Energy and spectroscopic parameters of neutral and cations isomers of the C_nH₂ (n = 2-6) families using high-level ab-initio approaches

Cationic species, previously detected from ion-induced desorption of solid methane by plasma desorption mass spectrometry (PDMS), and neutral species, are investigated using high-level ab-initio approaches. From a set of 25 cationic and 26 neutral structures belonging to C_nH₂ (n = 2-6) families, it was obtained the energy, rotational constants, harmonic vibrational frequency, charge distribution and excitation energies. The ZPVE-corrected energies, at CCSD(T)-F12; CCSD(T)-F12/RI/(cc-pVTZ-F12, cc-pVTZ-F12-CABS, cc-pVQZ/C) (n = 2-5) and CCSD(T)/cc-pVTZ (n = 6) levels, reveal that the topology of the most stable isomer vary with n and the charge. Out of 674 harmonic frequencies, those with maximum intensity are generally in the 3000-3500 cm^-1 range. Analysis of 169 vertical transition energies calculated with the EOM-CCSD approach, suggest three C₆H₂ species as potential carriers of the diffuse interstellar bands (DIB). Systematic comparison of properties between neutral and cationic species can assist in the structural description of complex matrices.