Trends in Inversion Barriers of Group 15 Compounds. 3. Are Fluorinated Pyridone Derivatives Planar or Nonplanar?

Fluorinated compounds of 4-pyridone are studied using the semiempirical PM3 method, and the ab initio HF and MP2 methods. The perfluorinated derivative of 4-pyridone is predicted to have a nonplanar ring structure with the fluorine ligand at the nitrogen atom lying above the pyridine ring. The inversion barrier for the pentafluoro-4-pyridone is predicted to be 26 kJ/mol similar to that found for NH(3). This distortion corresponds to a static second-order Jahn-Teller effect and is expected to be experimentally detectable at low temperatures. N-Fluoro-4-pyridone is predicted to be nonplanar and has a small inversion barrier of 0.2 kJ/mol at the MP2 level. However, the maximum point of this barrier lies below the lowest zero-point out-of-plane inversion vibrational mode ((1)/(2) 84 cm(-1) identical with 0.5 kJ/mol). This corresponds to a dynamic Jahn-Teller effect and thus is experimentally difficult to verify. The MP2 calculations indicate that at least one fluorine atom is required at the ring nitrogen in order to achieve nonplanarity. Schleyer's negative-independent chemical shift method (NICS) is applied, and the results are used to discuss aromaticity in fluorinated pyridones. The NICS values show that succesive fluorination increases aromaticity. The vibrational spectra of all fluorinated pyridone derivatives are predicted. The vibrational spectrum of 4-pyridone is discussed in detail using a normal-mode analysis defined within a set of nonredundant internal coordinates.