Structure of hydrogen peroxide

Both in the gas phase and in the solid phase, the H₂O₂ molecule adopts a skewed, non-planar conformation -- the two O-H bonds are twisted out of a single plane rather than lying flat, because of the repulsion between each oxygen's lone pairs of electrons and the O-H bonds nearby. H₂O₂ is, in fact, the smallest molecule known to show hindered rotation about a single bond (the O-O bond) -- rotation is possible but not free, because certain twist angles are energetically favoured over others.

Structurally, H₂O₂ is described as dihydroxyl: the two -OH groups do not lie in the same plane. A helpful mental picture is a partly-opened book, with the two hydrogen atoms lying on the two open "pages" and the two oxygen atoms running along the "spine" (the O-O bond).

The precise molecular dimensions differ measurably between the gas and solid phases (Figure 4.5):

• Gas phase: O-H bond length = 0.95 Å, O-O bond length = 1.47 Å, O-O-H bond angle = 94.8°, dihedral (inter-plane) angle = 111.5°.
• Solid phase: O-H bond length = 0.98 Å, O-O bond length = 1.46 Å, O-O-H bond angle = 101.9°, dihedral angle = 90.2°. …