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 semiempirica l PM3 method, and the ab initio HF and MP2 methods. The perfluorinated derivative of 4-pyridone is predicted to have a nonplanar ring struct ure with the fluorine ligand at the nitrogen atom lying above the pyri dine ring. The inversion barrier for the pentafluoro-4-pyridone is pre dicted to be 26 kJ/mol similar to that found for NH3. This distortion corresponds to a static second-order Jahn-Teller effect and is expecte d to be experimentally detectable at low temperatures. N-Fluoro-4-pyri done 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 barri er lies below the lowest zero-point out-of-plane inversion vibrational mode (1/2 84 cm(-1) = 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 negativ e-independent chemical shift method (NICS) is applied, and the results are used to discuss aromaticity in fluorinated pyridones. The NICS va lues show that succesive fluorination increases aromaticity. The vibra tional 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 coo rdinates.