Choosing Between Vibrational Methods: Infrared, Raman and Resonance Raman · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Choosing a method to watch vibrations

Science · Chemistry · ages 22-24
Name ______________________   Date ____________

  1. A motion changes polarizability but leaves the dipole fixed. Which method sees it?

    • Infrared absorption
    • Raman scattering
    • Neither method

  2. Why does an infrared absorption appear as a band of close lines?

    • Each vibration carries rotation companions along
    • The light source emits many colors at once
    • The molecule jumps two rungs at a time

  3. The symmetric stretch of carbon dioxide is strong in Raman and absent in infrared.

    Circle one:   True   False

  4. You must follow the O-H stretch of an alcohol dissolved in a dry, colourless solvent that does not absorb in the O-H region. Which technique fits best?

    • Infrared absorption
    • Resonance Raman
    • Plain Raman scattering

  5. When does Raman scattering grow enormously stronger?

    • When the sample is frozen before the run
    • When the spectrum is recorded twice as long
    • When the laser lands on an electronic absorption band

  6. A coloured protein holds a metal centre, and its colour comes from an electronic absorption at that centre. You want the metal-ligand vibrations, not the thousands of protein bands. Why does tuning the laser onto that colour band help?

    • It boosts every vibration in the sample equally
    • Only vibrations tied to the transition light up
    • It shifts every band to a clearer region

  7. Two analysts argue. One wants infrared for a polar stretch, the other resonance Raman for a colored impurity band. How do you settle it?

    • Match each band to the motion it changes
    • Pick infrared for both, since one instrument is simpler
    • Pick resonance Raman for both, since boosting always helps

  8. A student picks infrared to chase the symmetric stretch of carbon dioxide. What is the flaw, and what is the fix?

    • The band is too weak; record the infrared trace for longer
    • The sample is too pure; dilute it before measuring
    • The motion lacks a dipole change; switch to Raman
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Answer key

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

Choosing a method to watch vibrations W1-mt_YxLAvBIBAR-s1

  1. Raman scattering · Polarizability change is the Raman condition; infrared needs a dipole change.
  2. Each vibration carries rotation companions along · Rotation changes ride on the vibration, bunching lines into a band.
  3. True · The motion shifts polarizability without shifting charge to one side.
  4. Infrared absorption · The O-H stretch changes the dipole strongly, so it absorbs infrared well. Nothing in the dry solvent swamps it, and with no colour there is no resonance to use.
  5. When the laser lands on an electronic absorption band · Resonance with the electronic transition boosts tied vibrations up to a millionfold.
  6. Only vibrations tied to the transition light up · Resonance boosts only the vibrations tied to the metal-centred transition, so the protein's many other bands stay weak and the metal-ligand bands stand out.
  7. Match each band to the motion it changes · Each choice must name its motion and survive its rival.
  8. The motion lacks a dipole change; switch to Raman · No dipole change means infrared stays dark whatever the recording time.
Worksheet · LightMySky