Theoretical Study of the Negative Ion Photoelectron Spectrum of Cyclooctatetraene via Computation of Franck-Condon Factors.

The equilibrium structures and harmonic vibrational frequencies of the anion and the first triplet state of cyclooctatetraene were computed using the B3LYP, PBE0, and M06-2X approaches of the density functional theory associated with the aug-cc-pVTZ basis set. The first excited singlet state of cyclooctatetraene was calculated using the complete active space self-consistent field method. The photoelectron spectra of cyclooctatetraene anion were simulated for both the triplet and the excited singlet states via computing Franck-Condon factors. The adiabatic electron affinity was computed by extrapolation to the complete basis set limit from the energies calculated using CCSD(T)/aug-cc-pVXZ (X = D, T, Q). The simulated photoelectron spectrum and the calculated adiabatic electron affinity for the triplet state are in consistence with the experiment. The first excited singlet state, which plays a key role in the photochemistry of cyclooctatetraene, is predicted to possess vibrational structures in its photoelectron spectrum pertinent for experimental identification.