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 comparison with spectra of related compounds (ethynylbenzenes). AM1 calculations were also performed to confirm the assignment. We have deduced for the first time, solely on empirical grounds, the presence of orbital interactions between nitrogen lone pairs and in-plane π-orbitals of the ethynyl group. Relative Lewis basicities are predicted and rationalized.