NMR After Assignment: Relaxation, Distance and Exchange · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

What NMR says after assignment

Science · Chemistry · ages 22-23
Name ______________________   Date ____________

  1. What does a through-space enhancement measure?

    • The strength of the bond between them
    • How close they sit in space
    • The number of bonds between them

  2. What does a short longitudinal relaxation time report?

    • The nucleus is far from anything magnetic
    • It sheds energy fast to its surroundings
    • The nucleus has no neighbours at all

  3. A signal that broadens and then splits on cooling shows an exchanging process.

    Circle one:   True   False

  4. A nucleus near a paramagnetic centre relaxes very fast. Why?

    • Its unpaired electrons slow the tumbling
    • The magnet is too strong for that nucleus
    • Unpaired electrons speed energy loss

  5. Two assignments fit the shifts. Irradiating proton A enhances proton B. What follows?

    • The assignment putting A far from B
    • Neither assignment, the data is useless
    • The assignment putting A close to B

  6. Two peaks overlap hopelessly at your field. What is worth trying?

    • Another field strength
    • A longer run at the same field
    • Fewer scans at the same field

  7. No enhancement appears between two protons that shifts suggest are close. What should you suspect?

    • The protons are farther apart than assumed
    • The assignment is settled anyway
    • The shifts alone already prove they are close

  8. Two separate signals come from the two sites of an exchanging pair. On warming, both broaden. On cooling, both sharpen, and neither splits further. What is the best reading?

    • Slow exchange speeding up toward coalescence
    • Fast exchange that is speeding up even more
    • A sample that thickens and freezes as it cools
LightMySky · lightmysky.comW1-mt_dCV2EbNlF2-s1

Answer key

For grown-ups. Fold this page away before handing over the rest.

What NMR says after assignment W1-mt_dCV2EbNlF2-s1

  1. How close they sit in space · Cross-relaxation only reaches nuclei that sit close.
  2. It sheds energy fast to its surroundings · Fast energy loss means an efficient nearby pathway.
  3. True · Cooling slows the exchange until both partners resolve.
  4. Unpaired electrons speed energy loss · Short T1 reports an efficient surrounding, such as a paramagnetic neighbour.
  5. The assignment putting A close to B · Enhancement proves closeness, which only one candidate allows.
  6. Another field strength · Separation scales with field, so a new field can resolve them.
  7. The protons are farther apart than assumed · A missing enhancement challenges the closeness claim. The protons may be farther apart than the shifts suggested. For a medium-sized molecule the NOE can also pass through zero, so check at another field or with ROESY before concluding.
  8. Slow exchange speeding up toward coalescence · Temperature-dependent width tracks exchange near the measurement timescale.
Worksheet · LightMySky