Structural and Molecular Orbital Analyses of the Hydrazyl Cation, Radical, and Anion: A Paradigm for Stereomutations in Stereolabile Configurational Units

An analysis and classification of the stereochemical changes in stereolabile configurational units are presented, and the associated NMR consequences are discussed. It is shown that the barriers to conformational changes observed in various molecules are closely related to the electron occupancy in their π-systems. Thus, two-electron π-systems such as imines, amides, and nitrosamines have achiral torsional ground states and undergo achiral torsion (T_A) or achiral inversion (I_A). By contrast, four-electron systems including substituted hydroxylamines, sulfenamides, hydrazines, and other molecules with adjacent lone pairs undergo topomerization between ground structures that are chiral in principle, when maximally labeled, by either a torsion (Tc) or inversion (Ic) mechanism. The structural and MO analyses are described by means of the hydrazyl model system, NH₂NH, in which changes in electron occupancy alone (from the cation through the radical to the anion) bring about changes in stereochemical classification. The discussion is supported by ab initio self-consistent field MO calculations on the hydrazyl system.