A theoretical study of electronic spectra in the linear cationic chains NC2n+1N+ (n = 1-6)

Highlights: → → The NC2n+1N+ chains are more susceptible to fragmentation than NC2nN+ clusters. → The BLA character of X2Πg/u is more evident than corresponding 12Πu/g excited state. → CASPT2 method can give reliable results for vertical transition energy calculations. → The nonlinear λ -n relationship has been found in NC2n+1N+ (n = 1-6). The 22Πu/g <- X2Πg/u transition may be detected more easily in the further experiments. - Abstract: The B3LYP, CAM-B3LYP, and RCCSD(T) theories have been used to calculate the ground state equilibrium geometries of the linear cationic chains NC2n+1N+ (n = 1-6). Compared with the system NC2nN+, the odd-n cationic chains are more susceptible to fragmentation than the even-n cationic chains. The complete active space self-consistent-field method has been utilized to determine the stationary structure of the ground state (X2Πg/u) and first excited state (12Πu/g). The complete active space second-order perturbation theory has been used to compute the vertical excitation energies for the dipole-allowed (1, 2, 3)2Πu/g <- X2Πg/u transitions as well as the dipole-forbidden 12Φu/g <- X2Πg/u transitions. The calculated transition energies of 12Πu/g <- X2Πg/u in the gas phase are 2.61, 2.37, 2.07, 1.88, 1.64, and 1.34 eV, respectively, which accord well with the available experimental values. Moreover, the absorption spectra of 22Πu/g <- X2Πg/u may be detected more easily among the selected four transitions.