Theoretical study of the low-lying electronic states of CCCF radical and its ions

The low-lying electronic states of the CCCF radical and its ionic states have been investigated systematically using the complete active space self-consistent field (CASSCF) and multiconfiguration second-order perturbation theory (CASPT2) methods in conjunction with aug-cc-pVTZ basis set. To investigated the Renner-Teller effect on the CCCF radical, Cs symmetry was used for CCCF. The CCCF has been found to have a X2A′ ground state with rotational constant B¯ = 4500.5 MHz, which is in good agreement with the experimental values of 4555.8043 MHz. The calculations of vertical excitation energies of CCCF at 2.893 and 4.180 eV are attribute to the X2A′ → 32A″ and X2A′ → 52A′, respectively, which has larger oscillator strengths. The ionization potentials of CCCF are computed in order to provide a theoretical guidance to the photoelectron spectrum (PES) of the CCCF radical. The first adiabatic electron affinity (AEA) of CCCF is predicted to be 2.671 eV. A comparison of the geometries and bonding among the CCCX (X = F, Cl, and Br) radicals presents the ground state X2A′ of CCCX can be described as allenic structures with the unpaired electron on the C3 atom, while the excited state 12A″ (12Π) of CCCX have the linear acetylenic structures with the unpaired electron on the C1 atom. The barriers to linearity decrease as follows: ΔE (CCCF) < ΔE (CCCCl) < ΔE (CCCBr).