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

Dennis L. Lichtenberger, Lalitha Subramanian, Uwe Bunz, K. Peter C. Vollhardt

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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.

Original languageEnglish (US)
Pages (from-to)1351-1357
Number of pages7
JournalJournal of the Chemical Society, Perkin Transactions 2
Issue number6
StatePublished - 1994

ASJC Scopus subject areas

  • Chemistry(all)


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