The photoelectron spectra of methyl-substituted allenes and of tetramethyl-bisallenyl

Abstract

The split Δ = 0.6 eV of the first band at 10.5 eV in the photoelectron spectrum of allene ( 1) has been ascribed to a Jahn—Teller distortion by Baker and Turner. The influence of alkyl substitution on Δ and on the band position has been investigated by recording the photoelectron spectra of all methyl-substituted allenes 2 to 6, of 1,1-di-tert. butyl-allene ( 7) and of 1,2-cyclononadiene ( 8). It is shown that the dependence of Δ on substitution can be rationalized by assuming that it is due to a (pseudo) Jahn—Teller effect in 1 and in the symmetrically substituted derivatives and to the combined influence of this effect and a differential, inductive destabilization in the other cases. The photoelectron spectrum of tetramethyl-bisallenyl 9 can be explained on the basis of known parameters for semi-localized π-orbitals and for their through-space and through-bond coupling.