Interconversion Mechanisms in H₂N-O-NH₂: Rotamerism, Pyramidal Inversion, or Akamptisomerism?

Conformational isomerism is a fundamental aspect of molecular behavior, yet bond-angle inversion (akamptisomerism) remains a rare and poorly understood mechanism, with evidence largely limited to B-O-B-bridged macrocycles. In this study, the conformational landscapes of X-O-X systems (X = B, C, N, and O) were investigated to assess the generality of this process. Conformational searches and energy profiles were computed at the GFN2-xTB and B3LYP/def2-TZVP levels. The results indicate that akamptisomerism is not a viable pathway in these systems. A linear geometry was located but corresponds to higher-order saddle points rather than a true transition state and is associated with prohibitive energy costs (>70 kcal mol^-1). In contrast, H₂N-O-NH₂ undergoes interconversion via rotamerism and, more favorably, trigonal pyramidal inversion, with barriers of ∼15 and ∼8 kcal mol^-1, respectively. These findings indicate that akamptisomerism is not a general feature of X-O-X motifs and likely requires specific geometric constraints, such as those found in macrocyclic environments.