Coupling between substituents as a function of cage structure: synthesis and valence ionized states of bridgehead disubstituted parent and hexafluorinated bicyclo[1.1.1]pentane derivatives C5X6Y2.

He(I) photoelectron spectroscopy was used to examine the valence-shell electronic structure of three new and seven previously known bicyclo[1.1.1]pentane derivatives, 1,3-Y2-C5X6 (for X = H, Y = H, Cl, Br, I, CN; for X = F, Y = H, Br, I, CN). A larger series (X = H or F, Y = H, F, Cl, Br, I, At, CN) has been studied computationally with the SAC-CI (symmetry-adapted cluster configuration interaction) method. The outer-valence ionization spectra calculated by the SAC-CI method, including spin-orbit interaction, reproduced the experimental photoelectron spectra well, and quantitative assignments are given. When the extent of effective through-cage interaction between the bridgehead halogen lone-pair orbitals was defined in the usual way by orbital-energy splitting, it was found to be larger than that mediated by other cages such as cubane, and was further enhanced by hexafluorination. The origin of the orbital-energy splitting is analyzed in terms of cage structure, and it is pointed out that its relation to the degree of interaction between the bridgehead substituents is not as simple as is often assumed.