PHOTOELECTRON-SPECTROSCOPY OF THE ALLYL AND 2-METHYLALLYL ANIONS

The electron affinities of allyl, allyl-d(5), 2-methylallyl, and 2-met hylallyl-d(7) radicals have been determined from the 351 nm photoelect ron spectra of the allylic anions. The ions were prepared in a cooled helium flow reactor by the reaction of O- with the corresponding prope ne. The electron affinities found for the radicals listed above are 0. 481 +/- 0.008, 0.464 +/- 0.006, 0.505 +/- 0.006, and 0.493 +/- 0.008 e V, respectively. Extensive vibrational structure is observed in all th e spectra, as the CCC bending and symmetric stretching modes are activ ated upon photodetachment. Vibrational frequencies for these modes are obtained for all of the radicals. A second harmonic of an asymmetric CH2 rocking mode is observed in the spectra of the allyl, allyl-d(5), and 2-methylallyl anions. Hot bands are used to determine the CCC bend ing frequencies in the allylic anions. Photoelectron angular distribut ions were measured for the allyl and 2-methylallyl anions. The photoel ectron spectrum of allyl anion is calculated using a modeling procedur e that utilizes the vibrational frequencies and geometries from nb ini tio calculations. This spectrum agrees very well with that obtained ex perimentally. The measured electron affinities are used together with the previously reported gas-phase acidities of propene and 2-methylpro pene to determine the 298 K allylic C-H bond enthalpies for these hydr ocarbons, with DH298(CH2CHCH2-H) = 88.8 +/- 0.4 kcal/mol and DH298(CH2 C(CH3)CH2-H) = 88.3 +/- 2.3 kcal/mol. Methyl substitution at the 2-pos ition of allyl radical is found to have little effect on any of the pr operties determined in this study.