Microwave spectrum and structure of the 3,5-difluoropyridine⋯CO₂ van der Waals complex

The rotational spectrum of the weakly bound complex 3,5-difluoropyridine⋯CO₂ has been observed using pulsed-nozzle Fourier transform microwave spectroscopy. Spectroscopic constants are reported for the parent and ¹³CO₂ isotopologues. The data indicate a planar structure in which the nitrogen approaches the carbon of the CO₂ with either a C_2v or effectively C_2v geometry in the ground vibrational state. The N⋯C van der Waals bond distance is 2.8245(16) Å and the oxygen⋯ortho-hydrogen distance is 3.091(2) Å. The N⋯C van der Waals bond length is 0.027(8) Å longer than that previously determined for pyridine–CO₂, but is still considerably shorter than the 2.998 Å distance in HCN⋯CO₂. M06-2X/6-311++G(3df,3pd) calculations place the binding energy of the complex at 4.3 kcal/mol (4.1 kcal/mol with counterpoise correction). The calculations further indicate that a secondary interaction between the ortho-hydrogens of the ring and the CO₂ oxygens account for ∼50% of the total binding energy.