Vibrational Spectra: IR and Raman Spectroscopy: 22 flashcards
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1. What is a vibrational spectrum?
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The pattern of IR absorption or Raman scattering when a molecule moves between quantised vibrational levels.
2. Harmonic oscillator frequency formula
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$$\nu = \frac{1}{2\pi}\sqrt{\frac{k}{\mu}}$$
3. What does the force constant $k$ measure?
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Bond stiffness — stronger bonds have larger $k$ and higher vibrational frequencies.
4. What does reduced mass $\mu$ account for?
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The masses of the two vibrating atoms; heavier atoms give a larger $\mu$ and a lower frequency.
5. Why is a real bond anharmonic?
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The potential flattens on stretching and steepens on compression, so levels crowd together at higher energy.
6. What are overtones?
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Transitions to $v = 2, 3, \dots$ appearing at roughly integer multiples of the fundamental frequency.
7. What are combination bands?
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Bands from exciting two different vibrational modes simultaneously, at roughly the sum of their frequencies.
8. Vibrational mode count for a non-linear molecule
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$3N - 6$
9. Vibrational mode count for a linear molecule
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$3N - 5$
10. How many vibrational modes does water have?
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Three: symmetric stretch, asymmetric stretch, and bend.
11. Do stretches or bends appear at higher wavenumber?
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Stretches generally appear at higher wavenumbers than bends.
12. Name the four bending modes
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Scissoring, rocking, wagging, and twisting.
13. IR selection rule
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A vibration is IR-active only if it produces a change in dipole moment.
14. Raman selection rule
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A vibration is Raman-active only if it produces a change in polarizability.
15. Why is the symmetric stretch of $\text{CO}_2$ IR-inactive?
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It leaves the dipole moment unchanged, so it does not absorb IR — but it is Raman-active.
16. Which region shows O–H, N–H, and C–H stretches?
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Around $3000\ \text{cm}^{-1}$.
17. Which region is dominated by C=O and C=C stretches?
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$1600$–$1800\ \text{cm}^{-1}$.
18. What is the fingerprint region?
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The region below about $1500\ \text{cm}^{-1}$, useful for matching a spectrum to a known compound.
19. Why is Raman preferred for aqueous samples?
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Water interference is weak in Raman, whereas water absorbs strongly in IR.
20. Why do greenhouse gases absorb IR?
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Their vibrations change the molecular dipole, so they absorb outgoing infrared radiation.
21. What are coupled modes?
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Vibrations of two oscillators that share an atom and have similar frequencies, so they vibrate together.
22. Why is peak position alone insufficient for group identification?
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Intensity and width also matter, and small peaks may be overtones or combination bands.
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