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 C_s 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.