Computation of O-C-F and N-C-F systems: AB-initio calculations and a MM2 parameterization study. Theory vs. experiment

A parameterization scheme of Allinger's MM2 force field for the anomeric effect in O-C-F and N-C-F systems is presented. The scarcity of experimental data, in particular for the N-C-F case, dictated the use of ab-initio calculations to account for the energetic and structural manifestations of the effect. The resulting modified force field was tested against available X-ray, microwave and NMR results leading to a very good agreement between calculations and experiment. In addition, ab-initio results were used to demonstrate the role of the anomeric effect in lowering barriers to N-inversion and elevating barriers for rotation around single CN bonds. The results for the fluoro compounds, when juxtaposed to other systems, provide a complete treatment of the anomeric effect for the first row elements.