Question

In gas phase structure of $H_{2}O_{2}$ the dihedral angle is

• A. 94.5$^{\circ}$
• B. 101.9$^{\circ}$
• C. 111.5$^{\circ}$
• D. 90.2$^{\circ}$

Hint:

Remember: Gas phase = less crowding, larger dihedral angle (111.5$^{\circ}$); Solid phase = hydrogen bonded packing, smaller angle (90.2$^{\circ}$).

Answer 1

The Correct Option is C

Step 1: Concept
Hydrogen peroxide ($H_{2}O_{2}$) possesses a non-planar, open-book structure due to lone pair-lone pair repulsions, and its structural parameters vary between the gas phase and crystalline solid phase.

Step 2: Meaning
The dihedral angle (or torsional angle) is the angle between the plane containing the $H-O-O$ atoms and the plane containing the $O-O-H$ atoms.

Step 3: Analysis
In the gaseous phase, intermolecular forces are negligible, allowing the molecules to adopt a conformation that minimizes electronic repulsions, which leads to a larger dihedral angle of 111.5$^{\circ}$. In contrast, in the solid state, extensive intermolecular hydrogen bonding compresses this layout, bringing the dihedral angle down to 90.2$^{\circ}$.

Step 4: Conclusion
Since the question explicitly asks for the gas phase structure, the dihedral angle value is 111.5$^{\circ}$.

Final Answer: (C)