MCAT General Chemistry Question 258: Answer and Explanation

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Question: 258

7. Nitrate is best described by a resonance average of three structures:

What best describes the peaks in an IR spectrum that result from the three N—O bond stretches?

  • A. One peak at the double bond N=O stretch frequency and two peaks at the single bond N—O stretch frequency
  • B. One peak at the double bond N=O stretch frequency and one peak at the single bond N—O stretch frequency
  • C. One peak at the double bond N=O stretch frequency and one peak between the single N—O and double bond N=O stretch frequencies
  • D. One peak between the single N—O and double bond N=O stretch frequencies

Correct Answer: D

Explanation:

D A molecule essentially exists as an average of its resonance structures. No individual resonance structure exists in isolation. In nitrate there are three bonds of equal strength, containing about 33% N=O double bond character and about 67% N—O single bond character. Therefore, there will be only one stretch peak in the IR spectrum intermediate to the N—O and N=O stretches that accounts for all three of these bonds in nitrate.

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