The consequences of σ and π conjugative interactions in mono-, di- and triacetylenes. A photoelectron spectroscopic investigation

The HeI photoelectron spectra of mono‐, di‐, and triacetylenes are presented. In these compounds the two‐centre π‐orbitals of the ethynyl groups conjugate with the π‐orbitals of double bonds or benzene moieties, or with the Walsh orbitals of three‐membered ring systems. Assuming the validity of Koopmans' approximation, the observed energies of the radical cation states reached by electron ejection from π‐orbitals can be rationalized in terms of a simple LCBO‐MO model in those cases, where the molecule is planar. The corresponding numerical results for the ionization energies are in excellent agreement with experiment, if the three parameters of the model are properly calibrated. In contrast, the bands assigned to ejection from in plane π‐orbitals are shifted to lower energies by ca. 0.5 eV with respect to the expectation values derived from the above model, due to ‘through‐bond’ interaction with lower lying σ‐orbitals. Extensive σ/π mixing occurs in the non planar compounds for all orbitals. The assignments of the spectra of diethynylmethane, 1,4‐hexadiyne, 1,2‐diethynylethane and of cis‐ and trans‐diethynylcyclopropane are backed by semiempirical SCF calculations. The spectra of the cis and trans isomers of diethynylethyleneoxide and diethynylethylenesulfide are discussed by comparison with the corresponding hydrocarbons and with oxirane and thiirane respectively. Finally, the following topics are considered in detail: (a) The effect of spin orbit coupling on the spectrum of 1‐iodo‐1‐butyne‐3‐ene; (b) the effect of the essentially free internal rotation in divinylacetylene on the band shapes of its photoelectron spectrum and (c) the relationship between the conjugative properties of ethylenic π‐orbitals and of the Walsh‐orbitals of cyclopropane.