Using the MP2, CCSD(T), and DFT (B3LYP) methods, the structures and energies of the 2-butyl cation (C4H9+) were calculated. Energetically, the C-C hyper-conjugated structure 1 and hydrogen-bridged structure 2 were found to be almost identical at all levels. The C-13 NMR chemical shifts of 1 and 2 were computed by the GIAO-CCSD(T) method using different geometries. On the basis of calculated relative energies and calculated C-13 NMR chemical shifts, an equilibrium involving 1 and 2 (in a 50:50 ratio) seemed likely responsible for the experimentally observed C-13 NMR chemical shifts in superacid solutions at -80 degrees C. However, on the basis of computed and experimental frequencies the hydrogen-bridged structure 2 is most likely responsible for the experimentally observed frequencies in the solid state at -125 degrees C.