Kinetics and mechanisms of nucleophilic displacements with heterocycles as leaving groups. Part 25. X-Ray structure determinations, crystallographic evidence for steric crowding, and correlation with acceleration of rates

X-Ray crystallographic structures are determined for seven N-benzylpyridinium salts including compounds possessing free, five-ring fused, and six-ring fused α-phenyl groups. Significant bond lengths, non-bonding distances, torsional and interplanar angles are determined and compared for the eight structures (the triphenyl derivative was found in two crystal environments). The results show steric overcrowding to an extent that parallels the kinetic displacement rates. However, the steric strain existing in all these molecules is relieved to different extents by different distortions in the different molecules. Nevertheless, crystallographic interatomic distances and angles are shown to be of potential utility in determining the type of mechanistic changes to be undergone, provided the appropriate parameters are selected.