Photochemistry of fumaronitrile radical anion and its clusters

The photodetachment and photochemistry of the radical anion of fumaronitrile (trans-1,2-dicyanoethylene) and its clusters are investigated using photoelectron imaging and photofragment spectroscopy. We report the first direct spectroscopic determination of the adiabatic electron affinity (EA) of fumaronitrile (fn) in the gas phase, EA = 1.21 ± 0.02 eV. This is significantly smaller than one-half the EA of tetracyanoethylene (TCNE). The singlet-triplet splitting in fumaronitrile is determined to be δE _S-T ≤ 2.6 eV, consistent with the known properties. An autodetachment transition is observed at 392 and 355 nm and assigned to the ²B_u anionic resonance in the vicinity of 3.3 eV. The results are in good agreement with the predictions of the CCSD(T) and EOM-XX-CCSD(dT) (XX = IP, EE) calculations. The H₂O and Ar solvation energies of fn^- are found to be similar to the corresponding values for the anion of TCNE. In contrast, a very large (0.94 eV) photodetachment band shift, relative to fn^-, is observed for (fn)₂^-. In addition, while the photofragmentation of fn^-, fn ^-·Ar, and fn^-(H₂O)_1,2 yielded only the CN^- fragment ions, the dominant anionic photofragment of (fn)₂^- is the fn^- monomer anion. The band shift, exceeding the combined effect of two water molecules, and the fragmentation pattern, inconsistent with an intact fn^- chromophore, rule out an electrostatically solvated fn^-·fn structure of (fn)₂^- and favor a covalently bound dimer anion. A C ₂ symmetry (fn)₂^- structure, involving a covalent bond between the two fn moieties, is proposed.