The structures, rotational moments, vibrational normal modes, and infrared spectra of small to medium-size ionized carbon clusters C n + (n = 4−19) are investigated using density functional (DFT) and coupled cluster (CC) theories. Comparison is made with the neutral systems from which they derive. In contrast to previous restricted open-shell Hartree−Fock (ROHF) results by von Helden and co-workers, electron correlation is shown to strongly limit distortions of the structure upon an adiabatic ionization process. Nonetheless, for such a process, the C4 n +2 and to a lesser extent the C4 n +1 cyclic 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 C4 n and C4 n +3 rings. Similarly, linear carbon clusters, which can be regarded as mostly cumulenic in their neutral form, tend to become more polyynic after ionization. Rotational moments, IR spectra, and adiabatic ionization potentials as well should 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 absorption in certain regions of the spectrum (e.g., around 2036 cm-1).

No abstract

The 1h (one-hole) and 2h-1p (two-hole; one-particle) shake-up bands in the valence ionization spectrum of small carbon chains (C3,C5,C7,C9) are investigated up to 40 eV, using the one-particle Green’s function approach. Calculations have been performed at the second- and third-orders of an algebraic diagrammatic construction (ADC) scheme based on partial renormalization series, which incorporate static and dynamic electronic correlation consistently through those orders. The results obtained indicate a major or complete breakdown of the orbital picture of ionization extending down into the outer-valence energies for the largest chains (12.4 eV for C9). Cumulenic carbon chains represent the only case reported so far where outer-valence ionization lines of π character can be affected by severe fragmentation in shake-up sets. The one-hole states associated with the terminal carbon lone pairs are also very strongly affected by electronic relaxation.

Herein, we report a systematic theoretical investigation of the molecular and electronic properties of unsubstituted polytriacetylene (PTA) and iso-polytriacetylene (iso-PTA) oligomers, which are characterized by through and cross pi-conjugation pathways, respectively. The goal of the study is to compare through versus cross conjugation on the basis of the computed molecular geometries of the neutral, anionic, and cationic species, the electron affinities, ionization potentials, excitation energies, and nonlinear optical properties for oligomers up to the nonamer. Differences in the effective conjugation length are directly related to electron delocalization in cross- and through-conjugated pathways. As in the through-conjugated oligomers, that is, the PTAs, the frontier orbitals of the iso-PTA oligomers are delocalized along the entire carbon backbone, suggesting that pi-delocalization can extend through cross-linked carbon atoms. However, in contrast to the PTA oligomers, the bond lengths remain strictly constant and the reduction of the energy gap beyond the trimer is completely due to the correlation contribution. On the other hand, in the anions and cations, the bond lengths do change significantly with increasing chain length. Therefore, oxidation or reduction of the iso-PTA oligomer appears to switch on delocalization through cross-linked carbon atoms. Obviously, the effective conjugation length is specific and depends on the observable considered.

The 1h (one-hole) and 2h-1p (two-hole; one-particle) shake-up bands in the valence ionization spectrum of odd-membered carbon rings (C5, C7, C9, C11) are investigated by means of the third-order algebraic diagrammatic construction [ADC(3)] scheme for the one-particle Green's function. Despite a severe dispersion of the σ- and π- ionization intensity over intricately dense sets of satellites, the present study undoubtedly confirms that structural fingerprints in ionization spectra could be usefully exploited to discriminate the cyclic C5, C7, C9, and C11 species from their linear counterparts in plasma conditions. Such spectra could also be used to indirectly trace very fine details of the molecular structure, such as bond length alternations, out-of-plane distortions, or the strength of cyclic strains. Both structurally and electronically, the cyclic isomers of the C5 and C9 clusters must be described as even-twisted cumulenic tori, whereas the C7 and C11 cyclic species are simply planar polyynic rings. In comparison with their linear counterparts, all species display an intrinsically lower propensity to electronic excitations, marked by a rather significant increase of the fundamental HOMO−LUMO band gap. On the other hand, the lower symmetry of the cyclic clusters, C5 and C9 in particular, permits many more configuration interactions in the cation. The ultimate outcome of these two opposite factors is, overall, a severe enhancement of the shake-up fragmentation of ionization bands, compared with the linear isomers.