Photoelectron Spectroscopy of Heterocycles. Dipyridylethylenes

The photoelectron (PE) spectra of 1,2-di-(2-pyridyl)-ethylene 1, l-(2-pyridyl)-2-(3-pyridyl)-ethylene 2, 1-(2-pyridyl)-2-(4-pyridyl)-ethylene 3 and 1,2-di-(4-pyridyl) -ethylene 4 all in their trans conformation have been measured using He I radiation. For 1 — 4 Standard SCF LCAO MO calculations were performed changing the angle of twist # of the pyridine rings around their bonds to ethylene conrotatively from 0° to 60°. Comparison of PE spectra and these calculations allowed the assignment of all 7r-ionizations and indirectly also the nitrogen lone pair ionizations in 1 — 4 but not a determination of their conformation in the gas phase. There is only indication that $ in these molecules is rather small (30°).