Threshold Photoelectron Spectrum of m -Benzyne.

Due to their unusual electronic structure, the biradical m -benzyne, C 6 H 4 , and its cation are of considerable interest in chemistry. Here, the photoion mass-selected threshold photoelectron spectrum of the m -benzyne biradical is presented. An adiabatic ionization energy of 8.65 ± 0.015 eV is derived, while a vibrational progression of 0.10 eV is assigned to the ν 9 + ring breathing mode, in excellent agreement with computations. The experimental spectrum was reproduced well by Franck-Condon spectral modeling of the 2 A 1 ← X 1 A 1 transition, in which the cation retains a monocyclic C 6 framework. The energetically close-lying bicyclic 2 A 2 cation state exhibits low Franck-Condon factors, due to the large change in geometry, and thus cannot be observed.