We calculated the equilibrium geometries and harmonic vibrational frequencies of the ground state and five cationic states of dichloroketene using (TD‐)B3LYP, PBE0, and M06/M06‐2X approaches. The photoelectron spectra of dichloroketene were simulated by computing Franck‐Condon factors. The ionization energies were computed using the CCSD(T) approach with extrapolation to the complete basis set (CBS) limit. We propose two new CBS energy formulas (E = ECBS + Aexp(‐x) + B/(x−1) n, n = 2 or 3) and compare the performance of different CBS approaches. A new ionic state of dichloroketene belonging to the Cs point group is reported. This state is identified as the first excited state of Cl2CCO+ having a double‐well potential‐energy curve along the CCO bending mode with a barrier height of 1.335 eV. The simulated photoelectron spectra are in agreement with the experiment. The vertical ionization energies calculated via spectral simulation are more accurate compared with those obtained at the ground‐state structure. Among the CBS formulas used, the proposed ansatz with n = 2 performs best, with a mean absolute error of 0.021 and 0.012 eV for the adiabatic and vertical ionization energies, respectively.

The computation of Franck-Condon factors plays a key role for unraveling vibronic spectra and nonradiative process of molecules. In this work the photoelectron spectra of C122 were predicted by computing Franck-Condon factors. The equilibrium geometries and harmonic vibrational frequencies of five allotropes of C60C2C60 with dumbbell structures, including their cations and anions, were calculated at the B3LYP/6-31G(d) level. To facilitate the computation of Franck-Condon factors, a prescreening process for selecting transitions with stronger intensities was developed. The efficiency was further improved by rearranging the order of normal mode as the larger the maximum vibrational quantum number, the higher the order. The photoelectron spectra of C122- → C122 + e- and C122 → C122+ + e- were simulated. The simulated spectra are distinct for each molecule, indicating that the five allotropes of C122 can be identified by the experiment of photoelectron spectroscopy.

The structures and absolute configurations of two organic compounds, excoecoumarin A and excoecoumarin B, were studied by recording their electronic circular dichroism (ECD) spectra and performing quantum‐chemistry computations. The optical rotations and the nuclear magnetic resonance (NMR), ECD, vibrational circular dichroism (VCD), and ultraviolet (UV) spectra of candidate structures were computed using the B3LYP approach of the density functional theory. The structures and absolute configurations of excoecoumarins A and B are inferred from the agreement between the experimental and theoretical NMR, ECD, and UV spectra. Although a specific configuration can be assigned to excoecoumarin A, three structures are possible for excoecoumarin B. The computations indicated that the absolute configuration of excoecoumarin B can be further identified by acquiring the VCD spectra. In any case, the present study indicated that the structures reported previously should be revised.

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