Conformation properties of buta-1,3-diene-1,4-diones (bisketenes): computational and photoelectron spectroscopic studies

Abstract

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 Bu^tMe₂SiCHCO (6), the alkene Bu^tMe₂SiCHCH₂(7) and the bisketenes (Me₃SiCCO)₂(1) and (Bu^tMe₂SiCCO)₂(4) have also been calculated by ab initio methods, and are compared with experimentally measured photoelectron ionization energies. The spectra of the Bu^tMe₂Si compounds show a characteristic band associated with the Bu^t—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.