Electronic spectra of the linear polyyne cations HC(2n)H(+) (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 HC(2n)H(+) (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 identical withC-C identical withCcdots, three dots, centeredC identical withC-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 HC(2n)H(+) is less pronounced than the corresponding one of the neutral polyyne chains HC(2n)H. The CASPT2 approach has been employed to estimate the vertical excitation energies for the dipole-allowed (Acdots, three dots, centeredD) (2)Pi(u/g)<--X (2)Pi(g/u) transitions in HC(2n)H(+) (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(2n)H.
Zhang
Jinglai
J
Institute of Theoretical Chemistry, College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, People's Republic of China. jinglaizhang@126.com
Guo
Xugeng
X
Cao
Zexing
Z