The rotational spectrum of the pyridine–formaldehyde adduct generated in a supersonic expansion has been analyzed using Fourier transform microwave spectroscopy. The spectrum shows the quadrupole coupling hyperfine structure due to the presence of 14N. The spectra of the parent, 13C and 15N isotopologues have been observed to investigate its structure. The complex shows a 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)º). 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->pi* interaction together with a weak C-H...O bond.
