IR Spectra: N-O Nitro group: Why two peaks?

In summary, the IR spectra correlation chart for organic chemistry lists the stretch for a nitro N-O bond as occurring at 1550 and 1400cm-1 and appearing as "teeth". This is due to the symmetric and antisymmetric stretching modes. The rest of the functional groups on the chart only have one expected peak, so why does N-O have two? The answer lies in the analogous feature for CH3 groups at around 1430 cm^{-1}. Similarly, the carbon dioxide stretch is listed as a "doublet" around 2350 cm-1, but the reason for this is not due to bending or resonance, as CO2 does not have these properties. This raises the question of what could be causing
  • #1
nbd
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My IR spectra correlation chart for organic chemistry says that the stretch for a nitro N-O bond occurs at "1550 and 1400cm-1" and that it will look like "teeth". Why does N-O have two peaks? The rest of the functional groups on my chart list a range in which a single peak should appear, but why is N-O expected to generate two peaks?

Thanks!
 
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  • #2
nbd said:
My IR spectra correlation chart for organic chemistry says that the stretch for a nitro N-O bond occurs at "1550 and 1400cm-1" and that it will look like "teeth". Why does N-O have two peaks? The rest of the functional groups on my chart list a range in which a single peak should appear, but why is N-O expected to generate two peaks?

Thanks!

I suppose symmetric and antisymmetric stretching modes. Note the analogous feature for CH3 groups at around 1430 cm^{-1}.
 
  • #3
That makes sense, thank you! In the same vein, I notice that my correlation chart also has the carbon dioxide stretch listed as a "doublet" appearing around 2350 cm-1. Is that for a similar reason as N-O/CH3? In my mind it shouldn't be, because CO2 is not bent and does not have resonance, so the anti-symmetrical stretching would not be IR-active. If that's true, what could be causing the two bands for the carbon dioxide stretch?

Edit: Since this is kind of a new question, I posted it in its own thread here.
 
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FAQ: IR Spectra: N-O Nitro group: Why two peaks?

1. Why does the N-O nitro group have two peaks in an IR spectrum?

The N-O nitro group has two peaks in an IR spectrum because it has two distinct vibrational modes: the symmetric stretch and the asymmetric stretch. These modes are caused by the stretching and bending of the N-O bond, resulting in two different absorption frequencies.

2. How do the two peaks in the N-O nitro group differ from each other?

The two peaks in the N-O nitro group differ in their absorption frequencies. The symmetric stretch typically appears at a higher frequency, around 1350-1550 cm-1, while the asymmetric stretch appears at a lower frequency, around 1500-1600 cm-1. This difference is due to the different bond strengths and bond lengths of the N-O bond in each mode.

3. What information can be obtained from the two peaks in the N-O nitro group?

The two peaks in the N-O nitro group provide information about the chemical environment and bonding within the molecule. The frequencies and intensities of the peaks can help identify the type of nitro group present (e.g. primary, secondary, tertiary), as well as the presence of other functional groups in the molecule.

4. Are the two peaks in the N-O nitro group always present in an IR spectrum?

Yes, the two peaks in the N-O nitro group are always present in an IR spectrum. However, their intensities may vary depending on factors such as the concentration of the compound and the sensitivity of the instrument being used.

5. Can the two peaks in the N-O nitro group overlap with other functional groups?

Yes, the two peaks in the N-O nitro group can overlap with other functional groups in an IR spectrum. This is why it is important to consider the entire spectrum and not just one specific peak when interpreting IR data.

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