The effect of ortho-fluorination on intermolecular interactions of pyridine: Microwave spectrum and structure of the CO₂ – 2,6-difluoropyridine weakly bound complex

The weakly bound complex formed from CO₂ and 2,6-difluoropyridine (2,6-DFP) has been observed by chirped pulse and conventional cavity Fourier transform microwave spectroscopy. As in the related complexes with pyridine and 3,5-difluoropyridine, the carbon of the CO₂ approaches the nitrogen of the heterocycle in the plane of the ring. However, the CO₂−2,6-DFP complex is found to differ from the pyridine and 3,5-DFP analogues in several respects. First, the data indicate that the N⋯C weak bond distance is 2.9681(32) Å, a value that is ∼0.17 Å longer than that previously determined for CO₂−pyridine (2.798 Å) and 0.14 Å longer than that in CO₂−3,5-difluoropyridine (2.825 Å). Second, unlike the pyridine and 3,5-difluoropyridine complexes, the CO₂ oxygens in CO₂−2,6-DFP do not lie in the plane of the ring, i.e., fluorination of the pyridine in the ortho positions causes the CO₂ to rotate 90° out of plane. Moreover, the observed rotational constants indicate that the CO₂ moiety undergoes large amplitude vibrational motion with an average bending amplitude of ∼31° off this perpendicular geometry. Third, M06-2X/6-311++G(3df,3pd) and MP2/6-311++G(3df,3pd) calculations both indicate that in the equilibrium configuration, the carbon of the CO₂ is displaced from the C₂ axis of the 2,6-DFP such that axis forms an angle of 20–23° with the line joining the nitrogen and the CO₂ carbon. The current results support the previous conjecture that the anomalously short N⋯C distances in CO₂−pyridine and CO₂−3,5-difluoropyridine result from secondary attractive interactions between the oxygen atoms and the ortho hydrogens. These interactions are eliminated in the 2,6-DFP system by fluorination in the ortho positions, causing the weak bond distance to increase and the CO₂ to rotate out of the plane.