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Thread: Infrared spectroscopy puzzle

  1. #1 Infrared spectroscopy puzzle 
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    When obtaining an IR spectrum, a secondary amine should exhibit only one N-H stretch, however, when a student ran his sample, two N-H stretches appeared in the spectrum. What could account for this?


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    Bullshit Intolerant PhDemon's Avatar
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    Off the top of my head all I can think of is it is an isotope effect where some of the molecules contain nitrogen 14 and some contain nitrogen 15...


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  4. #3 IR puzzel 
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    Quote Originally Posted by PhDemon View Post
    Off the top of my head all I can think of is it is an isotope effect where some of the molecules contain nitrogen 14 and some contain nitrogen 15...

    What a head! I was thinking "A shoulder band usually appears on the lower wavenumber side in primary and secondary liquid amines arising from the overtone of the NH bending band: this can confuse interpretation."
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    How would the isotope affect work?
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  6. #5  
    Bullshit Intolerant PhDemon's Avatar
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    "A shoulder band usually appears on the lower wavenumber side in primary and secondary liquid amines arising from the overtone of the N–H bending band: this can confuse interpretation."
    This could be the answer (it is probably more likely than mine, I skipped the simple answer and went straight to complicated )
    As to the isotope effect if you think about the bond as a spring, changing the mass on the spring changes the frequency it vibrates at leading to a different wavenumber.

    You might find this page useful: ISOTOPE effect in IR Spectroscopy | PharmaTutor
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