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 RCH = C = O with a variety of representative substituents have been calculated by ab initio methods, and give good agreement wit h available experimental photoelectron ionization energies. The struct ures and orbital energies of the monoketenes Me3SiCH = C = O (5) and B u (t)Me2SiCH = C = O (6), the alkene Bu(t)Me2SiCH = CH2 (7) and the bi sketenes (Me3SiC = C = O)2 (1) and (Bu(t)Me2SiC = C = O)2 (4) have als o been calculated by ab initio methods, and are compared with experime ntally measured photoelectron ionization energies. The spectra of the Bu(t)Me2Si compounds show a characteristic band associated with the Bu (t)-Si bond. Comparison of the measured and calculated spectra provide s strong evidence that the bisketenes 1 and 4 exist predominantly in t wisted conformations, with dihedral angles 105-degrees in the former c ase and 120-degrees in the latter. Dipole moment measurements on 1 and 5 confirm this conclusion.