The ionization potentials, electron affinities, and π orbital shapes of 2-substituted norbornadienes have been determined by photoelectron spectroscopy, electron-transmission spectroscopy, and ab initio molecular orbital calculations, respectively. The deductions made about the electronic structures of 2-methoxy-, (trimethylsiloxy)-, chloro-, cyano-, (methoxycarbonyl)-, and phenylnorbornadienes permit a detailed interpretation of the reactivities and selectivities observed experimentally in carbene cycloadditions to these molecules. A substituent at C-2 of norbornadiene not only affects the 2-3 π bond but also influences the 5-6 π bond due to through-space interactions between π orbitals. The orbital energy changes and polarization induced by 2-substituents provide a compelling rationale of the variations in 1,2 and homo-1,4 cycloadditions of carbenes to these species, and confirm the electrophilic nature of both of these cycloadditions.