Photoelectron spectroscopy of the allyl and 2-methylallyl anions

The electron affinities of allyl, allyl-d(5), 2-methylallyl, and 2-methylallyl-d(7) radicals have been determined from the 351 nm photoelectron spectra of the allylic anions. The ions were prepared in a cooled helium flow reactor by the reaction of O- with the corresponding propene. 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 eV, respectively. Extensive vibrational structure is observed in all the spectra, as the CCC bending and symmetric stretching modes are activated 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 bending frequencies in the allylic anions. Photoelectron angular distributions were measured for the allyl and 2-methylallyl anions. The photoelectron spectrum of allyl anion is calculated using a modeling procedure that utilizes the vibrational frequencies and geometries from nb initio calculations. This spectrum agrees very well with that obtained experimentally. The measured electron affinities are used together with the previously reported gas-phase acidities of propene and 2-methylpropene to determine the 298 K allylic C-H bond enthalpies for these hydrocarbons, with DH298(CH2CHCH2-H) = 88.8 +/- 0.4 kcal/mol and DH298(CH2C(CH3)CH2-H) = 88.3 +/- 2.3 kcal/mol. Methyl substitution at the 2-position of allyl radical is found to have little effect on any of the properties determined in this study.