Knowledge Bank

In recent years it has been proposed that resonance-stabilized radicals (RSRs) may play an important role as intermediates in the formation of polycyclic aromatic hydrocarbons (PAHs). RSRs gain extra stability by delocalizing the unpaired electron through a neighboring conjugated $\pi$-system. Because of this extra stability, RSRs are able to build up in concentration, allowing for the creation of larger, more complex systems through their recombination with other RSRs. Mass-selective two-color resonant two-photon ionization spectra of two RSRs, phenylallyl and benzylallenyl radicals, have been recorded under jet-cooled conditions. These two radicals, while sharing the same radical conjugation, have unique properties. The phenylallyl and benzylallenyl radicals were respectively produced via discharge of trans-$\beta$-methylstyrene and benzylallene in argon prior to supersonic expansion. The D$\mathrm{<mrow\; data-mjx-texclass="ORD">0</mrow>}$-D$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ origin of the phenylallyl radical was found at 19204 $\backslash wn$ and was found to have a strong vertical ionization energy of 6.905(2) eV. By comparison, the benzylallenyl radical has an origin at 19703 $\backslash wn$ and, while showing similar Franck-Condon activity to phenylallyl, has an IP curve indicative of a large geometry change between the ground state and the ion ~7.50(2) eV. Visible-visible holeburning was used to show that each radical exists in one conformeric form in the expansion. The CH stretch region of each radical was taken using D$\mathrm{<mrow\; data-mjx-texclass="ORD">0</mrow>}$-Resonant Ion Dip Infrared Spectroscopy in a novel four laser experiment. A combination of this and DFT calculations was used to show that each radical exists in a trans geometry.