A photoionization study of 2-propyl and t-butyl radicals

We investigate the photoionization of the alkyl radicals 2-propyl (C3H7), termed also isopropyl, and t-butyl (C4H9). Both species were generated by flash pyrolysis from suitable precursors and ionized using vacuum ultraviolet synchrotron radiation. Photoelectron-photoion coincidence was applied to record photoion mass-selected threshold photoelectron spectra. Due to the large geometry change upon ionization, it is difficult to extract accurate ionization energies from the spectra. For 2-propyl, we observe vibrational bands 135 meV apart. By extrapolating this progression to lower energies, an ionization energy (IE) of 7.45 ± 0.03 eV could be determined, in good agreement with ab initio computations. A similar 93 meV progression yields an IE of 6.80 eV in t-butyl, which, although higher than previously reported photoelectron spectroscopy results, is probably still too low in the light of thermochemical cycle and computational results. The conformation of the methyl group changes upon ionization, and the neutral minima correspond to transition states in the cation. Thus, the 000 transition is not expected to exhibit measurable intensity, and the nuclear wave function overlap of the neutral with the cationic ground state is enhanced if the methyl rotors are torsionally excited. This significant vibrational population of hot neutrals leads to an onset of the spectrum at apparently lower energies.