Dioxidane - Structure and Properties

Structure and Properties

Hydrogen peroxide (H₂O₂) is not a flat molecule; it adopts a nonplanar (twisted) structure of C2 symmetry. Although chiral (the twist can be left or right-handed), the molecule undergoes rapid racemization, the result of which is that the left and right-handed twist forms cannot be isolated as they can quickly "flip" their handed-ness. The observed anticlinal "skewed" shape is a compromise between two conformers, called syn and anti. If the molecule had the flat shape of the anti conformer, it would minimize steric repulsions. However, if it had the 90° torsion angle of the syn conformer, there would be optimized mixing between the filled p-type orbital of the oxygen (one of the lone pairs) and the LUMO of the vicinal O-H bond. The compromise angle has the lowest energy state. The bond angles can also be affected by hydrogen bonding between molecules. As the molecules in gasses are too far apart for hydrogen bonding, the molecular structure of the gaseous and crystalline forms is different; indeed a wide range of values is seen in crystals containing H₂O₂.

Although the O−O bond is a single bond, the molecule has a relatively high barrier to rotation, of 29.45 kJ/mol; for comparison, the rotational barrier for the bulkier molecule ethane is 12.5 kJ/mol. The increased barrier is ascribed to repulsion between nonbonding electrons (lone pairs) of the adjacent oxygen atoms.