ELECTRONIC-STRUCTURE OF SOME PYRIDYLACETYLENES STUDIED BY HE 1 PHOTOELECTRON-SPECTROSCOPY - A WEAK ORBITAL INTERACTION BETWEEN THE NONBONDING ELECTRON-PAIR ON THE NITROGEN ATOM AND THE INPLANE PI ORBITAL IN THE ETHYNYL GROUP

The He I photoelectron spectra (PES) of 2- and 4-ethynylpyridine were measured, and the origin of each ionization band was clarified. For in stance, 4-ethynylpyridine shows five PES bands at 9.58, 9.9, 10.0, 10. 75 and 11.84 eV in the lower energy region, the 9.9 eV and 10.75 eV ba nds in particular being assigned to the ionizations from the aln orbit al of the pyridyl group and the in-plane b(2) pi orbital of the ethyny l group, respectively. Furthermore, with the aid of first order pertur bation theory, the 10.59 eV band of 2-ethynylpyridine has been ascribe d to the ionization from the a(2)-like pi orbital, and the 10.8 eV ban d to that from the in-plane b(2)-like pi one. Through the process of t he band assignments for the ethynylpyridines, it has been revealed tha t there is a weak interaction between the n orbital on the ortho-subst ituted nitrogen atom and the in-plane n orbital localized on the ethyn yl group of 2-ethynylpyridine. The photoelectron spectra of 1,4-bis(2- pyridyl)-1,3-butadiyne and 1,4-bis(4-pyridyl)-1,3-butadiyne, whose pi- electron systems are regarded electronically as double ones of the cor responding ethynylpyridines, were also measured, and we have succeeded in making band assignments of these compounds on the ground of the co rrelation energy diagram with the ethynylpyridines.