Energetics of the rearrangement of neutral and ionized perfluorocyclopropane to perfluoropropylene. Use of infrared multiphoton dissociation spectra to identify structural isomers of molecular ions

Infrared photodissociation spectroscopy is used to compare the structure of gas-phase C/sub 3/F/sub 6//sup +/ ions obtained by electron-impact ionization of two isomeric precursors: perfluoropropylene and perfluorocyclopropane. Photodissociation spectra are obtained by observing the extent of multiphoton dissociation as the CO/sub 2/ laser is tuned across the 925 to 1080 cm/sup -1/ wavelength range. Ions are formed, stored, and detected with the use of techniques of ion cyclotron resonance spectroscopy. Infrared multiphoton excitation is effected by using low-power, continuous-wave laser radiation. The fingerprint spectrum of the molecular ion of perfluorocyclopropane is identical with that obtained from perfluoropropylene, indicating rearrangement of the former to the latter. Photodissociation kinetics indicate that the entire perfluorocyclopropane molecular ion population isomerizes to the more stable perfluoropropylene structure. Thermochemistry of C/sub 3/F/sub 6/ and C/sub 3/F/sub 6//sup +/ isomers is discussed. Comparisons are made with the analogous C/sub 3/H/sub 6/ system. Photoionization mass spectroscopy results yield ..delta..H/sub f/(c-C/sub 3/F/sub 6/) = -233.8 kcal/mol. 4 figures.