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.
Bibliographical noteFunding Information:
This work was supported by the National Science Foundation (Grant #s CHE-1266320 and CHE-1563324 to the University of Minnesota and CHE-1111504 and CHE-1465014 to Amherst College). Computational work was done at the Minnesota Supercomputer Institute. RBM was supported by the University of Minnesota via a Lester C. and M. Krogh Graduate Fellowship during part of the course of this work. The authors thank Professor Jennifer van Wijngaarden for providing spectral data for Ar?2,6-DFP prior to publication.
- CO interactions
- Microwave spectroscopy
- Substituted pyridine
- Van der Waals interaction
- Weakly bound complex