Optimized structures and free energies dependence upon temperature of neutral - singlets and triplets - and singly charged ionic - anionic and cationic doublets - linear chains and monocyclic rings of eleven carbon atoms are obtained using density functional theory (B3LYP/6-31G*). Reaction pathways for the formation of linear chains and monocyclic rings are analyzed. Cyclization transition states and particular isomerization reactions are shown. C-11 rings can be formed at low temperatures and pressures as in the case of an interstellar medium. Electron affinities of C-11 linear chains and monocyclic rings - calculated with a larger basis set - match with measured values if temperature effects are taken into account.