ORBITAL INTERACTIONS IN ETHYNYLPYRIDINES

He I and He II photoelectron spectra of several isomeric ethynyl- and diethynylpyridines have been measured. The spectra were analyzed with the aid of empirical arguments: relative band intensities and comparis on with spectra of related compounds (ethynylbenzenes), AMI calculatio ns were also performed to confirm the assignment. We have deduced for the first time, solely on empirical grounds, the presence of orbital i nteractions between nitrogen lone pairs and in-plane pi-orbitals of th e ethynyl group. Relative Lewis basicities are predicted and rationali zed.