Theoretical studies on structures and electronic spectra of linear HC2n+1H+ (n = 2–7)

In this work, the odd-numbered linear hydrocarbon cations HC2n+1H+ (n = 2–7) have been investigated with the B3LYP, CAM-B3LYP, and RCCSD(T) calculations focusing on the ground-state geometries, as well as with the CASSCF calculation for the structural optimizations of the ground and first excited states. The present studies reveal that these cation radicals possess stable structures with the ground state of X2Πu when n is even or X2Πg when n is odd, featuring some sort of cumulenic character for the middle carbon chains. Consistent with the previous studies of HCnSi+ clusters, the odd-numbered HCnH+ chains are less stable than the even-numbered ones. The vertical excitation energies for the dipole-allowed 12Πg/u ← X2Πu/g transitions of HC2n+1H+ (n = 2–7), obtained by the CASPT2/cc-pVTZ level, are 2.59, 2.11, 1.87, 1.65, 1.49, and 1.35 eV, respectively, which mutually agree with the available experimental data of 2.48, 2.07, 1.78, 1.57, 1.42, and 1.29 eV. Particularly the corresponding absorption wavelengths are predicted to have the remarkably linear size dependence, as experimentally observed. In addition, the higher excited electronic transitions of HC2n+1H+ (n = 2–7) are also calculated, indicating that the absorption wavelengths for the 32Πg/u ← X2Πu/g transitions also exhibit similar linear relationship and the largest oscillator strengths make them accessible more easily in the further experiments.

Graphical abstractThe linear size dependences of the absorption wavelengths of the origin bands for HC2n+1H+ (n = 2–7) clusters by the experiments and CASPT2 calculations.Figure optionsDownload full-size imageDownload high-quality image (18 K)Download as PowerPoint slide