Theoretical studies on structures and electronic spectra of linear HC nN + ( n = 2–14)

Abstract

With unrestricted B3LYP and CAM-B3LYP calculations, we have investigated the linear HC n N + ( n = 2–14) clusters focusing on the ground-state geometries, vibrational frequencies, rotational constants, dipole moments, energy differences (Δ E n ), and incremental binding energies (Δ E I). The results indicate that the odd-numbered HC n N + clusters have polyacetylene-like structures with significant single–triple bond length alternation, while the even-numbered analogues exhibit some sort of cumulenic character, with the former being more stable than the latter. The CASPT2/cc-pVTZ approach has been employed to estimate the vertical excitation energies for the dipole-allowed (2, 3) 2Π ← X 2Π and dipole-forbidden 1 2Φ ← X 2Π transitions in HC n N + ( n = 5–14) clusters. The predicted excitation energies of 2 2Π ← X 2Π transitions for HC n N + ( n = 5–13) clusters are 2.27, 2.33, 2.04, 2.02, 1.84, 1.76, 1.62, 1.58, and 1.49 eV, respectively, in good agreement with the available observed values (2.12, 2.17, 1.84, 1.89, 1.61, 1.66, 1.42, 1.49, and 1.27 eV). In addition, the higher electronic transitions of HC n N + ( n = 5–14) are also calculated. We expect that calculations for 3 2Π ← X 2Π transitions in odd- n clusters are able to contribute to further experimental and theoretical researches because of their relatively higher oscillator strengths. Finally, in the paper are discussed the possible dissociation channels and corresponding fragmentation energies of HC n N + clusters.