Theoretical studies on structures and electronic spectra of linear HCnN+ (n=2-14)

With unrestricted B3LYP and CAM-B3LYP calculations, we have investigated the linear HCnN+ (n = 2-14) clusters focusing on the ground-state geometries, vibrational frequencies, rotational constants, dipole moments, energy differences (Delta E-n), and incremental binding energies (Delta E-I). The results indicate that the odd-numbered HCnN+ 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 CASFT2/cc-pVTZ approach has been employed to estimate the vertical excitation energies for the dipole-allowed (2, 3)(2)Pi <- X-2 Pi and dipole-forbidden 1(2)Phi <- X-2 Pi transitions in HCnN+ (n = 5-14) clusters. The predicted excitation energies of 2(2)Pi <- X-2 Pi transitions for HCnN+ (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 HCnN+ (n = 5-14) are also calculated. We expect that calculations for 3(2)Pi <- X-2 Pi 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 HCnN+ clusters. (C) 2011 Elsevier B.V. All rights reserved.