Reassignment of the Photoelectron Spectrum of Methylketene Using a Hybrid Model of Harmonic and Anharmonic Oscillators to Compute Franck-Condon Factors.

We constructed a hybrid model of harmonic and anharmonic oscillators to compute Franck-Condon factors and interpret the photoelectron spectrum of methylketene. The equilibrium structures of methylketene and its cation were optimized, and then, the harmonic and anharmonic vibrational frequencies were computed using the B3LYP, PBE0, APFD, and ωB97XD approaches of the density functional theory. The photoelectron spectrum of methylketene was simulated by computing the Franck-Condon factors with both the harmonic and hybrid models. The adiabatic ionization energy of methylketene was computed by using the CCSD(T) approach extrapolating to the complete basis set limit. The simulated photoelectron spectra are consistent with those from the experiment for both the harmonic and hybrid models. However, the error in band positions is reduced by using the hybrid model. The computed adiabatic ionization energies of methylketene are in agreement with the experiment, with the smallest error being 0.017 eV. Our interpretation based on the theoretical spectrum led to the reassignment of the experimental photoelectron spectrum of methylketene.

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.