Sigma-pi interactions in non-conjugated polyalkynes: A photoelectron spectroscopic study

Through-space and through-bond interactions between π-orbitals in the molecules hepta-1,6-diyne (A), 3,3-diethylpenta-1,4-diyne (B), 4,4-diprop-2-ynylhepta-1,6-diyne (C), 2,2-di(bromomethyl)-1,3-dibromopropane (D) and 4,4-diethynylhepta-1,6-diyne (E) have been studied using gas-phase HeI photoelectron spectroscopy. The assignments of the photoelectron bands are discussed in relation to the results of extended Hückel calculations. Mixing of the π orbitals with the σ bond framework of the molecules is revealed by broadened band profiles in the π ionization region. Detailed examination of the first ionization of A suggests that one conformation is predominant under the conditions of the experiment. The terminal π orbitals are separated by too great a distance for through-space interaction, so the spread of the ionization band is entirely from through-bond interactions. The low-energy ionizations of B correspond to the in-plane and out-of-plane symmetric and antisymmetric combinations of the four terminal π orbitals, each of which has a different interaction with the C-H and C-C bonds of the central carbon atom. The spectra of C and D are very similar to each other, even though C has only alkyne substituents and D has only bromine substituents. The spectrum of E is a complicated mix of some of the features of A and some of the features of B. Extended Hückel calculations help clarify the number of orbitals in this region and the nature of the orbital interactions.