Structural, rotational, vibrational, and electronic properties of ionized carbon clusters C-n(+) (n=4-19)

The structures, rotational moments, vibrational normal modes, and infrared spectra of small to medium-size ionized carbon clusters C-n(+) (n = 4-19) a re investigated using density functional (DFT) and coupled cluster (CC) the ories. Comparison is made with the neutral systems from which they derive. In contrast to previous restricted open-shell Hartree-Fock (ROHF) results b y von Helden and co-workers, electron correlation is shown to strongly limi t distortions of the structure upon an adiabatic ionization process. Noneth eless, for such a process, the C4n+2 and to a lesser extent the C4n+1 cycli c systems are found to evolve from an essentially regular (i.e., cumulenic) pattern to a more alternating (i.e., polyynic) structure in their ionized forms, whereas the opposite trend is observed for the C-4n and C4n+3 rings. Similarly, linear carbon clusters, which can be regarded as mostly cumulen ic in their neutral form, tend to become more polyynic after ionization. Ro tational moments, IR spectra, and adiabatic ionization potentials as well s hould provide specific markers of these contrasted characters and behaviors . It has been found that the linear cations show a much more pronounced IR intensity than the cyclic ones. Many of the studied species show a strong a bsorption in certain regions of the spectrum (e.g., around 2036 cm(-1)).