The influence of exocyclic lone pairs on the bonding and geometry of type A mesoionic rings

Abstract Based on structures determined by X-ray crystallography, ab initio MP2 calculations type A mesoionic rings give geometries in good agreement with observed values. study of four ring systems, each exocyclic oxygen, nitrogen or carbon groups, shows that the presence and configuration lone pairs significantly influences geometry configurational preference. Using a localised bond model NBO analysis, these effects are rationalised terms an anomeric interaction antibonding orbitals adjacent σ bonds. In experiment, similar calculated for pyran-2-imines.