Gas phase ion chemistry and photochemistry of ions generated from perfluoropropylene. Photodissociation of the perfluoroallyl cation

Abstract

The mass spectrometry, gas phase ion chemistry, and photochemistry of positive ions generated from perfluoropropylene are reported. Fragmentation mechanisms are formulated with the use of breakdown diagrams and metastable ion information. Trapped ion cyclotron resonance studies conducted with the major ions CF/sub 3//sup +/, C/sub 2/F/sub 4//sup +/, C/sub 3/F/sub 5//sup +/, and C/sub 3/F/sub 6//sup +/ demonstrate that CF/sub 3//sup +/ alone reacts by fluoride transfer to generate C/sub 3/F/sub 5//sup +/. The ions C/sub 2/F/sub 4//sup +/, C/sub 3/F/sub 5//sup +/, and C/sub 3/F/sub 6//sup +/ are unreactive with perfluoropropylene neutrals, making it possible to examine their photochemistry while trapped for periods up to three seconds and irradiated with a mercury-xenon arc lamp. Only the C/sub 3/F/sub 5//sup +/ ion, which is assumed to be the perfluoroallyl cation, is observed to dissociate upon photoexcitation, yielding CF/sub 3//sup +/ and C/sub 2/F/sub 2/. Interestingly, this decomposition pathway also corresponds to the most intense metastable peak observed in the ion kinetic energy spectrum. The CF/sub 3//sup +/ produced photochemically reacts to regenerate C/sub 3/F/sub 5//sup +/, leading to a photostationary state, which is analyzed in terms of the photodissociation and bimolecular reaction kinetics. The excitation function for photodissociation of C/submore » 3/F/sub 5//sup +/ obtained using a monochromator set for a bandpass of 75 A exhibits an onset at 295 +- 10 nm and lambda/sub max/ 270 nm at which wavelength a photodissociation cross section of sigma(lambda/sub max/) congruent to 0.2 A/sup 2/ is estimated. These results are compared to theoretical calculations of excitation energies for allyl cations and to absorption spectra measured for allyl cations in solution. Possible reasons for the failure to observe photodissociation of the parent ion are discussed.« less