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 RCHC
O 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 Me3SiCH
C
O (5) and ButMe2SiCH
C
O (6), the alkene ButMe2SiCH
CH2(7) and the bisketenes (Me3SiC
C
O)2(1) and (ButMe2SiC
C
O)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.