Conformational Preference of 2-(Halomethyl)- and 2-(Oxymethyl)Pyridines: Microwave Spectrum, Ab Initio, and MM3 Studies of 2-(Fluoromethyl)Pyridine.

One single conformer was assigned from the microwave spectrum of 2-(fluoromethyl)pyridine, investigated in the gas phase in the 26.0-39.0 GHz spectral region at about -10°C. Its Cα-F bond was found to be coplanar with the ring and anti to the N-C2 bond (syn to the C2-C3 bond). There was no indication in the microwave spectrum of the presence of other rotameric forms of the molecule. The results of the spectroscopic study were backed up by ab initio calculations at the MP2/6-31 G** (frozen core) level. These calculations predict that the assigned conformer is the only stable form of the molecule. The transition state was calculated to have the CH2 F group 180° from the stable anti conformation. The energy of the transition state was computed to be 20.2 kJ mol-1 higher than the energy of the anti rotamer. The results are interpreted in terms of a stereoelectronic effect, and the orbital overlaps responsible for the observed effect are discussed. It is shown that 2-(fluoromethyl)pyridine serves as a good model for 2-(alkoxymethyl)pyridines, previously found to show the same conformational preference.