for C2F5+, while C2F4+ and C3F7+ undergo only CID reactions. CID reactions proceed by simple bond scission except for C2F4+ which must undergo rearrangement processes since the dissociative products are CF3+, CF2+, and CF+ while are formed in the ratio of 4.8/1.4/1.0.