Rotational Characterization of an n -> pi* 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 N-14. The spectra of the parent species, C-13 and N-15 isotopologues, have been observed to investigate its structure. The complex shows C-s symmetry with the plane of pyridine bisecting the angle HCH angle of formaldehyde and the N atom located along the Burgi-Dunitz trajectory of nucleophile addition to a carbonyl group (r(N-C) = 2.855(4) angstrom, angle NC=O= 102.8(6)degrees). 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 center dot center dot center dot O bond.