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 C-13 derivatives. The data indicate a planar structure in which the pyridine nitrogen approaches the CO2 carbon with the C-2 axis of the pyridine perpendicular to the CO2. No evidence of internal rotation is observed. The zero point vibrationally averaged N center dot center dot center dot C van der Waals distance is 2.7977(64) angstrom and the O center dot center dot center dot ortho-hydrogen distance is 3.090(6) angstrom. The N-14 nuclear quadrupole coupling constants are consistent with a bending vibrational amplitude of the pyridine moiety of about 9 degrees 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 center dot center dot center dot N van der Waals bond distance is about 0.2 angstrom shorter than those observed in the related systems HCN center dot center dot center dot CO2 and H3N center dot center dot center dot CO2. (C) 2012 Published by Elsevier B.V.