Electronic spectra of H2C2nH+ radicals (n = 2–6): An ab initio study

The equilibrium geometries and vibrational frequencies of the carbon chain cations H2C2nH+ (n = 2–6) in the X1A1 ground states have been investigated by the DFT-B3LYP method. The structures of the X1A1 and 21A1 electronic states for H2C2nH+ (n = 2–5) have also been optimized by the CASSCF approach. The vertical excitation energies from the ground to selected excited states for H2C2nH+ (n = 2–6) have been evaluated at the CASPT2/cc-pVTZ level. The studies reveal that the vertical excitation energies for the 21A1 ← X1A1 transitions are 4.56, 3.44, 2.75, 2.15 and 1.62 eV, respectively. For H2C2nH+ (n = 2–4) molecules, our estimates are in reasonable agreement with the corresponding observed values of 4.34, 3.27 and 2.65 eV, respectively. In addition, the exponential-decay curves of these excitation energies are also suggested.