The molecular structure of allenes and ketenes. Part 19. Photoelectron spectra and conformations of donor-substituted allenes

Abstract

He^I photoelectron (p.e.) spectra for the donor-substituted allenes RCHCCH₂[R =(CH₃)₃C (1), N(CH₃)₂(2), OCH₃(3), OC₂H₅(5), OC(CH₃)₃(6), SCH₃(4), SC₂H₅(7), SC(CH₃)₃(8)] are reported and assignments of p.e. bands with energies I_v < 12.5 eV are presented. Substitutent effects on the p.e. band positions can be interpreted in terms of intrinsic electronic effects of the groups R, vibronic coupling effects (of the Jahn–Teller or pseudo-Jahn–Teller-type, respectively) being negligible. Interactions of the substituents with the remote allenic CC moiety are rationalized through Coulombic field effects and hyperconjugations. The lowest energy ionic states of (1)—(8) have geometries which deviate considerably from those of the neutral ground-state. In particular, (2⁺) and (8⁺) are shown to have bent CCC geometries. From the p.e. spectra it is inferred that compound (2) has a non-planar geometry. This is supported by MINDO/3 calculations. Furthermore, it is shown that all the radical cations exist in one largely preferred conformation. For the methyl and ethyl compounds (3), (5), (4), and (7) it is the s-cis conformation, whereas for the t-butyl molecules (6) and (8) the s-trans form is preferred.