Rotational barrier heights

Zeeman frequencies 10.5 and 21 MHz and Tmin located in the range 20-150 K, correspond to rotation barrier heights from 0.65 to 3.4kcal/ mol. 

R. Stolevik and P. Bakken, /. Mol. Struct., 220, 269 (1990). Conformational Structures and Energies Rotational Barrier Heights and Torsional Oscillations in Tetrahalodiphosphmes, Tetrahalohydrazines and X2-PX2 Molecules. 

If the isomerization proceeds via the open-structure complex (Figure 3.9b), the transition state energies for I and II appear to be the same due to the remoteness of the substituents in the ligand from the other ligands in the complex. Therefore, AG i = AG n -f AG or AG i > AG ii. Consequently, the steric factor should favor a decrease in the rotation barrier height of the oximate ligand provided that its coordination in the transition state occurs via one n-donor atom. 

S. Tsuzuki and K. Tanabe, /. Chem. Soc., Perkin Trans. 2, 1255 (1991). Basis Set and Electron Correlation Effects on die Internal Rotational Barrier Heights of Formamide and Acetamide. 

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