Using an energy-resolved electron beam, ionization potentials for the following free radicals have been measured: vinyl 8.95 V, allyl 8.07 V, benzyl 7.27 V. For vinyl and allyl ions new measurements of thresholds for dissociative ionization give AHf(C2H3+) = 266 kcal/mol and AHf(C3H5+) = 226 kcal/mol, leading to neutral radical heats of formation AHf(C2H3) =, 59.6 kcal/mol and AHr(C3H5) = 40 kcal/mol. The data for benzyl radical and ion give AHr(benzyl cat~on) = 213 kcal/mol.Canadian Journal of Chemistry, 49, 357 (1971)
IntroductionThe ionization potential values available for vinyl, allyl, and benzyl free radicals were measured some years ago, using conventional mass spectrometer ion sources. It is now well-known that, as a result of low resolution in energy, ionization thresholds determined in this way are generally higher than the adiabatic IP by 0.1-0.7 V, the error becoming larger with unfavorable Franck-Condon factors in the threshold ionization region. Recent electron impact measurements, either by the Retarding Potential Difference (RPD) method or by the use of true energy-resolved electron beams, are in satisfactory agreement with the more precise results obtainable by photoionization (PI) or photoelectron spectroscopy. The results of a recent study (1) of the ionization potentials of C,-C, alkyl radicals, using an energy-resolved electron beam from a two-stage double-hemispherical energy selector, show that good agreement now holds among the IP values for these radicals as measured by the RPD method, photoionization, and monoenergetic electron impact, even in cases where the ionization efficiency curves in the threshold region are decidedly non-linear. The present work reports new IP values for vinyl, allyl, and benzyl radicals using a monoenergetic electron beam with an energy width at half-height of about 0.07 V.For vinyl and allyl radicals, appearance potentials for formation of the corresponding C2H3+ and C3H5+ ions in dissociative ionization of suitable com~ounds have also been obtained. Heats of formation of the ionic and neutral