Restricted Rotation about Single Bonds between Atoms with Unshared Electron Pairs

It seems likely that the stable value of the dihedral angle for group VI elements is in general about 102°, and that the barrier (at 180°) is about 2 or 3 kcal/mole. The value 0.32 kcal/mole reported for H20 by microwave spectroscopy70 seems small in comparison with the values discussed above, although some decrease, perhaps 50 percent, might be expected because of the partial ionic character of the O—H bonds. 

It is hard to make a prediction about hydrazine, although the staggered configuration with the unshared electron pairs in the trans position might be expected from the foregoing considerations. The configuration has not yet been determined for the gas molecule the ar- 

—The helix with seven atoms in two turns, proposed as the repeating unit in the chains of fibrous sulfur. 

---