Characterizing the n ->pi* interaction of pyridine with small ketones: a rotational study of pyridine...acetone and pyridine...2-butanone

Complexes formed by pyridine and small ketones such as acetone and 2-butanone have been generated in a supersonic jet and characterized by broadband Fourier transform microwave spectroscopy combined with high-level theoretical computations. The spectra of the complexes show a quadrupole coupling hyperfine structure due to the presence of a nitrogen atom and the splittings owing to the low barriers of the internal rotation of the methyl groups bonded to the carbonyl group. The corresponding barriers have been determined from the analysis of the spectra. We show in both complexes that pyridine closes a cycle with a ketone carbonyl group through an N center dot center dot center dot C = O n ->pi* tetrel interaction and a C-H center dot center dot center dot O contact. The n ->pi* tetrel bond involves the pyridine N atom lone pair and the ketone carbonyl group with a geometry approaching the Burgi-Dunitz trajectory for the nucleophilic attack to a carbonyl group.