Interconversion Mechanisms in H ₂ N–O–NH ₂ : Rotamerism, Pyramidal Inversion, or Akamptisomerism?

High Resolution Image Download MS PowerPoint Slide 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, H 2 N–O–NH 2 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.