Rotational spectrum and structure of the pyridine-CO₂ van der Waals complex

The rotational spectrum of the pyridine-CO₂ van der Waals complex has been recorded in a cold supersonic jet using pulsed-nozzle Fourier transform microwave spectroscopy. Spectroscopic constants are reported for the parent species and for all four singly-substituted ¹³C derivatives. The data indicate a planar structure in which the pyridine nitrogen approaches the CO₂ carbon with the C₂ axis of the pyridine perpendicular to the CO₂. No evidence of internal rotation is observed. The zero point vibrationally averaged N⋯C van der Waals distance is 2.7977(64) Å and the O⋯ortho-hydrogen distance is 3.090(6) Å. The ¹⁴N nuclear quadrupole coupling constants are consistent with a bending vibrational amplitude of the pyridine moiety of about 9°away from the C_2v geometry. The observed structure is in excellent agreement with that previously calculated to correspond to the lowest of several nearly isoenergetic isomers, suggesting that it is, indeed, the global minimum-energy structure. Somewhat surprisingly, the C⋯N van der Waals bond distance is about 0.2 Å shorter than those observed in the related systems HCN⋯CO₂ and H₃N⋯CO ₂.