Electronic structure of docosahexaenoic acid studied by photoelectron spectroscopy

The gas-phase He(I) photoelectron spectra of docosahexaenoic acid (I) and its ethyl ester, docosahexaenoic acid ethyl ester (II), have been studied with the aid of MNDO calculations. These molecules have six CC double bonds and a carboxyl or an ethoxycarbonyl end-group. The six CC double bonds in these molecules are not directly adjacent to one another, but are placed with great regularity; that is, there is one methylene group between each pair of double bonds adjacent to one another. The first photoelectron spectral band groups of compound I show almost the same vertical ionization energy values as those of compound II and are ascribed to the ionizations from the six CC double bond π orbitals. The six CC double bonds of both compounds seem to be isolated from one another, as judged by their molecular structure, but the occupied π orbitals have been found to be delocalized over several CC double bond groups on the basis of the photoelectron spectra and the molecular orbital calculations. In compound II an independent second band is apparently well separated from the following band group. This second band group is ascribed to the carbonyl oxygen atom n band and the ethoxycarbonyl group π band overlapping each other.