Conformation properties of buta-1,3-diene-1,4-diones (bisketenes): computational and photoelectron spectroscopic studies
The highest occupied molecular orbital (HOMO) energies of a series of monoketenes RCHCO with a variety of representative substituents have been calculated by ab initio methods, and give good agreement with available experimental photoelectron ionization energies. The structures and orbital energies of the monoketenes Me3SiCHCO (5) and ButMe2SiCHCO (6), the alkene ButMe2SiCHCH2(7) and the bisketenes (Me3SiCCO)2(1) and (ButMe2SiCCO)2(4) have also been calculated by ab initio methods, and are compared with experimentally measured photoelectron ionization energies. The spectra of the ButMe2Si compounds show a characteristic band associated with the But—Si bond. Comparison of the measured and calculated spectra provides strong evidence that the bisketenes 1 and 4 exist predominantly in twisted conformations, with dihedral angles 105° in the former case and 120° in the latter. Dipole moment measurements on 1 and 5 confirm this conclusion.