The electronic spectrum of C-11 in its linear and cyclic conformation

Large-scale multi-reference configuration interaction calculations are empl oyed to differentiate in the electronic spectrum between the linear and cyc lic isomers of C-11 by certain fingerprints. The (1)Sigma (+)(u) <-- X (1)< Sigma>(+)(g) transition of the linear isomer is calculated at 4.02 eV in re asonable agreement with the recently measured 3.69 eV in a neon matrix. The yet to be observed (1)Pi (u) <-- X (1)<Sigma>(+)(g) transition is computed at 2.13 eV. The spectrum of cyclic C-11 is characterized by three strong t ransitions calculated at 3.94 eV (3(1)B(2)), 4.03 eV (3 (1)A(1)) and 4.59 e V (4 B-1(2)). In addition the first transition of cyclic C-11 occurs at 1.0 4 eV (1 B-1(1) <-- X (1)A(1)) but this transition is energetically close to the dipole-forbidden (1)<Delta>(u) state of the linear isomer to which tra nsitions become allowed in cis-bending displacements.