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MCAT Organic Chemistry Practice Question – IR Spectrum

While carbon dioxide exhibits four vibrational modes in space, its experimentally-obtained IR spectrum shows no more than two distinct peaks. This is partly because:

A. the two oxygen nuclei exhibit nuclear spins that are equal in magnitude but opposite in direction.

B. three of the vibrational modes are considered energetically degenerate.

C. one of the vibrational modes is not observable by IR spectroscopy.

D. two of the vibrational modes are not excitable by IR spectrometers of conventional power.

Explanation

The symmetric stretching mode of carbon dioxide does not induce a change in its molecular dipole. It is thus effectively invisible to IR spectroscopy, which functions by detecting changes in dipole moments, making C the correct answer.

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