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Question #44161

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The Question

The hydroperoxyl radical ( $\ce{HO2^\bullet}$ ) in the gas phase exhibits a magnetic moment of $\approx 1.73\text{ B.M.}$ and an O-O bond length of $1.33\text{ \AA}$ . In the crystalline solid state at $10\text{ K}$ , the bulk magnetic susceptibility drops to near zero, and the O-O bond length increases to $1.38\text{ \AA}$ , despite crystallographic data confirming that discrete $\ce{HO2}$ molecules remain intact (no covalent $\ce{H2O4}$ formation). Which of the following statements correctly identifies the primary chemical mechanisms responsible for these solid-state anomalies?

Options

A.
Intermolecular hydrogen bonding creates a pathway for antiferromagnetic superexchange, quenching the macroscopic magnetic moment, while the H-bond network withdraws electron density from the O-O bonding orbitals, lengthening the bond.
B.
The solid lattice compresses the molecules, forcing the unpaired electron from the $\pi^*$ to the $\sigma^*$ orbital, which pairs the electrons and drastically increases the bond length.
C.
Hydrogen bonding acts as a Lewis base, donating electron density into the O-O $\pi^*$ antibonding orbital, which pairs with the existing radical electron to form a diamagnetic species and lengthens the bond.
D.
The molecules undergo a localized Jahn-Teller distortion in the solid state, which lifts the degeneracy of the $\pi^*$ orbitals, pairing the electrons and increasing the bond length.

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

MOT and Magnetic Nature|Hydrogen Bonding Competition|Bond Energy vs Bond Length

Why This Question Is Tricky

Common Mistakes

The question tests the boundary between gas-phase Molecular Orbital Theory (MOT) and condensed-phase solid-state chemistry.

Students often assume that a radical must remain paramagnetic in the solid state unless it forms a covalent dimer.

The 'hidden variable' is antiferromagnetic coupling (superexchange) mediated by hydrogen bonds, which quenches the bulk magnetic susceptibility without forming a covalent bond.

Furthermore, the trap tests the electronic effect of H-bonding: acting as a Lewis acid (proton donor), it withdraws electron density from the oxygen lone pairs (which have partial bonding character), thereby weakening and lengthening the O-O bond.

Distractors exploit common misconceptions, such as applying Jahn-Teller distortion to already asymmetric molecules or misunderstanding the direction of electron flow in hydrogen bonds.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Chemical Bonding and Molecular Structure - Class 11 Part 1
Concept	MOT and Magnetic Nature
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free