A density functional study of the molecular structure and vibrational frequencies of the linear and planar monocyclic isomers of the C(n) (2 less-than-or-equal-to n less-than-or-equal-to 18) series was performed. A gradient corrected energy functional of the Becke-Perdew type and a triple-zeta valence basis with a polarization function was employed. The results show that for n less-than-or-equal-to 9 the linear chains are generally more stable than the ring structures. A possible exception is C6 where, in contrast to previous studies, the two forms were computed to be near isoenergetic. For n greater-than-or-equal-to 10 the cyclic structures dominate; however, only the C-10, C-12, and C-14 monocyclic planar rings were found to represent minimas on the energy surface. Excellent agreement for the computed vibrational frequencies with experiment-whenever available-was found. Therefore the vibrational data presented here may help to interpret spectra of yet unidentified C(n) species.