Gas-Phase Ion/Molecule Reactions in C5F8

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2005-02, Vol.109 (6), p.1049-1056
• Main Authors: Hiraoka, K, Fujita, K, Ishida, M, Ichikawa, T, Okada, H, Hiizumi, K, Wada, A, Takao, K, Yamabe, S, Tsuchida, N
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp040251k

Gas-phase ion−molecule reactions in octafluorocyclopentene (C5F8) were studied with a pulsed electron beam mass spectrometer. When a few Torr of major gas, CH4, Ar, or N2, containing ∼10 mTorr C5F8 was ionized by 2 keV electrons, C5F8 +, C5F7 +, C4F6 +, C4F5 +, and C3F3 + were formed as major fragment ions. The interaction between those ions and C5F8 is found to be a weak electrostatic interaction. The cation···C5F8 bonding energies are around 10 kcal/mol, which were reproduced well by (U)B3LYP/6-311+G(d) calculations. The proton affinity of C5F8 (=148.6 kcal/mol by B3LYP/6-311+G(d)) was found to be smaller than that of C2H4 (=162.8 kcal/mol). In the negative mode of operation, the intense signal of C5F8 - was observed during the electron pulse. This indicates that C5F8 has a positive electron affinity (1.27 eV by (U)B3LYP/6-311+G(d)). The C5F8 - ion was quickly converted to a complex C10F16 -. This complex did not react further with C5F8 down to 170 K. The theoretical calculation revealed that a C5F7−F-···C5F8 interaction mode in (C5F8)2 - was converted to a C5F7 •···C5F9 - one via fluoride-ion transfer. The F- ion was found to form a strong covalent bond with C5F8, but the interaction in F-(C5F8)- - -C5F8 is a weak electrostatic interaction due to the charge dispersal in F-(C5F8). The halide ions except F- interact with C5F8 only weakly. Thermochemical stabilities for the cluster ions I-(CH3I) n (n = 1, 2) were also determined.