Rotational Characterization of an n → π* Interaction in a Pyridine-Formaldehyde Adduct.

The rotational spectrum of the pyridine-formaldehyde adduct generated in a supersonic expansion has been analyzed using Fourier transform microwave spectroscopy. The spectrum shows a quadrupole coupling hyperfine structure due to the presence of 14N. The spectra of the parent species, 13C and 15N isotopologues, have been observed to investigate its structure. The complex shows Cs symmetry with the plane of pyridine bisecting the ∠HCH angle of formaldehyde and the N atom located along the Bürgi-Dunitz trajectory of nucleophile addition to a carbonyl group ( r(N-C) = 2.855(4) Å, ∠NC═O = 102.8(6)°). From this structure and with the help of ab initio computations and natural bond orbital analysis, it is shown unambiguously that pyridine links to formaldehyde, the smallest molecule bearing a carbonyl group, through an n → π* interaction together with a weak C-H···O bond.