Given below are two statements : One is labelled as Assertion A and other is labelled as Reason R.
Assertion A : The dihedral angles in H$$_2$$O$$_2$$ in gaseous phase is 90.2$$^\circ$$ and in solid phase is 111.5$$^\circ$$.
Reason R : The change in dihedral angle in solid and gaseous phase is due to the difference in the intermolecular forces.
Choose the most appropriate answer from the options given below for A and R.

Hydrogen peroxide $$\mathrm{(H_2O_2)}$$ has a non-planar (open-book) structure in both the gas phase and the solid phase.

Each oxygen atom is approximately $$sp^3$$ hybridized and contains two lone pairs. Due to lone pair-lone pair repulsion, the molecule does not adopt a planar arrangement and instead possesses a finite dihedral angle between the two $$\mathrm{O-H}$$ bonds.

In the gas phase:

• $$\mathrm{O-H}$$ bond length = $$95.0\ \mathrm{pm}$$
• $$\mathrm{O-O}$$ bond length = $$147.5\ \mathrm{pm}$$
• $$\mathrm{H-O-O}$$ bond angle = $$94.8^\circ$$
• Dihedral angle = $$111.5^\circ$$

In the solid phase (at $$110\ \mathrm{K}$$):

• $$\mathrm{O-H}$$ bond length = $$98.8\ \mathrm{pm}$$
• $$\mathrm{O-O}$$ bond length = $$145.8\ \mathrm{pm}$$
• $$\mathrm{H-O-O}$$ bond angle = $$101.9^\circ$$
• Dihedral angle = $$90.2^\circ$$

The variation in bond lengths and angles between the gas and solid phases arises from intermolecular interactions, particularly hydrogen bonding in the solid state.

Hence, hydrogen peroxide exists as a non-planar molecule with a dihedral angle of approximately $$111.5^\circ$$ in the gas phase and $$90.2^\circ$$ in the solid phase.