Through-bond and through-space interactions : The electronic spectrum of barrelene

The first theoretical calculations that reproduce the observed UV spectrum of bairelene are reported. Absorptions above 200 nm are assigned as π →σ^* while the first excitation that is essentially π → π^* is predicted at ca 180 nm. These results are interpreted in terms of through-space π - π interaction, which splits the π-levels, and through-bond interaction, which mixes σ into the π, and fills in the gap between the antibonding π-orbitals. The latter interaction is such that the σ-framework, in each of the three bridges, couples the ethylenic moieties that are based on the other two.