A theoretical study of the electronically excited states in linear and cyclic

Abstract

The electronically excited doublet and quartet states of the linear (D 1h ) and cyclic (C 2v ) C þ 6 ion were studied using high-level ab initio methods. For the linear ion it is found that the three lowest excited 2 Å g states, which contribute to the n 2 Å g -X 2 Å u transitions between 1.88 and 2.73 eV, are strongly coupled and form avoided crossings if the bond distances are varied. This leads to a centrosymmetric double minimum potential of the 2 2 Å g state. For the cyclic C 2v structures the lowest states have 2 A 1 and 2 B 2 symmetries. At their equilibrium geometries both states are almost degenerate and their energies are 0.3 eV lower than the minimum of the linear X 2 Å u state. The excitation energies are strongly affected by geometry relaxation effects. The adiabatic excitation energies of the cyclic 2 2 A 1 and the linear 1 2 Å g states are predicted to be about 1.9 eV, i.e. close to the observed band origin. Several electronically excited states-2 2 A 2 , 2 2 B 1 , 2 2 B 2 -of the cyclic structure and the 2 2 Å g state of the linear structure were calculated between 2.3 and 2.5 eV, i.e. higher than the observed band origin at 2.17 eV. Due to strong electronic and vibronic couplings a reliable prediction of relative intensities is presently not possible, and therefore the calculated transition moments cannot be used for the interpretation of the experimental spectrum.

Electronic and infrared absorption spectra of linear and cyclic C6+ in a neon matrix

“…The calculations overestimate the excitation energy to the 2 ⌸ g electronic state by 0.65 eV; the discrepancy is larger than the likely error quoted. 23 However, the results of extensive MRCI ab initio calculations agree better, predicting the energy of the allowed electronic transition at 1.88 eV 36 close to the experimental value of 2.18 eV.…”

“…23 The reason for this large difference appears to lie with geometry change in the two electronic states and the multiconfigurational complete active space self-consistent ͑CASSCF͒ method lead to a value of 2.39 eV for this transition. 36 In order to corroborate the above assignment of the electronic absorption bands to a specific form of C 6 ϩ , infrared spectra were also recorded. The section of the infrared ͑IR͒ spectrum around 2000 cm Ϫ1 of mass-selected C 6…”

Higher Excited Electronic Transitions of Polyacetylene Cations HC2nH+ n = 2−7 in Neon Matrixes

“…The congestion of the excited electronic states in these systems causes their vibrational bands to overlap. Theoretical calculations on C 6 + show the complexity arising in such open-shell systems by revealing several close-lying, mixed excited electronic states. , …”

3Σ-−X 3Σ- Electronic Transition of Linear C6H+ and C8H+ in Neon Matrixes

“…The influence of atomic orbital basis sets, active orbitals in CASSCF approach, and reference wave functions for internally contracted MRCI calculations on the precision for the electronic excitation energy and transition moment was previously investigated in an extensive theoretical study on C 6 + . 19 Because most of the excited states have strongly multiconfigurational character, accurate calculations of these properties in cumulenic carbon chains remain a difficult problem.…”