The electronic structural information from core orbitals of norbornadiene, norbornene and norbornane

Core molecular orbital (MO) topology and influence on the electronic structures of norbornadiene (NBD, C7H8, , X1A1), norbornene (NBN, C7H10, , X1A′) and norbornane (NBA, C7H12, , X1A2) in their ground electronic states are studied using both RHF/TZVP and B3LYP/TZVP models quantum mechanically. The present work focuses on the analysis of the topology, symmetry and implication on the electronic structures of the three structurally similar bicyclic hydrocarbons. The present work analyzes the changes in core orbitals of the molecules as a result of the CC double bond saturation. It has been demonstrated that the core orbital energy shift and wavefunction distortion of the strained molecular species are not subtle, even though they have similarities in geometries and bicyclic carbon frames. It is revealed in this work that the carbon atoms of the methano bridge bear large orbital energy changes (chemical shifts), due to the relaxation of the strained bridge angle ∠C(1)C(7)C(4), but remain inactive in momentum space, in the hydrogenation process of NBD and NBN. The core MOs in the ethano-ring experience obvious changes in both orbital energies (chemical shifts) and in particular, orbital momentum distributions (MDs), which is a direct indicator of electronic structural variation of the species. The distortion of molecular orbital MDs of NBD, NBN and NBA with respect to the corresponding carbon atomic orbital MDs can be considered as indicators of orbital polarization, imposed symmetry and chemical shifts within the individual molecular framework. The combination of electronic structural information in both configuration space and momentum space is capable of providing a comprehensive understanding of the electronic structural changes in the hydrocarbons.