Infrared Spectroscopy
High-Yield Summary
- Spectroscopy measures absorption of electromagnetic radiation in discrete amounts matching the energy gap between two quantized states. IR spectroscopy specifically probes vibrational transitions (bond stretching/bending).
- IR radiation is low-energy — not enough to excite electrons or ionize, but exactly matched to vibrational energy gaps. Each bond type absorbs a characteristic frequency because ΔE depends on bond strength and atomic mass.
- IR spectra plot % transmittance vs. wavenumber (cm⁻¹, reciprocal of wavelength, proportional to energy) over the standard 4000–400 cm⁻¹ range; a dip (peak) marks an absorbed frequency.
- Three peak characteristics to read: wavenumber (position — bond strength/atomic mass), intensity (height — size of dipole moment change), shape (broad vs. sharp — hydrogen bonding broadens peaks, e.g. alcohol O–H).
- Fingerprint region (below ~1500 cm⁻¹) is molecule-specific and not an MCAT focus; diagnostic region (~4000–1500 cm⁻¹) is where functional groups produce identifiable peaks.
Characteristic Carbonyl + O–H Diagnosis
C=O stretch ~1700–1750 cm⁻¹ + broad O–H ~2800–3200 cm⁻¹ → carboxylic acid
- C=O stretch alone = ~1700–1750 cm⁻¹ — aldehyde, ketone, or carboxylic acid
- Broad O–H, 3100–3500 = Alcohol O–H stretch (hydrogen bonding broadens it)
- Broad O–H, 2800–3200 = Carboxylic acid O–H stretch — lower and broader than alcohol O–H
- Sharp N–H, 3100–3500 = Amine N–H stretch (sharp, unlike the broad O–H bands)
- Combining two peaks narrows the functional group faster than reading either alone: carbonyl + broad low-range O–H = carboxylic acid specifically.
Key Terms
- Wavenumber
- Reciprocal of wavelength (cm⁻¹); proportional to energy — higher wavenumber means higher-energy vibration.
- Fingerprint region
- Below ~1500 cm⁻¹; dense, molecule-specific pattern of peaks, not individually diagnostic, useful for matching known vs. unknown samples.
- Diagnostic region
- ~4000–1500 cm⁻¹; where functional groups produce strong, distinct, interpretable peaks.
- Dipole moment change
- The property that determines peak intensity — bonds like O–H or C=O with large dipole changes absorb strongly; C–H with small change absorbs weakly.
Peak Characteristic → What It Tells You
| Characteristic | Driven by |
|---|---|
| Wavenumber (position) | Bond strength + atomic masses — stronger bonds/lighter atoms vibrate higher, absorb at higher wavenumber |
| Intensity (height) | Size of dipole moment change during vibration — bigger change, stronger absorption |
| Shape (sharp vs. broad) | Hydrogen bonding broadens peaks, e.g. alcohol/carboxylic acid O–H |
Common MCAT Trap
- A carbonyl peak (~1700–1750 cm⁻¹) alone doesn't distinguish aldehyde, ketone, or carboxylic acid — you need the accompanying O–H (carboxylic acid) or aldehydic C–H (~2700–2900 cm⁻¹, aldehyde) to narrow it down.
- Don't confuse alcohol O–H (broad, 3100–3500 cm⁻¹) with carboxylic acid O–H (broad, but lower range, 2800–3200 cm⁻¹) — both are broad from hydrogen bonding, but the range differs.
- The fingerprint region isn't 'noise' to ignore entirely — it's just not analyzed peak-by-peak on the MCAT; its value is as a whole-pattern match, not individual peak ID.
Quick Recall
Why doesn't IR radiation excite electrons or cause ionization?
A spectrum shows a strong peak at 1710 cm⁻¹ and a broad peak at 3000 cm⁻¹. What functional group is likely present?
Why does the O–H stretch in alcohols appear broad rather than sharp?
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