Gas-Phase Ion / Molecule Reactions in Octafluorocyclobutane

Abstract

Ion/molecule reactions in octafluorocyclobutane (c-C₄F₈) were studied using a high-pressure mass spectrometer. The thermochemical stabilities of the cluster ions of halide ions (X^-) with c-C₄F₈ were measured. While the F^- ion forms a covalent bond with c-C₄F₈, the interaction between other halide ions with c-C₄F₈ is mainly electrostatic. Theoretical calculation revealed that the halide ions interact not with the lowest unoccupied molecular orbital but with the next lowest unoccupied molecular orbital of the c-C₄F₈ molecule in the most stable cluster ions X^- (c-C₄F₈). The electron affinity of c-C₄F₈ was measured to be 24.2±2.3 kcal/mol (1.05±0.10 eV) by observing the equilibria for reaction of SF₆^-+c-C₄F₈ = c-C₄F₈^-+SF_6- While the sound equilibrium for that reaction was established in the temperature region from ∼350 K down to the lowest temperature measured (∼150 K), that was not established in the higher temperature region above 350 K. This was attributed to the existence of an isomer for c-C₄F₈^- whose electron detachment energy is smaller than 1.05 ev. By the measurement of thermochemical stabilitries of [(O2)m(c-C₄F₈)n]- (m+n = 1-3), the lower electron affinity of c-C4F8 was determined to be 12.0±1.2 kcal/mol (0.52±0.05 eV). The lower limit of the proton affinity of c-C₄F₈ was estimated to be 130 kcal/mol.