Torsional Barriers Resulting from Two‐electron and Four‐electron Interactions. The Hydrazyl Cation and Anion Models

The spectrum of rotations about single bonds has been divided into two different groups of processes, termed T_A and T_C. These describe interconversion of planar ground state conformations (as in amides), and interconversion of enantiomers (as in sulfenamides and hydroxylamines), respectively. A frontier molecular orbital analysis of the hydrazyl cation and anion reveals that a change in electron occupancy alone can bring about a change from one torsional process to the other. Thus, the four‐electron hydrazyl anion undergoes a T_C process, while the corresponding two‐electron cation undergoes a T_A rotation. Nonempirical SCF‐MO calculations of the model cation and anion with geometry optimization at the 4‐31G level confirm the results of the PMO analysis. The calculations also reveal a preference for an inversion (I_A) mechanism for the topomerization of the cation system over a torsional process.