MCAT Organic Chemistry Question 152: Answer and Explanation

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Question: 152

2. Oxygen (O2) does not exhibit an IR spectrum because:

  • A. it has no molecular motions.
  • B. it is not possible to record IR spectra of a gaseous molecule.
  • C. molecular vibrations do not result in a change in the dipole moment of the O2 molecule.
  • D. molecular oxygen contains four lone pairs overall.

Correct Answer: C

Explanation:

Because molecular oxygen is homonuclear (composed of only one element) and diatomic, there is no net change in its dipole moment during vibration or rotation; in other words, the compound does not absorb in a measurable way in the infrared region. IR spectroscopy is based on the principle that, when the molecule vibrates or rotates, there is a change in dipole moment. Choice (A) is incorrect because oxygen does have molecular motions; they are just not detectable in IR spectroscopy. Choice (B) is incorrect because it is possible to record the IR of a gaseous molecule as long as it shows a change in its dipole moment when it vibrates. Choice (D) is incorrect because lone pairs do not have an effect on the ability to generate an IR spectrum of a compound.

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