An electronic structural study of the ground electronic states for the chemically similar bicyclic norbornadiene (NBD, C7H8, X1A1), norbornene (NBN, C7H10, X1A′) and norbornane (NBA, C7H12, X 1A1) molecules is provided quantum mechanically. Initially, the unique orbital imaging capability of electron momentum spectroscopy is used to validate which of the quantum mechanical models available to us for these calculations best represents these species. Thereafter, individual molecular point group symmetry is incorporated in the calculations with energy minimization in the search for equilibrium geometries of the species using MP2/TZVP and B3LYWTZVP models. The optimized geometries compare favourably with available crystallographic results and also build confidence in cases where the crystallographic results are ambiguous. The present study aims to reveal the particular subtle structural deviation of the species, which results in significant molecular property differences among these organic compounds. This work intends to probe bonding information of the species and the impact, on the seven member carbon skeleton, as the C=C double bonds of NBD are progressively saturated by hydrogen atoms to give NBN and NBA. Significant changes observed through the present work include: (i) the seven member carbon skeleton tends to relax the strain whenever possible and (ii) the ethano ring experiences greater structural changes than the methano bridge. The methano bridge (C(1)-C(7)-C(4)) of the less symmetric NBN molecule (Cs) tilts to the single C-C bond side of the ethano ring of the molecule (rather than the C=C side), producing a dihedral angle of 8.7° between plane H-C(1)-C(4) (the yz-plane) and plane C(1)-C(7)-C(4). Our work suggests that it is this unique dihedral angle in NBN which causes the molecules exo-reactivity and is also responsible for the extra activity of its C=C bond. © 2004 Elsevier Ltd. All rights reserved.