Energetics of the Rearrangement of Neutral and Ionized Perfluorocyclopropane to Perfluoropropylene. Use of Infrared Multiphoton Dissociation Spectra To Identify Structural Isomers of Molecular Ions

Article abstract of DOI:10.1021/ja00404a001

Infrared photodissociation spectroscopy is used to compare the structure of gas-phase C3F6⁺ 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 CO2 laser is tuned across the 925-1080 cm^-1 wavelength range.Ions are formed, stored, and detected with the use of techniques of ion cyclotron resonance spectroscopy.Infrared multiphoton exitation is effected by using low-power, continuous-wave laser radiation.The "fingerprint" spectrum of the molecular ion of pefluorocyclopropane is identical with that obtained from perfluoropropylidene, 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 C3F6 and C3F6⁺ isomers is discussed.Comparisons are made with the analogous C3H6 system.Photoionization mass spectroscopy results yield ΔH_f(c-C3F6) = -233.8 kcal/mol.