We report density functional and coupled cluster calculations on numerous monocyclic and bicyclic (CH),2 isomers. At the RCCSD(T)/cc~pVDZ//UB3LYP/6~3l+G* level, a nearly planar, bond-equalized radical anion of 1,7-di-trans-[12]annulene (4a) is lowest in energy; several other isomers and conformations lie within 3 kcal/mol of 4a~. RCCSD(T)/AUG~cc~pVDZ//UB3LYP/6~3I+G* results place the all-cis isomer 3~ slightly below 4a in energy. Validation studies on the heptalene radical anion, [16]annulene radical anion, and tri-trans-[12]annulene radical anion indicate that electron spin resonance (ESR) hyperfine coupling constants (aH values) computed at the BLYPIEPR-III level on DFT geometries give much better agreement with experimental values than those computed using B3LYP/6-3 I G*. We were unable to locate any C,2H,f isomer that could account for the ESR spectrum previously attributed to a highly twisted structure for the 1,7-di-trans-[l2lannulene radical anion. Our computed energetic and ESR data for [12]annulene radical anions and their valence isomers suggest that 4a~ may have been made, yet its ESR spectrum was incorrectly assigned to the bicyclic isomer 6b. Finally, the computed `H NMR shift values of the dianion of 4 reveal a distinct diatropic ring current that should aid in its characterization. [ABSTRACT FROM AUTHOR]