The electronic structures of norbornadiene (NBD) and its methono-bridged hydrocarbons, norbornene (NBN), norbornane (NBA), [2.2.1]propellane (PPL), and quadricyclane (QC) ---, were studied in isolation and in CHCl3 solution. The chemical structures of the five compounds are shown in Figure , along with the carbon labels on NBN (where the double C=C bond in NBN is denoted as C2=C3). Structural optimization was performed using density functional theory (DFT) based on the B3LYP functional with Dunning's augmented correlation-consistent polarized valence triple zeta (aug-cc-pVTZ) basis set. No imaginary frequencies were detected, indicating that the hydrocarbon structures represent stable true minimum structures on their potential energy surfaces.