AQA A-Level› Organic Chemistry› 3.3.15 NMR Spectroscopy› Flashcards
3.3.15

Flashcards

Interactive study deck for ¹H and ¹³C NMR spectroscopy, integration, and spin-spin splitting.

NMR Spectroscopy: read all 18 questions and answers
  1. Why is tetramethylsilane (TMS) used as a reference standard in NMR?
    It produces a single, intense peak at 0 ppm (all 12 protons/4 carbons are equivalent) which is far right (upfield), it is inert, non-toxic, and highly volatile.
  2. What is the molecular formula and structure of TMS?
    Si(CH₃)₄, a silicon atom bonded tetrahedral-style to four methyl groups.
  3. How many carbon environments (peaks in ¹³C NMR) are there in propan-2-ol?
    Two (the two methyl carbons are equivalent, and the central −CH− carbon is different).
  4. How many carbon environments are there in pentan-3-one?
    Three (symmetrical molecule: 2 equivalent CH₃, 2 equivalent CH₂, and 1 C=O carbonyl carbon).
  5. What does the number of signals in a ¹H NMR spectrum represent?
    The number of non-equivalent hydrogen (proton) environments in the molecule.
  6. What does the relative area under a peak (integration curve) in ¹H NMR represent?
    The relative ratio of the number of protons in that specific environment.
  7. State the \(n+1\) rule for spin-spin splitting in ¹H NMR.
    If a proton has \(n\) adjacent (neighbouring) non-equivalent protons on the carbon(s) next to it, its NMR signal will split into \(n+1\) peaks.
  8. A ¹H NMR peak is split into a quartet. What does this tell you about the neighbouring carbon?
    The neighbouring carbon has exactly 3 protons attached to it (e.g. a neighbouring −CH₃ group).
  9. Why are deuterated solvents like CDCl₃ used to dissolve samples for ¹H NMR?
    Deuterium (²H) does not absorb in the ¹H NMR radiofrequency range, preventing solvent signals from obscuring the sample spectrum.
  10. What is the relative intensity ratio of peaks in a triplet splitting pattern?
    1:2:1 (based on Pascal's triangle).
  11. What is the relative intensity ratio of peaks in a quartet splitting pattern?
    1:3:3:1 (based on Pascal's triangle).
  12. Explain how adding D₂O (deuterium oxide) to a sample can identify O-H or N-H protons in ¹H NMR.
    The labile O-H or N-H protons exchange with deuterium in D₂O. Since deuterium does not resonate in the standard ¹H NMR range, the peak disappears.
  13. Why do O-H and N-H protons not show spin-spin splitting in standard ¹H NMR?
    They undergo rapid chemical exchange with other molecules, which averages out the coupling and causes them to appear as broad singlets.
  14. In what chemical shift region would you expect to find the carbon atom of a carbonyl (C=O) group in a ¹³C NMR spectrum?
    Around 160-220 ppm (highly deshielded region).
  15. In what chemical shift region would you expect to find a proton attached directly to a benzene ring in a ¹H NMR spectrum?
    Around 6.0-9.0 ppm.
  16. What type of splitting pattern would you expect for the protons in a −CH₂− group that is adjacent to a −CH₃ group and no other protons?
    A quartet (\(n = 3\), so \(n+1 = 4\)).
  17. What type of splitting pattern would you expect for the protons in a −CH₃ group that is adjacent to a −CH₂− group and no other protons?
    A triplet (\(n = 2\), so \(n+1 = 3\)).
  18. How many peaks are in the ¹³C NMR spectrum of benzene?
    One (all six carbons are completely equivalent due to symmetry).
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