Electronic spectra of the linear polyyne cations HC2nH+ (n=2-8): An ab initio study

The B3LYP, CAM-B3LYP, and RCCSD(T) calculations have been used to determine the ground-state geometries of the linear polyyne cations HC2nH+ (n = 2-8). The CASSCF method has also been used to optimize the ground and first excited states. The present results indicate that these linear cations generally have an acetylenic structure H-C C-C C center dot center dot center dot C C-H+ with the ground state of X (2)Pi(g) for even-numbered n or X (2)Pi(u) for odd-numbered n. Moreover, the bond length alternation of HC2nH+ is less pronounced than the corresponding one of the neutral polyyne chains HC2nH. The CASPT2 approach has been employed to estimate the vertical excitation energies for the dipole-allowed (A center dot center dot center dot D) (2)Pi(u/g)<- X (2)Pi(g/u) transitions in HC(2)nH(+) (n = 2-8) clusters. The predicted A (2)Pi(u/g)<- X (2)Pi(g/)u transition energies in the gas phase are 2.62, 2.14, 1.81, 1.52, 1.35, 1.22, and 1.10 eV, respectively, in excellent agreement with the corresponding observed values of 2.45, 2.07, 1.75, 1.52, 1.35, 1.20, and 1.08 eV. The present calculations show that the absorption wavelengths for the A (2)Pi(u/g)<- X (2)Pi(g/)u transitions exhibit notably linear size dependence, as shown in previous experimental studies, quite different from the nonlinear lambda-n relationship for origin bands in HC(2)nH. (C) 2009 American Institute of Physics. [doi:10.1063/1.3247289]