Abstract
Gas-phase ion-molecule reactions in octafluorocyclopentene (C5F8) were studied with a pulsed electron beam mass spectrometer. When a few Torr of major gases, CH4, N2, Ar, or He containing ∼10 mTorr C5F8 was ionized by 2 keV electrons, C5F8+, C5F7+, C4F6+, C4F5+, and C3F3+ were formed as major positive ions. The interaction between these ions and C5F8 is found to be electrostatic. The thermochemical stabilities for these ions with C5F8 were measured. The proton affinity of C5F8 may be smaller than that of C2H4. In the negative-mode operation, the strong C5F8- ion peak was formed during the electron pulse. This indicates that C5F8 has the positive electron affinity. The C5F8- ion was quickly converted to the polymerized dimer complex C5F8-·C5F8. This dimer complex did not react further with C5F8 down to 170 K, i.e., the polymerization reaction terminates with n = 1 in the cluster ion C5F8+(C5F8)n. The nature of bonding in F-(C5F8)n changes drastically from covalent with n = 1 to electrostatic with n ≥ 2. The F- ion was found to form a covalent bond with C5F8 but the interaction in F-(C5F8)…C5F8 becomes electrostatic. The halide ions except for F- interact with C5F8 electrostatically.
