HE-I AND HE-II STUDY OF THE SYMMETRICAL ISOMERS OF DIPYRIDYL DISULFIDE AND DISELENIDE

The He I and He II photoelectron spectra of the symmetrical isomers of dipyridyl disulfide and diselenide have been measured and interpreted in terms of a composite-molecule model. The sequence of the lowest va lence ionisation energies is assigned as: (pi8 - n(X))- greater-than-o r-equal-to (pi7 - n(X))- > n(N) approximately n(N) > (pi6 - deltan(X)) approximately (pi5 - deltan(X)) > (pi4 + n(X))- greater-than-or-equal -to (pi3 + n(X))+, which is consistent with He I/He II photoionisation cross-section ratios, a composite-molecule model and correlations wit h the assignment of the dimethyl chalcogenides and chalcogenobispyridi nes, but contrary to STO-3G and extended valence basis set calculatio ns.