The structural effects of fluorine substitution in pyridine,pyrimidine, and s-triazine: An ab initio study

The structures of 2-fluoropyridine, 3-fluoropyridine, 4-fluoropyridine, pentafluoropyridine, 2-fluoropyrimidine, 5-fluoropyrimidine, and fluoro-s-triazine have been evaluated by the ab initio gradient method with a 4-21 basis set augmented as needed with polarization functions on nitrogen. The structural effects of fluorination on the parent heterocycle are very similar to the effects of fluorination on benzene studied earlier. The ring angle is enlarged by about 2° at the point of fluorination and the adjacent ring bonds are shortened, much more for an adjacent C-N than for C-C. Fluorination of pyridine at the C₂ position causes an in-crease of bond localization in the ring. Rotational constants calculated from the structural parameters corrected with standard offset values are in exceptionally close agreement with experimental constants where these are known.